Because many species of animals live in diverse situations, it is unwise to generalise about what determines their population density. Some ecologists argue and others deny that the restricted fluctuations in numbers observed in nature can be explained only by some form of density-dependent regulation (Nicholson, 1954; cf. Andrewartha and Birch, 1954; see also Ehrlich and Birch, 1967). Some argue that animal numbers are usually limited by intraspecific competition for food (Lack, 1954a); others that once animals possess a piece of ground ‘they can do the actual food-getting in perfect peace and freedom, entirely without inter-ference from rivals’ (Wynne-Edwards, 1962, p. 12). Hairston, Smith and Slobodkin (1960) argued that because most green matter falls to the ground uneaten, the numbers of herbivores generally are not limited by food shortage; though in fact food may be limiting even when only a small fraction of the stock is eaten (Huf-faker, 1966). Many believe that ‘predation is centred upon over-produced young … upon what is identifiable as the more biologically expendable parts of the population’ (Errington, 1963, p. 184); yet Pearson (1971) suggests that carnivore predation alone is ‘responsible for the amplitude and timing of the microtine cycle’. Ehrlich and Birch (1967) expect regulatory factors to vary among populations of the same species and through time; while Chitty (1958), Watson and Moss (1970), and Krebs and Myers (1974) seek one ‘universal explanation’ for what prevents unlimited increase in small mammals. If there is any justification for these beliefs, they cannot be quite so incompatible as at first sight appears.
Weitere Kapitel dieses Buchs durch Wischen aufrufen
- Factors affecting population density in the wild rabbit, Oryctolagus cuniculus (L.), and their relevance to small mammals
Dr J. A. Gibb
- Macmillan Education UK