![]() ![]() With ambient food quality (no fertilizer, black symbols), spider predation and food limitation were compensatory: the same numbers of grasshoppers were recovered at the end of the 31-day experiment (Figure 9.8). This hypothesis was tested by Oedekoven and Joern (2000) who monitored grasshopper (Ageneotettix deorum) survivorship in caged prairie plots subject to fertilization (or not) to increase food quality in the presence or absence of lycosid spiders (Schizocoza spp.). Where food quantity or quality is higher, a given level of predation may not lead to a compensatory response because prey are not food-limited. Outcomes of predation may, therefore, vary with relative food availability. ![]() Indeed, whenever density is high enough for intraspecific competition to occur, the effects of predation on a population should be ameliorated by the consequent reductions in intraspecific competition. This was because the number of surviving pigeons was determined ultimately not by shooting but by food availability, and so when shooting reduced density, there were compensatory reductions in intraspecific competition and in natural mortality, as well as density-dependent immigration of birds moving in to take advantage of unexploited food.įigure 9.8 Trajectories of numbers of grasshoppers surviving (mean ± SE) for fertilizer and predation treatment combinations in a field experiment involving caged plots in the Arapaho Prairie, Nebraska, USA. Thus, in a classic experiment in which large numbers of woodpigeons (Columba palumbus) were shot, the overall level of winter mortality was not increased, and stopping the shooting led to no increase in pigeon abundance (Murton et al., 1974). The impact of predation is often limited by compensatory reactions amongst the survivors as a result of reduced intraspecific competition. (However, we have seen in other situations (see Section 9.2.5) that predispersal seed predation can profoundly affect seedling recruitment, local population dynamics and variation in relative abundance along environmental gradients and across microhabitats.) Hence, recruitment appears not to be limited by the number of seeds produced although whether it is limited by subsequent predation of seeds or early seedlings, or the availability of germination sites, is not clear (Crawley, 1989). Indeed, sowing 1000 thistle seeds per square meter also led to no observable increase in the number of thistle rosettes. For instance, the weevil Rhinocyllus conicus does not reduce recruitment of the nodding thistle, Carduus nutans, in southern France despite inflicting seed losses of over 90%. To deal with the second point first, if, for example, plant recruitment is not limited by the number of seeds produced, then insects that reduce seed production are unlikely to have an important effect on plant abundance (Crawley, 1989). Moreover, predation is least likely to affect prey dynamics if it occurs at a stage of the prey's life cycle that does not have a significant effect, ultimately, on prey abundance. In other words, whilst predation is bad for the prey that get eaten, it may be good for those that do not. Second, there may be compensatory changes in the growth, survival or reproduction of the surviving prey: they may experience reduced competition for a limiting resource, or produce more offspring, or other predators may take fewer of the prey. In the first place, the individuals that are killed (or harmed) are not always a random sample of the population as a whole, and may be those with the lowest potential to contribute to the population's future. However, these effects are not always so predictable, for one or both of two important reasons. Returning now to predators in general, it may seem that since the effects of predators are harmful to individual prey, the immediate effect of predation on a population of prey must also be predictably harmful. ![]()
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