10b.docx

Characteristic animals of the Arctic tundra are lemmings, caribou, musk-oxen, arctic foxes, arctic hares, ptarmigans, and (during summer) many migratory birds. Deserts are arid regions where rainfall is low (less than 25cm annually), and water evaporation is high. Deserts occur in two forms, the hot...

Characteristic animals of the Arctic tundra are lemmings, caribou, musk-oxen, arctic foxes, arctic hares, ptarmigans, and (during summer) many migratory birds. Deserts are arid regions where rainfall is low (less than 25cm annually), and water evaporation is high. Deserts occur in two forms, the hot deserts of the Sahara, and the cool Patagonian desert. Desert plants such as thorny shrubs and cacti have reduced foliage, drought-resistant seeds, and other adaptations for conserving water. Many large desert animals have developed remarkable anatomical and physiological adaptations for keeping cool and conserving water. Aquatic biomes Of all the water in the world, a mere 2.5% is freshwater. Most freshwater is contained in polar ice caps, or stored underground in aquifers and soil moisture, leaving only 0.01% of the world\'s inland waters available for aquatic life. Yet a quarter of the world\'s vertebrates and nearly half of fishes live in these fragile 'islands' of water. Inland waters are divided broadly into running-water, or lotic habitats, and standing-water, or lentic habitats. Lotic habitats follow a gradient from mountain streams to wide slow rivers. Brooks and streams with a high-velocity water flow are high in dissolved oxygen; organic input is mainly from organic detritus washed from terrestrial areas. More slowly moving rivers have less dissolved oxygen and more floating algae and plants; therefore, fauna are tolerant of lower oxygen concentration. Lentic habitats, such as ponds and lakes, have still lower concentrations of oxygen, particularly in deeper areas. Animals living on the bottom or on submerged vegetation (benthos) include snails, mussels, crustaceans, and a wide variety of insects. Depending on the nutrients available, a large contingent of small floating or weakly swimming plants and animals (plankton) can occur. Oceans represent by far the largest portion of the earth\'s biosphere, covering 71% of the earth\'s surface to an average depth of 3km-75km, the deepest areas being 11.5km below sea level. The marine world is relatively uniform compared with land, and in many respects it is less demanding on life forms. Oceans are the cradle of life, which is reflected by the variety of organisms, more than 200,000 species of unicellular forms, plants, and animals. The vast majority of these forms, about 98%, live on the seabed (benthic), and 2% live freely in open oceans (pelagic). Of the benthic forms, most occur in the intertidal or shallow areas of oceans. Less than 1% lives below 2000m. The most productive areas are along continental margins, and a few areas where waters are enriched by organic nutrients and debris is lifted by up welling currents into the sunlit, or photic, zone, where photosynthesis can occur. With few exceptions, all life below the photic zone is supported by the light 'rain' of organic particles from above. Life in the ocean is divided into regions, each with its distinctive life forms. The intertidal zone is the harshest, richest of marine environments. It is subjected to waves, sun, wind, rain, extreme temperature fluctuations, erosion, and sedimentation. Yet because of the diversity of available habitats and nutrients, animals such as barnacles, snails, limpets, mussels, sea urchins, and many others flourish. Below the intertidal zone is the subtidal zone (always submerged). It also supports a rich variety of animal life, as well as forests of brown algae. An estuary is a semi-enclosed transition zone where freshwater flows into the sea. Despite unstable salinity, an estuary is a nutrient-rich habitat supporting diverse fauna. The neritic, (shallow water) zone surrounds the continents and extends to the edge of the continental shelf (approx depth of 200m). This zone is more productive than open ocean, as it benefits from nutrients delivered by rivers, and by up welling at the edge of the continental shelf. Algal growth is prolific, which supports diverse animal life, including most of the world\'s fisheries. The vast open ocean (pelagic habitat) is relatively impoverished biologically, despite its size, (comprising 90% of the total oceanic area) due to the loss of organic material as it floats down out of the photic zone, into the bathypelagic zone. Areas of upwelling and convergence of ocean currents are vital sources of nutrient renewal for the surface photic zone. The enormously productive polar seas are an example. Below the surface, (epipelagic layers) of the pelagic area are the great ocean depths, characterised by enormous pressure, perpetual darkness, and a constant temperature near 0° C. The mesopelagic receives dim light and supports a varied community of animals. Below this area, perpetual darkness exists and deep-sea forms depend on that meagre portion of organic debris from above that escapes consumption by organisms in the water column. On the sea floor exists the benthos, represented by sea anemones, sea urchins, crustaceans, polychaete worms, and fishes. Most are deposit feeders characterised by very slow growth (because of scarcity of food) and long lives. In 2007, self-contained benthic communities of animals that are completely independent of solar energy and the rain of organic debris from above were discovered adjacent to vents of hot water issuing from rifts in the ocean floor. Researchers continue to look for new species. Animal distribution The study of zoogeography tries to explain patterns of animal distribution and dispersal. It is not always easy to explain why animals are distributed as they are, since similar habitats on separate continents may support quite different faunas. A particular species may be absent from a region supporting similar animals, because of barriers preventing mobility, or established populations of other animals prevent colonisation. The fossil record shows that animals once flourished in regions where they are now absent. For example, camels originated in North America, spreading during the Pleistocene epoch via Alaska to Eurasia and Africa, where they are represented today by true camels, and to South America, where descendants include llamas and alpacas, but they became extinct in North America about 10,000 years ago. The Earth\'s surface is undergoing constant changes, many areas that are now land were once seas, fertile plains turn to desert; impassable mountain barriers arise from plains, and ice fields give way to forests as climates warm. Thus, geological change is a powerful influence in shaping animal and plant distribution, responsible for much of the alteration in animal (and plant) distribution. A major problem for zoogeographers is to explain the numerous instances of discontinuous distributions; closely related species living in widely separated areas of a continent, or even the world. There are two possible ways for a discontinuous distribution to arise. Either a population moves from its place of origin to a new location (dispersal), across unsuitable territory, or the environment changes, breaking a once continuously-distributed species into geographically separated populations (vicariance). Vicariance can involve climatic changes that fragment areas of favourable habitat, or geological changes that move different populations of a species away from each other. Perhaps the most dramatic discontinuous distributing phenomenon in the Earth\'s history is continental drift, through which a once continuous landmass was sequentially broken into continents and islands were separated by ocean. All terrestrial and freshwater animal species that had spread across the initially continuous landmass became sequentially fragmented into many populations on different continents and islands separated by ocean. Dispersal involves emigration from one region and immigration into another. Dispersal is a one way, outward movement distinguished from periodic movement such as seasonal migrations. Dispersing animals may move actively under their own power, or they may be passively dispersed by wind, by float ing/rafting on water, or by hitching rides on other animals. Animals will expand their geographic distributions in this manner across all favourable habitats accessible to them. For example, as the last ice age ended, habitats favourable for temperate species became available on formerly glaciated territory so species that originated immediately south of the ice, prior to glacial retreat, expanded northward as new habitats appeared. Dispersal easily explains the movement of animal populations into favourable habitats that are geographically adjacent to their places of origin. This movement produces an expanded but geo graphically continuous distribution. In many groups of organisms, it is likely that both vicariance and dispersal have contributed to the evolution of discontinuous distributional patterns. According to the theory of plate tectonics (tectonics means 'deforming movement'\'), the Earth\'s surface is composed of six to ten rocky plates, about 100km thick, that shift position on a more malleable underlying layer. Earth\'s continents had been drifting like rafts following the break-up of a single great land-mass called Pangaea, approximately 200 million years ago. Two great super continents were formed: a northern Laurasia and a southern Gondwana. At the end of the Jurassic period (135 million years ago), the super continents began to fragment and drift apart. Laurasia split into North America, most of Eurasia, and Greenland. Gondwana split into South America, Africa, Madagascar, Arabia, India, Australia, New Guinea, Antarctica and numerous smaller fragments that now form Southeast Asia. The Arabian, Indian, and Southeast Asian fragments gradually moved across the ocean and eventually joined to Laurasia. This theory is supported by the appearance of a fit between the continents, by airborne paleomagnetic surveys, by seismographic studies, by the presence of mid-ocean ridges where the tectonic plates arise, and by a wealth of biological data. Continental drift explains several otherwise puzzling distributions of animals, such as the similarity of invertebrate fossils in Africa and South America, as well as certain similarities in present-day faunas at the same latitudes on the two continents. However, the continents have been separated for all of the Cenozoic era and probably for much of the Mesozoic era as well, much too long to explain the distributions of some modern organisms such as placental mammals. Continental drift theory is, nevertheless, enormously useful in explaining interconnections between flora and fauna of the past. The present distribution of marsupial mammals is an excellent example of the influence of continental break-up. Marsupials appeared in the middle Cretaceous period, about 100 million years ago, probably in South America. Because South America was at that time connected to Australia through Antarctica (then much warmer than it is today), marsupials spread throughout all three continents. They also moved into North America, but there they encountered placental mammals, which had dispersed from Asia. Marsupials evidently could not coexist with placentals and became extinct in North America. Placentals followed marsupials into South America, causing extinction. In the meantime, Australia drifted apart from Antarctica, barring entrance to placentals. Australia remained in isolation, allowing marsupials to diversify into the present varied fauna. Temporary land bridges have also been an important pathway, of dispersal. An important and well established land bridge, that no longer exists, connected Asia and North America across the Bering Strait. It was across this corridor that the placentals moved from Asia into North America. Today, a land bridge connects North and South America at the Isthmus of Panama. But from the mid Eocene epoch to the end of the Pliocene epoch, (three million years) the two continents were completely separated by water. During this long period, the major groups of mammals evolved in distinctive directions on each continent. When the lane bridge was re-established at the end of the Pliocene, mammals began to flow in both directions. For a period, both continents gained in mammalian diversity, but the extinction of large numbers of mammals on both continents soon followed. North American carnivores (raccoons, weasels, foxes, dogs, cats, and bears) began preying on South American mammals, which previously had evolved in an environment, free of carnivores. Other North American invaders included hoofed mammals (horses, tapirs, peccaries, llamas, deer, antelopes, and mastodons, rabbits, and several families of rodents). These replaced many South American residents in similar habitats. Conversely, only a few South American 'invaders' flourished in North America (porcupines, armadillos and opossums). Ecology Ecology is the study of a hierarchy of biological systems in interaction with their environments. At the base of the ecological hierarchy is an organism. To understand why animals are distributed as they are, ecologists must examine the varied physiological and behavioural mechanisms that animals use to survive, grow, and reproduce. Animals in nature coexist with others of the same species; these groups are known as populations. Populations have properties that cannot be discovered by studying individual animals alone. These properties include genetic variability among individuals (polymorphism), growth in numbers over time, and factors that limit the density of individuals in a given area. Ecological studies at population level help us to predict the future success of endangered species, and to discover controls for pest species. Just as individuals do not exist alone in nature, populations of different species coexist in more complex associations known as communities. The complexity of a community is measured by species diversity, the number of different species that coexist in a community. The populations of species in a community interact with each other in many ways, the most common being predation, parasitism, and competition. Predators obtain energy and nutrients by killing and eating prey. Parasites derive similar benefits from their hosts, but usually do not kill the hosts. Competition occurs when food or space is in limited supply and members of the same or different species interfere with each other\'s use of shared resources. Communities are complex, because all of these interactions occur simultaneously, and individual effects on the community often cannot be isolated. Ecological communities are biological components of even larger, more complex entities called ecosystems. An ecosystem consists of all populations in a community together with their physical environments. The study of ecosystems helps us to understand two key processes in nature, the flow of energy and the cycling of materials through biological channels. Environment and niche An animal\'s environment is composed of all the conditions that directly affect its survival and reproduction. These factors include space, forms of energy such as sunlight, heat, wind and water currents, and materials such as soil, air, water, and chemicals. The environment also includes other organisms, which can be an animal\'s food, its predators, competitors, hosts, or parasites. The environment thus includes both abiotic (non-living) and biotic (living) factors. Some environmental fac tors, such as space and food, are utilised directly by an animal and are called resources. The physical space where an animal lives, which contains its environment, is its habitat, which is defined by the normal activity exhibited by an animal, rather than by arbitrary physical boundaries. Animals of any species have certain environmental limits of temperature, moisture, and food within which they can grow, reproduce, and survive. A suitable environment therefore must meet all requirements for life. If we consider all environmental conditions that permit members of a species to survive and multiply, we can define the role of that species in nature as distinguished from all others. This unique, multidimen sional relationship of a species with its environment is called its niche. Dimensions of the niche can vary amongst members of a species, making the niche subject to evolution by natural selection. The niche of a species undergoes evolutionary changes over successive generations. Animals may be generalists or specialists with respect to tolerance of environmental conditions and food sources, e.g. brown bears are omnivores eating a very wide range of plant and animal food, whereas panda bears only eat certain species of bamboo. Populations A population is a reproductively interactive group of animals of a single species. Each population has a characteristic age structure, sex ratio, and growth rate. The study of these properties and the factors that influence them is called demography. Demographic characteristics vary according to lifestyles of the species under study. For example, some animals are modular. Animals, such as sponges and corals, consist of colonies of genetically identical organisms. Reproduction is by asexual cloning. Most colonies also have distinct periods of gamete formation and sexual reproduction. For these modular animals, age structure and sex ratio are difficult to determine. Changes in colony size can be used to measure growth rate, but counting individuals is more difficult than in unitary animals, which are independently living organisms. Most animals are unitary. However, even some unitary species reproduce by parthenogenesis. Parthenogenetic species are found in many animal groups, including insects, reptiles, and fish. Such groups contain only females, which lay unfertilised eggs that hatch into daughters whose genotype comes entirely from their mothers. Most Metazoans are biparental, and reproduction follows a period of organismal growth and maturation. Each new generation begins with a cohort of individuals born at the same time. For a population to retain its constant size from generation to generation, each adult female must replace herself with one daughter that survives to reproduce. Many animals survive to reproduce only once before they die, e.g. many insect species of the temperate zone. Here, adults reproduce before the onset of winter and die, leaving only their eggs to hatch over winter and repopulate their habitat the following spring. Similarly, Pacific salmon after several years return from the ocean to fresh water to spawn only once, after which all adults of a cohort die. However, other animals survive long enough to produce multiple cohorts of offspring that may mature and reproduce while their parents are still alive and reproductively active. Populations of animals containing multiple cohorts, such as robins, box turtles, and humans, exhibit age structure. Analysis of age structure reveals whether a population is actively growing, stable, or declining. Population growth Animals have much lower potential growth rates than bacteria, but huge growth rates are possible. Many insects lay thousands of eggs each year. A single codfish may spawn six million eggs in a season, and a field mouse can produce 17 litters of five to seven young each year. Obviously, unrestricted growth is not the rule in nature. Even in the most benign environment, a growing population eventually exhausts food or space. Exponential increases such as locust outbreaks or planktonic blooms in lakes must end when food or space is expended. The largest population that can be supported by the most limited resource (the one in shortest supply) in a habitat is called the carrying capacity of that environment. Ideally, a population will slow its growth rate in response to diminishing resources until it reaches the carrying capacity. Humans have the highest growth rate of population of any animal. It increased steadily from five million around 8000 BC, when agriculture was introduced, to 16 million, around 4000 BC. Despite the toll taken by famines, disease, and war, the population reached 500 million by 1650. With the coming of the Industrial Revolution in Europe and England in the eighteenth Century, followed by a medical revolution, discovery of new lands for colonisation, and better agricultural practices, the human carrying capacity increased dramatically. The population doubled to one billion in around 1850. It doubled again to two billion by 1927, to four billion in 1974, passed 6 billion in October 1999, and is expected to reach eight billion by the year 2030. Historical surveys provide hope that the growth of the human population is lessening. For example, between 1970 and 2000, the annual growth rate decreased from 1.9% to 1.33%. At 1.33%, it will take nearly 53 years for the world population to double rather than 36.5 years at the higher annual growth rate. The decrease was credited to better birth control. Nevertheless, as more recent surveys show, more than half the global population is still under 25 years old and most live in developing countries where access to reliable contraception is limited or nonexistent. Thus, despite a drop in growth rate, the greatest surge in population still lies ahead, with a projected three billion people added across each three decades. However, by the year 2030 more than half a million people will be added each day, in other words, less than ten days will be required to replace all people who inhabited the world in 8000 BC. Community ecology Populations of animals that form a community interact in a variety of ways that can be detrimental (-), beneficial (+), or neutral (0) to each species, depending on the interaction. For instance, we can consider a predator's effect on its prey as (-), because survival of the prey animal is reduced. However, the same interaction benefits the predator (+), because food obtained from prey increases a predator\'s ability to survive and to reproduce. Thus, the predator-prey interaction is + -. Ecologists use this shorthand notation to characterise interspecific interactions, because it shows the direction in which the interaction affects each species. Other + - interactions include parasitism, in which the parasite benefits by using the host as a home and source of nutrition and the host is harmed. Herbivory, in which an animal eats a plant, is another + - relationship. Commensalism is an interaction that benefits one species and, neither harms nor benefits the other (0+). Most bacteria that normally inhabit our intestinal tracts do not affect us (0), but the bacteria benefit (+) by having food and a place to live. Some evidence suggests that the harmless bacteria in our intestinal tracts may prevent the entry of more harmful bacterial species, in which case this commensalism grades into mutualism. A classic example of commensalism is the association of pilot fishes with sharks. These fishes get the 'crumbs' remaining when the host shark makes its kill, but we now know that some pilot fish also feed on ectoparasites of sharks. Therefore, this commensalism also grades into mutualism. Organisms engaged in mutualism have a friendlier arrangement than commensalistic species, because the fitness of both is enhanced (++). Some mutualistic relationships are not only beneficial, but also necessary for survival of one or both species. An example is the relationship between a termite and a protozoa inhabiting its gut. The protozoa can digest wood eaten by the termite, because the protozoa produce an enzyme, lacking in the termites, which digests cellulose; the termite lives on waste products of protozoan metabolism. In. return, the protozoa gain a habitat and food supply. Such absolute interdependence among species can be a liability if one of the participants is lost. Calvaria trees native to Mauritius have not reproduced successfully for over 300 years, because their seeds germinate only after being eaten and passed through the gut of a dodo bird, now extinct. Competition occurs when two or more species share a limiting resource. Simply sharing food or space with another species does not produce competition unless the resource is in short supply relative to the needs of the species that share it. Competing species may reduce conflict by reducing the overlap of their niches. Niche overlap is the portion of a niche\'s resources shared by two or more species. For example, if two species of birds eat seeds of exactly the same size, competition eventually will exclude one species from the habitat. This example illustrates the principle of competitive exclusion: strongly competing species cannot coexist indefinitely. To coexist in the same habitat, species must specialise by partitioning a shared resource and using different portions of it. Specialisation of this kind is called character displacement. Character displacement usually appears as differences in organismal morphology or behaviour related to the exploitation of a resource, e.g. Galapagos finches. Predators The relationship between predators and their prey causes co-evolution: predators get better at catching prey, and prey get better at escaping predators. This is an evolutionary race predators cannot afford to win. If a predator became so efficient that it exterminated its prey, the predator species would become extinct. The war between predators and prey reaches extremes in the evolution of defences by potential prey. Many animals that are palatable, escape detection (cryptic defences) by matching their background, or by resembling some inedible feature of the environment (such as a stick). In contrast, aposematic defences involve animals that are toxic or distasteful to predators actually advertising their strategy with bright colours and conspicuous behaviours. Food webs A food chain is the flow of energy from one organism to the next. Organisms in a food chain are grouped into trophic levels based on how many links they are removed from the primary producers (plants that fix solar energy into chemical energy). Trophic levels may consist of either a single species or a group of species that are presumed to share both predators and prey. They usually start with a primary producer and end with a carnivore. A food chain is a linear relationship between various organisms, e.g. osprey feed on northern pike, that feed on perch, that eat bleak, that feed on freshwater shrimp, which feed on phytoplankton (primary producer). It is often the case that biomass of each trophic level decreases from the base of the chain to the top. This is because energy is lost to the environment with each transfer. On average, only 10% of the organism\'s energy is passed on to its predator. The other 90% is used for the organism's life processes or is lost as heat to the environment. Some producers, especially phytoplankton, are so productive, and have such a high turnover rate that they can actually support a larger biomass of grazers. This is called an inverted pyramid, and can occur when consumers live longer and grow more slowly than the organisms they consume. Directly linked to this are pyramids of numbers, which show that as the chain is travelled along, the number of consumers at each level drops very significantly, so that a single top consumer (e.g. a polar bear will be supported by literally millions of separate producers (e.g. Phytoplankton). Food chains are overly simplistic as representatives of what typically happens in nature. The food chain shows only one pathway of energy and material transfer. Most consumers feed on multiple species and are, in turn, fed upon by multiple other species. The relations of detritivores and parasites are seldom adequately characterised in such chains as well. The food web below makes it possible to show much bigger animals (like a seal) eating very small organisms (like plankton). Food sources of most species in an ecosystem are much more diverse, resulting in a complex web of relationships. In this example, the grouping of Algae → Protozoa → Oligochaeta → Northern Eider → Arctic Fox is a food chain; the whole complex network is a food web. Throughout this course, a theme has emerged - that every organism (including humans) can be examined at a large range of levels, from the molecular to the global, and should not be assessed in isolation from these other aspects. A similar web to that above could be constructed to encompass the whole of zoology; each individual 'box' in isolation provides only a part of the story, it must be considered as a whole to provide complete understanding. Unfortunately there are far more 'boxes' in existence than can be included in a course of this size. Indeed, scientific knowledge has not filled all boxes as yet, and it is probable that there are many still waiting to be identified.