Life histories and the role of disturbance

Abstract

An alternative to the stability-time hypothesis explaining the high benthic faunal diversities in the deep sea ( Sanders, 1968; Slobodkin and Sanders, 1969) has been proposed by Dayton and Hessler (1972). According to Dayton and Hessler, nonselective predation reduces competition between species thereby allowing more species to coexist. Much controversy relating to the concept of diversity and what it implies can be resolved by realizing that an increase of within-habitat diversity is achieved by two entirely different and unrelated pathways. The resultant diversities are differentiated as follows: short-term, non-equilibrium, or transient diversity—induced by a low level or unpredictable physical or biological perturbation or stress resulting in biological ‘undersaturation’. Long-term or evolutionary diversity—increase in diversity is the product of past biological interactions in physical benign and predictable environments. Although predation may play a role in the evolution of deep-sea species, the life histories indicate that it does not seem a likely means for control of population size, regardless of whether predation is selective or non-selective. The known life history characteristics of deep-sea animals—small brood size, age-class structure not dominated by younger stages, probable slow growth rates—are features that neither would be expected nor have high survival value in predator-controlled communities or any environment where short-term or transient diversity is important. Non-selective cropping proposed by Dayton and Hessler as a mechanism for controlling population size of prey species would result in rapid extinction of species with relatively low reproductive rates. In addition to feeding behavior, niche diversification may be the product of biochemical specialization, biotic relationships and microhabitat specialization. Niche diversification may also result from adaptation to different parts of a temporal mosaic. The stability-time hypothesis does not state that disturbance plays no role in predictable environments. The relative predictability of the environment enables species to survive with lower reproductive rates, lower mortality rates, and smaller population size. Rates of competitive exclusion are lower and species are able to become more specialized on both biotic and physical components of the environment. Control of population size is seldom the result of changes in the physical environment or any disturbance, including predation, so that we may say that the community is biologically accommodated rather than physically regulated.