Organisms as food resources

Autotrophic organisms (green plants and certain bacteria) assimilate inorganic resources into packages of organic autotrophs and heterotrophs molecules (proteins, carbohydrates, etc.). These become the resources for heterotrophic organisms (decomposers, parasites, predators and grazers), which take part in a chain of events in which each consumer of a resource becomes, in turn, a resource for another consumer. At each link in this food chain the most obvious distinction is between saprotrophs and predators (defined broadly).

Saprotrophs - bacteria, fungi and detritivorous animals (see Chapter 11) - use other organisms, or parts of other organisms, as food but only after they have died, or they consume another organism's waste or secretory products.

Predators use other living organisms, or parts of other living organisms, as food. True predators predictably kill their prey. Examples include a mountain lion consuming a rabbit but also consumers that we may not refer to as predators in everyday speech: a water flea consuming phytoplankton cells, a squirrel eating an acorn, and even a pitcherplant drowning a mosquito. Grazing can also be regarded as a type of predation, but the food (prey) organism is not killed; only part of the prey is taken, leaving the remainder with the potential to regenerate. Grazers feed on (or from) many prey during their lifetime. True predation and grazing are discussed in detail in Chapter 9. Parasitism, too, is a form of predation in which the consumer usually does not kill its food organism; but unlike a grazer, a parasite feeds from only one or a very few host organisms in its lifetime (see Chapter 12).

An important distinction amongst animal consumers is whether they are specialized or generalized in their diet. Generalists (polyphagous species) take a wide variety of prey species, though they very often have clear preferences and a rank order of what they will choose when there are alternatives available. Specialists may consume only particular parts of their prey though they range over a number of species. This is most common among herbivores because, as we shall see, different parts of plants are quite different in their composition. Thus, many birds specialize on eating seeds though they are seldom restricted to a particular species. Other specialists, however, may feed on only a narrow range of closely related species or even just a single species (when they are said to be monophagous). Examples are caterpillars of the cinnabar moth (which eat the leaves, flower buds and very young stems of species of ragwort, Senecio) and many species of host-specific parasites.

Many of the resource-use patterns found among animals reflect the different lifespans of the consumer and what it consumes. Individuals of long-lived species are likely to be gener-alists: they cannot depend on one food resource being available throughout their life. Specialization is increasingly likely if a consumer has a short lifespan. Evolutionary forces can then shape the timing of the consumer's food demands to match the timetable of its prey. Specialization also allows the evolution of structures that make it possible to deal very efficiently with particular resources - this is especially the case with mouthparts. A structure like the stylet of an aphid (Figure 3.22) can be interpreted as an exquisite product of the evolutionary process that has given the aphid access to a valuable food resource - or as an example of the ever-deepening rut of specialization that has constrained what aphids can feed on. The more specialized the food resource required by an organism, the more it is constrained to live in patches of that resource or to spend time and energy in searching for it among a mixture of resources. This is one of the costs of specialization.

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