unit 6 notes - 6.1 Notes Area isolation and species...

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6.1 Notes Area, isolation, and species richness One implication of the species-area curve discussed in the previous unit is that the total spatial size of a habitat can have a direct effect on the number of species. This is because the number of niches may increase with habitat area, and greater space may decrease the intensity of competition, which would reduce the likelihood of competitive exclusion. How can we test these predictions on larger geographic scales? One approach that researchers have taken is to look at discrete geographic units such as islands; however, any discrete unit of land that is isolated, such as mountaintops or lakes, would work. Figure 22.5 on page 582 [559] in the textbook (upper panel) shows the relationship between island area and number of bird species for 346 marine islands around the world. Kalmar and Currie (2006) concluded that island area sets the upper limit for bird diversity, though extreme abiotic conditions such as temperature and moisture can reduce the actual diversity found on some islands. Similar results have been found for island area and number of carabid beetles ( Figure 22.5 on page 582 [559] in the textbook, lower panel ), montane area and number of mammal species ( Figure 22.6 on page 583 [560] in the textbook ), and lake area and number of fish species ( Figure 22.7 on page 583 [569] in the textbook ). One additional pattern that emerges when examining species richness on discrete geographic patches is isolation , or distance from a source of migrants. Simply put, species number often decreases with increasing isolation because of barriers to dispersal. Several examples of this pattern are described on pages 584 585 [561–562] of the textbook. Equilibrium model of island biogeography How do we explain the number of species on these discrete patches of land? MacArthur and Wilson (1963) proposed the equilibrium model of island biogeography , which is described in Figure 22.11 and pages 586 587 [563–564] of the textbook . How can we test MacArthur and Wilson’s hypothesis? First we need some predictions to test. MacArthur and Wilson predicted that (1) the number of species on an island of a given size should remain constant, and (2) species composition should change (i.e., species turnover ) as some species go extinct while others colonize. Jared Diamond (1969) tested these predictions by examining islands off the coast of southern California. He took advantage of a bird census done in 1917, and went back in 1968 to see if numbers of species had changed (Table 6.1). Because there was information on the identity of each species, he could tell which had gone extinct and which had immigrated. Keep in mind that the island biogeography model is viewed as being at equilibrium in terms of numbers only; the actual identity of species changes, and it is not possible to predict which species will go extinct and which ones will immigrate.

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