Island Biogeography and Geographic Ecology

• Geographic ecology is the search for patterns of plant and animal life that can be put on a map.
• Geographic ecology studies ecological structure and processes at large geographic scales.

Area, Isolation, and Species Richness

• Arrhenius developed a quantitative relationship between area and the number of species, pioneering geographic ecology.
• He worked on islands in Sweden, but at scales smaller than the geographic scale.
• Preston researched the relationship between species number and island area in the West Indies and found the fewest bird species live on the smallest islands, while most species live on the largest islands.
• Nilsson et al. determined that island area in Sweden was the best single predictor of species richness for woody plants, carabid beetles, and land snails.
• Climate warming at the end of the Pleistocene caused forest and alpine habitats to contract to the tops of high mountains across the American Southwest, resulting in island-like fragments, also known as montane vegetation.
• Montane animal distributions contracted, and species richness correlated with montane island area.
• Lakes can be considered habitat islands, with varying degrees of isolation.
• Tonn and Magnuson studied Wisconsin lakes and discovered the number of species increases with the area of the lake.
• Barbour and Brown studied 70 lakes globally and found a positive relationship between area and fish species richness.
• MacArthur and Wilson found that isolation reduces bird diversity on Pacific Islands.
• Williamson studied island area and species richness on the Azore and Channel Islands.
• Birds and pteridophytes show increased diversity with increasing island area.
• Bird diversity is influenced by isolation, unlike pteridophyte diversity, which reflects differences in dispersal abilities (pteridophytes produce light spores).
• Lomolino et al. studied montane mammal species on mountaintops across the American Southwest and found a negative correlation between isolation and species number, suggesting immigration's importance in maintaining diversity.

Equilibrium Model of Island Biogeography

• MacArthur and Wilson explained species diversity patterns on islands as a result of the equilibrium between immigration and extinction rates, known as the equilibrium model of island biogeography.
• Immigration rates are highest on new islands with no organisms, and decline as species accumulate because fewer arrivals constitute new species.
• Extinction rates rise with increasing numbers of species because there is a larger pool of potential extinctions, population size of each species diminishes, and competition between species increases.
• The number of species that will occur on an island is predicted by the point where immigration and extinction lines cross.
• The rate of immigration is mainly determined by distance from the source of immigrants, while the rate of extinction is mainly determined by island size.
• Large near islands support the highest number of species, while small far islands support the lowest number and small near and large far islands support intermediate numbers.
• The equilibrium model suggests that species composition on islands is dynamic.
• Changes in species composition are referred to as species turnover.
• Diamond found birds on nine Channel Islands in a stable equilibrium, due to approximately equal numbers of immigrations and extinctions, i.e. species turnover occurred.
• Simberloff and Wilson studied insect recolonization in Florida Keys and after one year, defaunated islands nearly returned to original species numbers, though the species composition changed.
• Rydin and Borgegård found that variation in species richness was positively correlated with island area, explaining 44 to 85% of species richness variation among islands.
• Small and medium islands continued to accumulate species, while large islands reached equilibrium and it is difficult to separate the effects of habitat diversity from area effects.
• Simberloff tested the effect of island area on species richness in mangrove islands.
• In all cases where the area was reduced, species richness decreased, whereas richness on control islands increased slightly, indicating that area has a positive influence on species richness.

Latitudinal Gradients in Species Richness

• Species richness generally increases from middle and high latitudes towards the equator.
• The pattern of increased number of species in the tropics is pervasive and dramatic which forms the basis for latitudinal gradient hypotheses.
• Brown (1988) grouped the hypotheses into six categories.
• Tropics have more species because they are older and disturbed less frequently, allowing more time for speciation and reducing the extinction rate.
• Additionally, there are assumptions the tropics have remained relatively stable, whilst middle and high latitudes have been disrupted by glaciation.
• Conversely, Connell (1978) proposed that tropical diversity is maintained by disturbance.
• High productivity contributes to high species richness, leading to more energy, more specialized consumers with larger populations, lower chance of extinction, and extinction rates.
• Tropics have more species because they are more heterogeneous, though most tropical species occur in fewer environments than high latitude species.
• Species in more diverse communities tend to subdivide the environment more finely.
• Tropics provide more favorable environments, while high latitude environments have more temperature variation and physically extreme environments restrict diversity.
• Several hypotheses concern relative niche breadths and interspecific interactions, such as tropical species being limited more by biological factors than physical factors and subject to controls by predators, pathogens, and parasites.
• Rosenzweig proposed immigration can be discounted at large scales, emphasizing the differences in speciation and extinction rates.
• The number of species in an area reflects the rate at which new species have been added minus the rate at which they have disappeared.

Historical and Regional Influences

• Long-term historical and regional processes can significantly influence species richness and diversity.
• There are differences in species richness that cannot be explained by area, such as the Cape region of South Africa, which is smaller than the California Floristic Province, yet has more species.
• Bond and Goldblatt attributed the Cape Floristic Region's unusually high species richness to continental drift, a wide variety of soil types, repeated expansion, contraction, and isolation of plant populations, and refuge areas reducing extinction rates.
• Latham and Ricklefs (1993) studied temperate zone trees and found that temperate forest biomes in Europe, Eastern Asia, and Eastern North America all have roughly equal area, but vastly different levels of biological diversity.
• Ralph (1985) found a negative correlation between foliage height diversity and bird species diversity in Argentina.
• Mountains in Europe form east-west oriented barriers, where during the the last ice age, temperate trees had their southward retreat largely cut-off, resulting in lower species richness as a consequence of a higher extinction rate.
• The Appalachian Mountains in North America run north-south, providing temperate trees an avenue of retreat as temperatures became colder.
• There were no mountain barriers in Asia, leading to the conclusion that most temperate tree taxa originated in Eastern Asia and dispersed to Europe and North America.
• After dispersal routes were cut, speciation continued in Asia.

Explore island biogeography, focusing on the correlation between island size and species richness. Understand geographic ecology through MacArthur's definition, factors affecting species richness, and the impact of post-Pleistocene climate warming on habitats. Discover how island size relates to species accumulation.