Spatial scaling laws may not apply to most animal species

Abstract

One of the central problems in ecology is the question of whether habitats are generally saturated with species or not, i.e., whether equilibrium or non-equilibrium conditions prevail. Evidence for saturation given by various authors is dense species packing leading to frequent interspecific competition as shown by complete or partial competitive exclusion or habitat shifts in the presence of other species, character displacement and particularly differences in the size of feeding organs of species using similar food resources (e.g., Krebs 1997), and an asymptotic relationship between local and regional species richness (Cornell and Lawton 1992). Evidence for a high degree of non-saturation is that many habitats are empty, i.e., that species are not densely packed (e.g., Rohde 1979, Lawton and Strong 1981, Lawton 1982), that many species are little or not affected by the presence of other potentially competing species, and that differences in the size of feeding organs do also occur when resources are in unlimited supply, suggesting that such differences may be fortuitous. Also, many examples of character displacement (of reproductive organs) can best be explained by reinforcement of reproductive barriers, and niche restriction may often be the result of selection to facilitate mating and not of competition (Rohde 1991). Furthermore, an asymptotic relationship between local and regional diversity may be a consequence of differential likelihoods of species to appear in a community because of different colonization rates and life spans (Rohde 1998). Reduced interspecific aggregation relative to intraspecific aggregation (Morand et al. 1999), little evidence for nestedness (Worthen and Rohde 1996), much more common positive than negative associations (Rohde et al. 1994), and the rare occurrence of hyperparasites of various degrees (e.g., Rohde 1989) has been used as evidence for non-saturation in parasites.