Spatial Heterogeneity and Population Stability: Some Evolutionary Consequences

Abstract

The theme of this review is ecological and evolutionary consequences of habitat heterogeneity (interpreted as different degrees of structural complexity of the habitat). A series of population dynamics models of predator-prey systems are considered. These models are developed in order to analyse changes with respect to stability as a result of (1) altering the difficulties with which predators are able to capture prey individuals (assumed to increase with increasing structural complexity of the habitat), (2) the existence of hiding places for prey individuals, and/or (3) the occurrence of dispersal between patches with differing availability of hiding places for prey individuals. From these analyses it is concluded that increased habitat heterogeneity leads to increased population stability. Applying the Red Queen Hypothesis, this conclusion, linking habitat heterogeneity with population stability, is used for arriving at the conclusion that an increased number of species is expected to exist in a spatially heterogeneous habitat as compared with a homogeneous one. A series of models of optimal life histories (e.g., reproductive rates and sex ratios) in habitats of different degrees of habitat heterogeneity are then reviewed. The prediction is that current reproductive rate is assumed to be higher, and to increase faster to its maximal level in a homogeneous habitat than in a more heterogeneous habitat; some qualifications to this prediction are, however, necessary. The sex ratio at birth is further predicted to be female biased in heterogeneous and patchy habitats; however, the condition for the female biased sex ratio to be optimal is that enough males are produced to ensure fertilization of all (or most) females. Thus, it is concluded that the biases may be only slight and consequently difficult to detect in field samples unless very large sample sizes are available; this is so because in heterogeneous habitats, both small family sizes and female biased sex ratios are expected to be optimal. In spatially homogeneous habitats, an approximately even sex ratio is predicted. The female biased sex ratio is assumed to be a result of closely related individuals mating with each other; therefore, models for analyzing under which circumstances inbreeding is expected to be optimal, are discussed. Throughout data from a variety of taxonomic groups relating to these issues are discussed.