Predators occupy the upper levels of food chains. This top position they occupy helps explain, especially in the case of predators feeding mainly or exclusively on other predators (as are a few birds ofprey specialized on snakes and lizards), their generally low density, their frequently spaced distribution, and their usually small total biomass. Behavioral peculiarities do often cause quite a low efficiency in the energy transfer from prey to predator, as the latter often consumes only selected or limited parts of the killed prey, whose remains are largely exploited by animals of other species.
In tracing a predator's role in the transfer of energy along the food web, it is also important to check to what extent the preys have actually grown in the same habitat where the predator consumes them and where it will eventually leave their unused remains or release its still exploitable excrements. Possible lack of correspondence between habitat of origin and habitat of destination of transferred energy is often due to the high mobility of predators, sometimes also to the peculiar habitat where they spend their life, at the interface between two largely different worlds. Bats and skaters, respectively, are good examples of the two situations. In the caves many of them use as shelters for their resting hours, bats can produce very large amounts of guano that will be exploited there, directly or indirectly, by organisms as diverse as moulds and beetles, but the actual source of this stuff is completely external to the cave habitat, as it consists of insects or other food gathered by the bats outside the cave. Skaters are long-legged bugs which spend most of their life skating on the surface of lakes and ponds into which they are likely to release their excrements, but the preys on which they feed are, to a large or very large extent, terrestrial insects accidentally ending up on the water surface, rather than aquatic or amphibious animals.
A key feature to be considered when placing a predator in the context of a food web is the degree to which it specializes on one or a restricted set of prey species. As in interspecific relationships generally, however, one must be aware that the range of preys actually exploited by an individual predator, or by a local population, is very often less diverse than the total prey spectrum acceptable to the predator species. As a rule, predators are less selective than parasites or parasitoids (which often specialize for just one host, or a very small number of similar host species). Nevertheless, there are singularly specialized predators, as are some beaked dolphins (Mesoplodon spp.), whose diet is restricted to squids falling within a specified, restricted range of size.
Mathematical models of optimal foraging theory predict that predators will be mostly opportunistic in respect to prey, as this strategy will release them from dependence on the changing and often unpredictable availability of individual kinds of prey. Food preferences often change seasonally and may temporarily include episodes of allotrophy, that is feeding on alternative food items, including food other than prey. Plant matters enter less than accidentally in the diet of animals - for example, many carnivores among mammals, and even some woodpeckers among birds - which nevertheless mainly behave as predators. Many birds of prey feed occasionally on carrion. Allotrophy is also widespread among insects, an example being the 7-spot ladybird, whose diet is mainly based on aphids (plant lice) but often includes pollen, too.
Sound knowledge of the degree of specialization of a predator is necessary, if we want to understand either the chances of survival of the predator in a changing ecosystem, or the possible consequences of its removal from the native ecosystem, or of its introduction into a novel one. Strictly specialized (stenophagous) predators can be successful in predictable and stable environments, but will obviously have little chance of survival in the case of unexpected environmental change. For broadly the same reason, we shall expect introduced predators to be more successful, when faced with the challenges of a new animal community, if they are food generalists (eurypha-gous) than otherwise. For these reasons, when looking for a natural agent capable of controlling outbreaks of an insect pest, the applied entomologist's first choice is generally a parasitoid rather than a predator; the second choice is an omnivorous predator, because it will probably be more effective than a stenophagous predator in suppressing prey population right because it can survive, on necessity, on alternative food items; this is by now well supported by observation and experiment alike.
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