Figure 8.9a and b describes cases in which a strong interspecific competitor invariably outcompetes a weak interspecific competitor. It is useful to consider this situation from the point of view of niche theory (see Sections 2.2 and 3.8). Recall that the niche of a species in the absence of competition from other species is its fundamental niche (defined by the combination of conditions and resources that allow the species to maintain a viable population). In the presence of competitors, however, the species may be restricted to a realized niche, the precise nature of which is determined by which competing species are present. This distinction stresses that interspecific competition reduces fecundity and survival, and that there may be parts of a species' fundamental niche in which, as a result of interspecific competition, the species can no longer survive and reproduce successfully. These parts of its fundamental niche are absent from its realized niche. Thus, returning to Figures 8.9a and b, we can say that the weak interspecific competitor lacks a realized niche when in competition with the stronger competitor. The real examples of interspecific competition previously discussed can now be re-examined in terms of niches.
In the case of the diatom species, the fundamental niches of both species were provided by the laboratory regime (they both thrived when alone). Yet when Synedra and Asterionella competed, Synedra had a realized niche whilst Asterionella did not: there was competitive exclusion of Asterionella. The same outcome was recorded when Cause's P. aurelia and P. caudatum competed; P. caudatum lacked a realized niche and was competitively excluded by P. aurelia. When P. caudatum and P. bursaria competed, on the other hand, both species had realized niches, but these niches were noticeably different: P. caudatum living and feeding on the bacteria in the medium, P. bursaria concentrating on the yeast cells on the bottom of the when interspecific competition is more important than intraspecific, the outcome depends on the species' densities when interspecific competition is less important than intraspecific, the species coexist fundamental and realized niches coexisting competitors often exhibit a differentiation of their realized niches tube. Coexistence was therefore associated with a differentiation of realized niches, or a 'partitioning' of resources.
In the Galium experiments, the fundamental niches of both species included both acidic and calcareous soils. In competition with one another, however, the realized niche of G. hercynicum was restricted to acidic soils, whilst that of G. pumilum was restricted to calcareous ones - there was reciprocal competitive exclusion. Neither habitat allowed niche differentiation, and neither habitat fostered coexistence.
Amongst Taniguchi and Nakano's salmonid fishes, the fundamental niches of each species extended over a broad range in altitude (and temperature) but both were restricted to a smaller realized niche (Dolly Varden at higher altitudes and white-spotted charr at lower altitudes).
Similarly, amongst Connell's barnacles, the fundamental niche of Chthamalus extended down into the Balanus zone, but competition from Balanus restricted Chthamalus to a realized niche higher up the shore. In other words, Balanus competitively excluded Chthamalus from the lower zones, but for Balanus itself, even its fundamental niche did not extend up into the Chthamalus zone: its sensitivity to desiccation prevented it surviving even in the absence of Chthamalus. Hence, overall, the coexistence of these species was also associated with a differentiation of realized niches.
The pattern that has emerged from these examples has also been uncovered in many others, and has been elevated to the status of a principle: the Competitive Exclusion Principle or 'Cause's Principle'. It can be stated as follows: if two competing species coexist in a stable environment, then they do so as a result of niche differentiation, i.e. differentiation of their realized niches. If, however, there is no such differentiation, or if it is precluded by the habitat, then one competing species will eliminate or exclude the other. Thus exclusion occurs when the realized niche of the superior competitor completely fills those parts of the inferior competitor's fundamental niche that are provided by the habitat.
When there is coexistence of competitors, a differentiation of realized niches is sometimes seen to arise from current competition (an 'ecological' effect), as with the barnacles. Often, however, the niche differentiation is believed to have arisen either as a result of the past elimination of those species without realized niches (leaving behind only those exhibiting niche differentiation - another ecological effect) or as an evolutionary effect of competition. In either case, present competition may be negligible or at least impossible to detect. Consider again the coexisting tits. The species coexist and exhibit differentiation of their realized niches. But we do not know whether they compete now, or have ever competed in the past, or whether other species have been competitively excluded in the past. It is impossible to say with certainty whether the Competitive Exclusion Principle was relevant. If the species do actually compete currently, or if other species are being or have been competitively excluded, then the Principle is relevant in the strictest sense. If they competed only in the past, and that competition has led to their niche differentiation, then the Principle is relevant, but only if it is extended from applying to the coexistence of 'competitors' to the coexistence of 'species that are or have ever been competitors'. Of course, if the species have never competed, then the Principle is of no relevance here. Clearly, interspecific competition cannot be studied by the mere documentation of present interspecific differences.
With Martin and Martin's warblers, on the other hand, the two species competed and coexisted, and the Competitive Exclusion Principle would suggest that this was a result of niche differentiation. But, whilst reasonable, this is by no means proven, since such differentiation was neither observed nor shown to be effective. Thus, when two competitors coexist, it is often difficult to establish positively that there is niche differentiation. Worse still, it is impossible to prove the absence of it. When ecologists fail to find differentiation, this might simply mean that they have looked in the wrong place or in the wrong way. Clearly, there can be very real methodological problems in establishing the pertinence of the Competitive Exclusion Principle in any particular case.
The Competitive Exclusion Principle has become widely accepted because: (i) there is much good evidence in its favor; (ii) it makes intuitive good sense; and (iii) there are theoretical grounds for believing in it (the Lotka-Volterra model). But there will always be cases in which it has not been positively established; and as Section 8.5 will make plain, there are many other cases in which it simply does not apply. In short, interspecific competition is a process that is often associated, ecologically and evolutionarily, with a particular pattern (niche differentiation), but interspecific competition and niche differentiation (the process and the pattern) are not inextricably linked. Niche differentiation can arise through other processes, and interspecific competition need not lead to a differentiation of niches.
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