Diet selection – an ecological perspective

Abstract

The traditional role of agricultural research has been to maximize the efficiency of food production. Agriculture has advanced by progressively removing nutritional, environ-mental and genetic constraints on production from organisms whose properties lend themselves to this endeavour. Shelter is provided for our domestic animals. Contact with natural enemies such as predators, parasites and disease organisms is minimized, as is competition for resources with other species. Seasonal shortages of food are avoided and foraging is greatly simplified. The need to seek out a mate is largely eliminated; indeed, large modern breeds of turkeys can no longer mate naturally. Competition with conspecifics for resources such as food, shelter and mates has largely disappeared. Weapons and rituals that once played essential roles in the search for social status, and the access to resources which status ensures, are now functionless relics of a more natural past. Natural selection for fit, competitive and resourceful animals has been replaced by selective breeding. One result of this is that domestic animals have smaller brains than their wild counterparts and the more recently evolved ('higher') parts of their brains have degenerated most (Herre & Rohrs, 1973). The properties of wild plants can be seen largely as consequences of two broad evolutionary pressures: their physical environment and interactions with other organisms. An important set of interactions is with animals which eat plants. Being eaten in part can be useful to plants in dispersing propagules. It can also, however, be disadvantageous and plants have evolved many defences against herbivores, defences such as spikes, toxins and digestion inhibitors. In turn, herbivores have evolved means for overcoming these defences. Such interactions have been regarded as a sort of evolutionary arms race. Each plant has limited resources. Resources allocated to the synthesis of defences or competing with other plants may not be available for growth and reproduction. A plant can defend itself and compete effectively with others in only a limited set of circum-stances, in its own ecological niche. The characteristics of a species are in general adapted to efficient exploitation of the ecological niche which it inhabits. 'Efficiency' in the ecological context is not, of course, the same as agricultural or productive efficiency: ecological efficiency is more closely allied to the concept of evolutionary fitness. The breeding of domestic plants for agriculture has, in part, been concerned with eliminating structures which were once ecologically useful but which do not now contribute to agricultural production. The metabolic resources which were once used to synthesize tall stems or toxic molecules have been channelled into more useful forms of protoplasm. Of course, many biologically active molecules with antibacterial, insecticidal or other useful properties form remedies, drugs and spices and so the evolutionary arms race has provided some weapons which we have captured and use to our own advantage. Most parts of most plants are inedible to most wild herbivores. Diet selection enables a wild animal to utilize the limited range of plant parts to which it is adapted. Domestic 72 K. MOSS animals do not have the same choices as wild ones, but they do show certain rcsidual preferences. To understand the evolution of these residua, we must look to the !iatural world. Diet selection and competition. Species which exist today survived and reproduced effectively in the past. To do this, individuals had to meet competition from other species and from individuals of the same species. This may help to explain why wild animals often eat certain foods whilst ignoring others which seem to be nutritionally adequate and which are eaten by closely related species, or even by different populations of the same species. An individual presumably shows preferences based on archetypes which ensured the genetic survival of its progenitors. This thought may be put teleologically by saying that an animal chooses foods which maximize its genetic fitness. A species may survive competition from other species by specializing, by becoming more proficient at foraging for a restricted range of foods and more efficient at digesting them than if it attempted to take a more catholic diet. Thus, in interior Alaska the closely-related willow ptarmigan (Lagopus lagupus) and rock ptarmigan (Lagopus m u m) co-exist on the same wintering grounds. The winter diet of the willow ptarmigan here is over 90% willow (Salix spp.), that of the rock ptarmigan over 90% birch Retula spp.). This occurs irrespective of whether both species are feeding close togelher or whether only one species is present and has the choice of willow or birch (Moss, 1974). Nonetheless, willow ptarmigan are quite capable of subsisting on birch and do so in parts of their range where little willow is available (Pulliainen & Iivanainen, 1981). Similarly, rock ptarmigan can subsist on willow; indeed, they prefer it to birch in Iceland, where there are no willow ptarmigan with which to compete (Gardarsson & Moss. 1970). In Alaska, then, it appears that interspecific competition has caused the observed differ-ences in diet selection and that the socially dominant willow ptarmigan (Moss, 1973) has gained access to the intrinsically more nutritious (Moss, 1983) willow. However, in Alaska each species has become adapted to its own particular diet, as shown by interspecific differences in bill sizes (Weeden, 1969), gut lengths and gizzard weights (Moss, 1974). It seems reasonable to suggest that birch-adapted birds eating birch are likely to perform less well than willow-adapted birds eating willow, but better than birch-adapted birds eating willow.