Habitats suitable to the predator's activity may be continuous over large distances, thus allowing survival of a predator's populations, but may be otherwise fragmented. In this case, the size of individual habitat patches as well as the existence of affordable corridors between patches can be critically important. Limited patch size may induce a predator to be less selective in the choice of prey items, in respect to the preferences it may show in less restricted conditions. This is one of the reasons why studies ofpredator-prey relationship cannot rely on comprehensive lists of acceptable preys utilized by the predator under different ecological conditions or in different geographical areas, but must be verified in the actual site and habitat conditions where the research is carried on. In the case of very fragmented habitats, the nature of the background matrix into which the relevant habitat patches are spread is also clearly of importance. Very poor background may temporarily cause unusual concentration of species in the residual higher-quality patches, thus releasing novel prey-predator relationships or unexpected forms of competition among predators.
There are several reasons for the accumulation of predators in structurally complex habitats. One is abundance of resources, especially prey, but also alternative food items to be exploited if required (see above about allo-trophy). Another reason is the reduced risk of falling prey to other predators or even to members of their own species. Additionally, a predator may find there better microclimatic conditions and shelter from unfavorable atmospheric events. Finally, structurally complex habitats may enhance foraging efficiency by allowing better use of specific capture strategies, especially at ambush. For example, in many habitats with stratified vegetation, the diversity of spider species is positively correlated with the geometrical complexity of the plant cover.
In addition to a possible direct effect by determining the habitat's structural complexity, plant species diversity can also have indirect effects on predators, by providing living conditions to more or less rich guilds of plant-eating preys.
Small-size predators such as phytoseiids (a family of mites often employed in the biological control of insect pests), ladybirds (larvae and adults), or the larvae of lacewings (neuropteran insects) are additionally affected by structural details such as the structure of leaf surface where, for example, the presence of hair may impair their walking in the search for prey.
One ofthe reasons why a predator cannot generally rely on only one prey species is the latter's seasonal availability, which rarely coincides with the temporal distribution of the predator's food requirements. Many small prey species, especially insects and other invertebrates, have several generations per year, and these do not necessarily overlap, especially in temperate regions. For examples, in large parts of Europe and North America, many butterflies have two or three generations whose flight times are separated by weeks during which no adult specimen can be found on the wing. Clearly, a predator eventually interested in this kind of prey cannot rely exclusively on it. No lesser problem is caused by potential preys prolonging their life cycles excessively, and showing up massively at very large intervals, as do cicadas, some North American species of which spend 13 or 17 years deep in the soil as juveniles, to eventually emerge as adults, in huge synchronized masses that will never show up again until their offspring will complete the next cycle.
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