Geographic Ecology

1. Geographic Ecology Chapter 22

2. Outline • Introduction • Island Area, Isolation, and Species Richness • Terrestrial • Aquatic • Equilibrium Model of Island Biogeography • Latitudinal Gradients in Species Richness • Historical and Regional Influences

3. Introduction • MacArthur defined geographic ecology as the search for patterns of plant and animal life that can be put on a map. • Geographic ecology is the study of ecological structure and process at large geographic scales.

4. Island Area and Species Richness • On islands and habitat patches on continents, species richness increases with area and decreases with isolation.

5. Island Area and Species Richness • Prestonfound fewest bird species live on smallest islands and most species on largest islands.

6. Island Area and Species Richness • Nilsson et.al. found island area was best single predictor of species richness among woody plants, carabid beetles, and land snails.

7. 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.

8. Lakes as Islands • Lakes can be considered as habitat islands. • Differ widely by degree of isolation. • Tonn and Magnuson found the number of species increases with the area of an insular environment. • Barbour and Brown found positive relationship between area and fish species richness.

9. Island Isolation and Species Richness • There is often a negative relationship between the isolation of an island and the number of species it supports. • An island that is very isolated for one group of organisms may be completely accessible to another group.

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

11. Marine Islands • Williamsonsummarized 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, and pteridophytes produce large quantities of light spores easily dispersed in the wind.

12. Isolation and Habitat Islands on Continents • Lomolino et.al. found a strong negative relationship between isolation and the number of montane mammal species living on mountaintops across the American Southwest.

13. Equilibrium Model of Island Biogeography • MacArthur and Wilson: Model explaining patterns of species diversity on islands as result of immigration and extinction rates. • 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.

14. Equilibrium Model of Island Biogeography • They predicted the 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 number of species increases, population size of each must diminish. • As number of species increases, potential for competitive interactions between species will increase.

15. 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. • Large near islands will support highest number. • Small far islands will support lowest number. • Small near and large far will support intermediate number.

16. Equilibrium Model of Island Biogeography • Predictions of the equilibrium model: • Island diversity is the outcome of a highly dynamic balance between immigration and extinction. • Rates of immigration and extinction are determined mainly by the isolation and area of islands. • Predicts that the species composition on islands is not static – it changes over time. • Species Turnover

17. Species Turnover on Islands • Diamond found birds in nine California Channel Islands in a stable equilibrium as a result of immigration and extinction.

18. Experimental Island Biogeography • Simberloff and Wilson studied insect recolonization in Florida Keys. • They chose 2 stands of mangroves as control islands, and 6 others as experimental islands which were defaunated.

19. Experimental Island Biogeography • Followed recolonization for 1 yr. • Species number stayed constant, but composition changed considerably.

20. Colonization of New Islands by Plants • Rydin and Borgegardfound variation in species richness correlated positively with island area and accounted for 44-85% of variation in species richness among islands. Small and medium islands continued to accumulate species. • Large islands attained equilibrium of immigration and extinction. • Difficult to separate effects of habitat diversity from area effects.

21. 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.

22. Island Biogeography Update • Brown and Kodric-Brown found higher immigration rates to near islands can reduce extinction rates. • Lomolinofound island area can have a significant effect on immigration rates. • Area and isolation are only two of several environmental factors affect island species richness.

23. Latitudinal Gradients in Species Richness • Species richness generally increases from middle and high latitudes to the equator. • Most groups of organisms are more species-rich in the tropics.

24. Latitudinal Gradients in Species Richness • Brown grouped hypotheses that explain geographic gradients into six categories: • Time Since Perturbation • More species in the tropics because tropics are older and disturbed less frequently. • More time for speciation, and less frequent disturbance reduces extinction rate.

25. Latitudinal Gradients in Species Richness • Productivity • High productivity contributes to high species richness. • More energy to divide among population. • Environmental Heterogeneity • More heterogeneity, thus more potential habitat areas and niches.

26. Latitudinal Gradients in Species Richness • Favorableness • Tropics have more favorable environments. • No extremes to limit diversity. • Niche Breadth and Interspecific Interactions • Brown suggests biological processes must play secondary role. • Ultimate causes must by physical differences.

27. Latitudinal Gradients in Species Richness • Differences in Speciation and Extinction Rates • Tropical species richness is greater because the tropics have experienced higher rates of speciation and/or lower rates of extinction.

28. Area & Latitudinal Gradients in Species Richness • Terborgh and Rosenzweig proposed that the greater species richness of the tropics can be explained by the greater area covered by tropical regions.

29. Area & Latitudinal Gradients in Species Richness • In addition, temperatures are more uniform across this tropical belt.

30. Continental Area and Species Richness • Brown analyzed the number of non-flying mammals on five major continents plus Madagascar and New Guinea. • Strong positive relationship between mammalian richness and the area of continents or large islands.

31. Continental Area and Species Richness • Rosenzweig found a strong positive relationship between area and species diversity in the tropics.

32. Historical and Regional Influences • Long-term historical and regional processes can significantly influence species richness and diversity. • Exceptions to the common patterns.

33. Historical and Regional Influences • Latham and Ricklefs: Reported striking contrast in diversity of temperate zone trees that cannot be explained by area effect.

34. Historical and Regional Influences • Temperate forest biome in Europe, Eastern Asia, and Eastern North America all have roughly equitable area, but support vastly different levels of biological diversity. • Eastern Asia: 3x North America and 6x Europe.

35. Historical and Regional Influences • Ralph found a negative correlation between foliage height diversity and bird species diversity.

36. Cape Floristic Region of South Africa • Bond and Goldblatt attributed unusually high species richness of the Cape floristic region to several historic and geographic factors. • Continental drift • Wide variety of soil types. • Repeated expansion, contraction, and isolation of plant populations. • Refuge areas

37. Diversity of Temperate Trees • Latham and Ricklefs: Must examine conditions trees in these regions faced during the last glacial period. • Mountains in Europe form east-west oriented barriers. • During last ice age, temperate trees had southward retreat largely cut-off. • Lower species richness as consequence of higher extinction rate.

38. Historical and Regional Influences • Appalachian Mountains in North America run north-south, thus temperate trees had an avenue of retreat as temperatures became colder. • Also no mountain barriers in Asia. • Concluded from various lines of evidence that most temperate tree taxa originated in Eastern Asia and dispersed to Europe and North America. • After dispersal lines were cut, speciation continued in Asia.

39. Bird Diversity in the Beech Forests of South America • Shrub habitats contain more bird species than the beech forests with a greater diversity of heights. • Shrub habitats occupy a much larger area than beech forest. • Beech forests isolated from tropical forests by arid habitat.