Assessment: some general features of interspecific competition

 

Individuals of different species can compete. This is hardly surprising. 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 spp.), or may coexist (as with the salamanders), perhaps by utilizing the habitat in slightly different ways (e.g. the barnacles and the second pair of Paramecium spp.).

But what about the tit study? Certainly the five species coexisted and utilized the habitat in slightly different ways. But did this have anything to do with competition? Lack thought so. In Connell’s phrase, he invoked 'the ghost of competition past'. In other words, he believed that they coexisted 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 favour 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 data is that there is no proof. We cannot go back ii time to check whether the species ever competed more than they do now, and it was not even part of the study to determine the extent o[ present-day competition. One plausible alternative interpretation, therefore, is that ' the species have, in the course of their evolution, responded to natural selection it 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, it might seem that the coexistence of the tits has nothing to do with competition. On the other hand, it may be that competition in the past eliminated a number of other species, leaving behind, only those that were different in their utilization of the habitat. In other words, we can still see the hand of the ghost of competition past, but it acted as an ecological force (eliminating species), rather than an evolutionary one (changing them),

The tit study 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, as we have seen, 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.

The second point, however, 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 salamanders and barnacles, indicate the presence of current competition, if, say, 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 independent evolution of non-competing species. In fact, for many sets of data, there are no easy or agreed methods of distinguishing between these explanations.

For now, though, what other general features emerge from our examples? As with intraspecific competition, a basic distinction can be made between interference and exploitation competition (although elements of both may be found in a single interaction). With exploitation, individuals interact with each other indirectly, responding to a resource level that has been depressed by the activity of competitors. The diatom work provides a clear example of this. By contrast, Connell's barnacles provide an equally clear example of interference competition. Balanus, in particular, directly and physically interfered with the occupation by Chthamalus of limited space on the rocky substratum.

Interference, on the other hand, is not always as direct as this. Amongst plants, it has often been claimed that interference occurs through the production of chemicals that are toxic to other species but not to the producer (known as allelopathy). There is no doubt that chemicals with such properties can be extracted
from plants, but their role is controversial. Allelopathy has been proposed with enthusiasm, welcomed and supported with enthusiasm, dismissed with great skepticism, treated as an established, uncontroversial fact and suggested soberly as a subject for serious enquiry. Amongst competing tadpole species, too, water-borne inhibitory products have been implicated as a means of interference (most notably, perhaps, an alga produced in the faeces of the common frog, Rana temporaria, inhibiting the natterjack toad, Bufo calamita, but here again their importance in nature is unclear. Of course, the production by fungi, actinomycetes and bacteria—especially those found in the soil—of chemicals that inhibit the growth of potentially competing micro­organisms is widely recognized—and exploited in the selection and production of antibiotics.

Interspecific competition (like intraspecific competition) is frequently highly asymmetric—the consequences are often not the same for both species. For instance, with Connell's barnacles, Balanus excluded Chthamalus from their zone of potential overlap, but any effect of Chthamalus on Balanus was negligible: Balanus was limited by its own sensitivity to desiccation. A closely analogous situation is provided by two species of cattail (reedmace) in ponds in Michigan, where one species, Typha latifolia, was rnostly found in shallower water, whilst the other, T.angustifolia, occurred in deeper water. Experimental manipulations suggested that amongst newly established plants, T.latifolia   normally excludes T. angustifolia from shallower water. But the distribution of T.latifolia is unaffected by competition with T.angustifolia. On the other hand, in the longer term, after establishment is complete, the asymmetry is largely reversed: at all but the shallowest depths, T. angustifolia expands at the expense of T. latifolia and is apparently unaffected by its presence.

On a broader front, it seems that highly asymmetric cases of interspecific competition (where one species is little affected) outnumber symmetric cases by around two to one in insects in herbaceous plants and more generally, too. The more fundamental point, however, is that there is a continuum linking the perfectly symmetric competitive cases to the 'perfectly' asymmetric ones. For instance, showed this in a study of 'overgrowth competition' amongst bryozoan species (colonial, modular animals) living on the undersurfaces of corals off the coast of Jamaica. For the pair-wise interactions amongst the seven most commonly interacting species, he found that the 'percentage wins' varied more or less continuously from 50% (perfect symmetry) to 100% (perfect asymmetry). Finally, it is worth noting that competition for one resource often affects the ability of an organism to exploit another resource. For example, Buss (1979), showed that in bryozoan overgrowth interactions, there appears to be an interdependence between competition for space and for food. When a colony of one species contacts a colony of another species, it interferes with the self-generated feeding currents on which bryozoans rely (competition for space affects feeding).
But a colony short of food will, in turn, have a much reduced ability to compete for space (by overgrowth).

Comparable examples are found amongst rooted plants. lf one species invades the canopy of another and deprives it of light, the suppressed species will suffer directly from the reduction in light energy that it obtains, but this will also reduce its rate of root growth, and it will therefore be less able to exploit the supply of water and nutrients in the soil. This in turn will reduce its rate of shoot and leaf growth. Thus, when plant species compete, repercussions flow backwards and forwards between roots and shoots. A number of workers have attempted to separate the effects of canopy and root competition by an experimental design in which two species are grown: (i) alone; (ii) together; (iii) in the same soil, but with their canopies separated; and (iv) in separate soil with their canopies intermingling. One example is a study of subterranean cloverand skeleton weed. The clover was not significantly affected under any circumstances (another example of asymmetric competition). However, the skeleton weed was affected when the roots intermingled (reduced to 65% of the control value of dry weight) and when the canopies intermingled (47% of the control). When both intermingled, the effect was multiplicative, dry weight being reduced to 31% of the control, compared with the 3O.6% (65 x 47%) that might have been expected.

 

THE NATURE OF PREDATION