1 Selective predators are likely to act to enhance species richness in a community if their preferred prey are competitively dominant and in situations where community productivity is high. It seems likely that there is some general correlation between palatability to predators and high growth rates. If the production of chemical and physical defenses by prey requires a sacrifice of resources used in growth and reproduction, we might expect species that are competitive dominants in the absence of predators (and hence, which devote resources to competition rather than defense) to suffer excessively from their presence. Thus, selective predators may often enhance
Figure 19.22 Manipulation of the density of cockles at the sediment surface, mimicking variation in infection levels by the trematode Curtuteria australis, and its effect on intertidal communities. Mean (± 1 SE) species richness of macrofaunal invertebrate species per plot and mean density of several invertebrate taxa in three experimental treatments (0, 30 and 100 cockles added to a 1m2 plot). All means are derived from five core samples taken per plot, with seven plots per treatment. (After Mouritsen & Poulin, 2005)
species richness. If the predators act in a frequency-dependent manner their action should be even stronger. Even very generalist predators may increase community diversity through exploiter-mediated coexistence, because if prey are attacked simply in proportion to their abundance, it will be those species that are assimilating resources and producing biomass and offspring most rapidly (the competitive dominants) that will be most abundant, and will therefore be most severely set back by predation. Note, however, that predators seem just as frequently to cause a reduction in species richness, or to have no effect.
2 An intermediate intensity of predation is most likely to be associated with high prey species richness, since too low an intensity may not prevent competitive exclusion of inferior prey species, whilst too high an intensity may itself drive preferred prey to extinction. (Note, however, that 'intermediate' is difficult to define a priori.)
3 The role of predators and parasites in shaping community structure may often be least significant in communities where physical conditions are more severe, variable or unpredictable (Connell, 1975). In sheltered coastal sites, predation appears to be a dominant force shaping community structure
(Paine, 1966), but in exposed rocky tidal communities where there is direct wave action, predators seem to be scarce and to have a negligible influence on community structure (Menge & Sutherland, 1976; Menge et al., 1986). Deep-sea hydrothermal vents provide an exception to these generalizations, probably because the physically severe circumstances close to the vents also engender very high levels of productivity.
4 The effects of animals on a community often extend far beyond just those due to the cropping of their prey. Burrowing animals (such as earthworms, rabbits and porcupines) and mound-builders (ants and termites) - and parasitized cockles - all create disturbances and act as ecosystem engineers (by modifying the physical structure of the environment) (Wilby et al., 2001). Their activities provide local heterogeneities, including sites for new colonists to become established and for microsuccessions to take place. Larger grazing animals introduce a mosaic of nutrient-rich patches, as a result of dunging and urinating, in which the local balance of other species is profoundly changed. Even the footprint of a cow in a wet pasture may so change the microenvironment that it is now colonized by species that would not be present were it not for the disturbance (Harper, 1977). The predator is just one of the many agents disturbing community equilibrium.
5 Carnivores that also feed at other trophic levels (omnivores) may have particularly far-reaching consequences for the community. For example, omnivorous freshwater crayfish can influence the composition of plants (which they consume), herbivores and carnivores (which they consume or with which they compete), and even detritivores because their extreme omnivory includes feeding on dead plant and animal material (Usio & Townsend, 2002, 2004). Moreover, they can also act as ecosystem engineers by dislodging animals and detritus as they move or burrow through the substrate (Statzner et al., 2000).
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