How Size Affects Population Dynamics

The characteristics that determine the demography of a population collectively make up its life history pattern. They include, for example, age at first breeding, mating system, litter size, average number of litters per season and per lifetime, amount of parental care invested in each litter, expectation of life at birth, and so on.

Life history patterns are determined mainly by two things: body size and the permanence of the habitat relative to the lifetime of an individual. Small animals can survive in small patches of the environment that cannot support large animals, which tend to range across many different patches. However, since small patches tend to be unreliable and often short-lived, small animals must also be good at dispersing from one patch to the next as local food supplies run out.

For example, an overgrown corner of a field containing many voles provides an excellent temporary home for a common or least weasel, although after a while the weasel and its young will probably have to move on to find another field. By contrast, the whole field and a large area around it may be the stable and permanent home of generations of foxes or badgers (King 1983c). On the other hand, because small animals generally have small home ranges, they can, reach greater population densities than larger animals of similar food habits. For example, common weasels are much more abundant than wolves, and vary in numbers from year to year much more. In Bialowieza Forest in Poland, during an irruption of forest rodents, the density of common weasels went from two per km2 in spring to ten per km2 in summer 1990, falling to less than one per km2 in spring 1991; but the density of wolves averaged two to three per 100 km2 throughout (Jgdrzejwska & Jgdrzejewski 1998).

The suite of characteristics typical of small, opportunistic animals living in variable and unstable habitats make up the "r-strategy," so named decades ago because the net effect of that combination of characteristics is to maximize r, the rate of population increase. The opposite characteristics typical of large animals living near carrying capacity (K) in relatively stable and reliable habitats make up the "K-strategy," which tends to maximize sustainable population density. For K-strategists, the rate of increase is far less important than producing a few high-quality young, well developed and prepared to compete with others for a place in the adult population. Between the two extremes are intermediates that have these characteristics developed to different degrees.

The idea of the r—K dichotomy is obviously a gross oversimplification of how real animals live. Nevertheless, it is a robust generalization that has been immensely helpful in the past, and it does capture some important general truths explaining broad-scale variations in population dynamics. For example, a set of related species can be arranged along a spectrum, from extreme opportunists at one end through all shades of intermediates to extreme equilibrium species at the other. Their positions on the spectrum are determined from the relative importance for each species of the characteristics that most augment the rate of population increase, which are early maturity, more than one litter per year, large litter size, and more than one litter per female's lifetime, in that order.

Mustelids demonstrate this idea well (King & Moors 1979b). Least and common weasels are the most variable in numbers of all the mustelid family, and stoats and longtails are next. In Britain, Europe, and Asia, the badger is the largest mustelid species, well adapted to relatively stable woodland conditions. In North

America, the wolverine is the largest mustelid, and populations of wolverines, where undisturbed, remain much the same from year to year.

Common and least weasels have all the characters of determined opportunists. They mature early and bear small young with a short life expectancy. In seasons of abundant food, spring-born young can breed in the season in which they are born, while adults can produce a second litter. Their populations fluctuate wildly from year to year and are governed mainly by food supplies. The parent animals most likely to pass their genes to the next generation tend to be those that produce and rear to independence the most young most quickly.

Common and least weasels concentrate on quantity rather than on quality. They spend the least possible time on parental care, because, if food is abundant, the effort spent on training one litter could better be spent on producing another one. Besides, training individual young is not a good investment of parental energy for the small weasels—when food resources are unpredictable, chance is probably as important as hunting or fighting prowess in determining whether a young common or least weasel finds a suitable home range to settle and hunt. The more young are produced, the greater the chances are that one of them will survive and breed in its turn. On top of that, the mortality rate of adults is so high that few live through more than one breeding season, so they have every reason to make the most of the present one.

Stoats and longtails have similar traits, developed to a lesser extent. Females mature and are mated at a few months old (Chapter 9), but then are prevented by a long period of compulsory delayed implantation from producing the litter until the following season. Young males cannot mate until 13 or 14 months old for stoats, and about 15 months old for longtails.

Individual stoats and longtails of both sexes are incapable of producing young in their first year, and only one litter a year is possible in older ones, however abundant the food supplies. Hence, the potential rate of population increase for stoats and longtails is considerably less than that for common and least weasels. On the other hand, although the mortality of the first-year young is very high for stoats (and probably also for longtails), those that survive to their second year in an untrapped population have some chance of living for 3 to 6 years, and breeding several times (see Tables 11.1 and 11.2) .

Populations of stoats and longtails are unstable, but not to the same extent as those of common and least weasels, not only because stoats and longtails are longer lived but also because they can catch a wider range of prey, switching from one to another if necessary. They spend rather longer on parental care, partly because they cannot produce another litter until the following year anyway, and perhaps also partly because greater investment at this stage has some chance of being repaid with greater success later.

For example, larger than average size is an advantage to a male stoat or longtail, because large males are more likely to be dominant and to be successful breeders (Erlinge 1977a). Among the things that determine whether a male will grow to his full potential size is whether his mother feeds him well (Ralls & Harvey 1985). In litters of weasels raised in captivity on guaranteed food supplies, the young males grow larger than wild-reared young of the same age (East & Lockie 1965; Hayward 1983). In habitats with feast-or-famine food supplies, young born in years of abundant food grow larger than young born in other years (Powell & King 1997).

As the theory predicts, the differences in population dynamics and parental care among weasel species, and between all weasels and the larger predators, are related to their small size and fast-paced lives.

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