The notion that ecological niches cannot be infinitely similar to each other, and the knowledge that ecological space is heterogeneous and that total number of resources available to a community is always limited, has led to an idea that for a given environment there is a limited number of available niches which could be potentially occupied. An environment then could be seen as a set of empty niches, which could - but may not - be filled with species. Consequently, we might ask whether in a particular case the niche space is or is not saturated with species.
There are two facets of the problem, which are sometimes confused. First, there is no doubt that the limited amount of resources in an ecosystem can sustain only limited total number of individuals (assuming a given body size distribution). Therefore, there is always a limited potential for the whole community size determined by total amount of resources, and thus also for a limited number of species (given that each species needs some viable population size). If this potential is fully utilized, we speak about biotic saturation of the community. However, biotic saturation does not imply that the number of ecological niches is fixed and that all possible niches are occupied. Such a statement would be much stronger and would require at least some level of discreteness of ecological niches, that is, ecological space cannot be divided into an infinite number of subtly different niches with arbitrary positions. Is there any reason to believe that niches are discrete and their number within an environment is limited?
Apparently, there is a considerable level of environmental heterogeneity in resource distribution and abundance; resources are more abundant for some combinations of parameters than for others. Environmental heterogeneity would not be, however, a sufficient condition for discreteness of ecological niches if species could utilize equally easily several different resources. The discreteness of ecological niches comes out from the existence of 'tradeoffs' in resource utilization: resources can be always potentially utilized by many ways, but some ways are mutually exclusive. A Galapagos finch from the genus Geospiza can have either a big beak appropriate for cracking big seeds, but then it can crack small seeds with much more difficulty, and vice versa. A plant can either invest to its rapid growth and so quickly utilize resources, or it can invest into woody trunk which enables it to grow higher and sustain longer - but at a cost associated with a slower growth. Moreover, some methods of resource utilization are less effective than others and natural selection supports phenotypes better utilizing available resources, some phenotypes being suboptimal. Consequently, evolution leads to utilization of only a restricted spectrum of resources.
In the presence of tradeoffs, there is only a limited number of mutually exclusive ways to utilize resources, and thus a limited number of available niches. However, as the discreteness of niches follows from the tradeoffs between adaptations, and since all the tradeoffs are determined by unique properties and constraints of given organisms, it makes sense to speak about available niches only in relation to organisms which already inhabit the environment. A habitat without its inhabitants can provide a potentially infinite number of opportunities for existence, and this landscape of opportunities changes with each new inhabitant. For the organism in an environment, the number of possible niches is determined by the number of possible ways to utilize the resource - with all constraints and tradeoffs of the given organism.
Therefore, it is likely that there are always more niches than the current number of species, because each species has several mutually exclusive possibilities of future adaptive evolution arising from the tradeoffs - unless all niche changes require a corresponding niche change in other species.
In some cases, the number of available niches can be predicted from the knowledge of resource heterogeneity and the possibilities of resource utilization for given taxon. The number of Galapagos finches occurring on each island is reasonably well predicted by the number of peaks of the 'landscape' constructed using the knowledge of frequency distribution of seed size, the general relation between finch and seed biomass, and the relation between preferred seed size and beak depth (Figure 2). Similarly, using the knowledge of the relationship between beak shape of crossbills (Loxia curvirostra) and their foraging efficiency in obtaining cone seeds from cones of various coniferous tree species, it is possible to construct a resource utilization function related to different morphologies, and find out how many optimal shapes do exist. And again, it has been found that there are several ecomorphs of crossbills, each of them occupying one adaptive peak (optimum) in the morphological space.
There is another evidence that ecological niches are partially predictable - the phenomenon of community convergence. Animal or plant communities occurring on different continents or biotic provinces often comprise similar morphological types utilizing similar types of resources. Anolis lizards, for instance, have evolved independently into several well-recognizable ecomorphs on each Caribbean island, with known sequence of this evolution, repeated on every island. However, there can be more than one species within each ecomorph, and thus this convergence does not imply that the number of species-specific niches is predictable. This is quite typical for most cases of community convergences: they provide a clue to our understanding of how many possibilities are there for utilizing resources within given habitat and for given taxon, but not to the prediction of how many species can actually coexist there. The total potential number of species within an environment is given by the total amount of resources determining the total number of all individuals, regardless of the level of discreteness of ecological niches.
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