In spite of all the difficulties of making a direct connection between interspecific competition and niche differentiation, there is no doubt that niche differentiation is often the basis for the coexistence of species.
There are a number of ways in which niches can be differentiated. One is resource partitioning or, more generally, differential resource utilization. This can be observed when species living in precisely the same habitat nevertheless utilize different resources. Since the majority of resources for animals are individuals of other species (of which there are literally millions of types), or parts of individuals, there is no difficulty, in principle, in imagining how
... that is almost certainly wrong the answer? it depends niche differentiation
... easy to imagine in animals, less easy in plants . . .
competing animals might partition resources amongst themselves. Plants, on the other hand, all have very similar requirements for the same potentially limited resources (see Chapter 3), and there is much less apparent scope for resource partitioning (but see below).
In many cases, the resources used by ecologically similar species are separated spatially. Differential resource utilization will then express itself as either microhabitat differentiation between the species (e.g. different species of fish feeding at different depths), or even a difference in geographic distribution. Alternatively, the availability of the different resources may be separated in time; for example, different resources may become available at different times of the day or in different seasons. Differential resource utilization may then express itself as a temporal separation between the species.
The other major way in which niches can be differentiated is on the basis of conditions (Wilson, 1999). Two species may use precisely the same resources, but if their ability to do so is influenced by environmental conditions (as it is bound to be), and if they respond differently to those conditions, then each may be competitively superior in different environments. This too can express itself as a microhabitat differentiation, or a difference in geographic distribution or a temporal separation, depending on whether the appropriate conditions vary on a small spatial scale, a large spatial scale or over time. Of course, in a number of cases (especially with plants) it is not easy to distinguish between conditions and resources (see Chapter 3). Niches may then be differentiated on the basis of a factor (such as water) which is both a resource and a condition.
There are many examples of the separation of competing species in space or time involving both animals and plants. For example, tadpoles of two anuran species in New Jersey, USA (Hyla crucifer and Bufo wood-housii), have their feeding periods offset by around 4-6 weeks each year, apparently, though not certainly, associated with differential responses to environmental conditions rather than seasonal changes in resources (Lawler & Morin, 1993). Two coexisting species of spiny mice in rocky deserts in Israel partition activity on a diel basis: Acomys cahirinus is nocturnal and A. russatus is diurnal, although the latter becomes nocturnal if its congener is removed (Jones et al., 2001). Two phloem-feeding bark beetles, Ips duplicatus and I. typographus, on Norway spruce trees, in Norway, are separated in their feeding sites on a small spatial scale by trunk diameter, although the reason for this is not at all clear (Schlyter & Anderbrandt, 1993). But, it is amongst plants and other sessile organisms, because of their limited scope for differential resource utilization at the same location and instant, that spatial and temporal separation are likely to be of particular significance (see Harper, 1977). Although, as ever, it is one thing to show that
species differ in their spatial or temporal distribution - it is quite another to prove that this has anything to do with competition. The cattails in Section 8.3.3 provide one example of competing plants separated spatially. Another is shown in Figure 8.31, concerning the annuals Sedum smallii and Minuartia uniflora that dominate the vegetation growing on granite outcrops in southeastern USA. The adult plants exhibit an especially clear spatial zonation associated with soil depth (itself strongly correlated with soil moisture), and further experimental results reinforce the idea that it is competition rather than mere differences in tolerance that gives rise to this zonation.
Describing the outcome of competition, however - 'one species coexists with or excludes another' - and even associating this with niche differentiation, whether based on resources themselves, or conditions or merely differences in space or time, actually provides us with rather little understanding of the competitive process. For this, as we have seen repeatedly in this chapter, we may need to focus more on the mechanisms of exploitation. How, precisely, does one species outexploit and outcompete another? How can two consumers coexist on two limiting resources, when both resources are absolutely essential to both consumers?
... based on resources and conditions spatial and temporal separation
Furthermore, as Tilman (1990) has pointed out, whilst monitoring the population dynamics of two competing species may give us some powers of prediction for the next time they compete, it will give us very little help in predicting how each would fare against a third species. Whereas, if we understood the dynamics of the interaction of all the species with their shared limiting resources, then we might be able to predict the outcome of exploitative competition between any given pair of the species. We therefore turn now to some attempts to explain the coexistence of species competing for limiting resources that explicitly consider not only the dynamics of the competing species but also the dynamics of the resources themselves. Rather than going into details, we examine the outlines of models and some major conclusions.
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