The relation between density regulation and natural selection

Abstract

Darwin (1859) introduced the notion of natural selection by showing that if the density of a species is to remain steady, most of the individuals in each generation must die prematurely. Nicholson (1954) has analyzed the factors determining the density of a species in great detail. Haldane (1953) did so more summarily. It is simplest to consider annual plants or animals. Their increase or decrease from one year to another is a function, among other things, of their density. A factor which leads to increase as the density increases is called density-disturbing (Nicholson), or positive density-dependent (Haldane). Such factors include all forms of mutual aid, from the mutual protection of trees from storms and the greater ease of finding mates if the population is not too sparse, to various forms of social behaviour. Density-regulating (Nicholson) or negative density-dependent (Haldane) factors include all forms of competition, including competition for food and space, and disease facilitated by overcrowding. Other factors, which Nicholson calls density-legislative and density-inactive, are independent of density. Examples are the effects of heat and cold, in so far as they are not modified by competition or co-operation. Clearly at high densities some negative density-dependent factor must come into action, or density would increase indefinitely. Positive density-dependent factors maybe important at low densities. If so there may be an unstable equilibrium, and populations which fall below it decrease further and die out, for example, sessile bisexual or self-sterile organisms with juvenile dispersal. They always make for instability of equilibrium. Negative density-dependent factors may do so if their effect increases very sharply with density, or if it is delayed (Nicholson & Bailey 1935).