Predator Prey Interactions

An example of the ways spatial patterns influence ecological processes is provided by the dynamics between predators and their prey. Spatial structuring of populations may rescue prey from extinction, in many cases where conventional models may provide little hope for their survival. An example is given by the two-spotted spider mite (Tetranychus urticae), which is an important pest in commercial greenhouses. Many growers control this pest species by introducing predatory mites, such as Phytoseilus persimilis, which maintain spider mites at low densities without eradicating them completely. Though the introduced mites are voracious predators, spatial structuring on the leaves and plants of the greenhouse maintains the coexistence and persistent survival of both the spider mites and their predators. Small populations of the spider mites survive in temporary 'refugia' on the leaves of plants, even in greenhouses where the overall density of predatory mites is otherwise large enough to drive the spider mites to extinction.

A large-scale example of the influence of spatial patterns on the outcome of predator-prey dynamics is given by the population dynamics of microtine voles. Vole populations (mainly species of Microtus and Clethrionomys) in Fennoscandia exhibit a wide range of population dynamics patterns, from regular multiannual cycles in the north gradually shifting to stable or biennially fluctuating populations toward the south. Several explanations have been proposed to explain these dynamics, those attracting most attention in Fennoscandia being interactions with predators.

These hypotheses seek to explain spatial differences in dynamics between northern and southern populations by variations in predator composition and density, together with changes in landscape structure. The basic idea is that the specialist predators which are common in northern regions, such as mustelids (Mustela sp.), can generate population fluctuations since their numbers are strongly coupled to those of their prey. This means that when there are few voles, there will be few predators. This allows vole populations to grow rapidly, followed by an increase in the number of predators, and so on.

Generalist predators, on the other hand, may switch to other prey types when vole densities are low, and hence their population numbers are much less strongly coupled to the density of voles. These predators are thus thought to stabilize the density fluctuations of the microtines.

Habitat structure in Fennoscandia may also influence the population dynamics of voles. Whereas the landscape in northern Scandinavia is characterized by large tracts of homogenous habitat, southern regions are dominated by agricultural land with multiple patches of different habitat types. The variation in habitats in the south allows a greater number of prey and predator species to exist, and favors generalist predators which stabilize the population fluctuations of the prey.

An added effect of habitat fragmentation is that the isolated habitat patches occurring in fragmented landscapes each supports separate subpopulations of voles, with relatively independent population dynamics. This independence prevents the abundances of predator species from tracking those of their prey too closely, and also facilitates local outbreaks in prey abundance in those patches where the subpopulations have gone undiscovered by predators. However, even though the prey density of local patches shows high temporal variability, the asynchrony between such localized outbreaks ensures that average density, when viewed at a large spatial scale, remains relatively constant over time.

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