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predators, grazers and parasites

‘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 (both herbivorous predators), and even a pitcher plant 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 to live on with the potential to reproduce or regenerate. Also, grazers feed on (or from) many prey during their lifetime. Cattle and sheep are grazers of plants, but blood‐sucking flies, for example, are carnivorous grazers. 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. Chapter 12 is devoted to parasitism.

specialists and generalists

An important distinction amongst animal consumers is whether they are specialised or generalised 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. Some specialists consume only particular parts of their prey though they range over a number of species. This is most common among herbivores because, as we saw in Figure 3.26 and shall see again in Figure 3.29, different parts of plants are quite different in their composition. Thus, many birds specialise in 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.

Figure 3.29 The composition of various plant parts and of the bodies of animals that serve as food resources for other organisms.

Data from various sources.

the importance of lifespan

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 generalists: they cannot depend on one food resource being available throughout their life. Specialisation 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. Specialisation also allows the evolution of structures that make it possible to deal very efficiently with particular resources. This is especially the case with mouthparts. Darwin’s hawkmoth, Xanthopan morganii praedicta, with its 20 cm long proboscis, is alone in being able to take nectar and pollen from the Madagascan orchid, Angraecum sesquipidale, with its near‐30 cm long nectary. (It is called Darwin’s hawkmoth, because Charles Darwin predicted its existence on seeing the flower 20 years before the moth itself was discovered.) This can be interpreted as an exquisite product of the evolutionary process that has given the moth access to a valuable food resource – or as an example of the ever‐deepening rut of specialisation that has constrained what the moth can feed on. The more specialised 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 specialisation. We return to food preferences and diet widths in Section 9.2.