The Ecological Regulation of Species Diversity

A model is proposed to account for the level of diversity supported by any ecological community. If we begin with a hypothetical increase in the stability of the physical environment, the following consequences ensue. With greater environmental stability less energy is required for regulatory activities, that is, those which counter the challenges offered by the environment. Therefore, more energy is allocated for net productivity, that is, growth and reproduction. With increased net productivity, larger populations are supported. Larger populations provide more opportunities for the formation of interspecific associations; they also maintain greater genetic variety. Animals in more productive communities are more sedentary so that the species tends to be broken into many semi-isolated populations. As a result, speciation is favored with the interspecies associations providing the new adaptive opportunities. Plants which are pollinated by animals would be in the same situation. The new species tend to be more' specialized and to have, initially, smaller populations. These events are shown in Steps 1 to 5 in figure 1. in the early stages of the evolution of a community, positive feedback mechanisms would operate, ever-increasing the rate of speciation. The evolution of large heterotrophs (animals) would increase the rate of cycling of mineral nutrients, which would augment the net productivity. As more complex food webs developed with the increase in the number of species, community stability would increase, augmenting the stability of the physical environment. The clothing of the earth's surface with larger plants would tend to damp the fluctuations in climate, also increasing stability (Step 6). In the later stages, the tendency toward overspecialization and smaller populations would decrease community stability and provide a negative feedback control on the whole process (Steps 7 and 8). Even under very stable conditions, productivity may be limited directly by the short supply of various factors such as light, water, heat, etc. Population size may also be limited directly by a restriction in the area of suitable habitat or by a larger body size. Any such limitations would result in a lower diversity of species. This model applies only to equilibrium conditions. In certain situations, such as during the short favorable periods in the arctic or desert, diversity may be temporarily increased; care must be exercised in comparing this to the diversity maintained all the year in the wet tropics. We feel that, although some niches are determined by physical variations in the environment, most of the dimensions of the niche are a result of interaction between organisms. For this reason, it is impossible to predict the number of niches (and therefore species) from environmental complexity alone. We also discard the idea that rigorousness per se limits diversity. Lastly, the hypothesis that the tropics are closer to equilibrium while the temperate zone is in a "successional" state of development of diversity is not accepted, for theoretical reasons and for lack of evidence.