The widths and compositions of diets

Consumers can be classified as either monophagous (feeding on a single ргеу-type), oligophagous (few prey-types) or polyphagous (many prey-types). Often, an equally useful distinction is between specialists (broadly, monophages and oligophages) and generalists (polyphages). Herbivores, parasitoids and true predators can all provide examples of monophagous, oligophagous and polyphagous species. But the distribution of diet widths differs amongst the various types of consumer. True ргеdators with specialized diets do exist; for instance the Everglades kite (Rostraharmis sociabilis) feeds almost entirely on snails of the genus Pomacea— but most true predators have relatively broad diets. Parasitoids, on the other hand, are typically specialized and may even be monophagous. Herbivores are well represented in all categories, but whilst grazing and 'predatory' herbivores typically have broad diets, 'parasitic' herbivores are very often highly specialized. For instance, Janzen (1980) examined 110 species of beetle that feed as larvae inside the seeds of dicotyledonous plants in Costa Rica ('parasitizing' them) and found that 83 attacked only one plant species, 14 attacked only two, nine attacked three, two attacked four, one attacked six and one attacked eight. This was in spite of there being 975 plant species present in the area.

Food preferences

It must not be imagined that polyphagous and oligophagous species are indiscrimi­nate in what they choose from their acceptable range. On the contrary, some degree of preference is almost always apparent. An animal is said to exhibit a preference for a particular type of food when the proportion of that type in the animal's diet is higher than its proportion in the animal's environment. To measure food preference in nature, therefore, it is necessary not only to examine the animal's diet (usually by the analysis of gut contents) but also to assess the 'availability' of different food types. Ideally, this should be done not through the eyes of the observer (i.e. not by simply sampling the environment), but through the eyes of the animal itself.

The exact quantification of preference is therefore fraught with difficulties—but it is generally much easier at least to establish that a preference does exist. For instance, the results of an accidental field experiment, in which deer broke into a tree plantation, provide a nice example. The plantation contained equal numbers of four species arranged at random: white pine, red pine, jack pine and white spruce. As theTable shows, the deer, with free access to all four species, exhibited a fairly consistent preference for jack pine, followed by white pine, with red pine being only lightly browsed and white spruce ignored.

A food preference can be expressed in two rather different contexts. There can be a preference for items that are the most valuable amongst those available, or for items that provide an integral part of a mixed and balanced diet. These will be referred to as ranked and balanced preferences, respectively.

A feeding preference: the percentages of various planted trees browsed by deer when they broke into a plantation in which the four species of tree were equally abundant and arranged at random. (After Hortrin, 1964.)

White pine                       Red pine Jack pine White spruce

Winter 1956-1957         31       19       84   0

Winter 1958-1959          9        1         48 0

Winter 1960-1961          17      0       70   0

 

There are two important reasons why a mixed diet may be favoured. First, consumers may accept low-quality items simply because, having encountered them, they have more to gain by eating them (poor as they are) than by ignoring them and continuing to search. Second, consumers may benefit from a mixed diet because each food type contains a different undesirable toxic chemical. A mixed diet would then keep the concentrations of all of these chemicals within acceptable limits. It is certainly the case that toxins can play an important role in food preference. For instance, one study examined the winter food of a variety of Arctic animals: three species of ptarmigan, three grouse, capercaillie, two kinds of hare and the moose (Bryant & Kuropat, 1980). In each case, the conclusions were the same: animals ranked their foods on neither energy nor nutrient content. Instead, preference was strongly and negatively correlated with the concentrations of certain toxins. Overall, however, it would be quite wrong to give the impression that all preferences have been clearly linked with one explanation or another. For example, Thompson (1988) has reviewed the relation­ship between the oviposition preferences of phytophagous insects and the perfor­mance of their offspring on these foodplants in terms of growth,- survival and reproduction. A number of studies have shown a good association (i.e. females preferentially oviposit on plants where their offspring perform best), but in many others the association is poor. In such cases, there is generally no shortage of explanations for the apparently unsuitable behaviour, but these explanations are, as yet, often just untested hypotheses.

Switching

The preferences of many consumers are fixed, i.e. they are maintained irrespective of the relative availabilities of alternative food types. But many others switch their preference, such that food items are eaten disproportionately often when they are common and are disproportionately ignored when they are rare. One study shows the fixed preference exhibited by predatory shore snails when they were presented with two species of mussel prey at a range of proportions. The assumption was made that they exhibited the same preference at all proportions. This assumption is clearly justified: irrespective of availability, the predatory snails showed the same marked preference for the thin-shelled, less protected Mytilus   edulis, which they could exploit more effectively. By contrast, when guppies (a species of fish) were offered a choice between fruit-flies and tubificid worms as prey they clearly switched their preference, and consumed a disproportionate number of the more abundant prey-type.

There are a number of situations in which switching can arise. Probably the most common is where different types of prey are found in different microhabitats, and the consumers concentrate on the most profitable microhabitat.

Switching in a population often seems to be a consequence not of individual consumers gradually changing their preference, but of the proportion of specialists changing. This is illustrated by a study of switching in woodpigeons feeding on maple peas and tic beans. When the two were equally abundant there was a slight preference for maple peas; but when there were 82% tic beans on offer, the birds switched to an average of 91% tic beans in their diet. This average, however, included two birds that specialized on the rarer maple peas, taking only 5% and 0% tic beans.

 

Part III

Task 1