ZOOV112 Course notes, 2024.pdf
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Course notes Animal patterns across space and time 2024 Department of Zoology @ Mandela 1 Prepared by Marietjie Landman, Graham Kerley, Anton McLachlan & Matthys P...
Course notes Animal patterns across space and time 2024 Department of Zoology @ Mandela 1 Prepared by Marietjie Landman, Graham Kerley, Anton McLachlan & Matthys Potgieter CONTENTS 1. Introduction 3 2. Global distribution of terrestrial fauna 8 3. Processes of animal distribution across continents 22 4. Other patterns of distribution of terrestrial fauna 34 5. Explaining patterns of species richness using Island Biogeography Theory 40 6. Applying Island Biogeography Theory to the design of nature reserves 45 7. Global distribution of marine fauna 50 8. Other patterns of distribution of marine fauna 58 9. South African marine zoogeography 62 10. Extinctions and the patterns of mammal distribution 70 11. African mammal zoogeography 78 12. South African mammal zoogeography 85 13. Contemporary climate change and faunal distributions 91 14. Contemporary climate change and faunal distributions in South Africa 96 Department of Zoology @ Mandela 2 1. INTRODUCTION Understanding the geographical distribution of animals is a central issue in ecology and at the cornerstone of many biodiversity management and conservation strategies. These patterns of distribution vary at a variety of spatial scales (from continents to habitats and microhabitats) and reflect a complex interaction between historical and current processes. Considering anthropogenic threats to biodiversity, it is likely that the distribution of many animal species will continue to change in the future. The study of the geographical distribution of animals – Zoogeography – integrates several disciplines, including geology, paleontology, phylogenetics and ecology, and seeks to answer two main questions: What are the main patterns of animal distribution? Why and how did these patterns form? We will expand these concepts by developing an understanding of how these patterns are likely to change in the future. There are two basic approaches to zoogeography: historical and ecological. Historical zoogeography is the more traditional form and incorporates the accumulation of faunal lists between regions, the comparison of these lists for similarities and the tracing of animal distributions into the past based on paleontological studies. Such lists have enabled historical zoogeographers to divide the world into zoogeographical regions, each with distinct fauna. Ecological zoogeography is the more modern approach. Here the emphasis is placed on different environments (or biomes) and in comparing faunas; i.e., the environments themselves are evaluated. Whereas historical zoogeography forms the basis for the following discussions on terrestrial faunas, a more ecological approach will be adopted for the marine zoogeography sections. In the case of the latter, factors such as temperature and salinity will be examined to understand faunal distributions. For analysis of terrestrial zoogeography, certain taxonomic groups must be selected, otherwise the body of information is too vast to handle. The following discussions will focus on vertebrates since they are well studied (see figure below). The taxonomic level will be the Family, since families are units that adequately show how faunas are distributed; at Order level the differences are too few, while at Genus level there are too many taxa to make sense. We will further focus on non-volent mammals, but birds and other vertebrates will also be mentioned. Department of Zoology @ Mandela 3 Traditional classification of mammals Phylum: Chordata Subphylum: Vertebrata Superclass: Gnathostomata Infraclass: Tetrapoda Class: Mammalia Subclass I: Prototheria (egg-laying mammals) Subclass II: Theria (live-bearing mammals) Infraclass I: Metatheria (marsupials – 7 Orders) Infraclass II: Eutheria (placentals – 20 Orders) Irrespective of whether a species’ distribution is considered on a continental, habitat, or microhabitat scale, it is surrounded by areas where the species cannot maintain a population. In comparing the faunas of different areas, two related phenomena influence the degree of similarity between the faunas of regions, including: barriers that separate faunas between regions, and dispersal that allows faunal exchange (resulting in similarity) between regions. On land, there are four main barriers to distribution, including: Climate (temperature, rainfall) Most animals are adapted to live within narrow temperature ranges; rainfall affects vegetation (habitat) (see figure below). Vegetation Many animals have adapted to specific vegetation types (habitats) depending on their specific ecological requirements, e.g., koala bears feed on Eucalyptus leaves and sugarbirds feed on Protea nectar. We will use the Biomes of the world to develop an understanding of some of the ecological drivers of the distribution of animals (see figure below). Other animals Animals compete with, prey on, and parasitize each other, which influence distribution. Physical barriers mountains, deserts, rivers, and seas often form impenetrable barriers. Department of Zoology @ Mandela 4 Global climatic regions Polar Zone 60ON Temperate Zone 30ON Tropical Zone Equator 0O Tropical Zone 30OS Temperate Zone 60OS Polar Zone Department of Zoology @ Mandela 4 In the sea, the main barriers to distribution include the following: Climate (temperature, salinity) Physical barriers Continents and oceanic ridges The geographical distribution of a species is the distributional range (frequently expressed in the form of a map) over which the species occurs. This distribution is discontinuous when the range of the species is fragmented. A discontinuous distribution can arise in two ways: Disjunct populations Formed when dispersal across a barrier occurs, resulting in two or more separate populations. Vicariant populations Formed when a previously continuous population is split through the development of a barrier (e.g., a river), resulting in two or more separate populations. Different species are faced with different opportunities or barriers to dispersal. For example, amphibians and freshwater fish are very limited in their distributions since they cannot leave the water or enter the sea. Most reptiles can swim and birds, being able to fly, can cross many barriers. Many mammals can swim, bats can fly, and rats are accomplished rafters (i.e., the process of crossing the sea between landmasses using patches of floating debris or vegetation). Department of Zoology @ Mandela 5 What are Biomes? Biomes are climatically determined communities of plants and animals that are distributed over a Major terrestrial biomes of the world wide area. For plant communities, a biome may be defined by factors such as plant structure (trees, shrubs, and grasses), leaf types (needle-like, broadleaved), and plant spacing (forest, woodland, savanna). © Creative Commons 18 Major biomes have been mapped across the world, including tundra, broadleaf forests, coniferous forests, taiga, © Ville Koistinen @ Wikipedia.org grasslands, savannas, shrublands, deserts, and mangroves. Department of Zoology @ Mandela 6 Geological timeline EON ERA PERIOD EPOCH MYA Holocene 0.01 Quaternary Pleistocene 2.4 Pliocene 5 Cenzoic Miocene 25 Tertiary Oligocene 36 Eocene 54 Phanerozoic Paleocene 65 Mesozoic Cretaceous 146 Jurassic 208 Triassic 245 Permian 290 Carboniferous 362 Paleozoic Devonian 408 Silurian 440 Ordovician 510 Cambrian 570 Proterozoic 2500 Department of Zoology @ Mandela 7 2. GLOBAL DISTRIBUTION OF TERRESTRIAL FAUNA The theory of biogeography grew out of the work of Alexander von Humboldt (1769–1859, founder of plant geography), Alfred Wallace (1823–1913), Charles Darwin (1809–1882), Philip Sclater (1829–1913) and many other biologists and explorers. Although both Darwin and Wallace were the first to suggest that animals were related to one another by evolution, Wallace was the greatest contributor to the fundamentals of zoogeography. Using ancestral relationships between bird species, Sclater (1857) was the first biologist to divide the world into Zoogeographic regions (see figure below). These regions are still recognized today and include: Nearctic region New world Neotropical region Palearctic region Ethiopian region Old world Oriental region Australian region Since the work of Sclater, the terrestrial zoogeographic regions of the world have been revised on several occasions. Using information on the ancestral relationships and contemporary distributions of the world’s amphibians, terrestrial birds, and non-marine mammals, Holt and colleagues (2013) currently recognize 11 broad zoogeographic regions in the terrestrial environment. If you are interested, you can follow Holt’s work here. Each region possesses similarities and distinctions in the animals that it supports with other regions. Animals that are distributed throughout these regions are referred to as being cosmopolitan, while those that have their distributions limited to a particular region are referred to as endemic. Organisms that occur naturally in a region are indigenous to the region, while those that have been introduced by man are exotic to that region. 2.1 Palearctic region This is the largest region and includes Europe, northern Africa, the central parts of the Arabian Peninsula, and Asia north of the Himalayas (see figure below). It is bounded by the sea in the north, east and west and by the Himalayas, Sahara and Arabian Deserts and Atlas Mountains in the south. It is in continuous land connection with the Oriental and Ethiopian regions. The climate is generally temperate, but the northern parts reach into the Arctic Circle where life is scarce. Only a few species (e.g., Arctic wolf Canis lupus arctos, wolverine Gulo gulo, Arctic fox Department of Zoology @ Mandela 8 Terrestrial zoogeographical regions of the world Department of Zoology @ Mandela 9 Vulpes lagopus, brown bear Ursus arctos, reindeer Rangifer tarandus, roe deer Capreolus capreolus, hares, lemmings, and voles) live here year-round and these have special adaptations (e.g., larger body sizes, shorter limbs, more fur and blubber, huddling behavior) to cope with extreme climatic conditions (air < sea temperatures, 24h winter darkness). South of the Arctic Circle is the tundra, snow covered in winter and bare during the short summer. The subsoil (25– 90 cm) is permafrost (permanently frozen) and only a few hardy animals survive there. Further south, the tundra is bounded by boreal coniferous forests (the taiga) that support many large herbivorous mammals. These forests are bounded by the steppes, grassy plains which are cold in winter and very hot in summer. Here many animals hibernate in winter. Further south the climate is more moderate and finally deserts occur in the extreme south. In inland parts, the climate is drier. Fauna of the Palearctic The animal life of the Palearctic is not as zoned as might be expected from the zoned distribution of the climate and vegetation. The endemic vertebrate fauna is not very rich (see table below), a consequence of its extended contact with other Old world regions preventing it from developing a distinct fauna. Many birds belong to cosmopolitan families. The region has one endemic bird family (Prunellidae – passerines) and 31 endemic genera. Most birds are shared with the Nearctic or Old world regions. Many of the birds migrate annually, patterns that developed during the Miocene glaciation (16 million years ago). Sub-regions British Isles – a continental island with a fauna very similar to that of the continent due to a long period of land connection. The fauna is, however, depauperate compared to the continent due to extinctions after isolation, many of which have been anthropogenic. Mediterranean – separated from the rest of Europe by the Pyrenees, Alps, and Balkans. The area suffered little glaciation during the Pleistocene ice ages (1.6 million years ago) and is one of the hotspots for plant and vertebrate endemism in the world. Mammals typical of this region include fallow deer (Dama), the elephant shrews of North Africa (Macroscelides), genets of Southwest Europe and Africa (Genetta) and the only true Old world monkey in the region (Macaca). Department of Zoology @ Mandela 10 Number of mammal families Total number of families (excl. bats) 28 Lagomorpha: Leporidae (rabbits) Cosmopolitan families 3 Rodentia: Muridae (mice) Carnivora: Canidae (dogs) Rodentia: Spalacidae (mole rats - only the Endemic families 2 Genus Spalax ) Carnivora: Ailuridae (red pandas) Absent Monotremes Number of shared families New world 1 Carnivora: Procyonidae (racoons & pandas) Rodentia: Talpidae (true moles) Castoridae (beavers) Nearctic 4 Zapodidae (jumping mice) Lagomorpha: Ochotonidae (pikas & mousehares) Neotropical 1 Artiodactyla: Camelidae (camels) Rodentia: Dipodidae (jerboas) Gliridae (dormice) Ethiopian 4 Hyracoidea: Procaviidae (dassies) Perissodactyla: Equidae (horses) Carnivora: Ursinidae (bears) Nearctic, Neotropical, Oriental 2 Artiodactyla: Cervidae (deer) Nearctic, Ethiopian, Oriental 1 Artiodactyla: Bovidae (goats, etc.) Insectivora: Erinaceidae (hedgehogs) Rodentia: Hystricidae (porcupines) Ethiopian, Oriental 5 Carnivora: Viverridae (civets) Hyaenidae: (hyenas) Artiodactyla: Suidae (pigs) Summary The mammal fauna of the Palearctic is not very rich and shows strong relationships with the Ethiopian, Oriental and Nearctic regions. Department of Zoology @ Mandela 11 2.2 Nearctic region The Nearctic region includes North America and Greenland (including the Aleutian Islands) as far south as the Mexican Plateau. Except for the Neotropical region, it is currently cut off from all other regions by sea. The vegetation and climate are very similar to the Palearctic region and range from tundra (in northern Canada) to deserts (e.g., Chihuahuan, Sonoran and Mojave) in the American west. Fauna of the Nearctic Numerous animal species evolved in the Palearctic region and later spread to the Nearctic. Hence, the Nearctic and Palearctic regions share many animal (and plant) species such that it is often considered a single region, the Holarctic. Examples of mammal taxa that spread in this way include the brown bears (known in North America as grizzly bears), red deer (known in North America as elk or wapiti), bison (Bison bison in North America and Bison bonasus in Europe) and reindeer (known in North America as caribou). Number of mammal families Total number of families (excl. bats) 24 Shrews, rabbits, squirrels, cricetid mice, murid mice, Cosmopolitan families 12 dogs, cats, mustelids, bears, deer, bovids, procyonids Rodentia: Geomyidae (gophers) Aplodontidae (sewellel) Heteromyidae (pocket mice, Endemic families 4 kangaroo rats) Artiodactyla: Antilocapridae (pronghorn antelope — monotypic family) Hedgehogs, Absent hyenas, pigs, tapirs Number of shared families Marsupialia:Didelphidae (opossum) Edentata:Dasypodidae (armadillos) Neotropical 4 Rodentia:Caviomorpha (tree porcupine) Artiodactyla: Tayassuidae (peccaries) Rodentia:Talpidae (true moles) Castoridae (beavers) Palearctic 4 Zapodidae (jumping mice) Lagomorpha: Ochotonidae (pikas & mousehares) Old World, Australian No shared families Department of Zoology @ Mandela 12 The Nearctic has only one bird family (Timaliinae – wrentits) that is endemic to the region. Reptiles and tailed amphibians (newts, salamanders) are abundant. Summary The Nearctic region is closely related to the Palearctic and its mammal fauna is a complex of Old World temperate and New World tropical forms. 2.3 Ethiopian region The Ethiopian region includes Africa, south of the Atlas Mountains and the Sahara Desert, and the southern corner of Arabia and Madagascar. The region is isolated on all sides by sea (except to the north) and is separated from the Palearctic by the Sahara and Arabian Deserts. The climate from north to south ranges from northern subtropical to equatorial to southern subtropical to southern temperate. Rainfall exerts a more important influence than temperature and is highest at the equator, decreasing eastwards and north and south. Thus, the central west is tropical rain forest, the south is mainly grassland, and the rest is veld with (savanna) or without trees and shrubs. There tends to be separate forest and plains faunas, with the former being the oldest. Fauna of the Ethiopian The Ethiopian has the most varied of all vertebrate faunas, mainly due to its long separation from other regions, variations in climate, and ecological diversity. A special feature of the Ethiopian fauna is the presence of a unique endemic group – Afrotheria – that evolved in Africa 100 million years ago at the time of the break-up of Gondwanaland. This group comprises a unique assemblage with diverse morphological features, including the golden moles, tenrecs (Tenrecidae), elephant-shrews, aardvarks, hyraxes, elephants and dungongs and manatees (Serinidae). Birds are numerous and have strong affinities with the Oriental region. There are six endemic bird families, including the ostriches, secretary birds, colys (muisvoëls), hamerkops, loeries (turacos) and helmet shrikes. There is a rich reptile fauna. Department of Zoology @ Mandela 13 Sub-regions Madagascar and other Indian Ocean islands – distinct fauna from the mainland and a high number of endemic taxa (e.g., lemurs). Number of mammal families Total number of families (excl. bats) 38 Shrews, rabbits, squirrels, cricetids, murids, dogs, Cosmopolitan families 14 mustelids, cats, bovids, hedgehogs, porcupines, civets, hyenas, pigs Insectivora: Potamogalidae (otter shrews) Chrysochloridae (golden moles) Macroscelidea: Macroscelididae (elephant Rodentia: Pedetidae (spring hares) Anomaluridae (flying squirrels) Bathyergidae (mole rats) Endemic families 12 Thryonomyidae (cane rats) Petromuridae (rock rats) Ctenodactylidae (gundis) Tubulidentata: Orycteropidae (aardvark) Artiodactyla: Hippopotamidae (hippos) Giraffidae (giraffe) Beavers, Absent bears and camels Number of shared families Rodentia: Dipodidae (jerboas) Gliridae (dormice) Palearctic 4 Hyracoidea: Procaviidae (dassies) Perissodactyla: Equidae (horses) Primates: Lorisidae (lorises) Cercopithecidae (old world Pongidae (apes) Pholidota: Manidae (pangolins) Oriental 8 Rodentia: Rhizomyidae (bamboo rats) Proboscidea: Elephantidae (elephants) Perissodactyla: Rhinocerotidae (rhinoceros) Artiodactyla: Toagulidae (chevrotains) New world No shared families Department of Zoology @ Mandela 14 Summary This region has the most varied vertebrate fauna and many endemic families. The birds and mammals are closely related to those of the Oriental and Palearctic regions. 2.4 Oriental region The Oriental region extends across most of south, southeast, and southern east Asia, including Pakistan, India, lowland southern China, Indonesia, Thailand, and including Java, Bali and Borneo and the Philippines. The region is bounded by the Himalayas in the north and the Indian and Pacific Oceans to the east and west. There is no specific boundary at the southeastern corner where the islands of the Malay Archipelago stretch out to Australia. The climate is mainly tropical and much of the area was originally covered with forests. Fauna of the Oriental The fauna of the Oriental region is very similar to that of the Ethiopian but has fewer species. For example, the Oriental region differs from the Ethiopian by having moles, bears, tapirs, and deer, but lacks jerboas, dassies and horses. Large mammals that are characteristic of the Oriental region include leopards, tigers, water buffalos, Asian elephant, Indian rhinoceros, Javan rhinoceros, Malayan tapir, orangutans, and gibbons. Families shared with the Ethiopian region are, however, different at genus level. For example: Elephants: Elephas (Oriental) versus Loxodonta (Ethiopian) Great apes: Hylobates and Pongo (Oriental) versus Pan and Gorilla (Ethiopian) The region supports a rich bird fauna, including endemic leafbirds and fairy bluebirds and many pheasants, pitas, Old world babblers and flowerpeckers. There is a rich reptile fauna. Department of Zoology @ Mandela 15 Number of mammal families Total number of families (excl. bats) 30 Shrews, moles, rabbits, squirrels, cricetids, murids, Cosmopolitan families 12 dogs, cats, mustelids, bears, deer, bovids Dermoptera: Cynocephalidae (colugo, flying lemurs) Primates: Tupaiidae (tree shrews) Endemic families 5 Tarsiidae (tarsiers) Hylobatidae (gibbons) Rodentia: Platacanthomyidae (spiny dormice) Number of shared families Neotropical 1 Perissodactyla: Tapiridae (tapirs) Old world 5 Hedgehogs, porcupines, civets, hyenas, pigs Lorises, old world monkeys, apes, pangolins, Ethiopian 8 bamboo rats, elephants, rhinos, chevrotains Summary A rich fauna with strong affinities with the Ethiopian region; both are Old World tropical regions. 2.5 Australian region The Australian region comprises Australia, Tasmania, New Guinea, and some smaller islands of the Malay Archipelago. New Zealand and the Pacific islands are generally not included. The region has no current land connection with any other region. The climate is variable. The north is tropical and includes rain forests, while most of the central area is desert or semi-desert. The south is temperate and dominated by grass steppes and forests in Tasmania. Fauna of the Australian The vertebrate fauna is notably poor, but unique. The region is dominated by marsupial mammals, some of which are shared with the Neotropical region. Recent introductions by man include rabbits, foxes, rats, mice, camels, donkeys, water buffalo, cervids, pigs, and dingo dogs. Department of Zoology @ Mandela 16 Number of mammal families Total number of families (excl. bats) 9 Cosmopolitan families 1 Murid mice Monotremata: Tachyglossidae (spiny anteater or echidna) Ornithorhynchidae (platypus) Marsupialia: Notoryctidae (pouched moles) Dasyuridae (marsupial mice, Endemic families 8 cats and anteaters) Peramelidae (bandicoots) Phascolomidae (wombats) Macropodidae (kangaroos and wallabies) Phalangeridae (phalangers) Marsupial bats, seals or Absent whales; replaced by placentals Most bird families have a wide range, but there are 10 endemics, including flightless cassowaries, emus and honeysuckers, lyre birds, birds of paradise and bowerbirds. The reptile fauna is poor. Wallacea – Between two Worlds When the zoogeographical regions were proposed, it was not known where to draw the line between the string of islands separating the Oriental and Australian regions. It was obvious that the large islands of Sumatra, Java and Borneo belonged to the Oriental region, while New Guinea belonged to the Australian region. However, the fate of the string of islands in between was uncertain. In the 1850’s, AR Wallace was working in this area on mammals, birds, and mollusks when he noticed a vast difference between the faunas of the two small islands of Bali and Lombok (which are only 30 km apart). He then visited numerous other islands to sample the fauna, and as a result drew up a line – Wallace’s Line (1863) – that divided the Oriental and Australian regions (see figure below). Some years later another line – Weber’s Line – was drawn separating the two regions based on mollusks and mammals. Department of Zoology @ Mandela 17 Sulawes The zoogeographical division between the Oriental and Australian regions, showing Wallace’s line and Weber’s line. The positions of these lines are as follows: Wallace’s Line separates the Asian continental shelf from the oceanic islands in the east, separating Bali from Lombok, Borneo from Sulawesi, and Palawan from the remainder of the Philippines (see 100 fm isobath). Weber’s Line marks the western limit of the Australian continental shelf. The islands in between do not strictly belong to either of the regions and were under the sea for most of the Cenozoic period (70 million years). Some zoogeographers call this area Wallacea, a transition zone between the placental mammals of Asia and the marsupials of Australia. Wallacea is one of the most intriguing areas of the world (hence, the “birthplace” of biogeography), is extremely rich in endemic species, and thus recognized as a hotspot for biodiversity. The phalangers (genus of possum) are the only marsupials that have dispersed into the Wallacea. Department of Zoology @ Mandela 18 Summary The Australian mammal fauna is poor but unique – it has the highest degree of endemicity of all the regions (89% of the mammal families are endemic). No strong relationships with other regions, but the birds have some ties with the Oriental. South America also has marsupials. 2.6 The Neotropical region The Neotropical region includes South and Central America, most of Mexico, and the West Indies, and is bordered on all sides by the sea (except Mexico). The Andes mountains are a major feature and extend from the north to the south of the region. The climate is mostly tropical, with only the southern parts and high latitudes extending into the temperate zone. Relative to the other zoogeographic regions, the Neotropical region comprises the greatest coverage of tropical rainforest. These forests extend from southern Mexico through Central America to southern Brazil and include the Amazon. The northern tropical forests give way to grassy plains and small semi-deserts in the south. Fauna of the Neotropical The fauna of the Neotropical region is varied and distinct, and distinct from that of the Nearctic. This can largely be ascribed to the long separation (until 15 million years ago) of the two American continents. Thus, decedents from the Nearctic (e.g., llamas and tapirs) that currently occur in the Neotropical are relatively recent. The only Neotropical mammals to survive in the Nearctic are the armadillos, opossums and porcupine. The Neotropical lacks the plains ungulates of the Nearctic and Ethiopian regions. The bird fauna is extremely diverse: South America has been called the bird continent. Half of the bird families and two orders are endemic, i.e., toucans, trumpeters, hoatzins, and macaws; hummingbirds are typical but not endemic. The reptiles are numerous and diverse. Department of Zoology @ Mandela 19 Number of mammal families Total number of families (excl. bats) 32 Shrews, rabbits, squirrels, murids, dogs, bears, Cosmopolitan families 10 mustelids, raccoons, cats, deer Paucituberculata: Caenolestidae (mouse-like Endemic families 16 opossums) Primates: Cebidae (New World monkeys) Callithricidae (Marmosets) *Edentata: Myrmecophagidae (S. Am. Anteaters) Bradypodidae (sloths) *armadillos are not endemic Rodentia: 11 Families (e.g., capybara, chinchilla, guinea pig, spiny rat) Also, 5 endemic bat families, including vampire bats. Absent No Number of shared families Oppossums, armadillos, tree porcupines, peccaries Nearctic 4 Artiodactyla: Camelidae (llamas), but these Palearctic 1 have a discontinuous distribution Perissodactyla: Tapiridae (tapirs), but these Oriental 1 have a discontinuous distribution Summary The Neotropical region is the richest in the world in terms of endemic families (16 families) and ranks second only to the Australian region in terms of the degree of endemicity (50%). Of the endemic families, it shares many with the Nearctic region and one with the Old world tropical regions. Department of Zoology @ Mandela 20 2.7 Antarctica, the forgotten zoogeographic region? There are large inconsistencies in our understanding of the zoogeography of Antarctica. During geological time, Antarctica did not always have a polar location, had a warm climate during the Cretaceous period (100 million years ago) and was covered with forests. During this period, both marsupials and placentals occupied the portion of the Antarctic Peninsula that stretched towards South America. However, because Antarctica is largely inaccessible and covered with ice, we have no fossil record of the mammals there. Recent genetic information suggests that marsupials had spread from South America, across the Antarctic continent to Australia, and that this group was later separated by the glaciation of Antarctica (as the poles shifted) (this also resulted in the mass extinction of fauna). Currently, Antarctica supports no indigenous terrestrial mammals or birds, but supports the highest diversity of seals (family Phocidae). The glaciation of Antarctica explains its depauperate fauna. The most successful Wanderers Some faunal families were extremely successful in dispersing across the world. For example: The soricids (shrews), sciurids (chipmunks, squirrels, marmots), cricetids (hamsters, lemmings, voles, field mice), leporids (hares, rabbits), cervids (deer), ursids (bears), canids (dogs), felids (cats), and mustelids (weasels, badgers, skunks) dispersed to all the zoogeographical regions, except the Australian. The bovids (i.e., cattle, sheep, plains game) dispersed everywhere, except the Neotropical and Australian regions. Department of Zoology @ Mandela 21 3. PROCESSES OF ANIMAL DISTRIBUTION ACROSS CONTINENTS Using Darwin’s theory of evolution, many early biogeographers assumed that species evolved in a particular area and dispersed from there to create the patterns of distribution that we observe today – the concept of dispersalism. Thus, when two related taxa were found on either side of a barrier, it is because they were able to cross that barrier after it had formed. Although some groups did follow this route (see figure below), this concept proved inadequate to explain all patterns of distribution, particularly the presence of marsupials (but not placentals) in Australia. Monotremes Platypus, Echidna Marsupials Opossum, Kangaroo Eurasia Modern insectivores, Bats, Pangolins, Carnivora, Perissodactyls, Artiodactyls, (Cetacea – marine) North America Rodents, Lagomorphs, Flying lemurs Tree shrews, Primates Placentals South America Sloths, Anteaters, Armadillos Africa Golden moles, Tenrec, Elephant shrews Aardvark, Sirenia, Hyrax, Elephants The relationship between mammalian evolution and biogeography. (modified from Cox & Moore 2010) Department of Zoology @ Mandela 22 Hence, many others believed that the continents, particularly those of the southern hemisphere (i.e., Australia, South America, and Africa), were once connected by narrow land bridges, which allowed faunal exchanges between the continents, but later subsided beneath the present Atlantic and Indian Oceans. Examples of such land bridges included: Lemuria Linking Africa, Madagascar, and India, and explaining the distribution of lemurs in Madagascar and their close relatives, the Lorises, in Africa and India. South Atlantis Linking South America and Africa, and explaining the shared distribution of porcupines, lungfish, and monkeys. A large bridge from South America, through Antarctica to Australia, and explaining the shared distribution of flightless birds, marsupials and side-necked turtles. These hypothetical Cenozoic land bridges have been discredited as no geological evidence has been found to support them. Alternative better supported hypotheses are currently accepted. It is now thought that the continents themselves have changed their positions over time – the theory of continental drift (or Plate tectonics) – and that this together with the climatic changes of the Cenozoic period determined the faunal distributions that we observe today. 3.1 Continental drift (or Plate tectonics) The theory of continental drift is attributed to Alfred Wegener who, at the beginning of the 1900s, did more than anyone else to establish the idea. However, his theory was only formally accepted during the early 1960s when the mechanisms by which continents split and moved apart became clearer. Wegener suggested that all the continents had once been part of a single supercontinent, Pangaea. Approximately 160 million years ago this supercontinent divided when Gondwanaland (comprising Antarctica, South America, Africa, Australia and India) separated from Laurasia (comprising North America and Eurasia) (see figure below). The Tethys Sea was the only ocean that existed between Gondwanaland and Eurasia before the opening of the Indian Ocean. Over the last 400 million years the histories of the northern and southern hemisphere continents have been substantially different, influencing the faunal communities on these continents. For example, the southern continents followed a history of continual fragmentation as Gondwanaland was progressively broken up into several tectonic plates, each with its own separate continent. Some of these continents moved northwards, exposing their faunal and floral communities to a range of latitudes and climates to which they had to adapt. The relatively recent collision of India with Asia (50 million years ago) allowed for the movement of biota into Asia, while the placentals were able to move into India (however, this collision also caused the formation of the Himalayan mountains and the raising of the Tibetan plateau, which would later pose a challenge to the Department of Zoology @ Mandela 23 The effects of continental drift on Laurasia and Gondwanaland. Department of Zoology @ Mandela 24 dispersal of some species). Similarly, the movement of Australia towards southeast Asia allowed for the limited interchange of biota in the area of Wallacea. In contrast to the complex history of the southern hemisphere, the mayor continents of the northern hemisphere (i.e., North America and Eurasia) were never far apart, such that the dispersal of biota between them occurred reasonably easily. However, the expansion and contraction of shallow continental seas (associated with the rise and fall of sea-levels) and the rise or erosion of mountains did influence the dispersal of fauna between these landmasses. Evidence for continental drift Several different lines of evidence led to the proposal of the theory of continental drift. One of the first noticed and most obvious indications that the continents had drifted apart was the fact that the coastlines of eastern South America and west Africa fitted together so well (see figure below). Various people tried different ways of fitting the continents together and the agreement was so good that it was hard to accept that it was pure coincidence. Another line of evidence tying in with the continental outlines was the similarity of geological formations of parts of the continents that fitted together; the same was found for fossils. Ancient location of the continents showing the near-perfect fit of South America and west Africa. Department of Zoology @ Mandela 25 Further, geological evidence has indicated that in the distant past the land masses had different climates to what they have today. For example, Africa was once covered with ice and northern Europe and America were deserts. South Africa was glaciated between 250 million and 350 million years ago. The most obvious explanation is that the continents had moved in relation to the poles. Another powerful line of evidence was geomagnetism. Rocks containing iron minerals become magnetized during their formation. Volcanic rocks become magnetic when the iron particles align themselves with the prevailing magnetic field of the earth at their time of formation. The cooling of the rock fixes the particles in that alignment and leaves a permanent magnetic ‘footprint’. Sedimentary rocks are usually made of particles that are already magnetic and align themselves with the prevailing magnetic field during sedimentation. Since the magnetic field of the earth periodically changes by a predictable amount, the age of the rocks may be determined. It is also possible to determine where the rock relates to the earth’s magnetic poles. If a plot is made of the position of the North Pole relative to North America and Europe for the past 400 million years, two quite separate ‘polar wandering curves’ are obtained (see figure below). This means that either there were two north magnetic poles or that the drifting of the continents is responsible. The same may be done for the South Pole. The North American and European polar wandering curves. Department of Zoology @ Mandela 26 The mechanisms of continental drift It has recently been discovered that along the centres of most oceans are large sub-marine ridges that are the sites of volcanic activity. The continents themselves (or crust – sial) float on a deeper layer (mantle) of iron and magnesium silicates (sima), which appear to be solid but are in fact fluid. The central rift valleys in these ridges are the points where activity occurs, thrusting up new volcanic or igneous rocks from the mantle beneath. This forms new sea floor which gradually moves away on both sides. As it does so it becomes covered in sediments and subsides. The points furthest from the ridges can have sediments over 1 km thick. In this way volcanic activity along rift valleys forms new crust and pushes apart large parts of the earth’s surface called tectonic plates. The rocks formed at the rifts take on a magnetism similar to that of the earth at that time. Knowing the times when the earth’s magnetism changed, scientists can draw lines on either side of the ridge joining crust of the same age. This allows the calculation of the rate of spreading of the sea floor, which is generally in the range of a few centimeters per year. It is obvious that if crust is being pushed apart and sliding over the mantle in some places, it must be colliding at other places. Where a collision occurs one plate slides under the other and is reabsorbed into the mantle. It is along these zones (trenches) that earthquakes occur. The great heat and pressure drive off all volatiles and this gives rise to volcanism. The mechanism of continental drift. Department of Zoology @ Mandela 27 The processes involved in the spreading of the sea floor. Department of Zoology @ Mandela 28 What evidence is there for the role of continental drift in the distribution of fauna? Case study: Australian marsupials and New Zealand mammals Marsupial remains have been found all over Europe, North America, and Asia in rocks 100 million years old. Between 70 and 80 million years ago, these marsupial ancestors had a worldwide distribution. This indicates that they could move relatively easily between all the continents (thus, supporting the idea of these supercontinents). Shortly after this the continents started separating and diverse marsupial assemblages evolved on each continent. Placentals then evolved in Europe and Asia (60–70 million years ago) and spread into North America as these regions were still connected. These new placentals outcompeted the marsupials. Further, many were carnivorous and preyed on the marsupials. Marsupials were eliminated in the northern hemisphere and Africa. In South America, the marsupial fauna survived intact until 30–40 million years ago when it joined with North America. Placentals invading from the north caused the extinction of most of the South American marsupials. Australia, however, remained completely isolated throughout this period and the marsupial fauna survived intact. New Zealand separated from Australia before 100 million years ago (before the marsupials had evolved) and thus never acquired a mammalian fauna. It has, however, retained some oddities, e.g., flightless birds including the moa (now extinct) and the last survivor of the lizard group Rhynchocephalia sphenodon. New Zealand is thus an extremely ancient continental island. Department of Zoology @ Mandela 29 3.2 Climate The diversity of fauna that we observe on continents today is the result of a complex interaction between several factors, including the species’ origin, the fragmentation of continents, and the climatic changes of the Cenozoic. Much of the early climatic changes coincided with the fragmentation and movement of the continents as global land and ocean areas were reconfigured (influencing atmosphere-ocean circulation and heat and moisture transfer across the globe) and mountains were formed. Thus, the earth experienced cycles of extensive warming and cooling throughout its history, which either facilitated or restricted faunal movements. For example, the cold climates of the late Cenozoic (e.g., Pleistocene ice ages) resulted in extensive glaciation, which facilitated the movement of cold-adapted animals across high-latitude land-bridges. In contrast, these cooling events also caused the extinction of groups adapted to warmer climates. Similarly, changing climates would have changed sea-levels and caused the expansion and contraction of habitats, each influencing opportunities for movement and distribution. Below we discuss some of the major faunal exchanges between continents, facilitated by changing climatic regimes. Between Eurasia and North America Throughout geological time, Eurasia and North America were never isolated for any great length of time (therefore few endemic species) and animals were able to move between these continents with reasonable ease. Furthermore, both continents were located at similar latitudes, such that their faunas had similar adaptations, which facilitated their persistence in new regions. Consequently, the effect of continental drift on the fauna of the northern hemisphere was far less dramatic when compared to that of the south. During the Eocene, North America/Greenland and Eurasia were connected via a long ridge of land (exposed by lower sea-levels, but now submerged), which filtered the exchange of mammals between the continents. More recently, during the Pleistocene ice ages (2.4 million– 11 000 years ago), numerous animal species that evolved in Eurasia (but also some that moved from Africa to Eurasia) were able to disperse to North America. This movement was facilitated by the Bering land bridge, which joined present-day Alaska and eastern Siberia. Because the formation of the Bering land bridge was associated with cooler climates, animals adapted to warmer climates (e.g., the apes and giraffe of Africa) could not reach North America. At the end of the Pleistocene the restrictions imposed by the colder climates meant that only the larger and hardier animals (e.g., mammoths, bison, mountain sheep, mountain goat, musk ox, and humans were able to reach North America. Department of Zoology @ Mandela 30 Between North and South America In contrast to North America, the effects of continental drift dominated the history of the South American fauna. Thus, in addition to the initial effects of the fragmentation of Gondwanaland, the South American fauna were exposed to cycles of immigration, isolation, evolution, and extinction as the land bridge that joined North and South America – Isthmus of Panama – went through cycles of being submerged or exposed (in response to changing sea levels). Some cycles of isolation lasted c. 180 million years, causing the evolution of distinct faunal communities. The Panama Isthmus finally became a complete land connection during the Pleistocene (2–3 million years ago), thus precipitating the Great American Interchange (see Figure below). During the Interchange, c. 60% of both the North and South American faunas, mostly those adapted to open savanna habitats, dispersed between the continents. N-America S-America Movement of the mammal fauna during the Great American Interchange. (modified from Cox & Moore 2010) Although similar proportions of the two faunas emigrated, the North American emigrants were more successful than those from South America. The most successful immigrants to South America were the cricetid rodents (diversifying into 45 genera), canids, horses, camelids (evolving into llamas) and peccaries. The influx of fauna into South America also caused several extinctions. The most notable of these extinctions was the extinction of 13 genera of endemic ungulates, presumably unable to cope with the North American carnivores or with competition from immigrating North American ungulates. It has also been suggested that the North American mammals were generally at a competitive advantage because, at the time of immigration, they had already survived millions of years of competition with mammals across the entire northern hemisphere (i.e., including Eurasia). In contrast, South American mammals were protected on an isolated continent. Furthermore, the Department of Zoology @ Mandela 31 mammals of southern North America comprised many lineages adapted to the open savanna habitats that were spreading in the region due to the general cooling of the climate. At the same time, many South American mammals were still adapted to and occupying forested habitats that were in a process of fragmenting and changing to savanna habitats. Thus, the South American mammals were exposed to environmental changes that favored the North American groups and facilitated their success. Between Africa and Eurasia Several faunal exchanges occurred between Africa and Eurasia as the shallow seas that separated the continents dried-up during warmer periods. The changing climates further resulted in the expansion and contraction of habitats, which either extended or reduced the distributions of some groups across these continents. Recent unique climatic and geological events also caused the development of the highly diverse and unique fauna of the Mediterranean countries. During the late Miocene (5.5–6 million years ago), the straits of Gibraltar (connecting the North Atlantic Ocean with the Mediterranean Sea, the latter separating Spain in Europe from Morocco in Africa) closed as the African continental shelf subsided beneath the European shelf, thus isolating the Mediterranean Sea from the Atlantic Ocean. This, together with the generally dry climatic conditions at the time, caused the Mediterranean Sea to evaporate into a deep (3–5 km below the world ocean level) dry basin – Messinian salinity crisis. As a result, many African species (e.g., antelope, elephant, hippo) could migrate into the empty basin to reach the wetter cooler highlands of Spain. Around 5.5 million years ago, less dry conditions caused increased freshwater inflow into the basin and the straits of Gibraltar opened, thus filling the basin with water from the Atlantic Ocean (Zanclean flood). This prevented any further migration of animals between Africa and Europe, but also caused some of the migrating species (elephant, hippo) to become trapped on islands in the Mediterranean basin where they underwent island dwarfism. During the Pliocene (2.4–5 million years) and the Miocene (5–25 million years), North Africa, Arabia and India shared a moist climate and were covered with forest. Fossils of this period are related to the Oriental region since a continuous belt of forests connected the two regions. These forests have gradually withdrawn in drier times and now only remain in central west Africa and as belts along mountains and rivers. The forest fauna thus largely went extinct on the grasslands except for a few species that could adapt, e.g., baboon, man. The contraction of the forest habitats resulted in the expansion of savanna habitats in Africa and the radiation of the African plains ungulates. But what if there is no bridge? Rafting has played an important role in the colonization of many islands and some continents during periods of isolation. For example, New world monkeys, caviomorph rodents (i.e., guinea Department of Zoology @ Mandela 32 pigs, capybara) and at least two related genera of skink (i.e. Mabuya and Trachylepis) appear to have reached South America by rafting across the Atlantic Ocean from Africa at a time when the continents were much closer. Skinks from the same genera have also rafted from Africa to Madagascar, the Seychelles and Comoros. Department of Zoology @ Mandela 33 4. OTHER PATTERNS OF DISTRIBUTION OF TERRESTRIAL FAUNA 4.1 Latitudinal gradients As we have seen, animals (and their habitats) are not distributed uniformly across the surface of the earth mostly because of differences in patterns of species evolution, geological events, and the effect of changing climates. However, in addition to these broad-scale patterns of distribution, there also exist latitudinal gradients in species richness (i.e., number of species in an area). For many marine, terrestrial (e.g., butterflies, birds, and mammals), and freshwater ecosystems in both the northern and southern hemisphere, species richness are higher in the tropics (300 north and south of the equator) than an equivalent area at higher latitudes. These patterns of distribution are also repeated across altitudinal gradients (i.e., up and down mountains), where species richness decreases with increasing altitude. What causes latitudinal gradients in biodiversity? Although numerous hypotheses have been proposed that explain these latitudinal patterns, no consensus has yet been reached. Some of the better supported hypotheses are: Geographical area hypothesis – The increased species richness of the tropics can be explained by the greater area (water and land) covered by these regions, suggesting that the area can support more species. In addition to the greater surface area, temperatures in this region are also more uniform – above and below the tropics, mean annual temperatures decrease linearly with latitude. More space allows species to have larger ranges, facilitated by regular temperatures, and consequently larger population sizes. Thus, species with larger ranges also have lower extinction rates and higher rates of speciation (more geographic barriers). The combined effect of higher rates of speciation and lower rates of extinction causes high species richness. Habitat diversity hypothesis – Large geographic regions are likely to comprise greater topographic and climatic variability, and therefore a greater diversity of habitats. More habitats create more opportunities for speciation. However, in some highly diverse communities (e.g., birds in the Caribbean and Panama) some species may restrict their use of habitats in the presence of more species, such that the habitat diversity hypothesis does not always hold. Productivity hypothesis – This hypothesis suggests that the productivity of a system sets limits to the number of species it can support. Thus, increased solar energy and water at the tropics causes increased primary productivity (resources), which can be divided Department of Zoology @ Mandela 34 between more individuals. Since larger populations have lower extinction probabilities, extinction rates should be lower in regions with high productivity. The hypothesis is often critiqued by the fact that increased productivity does not always cause increased species richness (e.g., areas with nutrient enrichment). Climate harshness hypothesis – Latitudinal gradients in species richness may exist because fewer species are able to tolerate the physiologically extreme conditions (i.e., cold, dry) of high latitude regions. Historical perturbation hypothesis – There are more species in the tropics because the region is older and has been disturbed less frequently. At high latitudes there has been insufficient time for species to colonize or re-colonize due to extensive historical perturbations such as glaciation. Thus, species richness in the tropics is still increasing and has not yet reached equilibrium. Interspecific interactions hypothesis – This hypothesis suggests that species interactions such as competition, predation, mutualism, and parasitism are stronger in the tropics, which promote the specialization and coexistence of species, hence causing an increase in speciation. Because these species interactions are generally difficult to test and do not explicitly address the causes of difference in species richness between regions, this hypothesis has generally received little support. Several recent studies have failed to observe consistent changes in species interactions with latitude. Summary Many factors may contribute to the higher species diversity of the tropics. Most of the evidence, however, supports the hypothesis that geographical area plays the most important role in explaining the patterns of diversity across latitudes. More locally, patterns of diversity are often determined by factors such as habitat diversity, disturbance, and age of the community. However, there are many anomalies in these patterns that can only be explained by unique historical and geographic features. Department of Zoology @ Mandela 35 4.2 Island fauna Islands are particularly interesting places for biogeographers as they represent centers of rapid speciation (therefore many endemic species) and extinction (due to habitat transformation, effects of introduced species, and overexploitation). Charles Darwin (1859) classified islands as being either continental or oceanic based on his observation that distance from the mainland alone could not explain differences in fauna between islands. He argued that continental islands (e.g., British Isles, Japan, Greenland, Madagascar, New Zealand) were once part of a larger land mass (continent) and have since lost that connection. The fauna of these islands are generally similar to those of the mainland and the degree of difference depends on how long they have been separated. Example of a continental island – Madagascar: Madagascar has a particularly complex history of continental drift as it first separated from Africa during the Jurassic (160 million years ago) and later separated from India when it finally became an island (80 million years ago). This long period of isolation is thought to explain the high number of endemic plants (> 11 000 sp.) and vertebrates (> 1000 sp.) on the island. Madagascar’s mammalian fauna comprises 155 species, nearly all belonging to only four groups: Insectivores (tenrecs) Primates (lemurs) Carnivores (fossa Cryptoprocta ferox) Rodents Most mammals reached Madagascar from Africa by rafting across the Mozambique Channel, while rodents and chameleons reached the island from Asia (later invading Africa). The now extinct mammals of Madagascar include tree sloths and ground sloths of South America, koalas of Australia, gorillas, dwarf hippopotamus (3 sp.), the river hog and the herbivorous flightless elephant bird (Aepyornis sp. and Mullerornis sp. related to the ostrich). In contrast to continental islands, Oceanic islands (e.g., Galapagos, Azores, St. Helena, Tristan da Cunha) never had land connections and are of volcanic origin, emerging from the ocean floor fairly recently. The fauna of these islands are derived by dispersal from across the sea and is, therefore, generally poor. Example of an oceanic island – Galapagos Islands: The Galapagos Islands lies on the equator in the eastern Pacific Ocean (900 km from the coast of Ecuador) and consists of 15 small volcanic islands with arid coastal areas, mixed open country, and inland forests. The islands are geologically recent and have a great diversity of Department of Zoology @ Mandela 36 endemic species, first studied by Charles Darwin during the Voyage of the Beagle (1831). It appears that all the vertebrate fauna of the Galapagos colonized the islands from across the sea, hence the absence of amphibians and freshwater fish. The fauna of the Galapagos comprises more than 9000 species, including: Terrestrial mammals: a single species of bat and rice rats. Terrestrial birds: 26 species, including flightless cormorants, the Galapagos penguin, and the endemic Darwin’s finches Geospiza sp. These finches are descended from a flock of South American finches that arrived on the Islands 2–3 million years ago and evolved into 14 different species, each with a distinct beak. Differences in beak size have allowed the species to be more specialized, focusing on the seeds it is best adapted to. For example, slim beaked species feed on cacti, while broad beaked species feed on the ground. Giant tortoises and large iguanas are common. Irregular evolution on islands Irregular patterns of evolution may occur on isolated islands. For example, many large mammals become smaller – island or insular dwarfism – than their mainland relatives over evolutionary time, while smaller animal species become larger – island gigantism (the Island rule). Generally, rodents, birds and reptiles tend towards gigantism, while mammalian carnivores, lagomorphs (rabbits, hares), proboscideans (e.g., elephants) and artiodactyls (even-toed ungulates – e.g., deer, hippos) are more likely to become dwarfed. Island birds typically develop into large flightless forms, while insects become wingless to prevent being blown out to sea. Although there are several proposed explanations for the mechanisms that produce these irregular forms, it is most likely that small species grow larger in the absence of predators on islands, while large mammals become smaller in the absence of food resources. Thus, large mammalian predators are often absent from islands due to their large range requirements and the difficulties of dispersing; in their absence, birds and reptiles grow larger, thus filling the ecological niches of these large carnivores. In contrast, limited space and food limits large mammal body sizes on islands because smaller animals require fewer resources to survive and reproduce. Department of Zoology @ Mandela 37 Examples of island dwarfism and gigantism include the following: Dwarf elephants (extinct) Mediterranean islands (Cyprus, Malta, Crete, Elaphas sp. Sicily, Sardinia) Weighed c. 200 kg (2% of its 10 000 kg ancestor). When sea-levels were low, these islands were colonized and re- colonized, such that the elephants were taxonomically Island dwarfism different on each island. Relatives: continental straight-tusked elephant, Elaphas sp. Dwarf hippopotamus (extinct) Mediterranean islands, Madagascar (3 sp.), and e.g., Hippopotamus minor the British Isles Pygmy mammoth (extinct) Channel Islands (California, Sardinia) Mammuthus exilis Within 5000 years, these mammoths dwarfed from 6000 to 2000 kg, providing evidence that island species evolve faster than mainland species. Relatives: continental Columbian mammoth Mammuthus columbii Dodo (extinct) Endemic to Mauritius Raphus cucullatus 20 kg flightless, ground-nester Ancestors: pigeons and doves from Southeast Asia, Wallacea Moa (extinct) Endemic to New Zealand Dinornis sp. (11 spp.) 230 kg, 4 m tall flightless bird – dominant herbivores in New Zealand’s forests. Relatives: African ostrich, Australasian emu, cassowary, and South American rheas Island gigantism Ancestors: Northern Hemisphere ground-feeding birds Kakapo Nestor sp. Endemic to New Zealand 3 kg flightless, nocturnal parrot Ancestors: became isolated from the remaining parrot species when New Zealand broke away from Gondwanaland. Giant tortoises Galapagos Islands, Seychelles, and Mascarenes 300 kg, 1.3 m long Ancestors: tortoises from the Seychelles derive from Madagascar, while those from the Galapagos derive from Ecuador. Komodo dragon Varanus Indonesian islands of Komodo, Rinca, and Flores komodioensis 70 kg, 3 m long Relatives: members of the monitor lizard family, e.g., water and rock monitors (leguaans) Department of Zoology @ Mandela 38 In addition to the patterns of island dwarfism and gigantism, the phenomenon of deep-sea gigantism (or abyssal gigantism) has been recognized whereby deep-sea animals (mostly crustaceans and other invertebrates) show larger sizes than their shallow-water relatives. It has been hypothesized that this may reflect an adaptation to scarcer food resources at depths (thus in contrast to island gigantism), greater pressure or an advantage in the regulation of body temperature and a diminished need for constant activity (key features of animals with a low surface area to mass ratio). Examples of deep-sea gigantism include: Giant isopod Atlantic and Pacific Oceans at depths 170–2140 m Bathynomus giganteus 19-36 cm long, with shallow water species between 1–5 cm Colossal squid Circum-Antarctic Southern Ocean at depths of 2200 m Mesonychoteuthis hamiltoni 12–14 m long, with shallow water species average at 60 cm Department of Zoology @ Mandela 39 5. EXPLAINING PATTERNS OF SPECIES RICHNESS USING ISLAND BIOGEOGRAPHY THEORY To explain the factors that determine the patterns of species richness of natural communities, McArthur and Wilson published the Theory of Island Biogeography in 1967. Although they developed this theory using the example of islands (oceanic and continental), the theory could also be applied to any isolated patches (islands) of habitat on continents (e.g., mountains surrounded by desert, lakes surrounded by dry land or forests surrounded by grassland). The Theory of Island Biogeography developed from observations that larger islands generally had higher species diversity. To explain this phenomenon, various hypotheses were proposed: Hypothesis 1: Sampling-effect Number of species Bigger area = more species ‘sampled’ Area of the island Hypothesis 2: Habitat heterogeneity Bigger area, more chance of different habitats being present, hence more niches to be filled by more species. Department of Zoology @ Mandela 40 Hypothesis 3: Equilibrium Theory of Island Biogeography The number of species occurring on any island represents a balance (equilibrium) between the rate of extinction and the rate of immigration. Immigration rate is the number of new species that establish on an Rate of immigration island per unit time. Immigration rate decreases as the number of species increases because new arrivals will increasingly represent species already present on the island. There is an upper limit (p) to the number of immigrant species that can colonise the island, this being the total pool of species that occur Number of species p in the region. Extinction rate is the rate at which species die-out or go extinct from an island. Extinction rate increases as the number of species increases. This could happen for three reasons: Extinction 1. With more species there is a greater chance of a species going extinct simply because there are more species. 2. With an increase in the number of species the population sizes of each must Number of species decrease. 3. With an increase in the no. of spp., competitive interactions will increase. Department of Zoology @ Mandela 41 Hypothesis 3: Equilibrium Theory of Island Biogeography (continue) Extinction/Immigration rate Species immigration Species extinction The equilibrium number of species (s) on the island is the point where the rate of immigration and extinction cross. s Number of species What are the assumptions of the Equilibrium Theory of Island Biogeography? The Equilibrium Theory of Island Biogeography makes some important assumptions which can influence estimates of species richness: 1. The theory tells us nothing about the nature of the species present. All species are assumed equal. 2. The theory ignores anthropogenic extinctions or introductions. Species turnover occurs as species go extinct or new species establish. 3. The theory assumes constant environmental conditions. 4. The theory ignores the evolutionary history of the species. We, therefore, need to understand the effects of habitat availability and species interactions (competition, predation, facilitation) to resolve the actual community composition on an island. Department of Zoology @ Mandela 42 How does the equilibrium number of species vary between islands? Two factors influence the equilibrium number of species on an island: island size and isolation. As island size decreases, the rate of extinction increases. This is Immigration rate because smaller areas support smaller populations and small Small Large populations are more likely to go islands islands Rate extinct than large populations. Consider a set of islands where the rate of immigration is constant (for convenient discussion’s sake), but with island size varying and affecting extinction rates. We can graphically demonstrate that large islands will support more species No. resident species As isolation (or the distance between the island and mainland) Extinction rate increases, the rate of immigration decreases. This is because fewer species can successfully cover the greater distance. Near islands Rate Consider a set of islands where the rate of extinction is constant, but with varying distance Far to the mainland. We can islands graphically demonstrate that more isolated islands will support fewer species at equilibrium. equilibrium. No. resident species Department of Zoology @ Mandela 43 Evolution of the Theory of Island Biogeography Since McArthur and Wilson first proposed the Theory of Island Biogeography nearly 60 years ago, many new discoveries have been made, which influence our understanding of the theory. These include: Species richness is not in equilibrium on many islands. Species vary in their rates of speciation, colonization, and extinction, which influence diversity on islands. High rates of immigration to near islands reduce the rates of extinction on those islands. Thus, contrary to McArthur and Wilsons’ model, island isolation can also influence the rate of extinction. Finally, it has now been shown that island size influences the rate of immigration, such that larger islands have a greater rate of immigration. Notwithstanding these constraints, the Equilibrium Theory of Island Biogeography is still a powerful conceptual tool to explore the patterns of species richness and dynamics on islands. It furthermore has application in the field of conservation (see later). Department of Zoology @ Mandela 44 6. APPLYING ISLAND BIOGEOGRAPHY THEORY TO THE DESIGN OF CONSERVATION AREAS The parallels between islands and conservation areas (or nature reserves) are obvious when you consider that nature reserves represent islands of protected habitat surrounded by a sea of transformed habitat. Despite this obvious similarity, many researchers disagree about the extent to which conservation areas follow the patterns of islands, and thus the extent to which Island Biogeography Theory can be applied in the design of nature reserves. This largely reflects the fact that the theory provides general rules for species occurrences, ignoring any species interactions, while nature reserve design is concerned with entire ecosystems, including species interactions. Nevertheless, Island Biogeography Theory provides key insights into conservation biology (i.e., the science of conservation management to protect biodiversity) that may be used to guide nature reserve design. At least two of the predictions of Island Biogeography Theory appear to be particularly useful: 1. The dynamic nature of the Equilibrium Theory of Island Biogeography suggests that there will always be a probability of extinction of species in conservation areas. Thus, it would be critically important to ensure that a species is represented in more than one reserve to allow for reintroductions in the event of extinctions. 2. Species richness on islands increases with island size. According to this prediction, large reserves are more likely to support more species of plants and animals. This is usually the consequence of lowered extinction rates in large areas (due to larger population sizes) and the probability that larger areas comprise a greater diversity of habitats. The relationship between species richness and area suggests that at least 50% of species will be extinct when 90% of the habitat is lost. It has been shown that the extinction of particularly larger body-sized species (e.g., carnivores, artiodactyls) is related to reserve area. In fact, area is the single best predictor of the diversity of large mammals in conservation areas, being more important than other variables such as latitude or the diversity of plants. The role of habitat diversity, however, may override the effects of area alone. How large should reserves be? This issue is particularly complex and may be influenced by many factors that operate at a variety of scales. In its simplest form, reserve size may depend on the space requirements of animals, influenced by their ecological specialization. For example, large mammals and birds have larger home ranges than amphibians and butterflies. Larger population sizes (and hence larger reserves) may also be required to maintain sufficient genetic variability to avoid inbreeding Department of Zoology @ Mandela 45 and ensure the persistence of the population in the presence of environmental change. Where large population sizes are maintained in relatively small areas (e.g., large herbivores), effects on other components of biodiversity may intensify. Thus, it may be tempting to assume that larger conservation areas are better than smaller ones. However, this may not always be the case and has led to the SLOSS-debate, i.e., Single Large Or Several Small reserves? There exist several advantages to having larger reserves, including: Large reserves are likely to support larger populations of most species, causing them to be less vulnerable to extinction. Local ecological variations are unlikely to affect all areas occupied by a species within a large reserve. Larger reserves are likely to have smaller edge effects (the transitional habitats on the periphery of reserves). Although these transitional habitats may be occupied by species that are absent from core habitats, thus increasing overall species diversity, this may be at the expense of the more important core species that could be less successful in transitional habitats. Transitional habitats reduce effective reserve size and tend to cover a larger proportion of smaller reserves. In contrast, species in larger reserves may be vulnerable to the effects of fire, disease or the presence of a particularly competent predator or competitor that may exterminate all individuals. Such factors are unlikely to affect all individuals scattered among many small reserves. Several smaller reserves are also likely to comprise a greater diversity of habitats, thus supporting a greater diversity of species. However, small reserves suffer the disadvantage of being isolated from one another or from neighboring large reserves (hence, limited gene-flow and increased chance for inbreeding). Close linkages between reserves will allow for the immigration of species between reserves. This can either be achieved by establishing corridors or by actively moving animals to avoid local extinctions. The SLOSS-debate is largely academic and has now been overtaken by the principle of systematic conservation planning. This approach focuses not just on the size of reserves (which is important for the populations within them) but also the location of reserves (which is important for the representation of biodiversity) and their replication (which is important for the management of extinction). How much does size really matter? Although reserve size is a key driver in species conservation, the issue of species persistence is far more complex. Thus, having a clear understanding of species-specific requirements may be more important than size alone. For example, if suitable habitat is not available, the species will not survive. Furthermore, it may be important to estimate the size of the minimum population size that is likely to ensure the long-term survival of the species. Department of Zoology @ Mandela 46 For some communities, the presence of a keystone species may be crucial to the survival of other species in the community. In fact, the disappearance of such a keystone species would cause a fundamental change in the nature of the community. Conservation areas with keystone species should therefore be large enough to also support minimum viable populations of these species. Reserve design in the presence of global climate change As we have seen, climate is one of the primary determinants of the natural distribution of species. Thus, it follows logically that changing climates should have a profound influence on species’ range expansion and contraction as the limits of suitable habitat expand and contract. This also suggests that contemporary climate change will have a significant impact on species distributions (Section 13), which should be recognized and incorporated in reserve design. Important questions that emerge in this regard include: How will the climate shift in the future? Where will species move? Will a species habitat (plants for herbivores and prey for predators) move with it? Which species will benefit from a change in climate? What are the potential barriers to species range shifts? A recently published study provides some insights into these questions, indicating that nearly 80% of the 2000 species for which there were data shifted their ranges in the direction predicted by global climate change. These range shifts averaged: c. 17 km per decade towards the poles, which is the same as c. 150 km towards the poles for every degree Celsius change in temperature. Many species are colonizing regions that had previously been cooler. c. 11 km per decade towards higher altitudes, which is the same as c. 100 m towards higher latitudes with every degree Celsius change in temperature. This means that areas or habitats included in conservation areas today may not necessarily be suitable for similar species in the future (see figures below). This questions current reserve placement and indicates that larger reserves and corridors may play an increasingly important role in future conservation strategies. Department of Zoology @ Mandela 47 Untransformed habitat Nature Reserve Habitat transformation due to human activity Nature Reserve Nature Reserve Potential consequences of shifting species’ range limits due to climate change on the effectiveness of established nature reserves Department of Zoology @ Mandela 48 Predicted shift in species (A, B, C & D) distributions in relation to altitude due to climate change. Note: Altitudinal shift is not available to Species A. These species will, therefore, lose their habitat and risk extinction. Species at lower altitudes (B, C & D) will suffer from the fragmentation of habitat and reduced habitat area. The effects of both factors will be an increase in the risk of extinction. Department of Zoology @ Mandela 49 7. GLOBAL DISTRIBUTION OF MARINE FAUNA Marine zoogeographic provinces, each with their own distinct faunal communities, were first recognised over 150 years ago by Edward Forbes. Since Forbes’ work, these provinces have been characterised and mapped. As more information becomes available, for example on the phylogenetic relationships between faunal species, these maps are reevaluated and realigned. Very few species are distributed throughout the world’s oceans, and most have much smaller ranges. The boundaries between faunal regions are often associated with continents (or vast open oceans) and strong ecological gradients. The most obvious barriers to the dispersal of marine fauna are geographic, including for example: Africa-Eurasia barrier Approximately 30 million years ago Africa touched Eurasia, closing the Tethys Sea, and with this the constant flow of warm water around the globe. T
