I.1 Ecological Niche
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I.1 Ecological Niche Thomas W. Schoener OUTLINE 1. Three concepts of the ecological niche 2. The recess/role niche and seeking ecological equivalents 3. The population-persistence niche and mechanistically representing competition 4. The resource-utilization niche and understanding the evolution of species differences 5. Environmental niche modeling and analyzing niches on a macroscale 6. Conclusion It may come as something of a surprise that ecological niche, a term so common in the popular media, has three distinct meanings among scientists, each with an associated conceptual basis: these are the recess/role niche, the population-persistence niche, and the resource-utilization niche. GLOSSARY character displacement. The situation in which two species are more different in geographic locations where they overlap than between locations where they occur alone community. Thosespeciespopulationsoccurringatsome location competition. Ecologicalinteractioninwhichtwoormore species negatively affect one another by consuming common resources or by other harmful means convergence. Development of increasing similarity over time, usually applied to species somewhat unrelated evolutionarily niche dimension. Environmental variable along which a species’ niche is characterized, e.g., food size, and typically represented as the axis of a graph polymorphism. The existence of two or more forms, differing in morphology or some other way, in the same population population. Those individuals of a species occurring at some location population growth rate r. The per capita rate at which a population changes size, typically computed as the birthrate minus the death rate 1. THREE CONCEPTS OF THE ECOLOGICAL NICHE The Recess/Role Niche The first use of ‘‘ecological niche’’ appeared in a report on ladybugs written by R. H. Johnson nearly a century ago, although the term was used shortly thereafter by the zoologist Joseph Grinnell, who is generally given credit for its original development. The meaning was very close to figurative usage: the ecological niche of a species is its ‘‘role,’’ ‘‘place,’’ or more literally ‘‘recess’’ (in the sense of a ‘‘nook’’ or ‘‘cubbyhole’’) in an ecological community. Thus, the California thrasher, one of Grinnell’s major examples, is a bird of the chaparral community that feeds mostly on the ground by working over the surface litter and eating both animal and plant items of a suitable size. Escape from predators is similarly terrestrial, with the well-camouflaged bird shuffling off through the underbrush on the rare occasions when it is threatened. The idea that there exists a set of characteristic habitatandfoodtypeswithaccompanyingbehavioral ,morphological , and physiological adaptations leads to the notion of ecological equivalents. These are defined as two or more species with very similar niche characteristics that occur in completely different localities. An example from Grinnell’s writings is the kangaroo rat of North America, which ‘‘corresponds exactly’’ to the jerboa (another desert rodent species) of the Sahara. The existence of ecological equivalents would imply that rather invariant rules determine the niches available for occupancy in a particular kind of environment, e.g., a desert. Moreover, niches can be empty in the sense that a suitable species does not occur within a locality, perhaps because it never got there or was unable to evolve in situ. But to what extent do ecological equivalents really exist? Decades after Grinnell’s work, we now know (section 2, below) that although some examples can certainly be found, perhaps more commonly, species of similar environments (e.g., deserts) among distant localities are neither identical nor often even similar. Perhaps such considerations helped to engender the two other meanings of ecological niche, each with its accompanying set of ideas about how the ecological world works. The Population-Persistence Niche The population-persistence niche has its roots in papers written in the mid-twentieth century by the ecologist and limnologist G. E. Hutchinson. This concept focuses on the species, in this case its population, rather than on the environment. Hutchinson formulates the ecological niche as a quantitative description of the range of environmental conditions that allow a population to persist in some location; the term persist means having a positive or at least zero (break-even) population growth rate, r (if r is negative, the population dwindles away to extinction). An example of an environmental condition is temperature; a second example would be humidity (for organisms on land) or salinity (for organisms in water). If we represent an environmental condition by the axis of a graph, a range is an interval along that axis, e.g., temperature from 08C to 308C (figure 1). A second interval, say for relative humidity, might range from 20% to 80% along...