These examples show that individuals of different species can compete. This is hardly surprising. The field experiments with barnacles and warblers also show that different species do compete in nature (i.e. there was a measurable interspecific reduction in abundance and/or fecundity and/or survivorship). It seems, moreover, that competing species may either exclude one another from particular habitats so that they do not coexist (as with the bedstraws, the diatoms and the first pair of Paramecium species), or may coexist, perhaps by utilizing the habitat in slightly different ways (e.g. the barnacles and the second pair of Paramecium species).
But what about the story of the coexisting tits? Certainly the five bird species coexist and utilize the habitat in slightly different ways. But does this have anything to do with competition? It may do. It may be that the five species of tit coexist as a result of evolutionary responses to interspecific competition. This requires some further explanation. When two species compete, individuals of one or both species may suffer reductions in fecundity and/or survivorship, as we have seen. The fittest individuals of each species may then be those that (relatively speaking) escape competition because they utilize the habitat in ways that differ most from those adopted by individuals of the other species. Natural selection will then favor such individuals, and eventually the population may consist entirely of them. The two species will evolve to become more different from one another than they were previously; they will compete less, and thus will be more likely to coexist.
The trouble with this as an explanation for the tit story is that there is no proof. We need to beware, in Connell's (1980) phrase, of uncritically invoking the 'ghost of competition past'. We cannot go back in time to check whether the species ever competed more than they do now. A plausible alternative interpretation is that the species have, in the course of their evolution, responded to natural selection in different but entirely independent ways. They are distinct species, and they have distinctive features. But they do not compete now, nor have they ever competed; they simply happen to be different. If all this were true, then the coexistence of the tits would have nothing to do with competition. Alternatively again, it may be that competition in the past eliminated a number of other species, leaving behind only those that are different in their utilization of the habitat: we can still see the hand of the ghost of competition past, but acting as an ecological force (eliminating species) rather than an evolutionary one (changing them).
The tit story, therefore, and the difficulties with it, illustrate two important general points. The first is that we must pay careful, and separate, attention to both the ecological and the evolutionary effects of interspecific competition. The ecological effects are, broadly, that species may be eliminated from a habitat by competition from individuals of other species; or, if competing species coexist, that individuals of at least one of them suffer reductions in survival and/or fecundity. The evolutionary effects appear to be that species differ more from one another than they would otherwise do, and hence compete less (but see Section 8.9).
The second point, though, is that there are profound difficulties in invoking competition as an explanation for observed patterns, and especially in invoking it as an evolutionary explanation. An experimental manipulation (for instance, the removal of one or more species) can, as we have seen with the warblers, indicate the presence of current competition if it leads to an increase in the fecundity or survival or abundance of the remaining species. But negative results would be equally compatible with the past elimination of species by competition, the evolutionary avoidance of competition in the past, and the independent evolution of noncompeting species. In fact, for many sets of data, there are no easy or agreed methods of distinguishing between these explanations (see Chapter 19). Thus, in the remainder of this chapter (and in Chapter 19) when examining the ecological and, especially, the evolutionary effects of competition, we will need to be more than usually cautious.
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