As discussed earlier in this chapter, the availability of particular resources determines the presence of associated species. If a limiting resource (host) becomes more abundant, then associated species also become more abundant until some other factor(s) become limiting. For example, Siemann (1998) reported that experimental fertilization of grassland plots increased arthropod species richness and abundance (Fig. 9.11).
Limiting resources may preclude any single adaptive strategy from becoming dominant and thereby maintain high species richness. Rosenzweig and Abramsky (1993), Tilman and Pacala (1993), and Waide et al. (1999) concluded that species richness is not always linearly related to productivity. Intermediate levels of productivity often support the highest diversity because higher productivity favors dominance by the most competitive species. Mittelbach et al. (2001) compiled 171 published studies relating species richness and productivity for aquatic and terrestrial plants and animals. Hump-shaped relationships were most common, indicating that productivity generally was higher at intermediate levels of species richness.
A number of studies have compared species richness between relatively homogeneous and heterogeneous environments (e.g., Cromartie 1975, Risch 1980,1981, Root 1973, D. Strong et al. 1984,Tahvanainen and Root 1972). Because organisms have greater difficulty maintaining energy and nutrient balance when resources are scattered (see Chapter 4), the abundance of individual species generally decreases with increasing resource heterogeneity, precluding exclusive use
None Low High Historical fertilization
600 120 -110 100 -90
i i Sepcies richness i i Abundance
| Responses of arthropod species richness and abundance to historical (a) and experimental (b) fertilization treatments. Vertical lines represent 1 SE. From Siemann (1998) with permission from the Ecological Society of America. Please see extended permission list pg 571.
of the niche and permitting species richness to increase. By contrast, homogeneous resources facilitate rise to competitive dominance by the best-adapted species, leading to reduced species richness. Extensive planting of agricultural or silvicultural monocultures establishes the conditions necessary for some species to reach epidemic population levels across landscapes (see Chapter 7), reducing availability of resources shared with other species but providing prey resources for predators (Polis et al. 1997a; see also Chapter 8).
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