The removal of a species (experimentally, managerially or naturally) can be a powerful tool in unraveling the workings of a food web. If a predator species is removed, we expect an increase in the density of its prey. If a competitor species is removed, we expect an increase in the success of species with which it competes. Not surprisingly, there are plenty of examples of such expected results.
Sometimes, however, removing a species may lead to a decrease in competitor abundance, or the removal of a predator may lead to a decrease in prey abundance. Such unexpected effects arise when direct effects are less important than the effects that occur through indirect pathways. Thus, the removal of a species might increase the density of one competitor, which in turn causes another competitor to decline. Or the removal of a predator might increase the abundance of a prey species that is competitively superior to another, leading to a decrease in the density of the latter. In a survey of more than 100 experimental studies of predation, more than 90% demonstrated statistically significant results, and of these about one in three showed unexpected effects (Sih et al., 1985).
These indirect effects are brought especially into focus when the initial removal is carried out for some managerial reason - either the biological control of a pest (Cory & Myers, 2000) or the eradication of an exotic, invader species (Zavaleta et al., 2001)
- since the deliberate aim is to solve a problem, not create further, unexpected problems.
For example, there are many islands mesopredators on which feral cats have been allowed to escape domestication and now threaten native prey, especially birds, with extinction. The 'obvious' response is to eliminate the cats (and conserve their island prey), but as a simple model developed by Courchamp et al. (1999) explains, the programs may not have the desired effect, especially where, as is often the case, rats have also been allowed to colonize the island (Figure 20.1). The rats ('mesopredators') typically both compete with and prey upon the birds. Hence, removal of the cats ('superpredators'), which normally prey upon the rats as well as the birds, is likely to increase not decrease the threat to the birds once predation pressure on the mesopredators is removed. Thus, introduced cats on Stewart Island, New Zealand preyed upon an endangered flightless parrot, the kakapo, Strigops habroptilus (Karl & Best, 1982);
Figure 20.1 (a) Schematic representation of a model of an interaction in which a 'superpredator' (such as a cat) preys both on 'mesopredators' (such as rats, for which it shows a preference) at a per capita rate |lr, and on prey (such as birds) at a per capita rate |lb, while the mesopredator also attacks prey at a per capita rate nb. Each species also recruits to its own population at net per capita rates rc, rr and rb. (b) The output of the model with realistic parameter values: with all three species present, the superpredator keeps the mesopredator in check and all three species coexist (left); but in the absence of the superpredator, the mesopredator drives the prey to extinction (right). (After Courchamp et al., 1999.)
but controlling cats alone would have been risky, since their preferred prey are three species of introduced rats, which, unchecked, could pose far more of a threat to the kakapo. In fact, Stewart Island's kakapo population was translocated to smaller offshore islands where exotic mammalian predators (like rats) were absent or had been eradicated.
Further indirect effects, though not really 'unexpected', have occurred following the release of the weevil, Rhinocyllus conicus, as a biological control agent of exotic thistles, Carduus spp., in the USA (Louda et al., 1997). The beetle also attacks native thistles in the genus Cirsium and reduces the abundance of a native picture-winged fly, Paracantha culta, which feeds on thistle seeds - the weevil indirectly harms species that were never its intended target.
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