Summary

In interspecific competition, individuals of one species suffer a reduction in fecundity, growth or survivorship as a result of resource exploitation or interference by individuals of another species. Competing species may exclude one another from particular habitats so that they do not coexist, or may coexist, perhaps by utilizing the habitat in slightly different ways. Interspecific competition is frequently highly asymmetric.

Although species may not be competing now, their ancestors may have done so in the past. Species may have evolved characteristics that ensure they compete less, or not at all, with other species. Moreover, species whose niches appear differentiated may have evolved independently and, in fact, never have the more limiting resources there are, the more species may coexist

Figure 8.37 Variation in phytoplankton species diversity (Simpson's index) with depth in 2 years in three large lakes in the Yellowstone region, USA. Shading indicates depth-time variation in a total of 712 discrete samples: dark orange areas denote high species diversity, and gray areas denote low species diversity. (After Interlandi & Kilham, 2001.)

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Measured limiting resources

Figure 8.38 Phytoplankton diversity (Simpson's index; mean ± SE) associated with samples with different numbers of measured limiting resources. It was possible to perform this analysis on 221 samples from those displayed in Figure 6.14. The number of samples (n) in each limiting resource class is shown. (From Interlandi & Kilham, 2001.)

competed, now or historically. An experimental manipulation (for instance, the removal of one or more species) can 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.

Mathematical models, most notably the Lotka-Volterra model, have provided important insights into the circumstances that permit the coexistence of competitors, and those that lead to competitive exclusion. However, the simplified assumptions of the Lotka-Volterra model limit its applicability to real situations in nature. We know from other models and experiments that the outcome of interspecific competition can be strongly influenced by heterogeneous, inconstant or unpredictable environments. Coexistence of a superior and an inferior competitor on a patchy and ephemeral resource can occur if the two species have independent, aggregated distributions over the available patches.

We describe the range of approaches used to study both the ecological and evolutionary effects of interspecific competition, paying particular attention to experiments in the laboratory or field (e.g. substitutive, additive, response surface analysis) and natural experiments (e.g. comparing niche dimensions of species in sympatry and allopatry). The important question of whether a minimum amount of niche differentiation is required for stable coexistence is much easier to pose than answer.

The chapter concludes by acknowledging the need to consider not just the population dynamics of the competing populations but also the dynamics of the resources for which they are competing, if we wish to achieve a full understanding of interspecific competition and species coexistence.

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