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Geographic Ecology

Geographic Ecology. On islands and habitat fragments on continents, species richness increases with area and decreases with isolation. Species richness on islands can be modeled as a dynamic balance between immigration and extinction of species.

duncan-bernard
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Geographic Ecology

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  1. Geographic Ecology • On islands and habitat fragments on continents, species richness increases with area and decreases with isolation. • Species richness on islands can be modeled as a dynamic balance between immigration and extinction of species. • Species richness generally increases from middle and high latitudes to the equator.

  2. Introduction • MacArthurdefined geographic ecology as the search for patterns of plant and animal life that can be put on a map.

  3. Island Area and Species Richness • Preston found fewest bird species live on smallest islands and most species on largest islands. • Nilsson et.al. found island area was best single predictor of species richness among woody plants, carabid beetles, and land snails.

  4. Habitat Patches on Continents: Mountain Islands • As Pleistocene ended and climate warmed, forest and alpine habitats contracted to the tops of high mountains across American Southwest. • Woodlands, grasslands, and desert scrub, invaded lower elevations. • Once continuous forest converted to series of island-like fragments associated with mountains: montane.

  5. Kikuchi’s field vole Fig. Neighbor-joining tree based on 703 bp of mtDNA control region sequences.

  6. Number of species increases with area sampled. • Arrhenius first formalized the species-area relationship as: S = cAz where: S = number of species encountered A = area c and z are fitted constants • After log transformation, the relationship is linear: logS = logc + zlogA

  7. Species-Area Relationships • Analyses of many species-area relationships have shown that values of the slope, z, fall within the range 0.20 - 0.35. • For continental areas, z is often between 0.15 and 0.25, which is lower than insular situations, where z is often between 0.20 to 0.40 • Are species-area relationships artifacts of larger sample size (more individuals) in larger areas? • comparisons of species numbers in different areas where samples of similar size were used still reveals a species-area relationship

  8. Why do larger areas have more species? • Larger areas have greater habitat heterogeneity. • For islands, size per se makes the island a better target for potential immigrants from the mainland. • Larger islands support larger populations, which persist because they have: • greater genetic diversity • broader distributions over habitats • numbers large enough to prevent stochastic extinction

  9. Marine Islands • MacArthur and Wilson found isolation reduces bird diversity on Pacific Islands.

  10. Williamson summarized data from relationship between island area and species richness in Azore Islands: • Birds show clear influence of isolation on diversity, pteridophytes do not. • Land birds fly across water barriers, pteridophytes produce large quantities of light spores easily dispersed in the wind. *dispersal ability

  11. Effect of distance on island immigration • Tabulated land birds on island close to the source area (New Guinea), and assumed 100% of the available birds • Documented drop-off in species with increasing distance to New Guinea

  12. Equilibrium Model of Island Biogeography • MacArthur and Wilson: model explaining patterns of species diversity on islands as result of immigration and extinction rates. • Reasoned rates of immigration would be highest on new island with no organisms. • As species began to accumulate, rate of immigration would decline since fewer arrivals would be new species.

  13. Equilibrium Model of Island Biogeography • Predicted rate of extinction would rise with increasing number of species on an island for three reasons: • Presence of more species creates a larger pool of potential extinctions. • As # of species increases, population size of each must diminish. • As # of species increases, potential for competitive interactions between species will increase.

  14. Equilibrium Model of Island Biogeography • Point where two lines cross predicts the number of species that will occur on an island. • Proposed rates of extinction on islands would be determined mainly by island size. • LG, near islands will support greatest #. • SM, far islands will support lowest #. • SM near and LG far will support intermediate #.

  15. Species Turnover • Equilibrium model predicts spp. composition on islands is fluid: • Change referred to as species turnover. • Rate of species lost and other gained • Diamond found birds in nine CA Channel Islands in a stable equilibrium as a result of immigration and extinction. - compared his list to that of A. B. Howell (1917) - 5-10 species per island were no longer present, but just as many species had not listed by Howell had apparently colonized the island – indicating turnover

  16. Experimental Island Biogeography • Simberloff and Wilson studied insect recolonization in Florida Keys. • Chose 2 stands of mangroves as control islands, and 6 others as experimental islands. • Defaunated islands • Followed recolonization for 1 yr. • Species number stayed constant, but composition changed considerably.

  17. Colonization of New Islands By Plants Rydin and Borgegard found variation in spp. richness correlated positively with island area and accounted for 44-85% of variation in spp. richness among islands. Small and medium islands continued to accumulate species. Large islands attained equilibrium of immigration and extinction. Area or habitat effects? Difficult to separate effects of habitat diversity from area affects.

  18. Manipulating Island Area • Simberloff tested effect of island area on species richness. • In all cases where area was reduced, species richness decreased. • Richness on control island increased slightly. • Islands with reduced area lost species with each reduction in area. • Showed area has positive influence on species richness.

  19. Modifications to MacArthur & Wilson’s theory model

  20. Theory of Nature Reserve Design • Larger is better than smaller • One large area is better than several smaller areas (SLOSS) • Corridors connecting isolated areas are desirable • Circular areas are better than elongate ones with much edge

  21. Latitudinal Gradients in Species Richness

  22. Latitudinal Gradients in Species Richness • Most groups of organisms are more species-rich in the tropics. Hot Spot

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