Consequences of limited seed dispersal within simulated annual populations

Abstract

Theoretical analyses of the consequences of dispersal have been almost entirely limited to studies of colonization. A simulation model is constructed and analyzed to study the effects of dispersal within local plant populations. Rules for the growth and reproduction of individuals are deterministic and related only to competition with other plants. Rules for the dispersal of individual seeds are stochastic; the generalized seed shadow is negatively exponential. The analysis has several interesting results. 1) In small, dense populations of annuals, mortality and growth behavior are significantly affected by small-scale changes in dispersal. 2) Stable population size, at any life stage, is not environmentally fixed (e.g. by the number of safe sites) but is also a function of reproductive capacity and dispersal. 3) There is a minimum productive capability which must be exceeded for a population to be able to respond to increased dispersal. The responsive range of the parameters may be related to the grain of safe site distribution. 4) When the total number of seeds per generation is large, high dispersal decreases the number of germlings that are able to establish. 5) For plants that reach the growing stage, however, expected reproduction can exceed that of genotypes with greater reproductive capacity but less dispersal. 6) An uneven distribution of seeds is essential to a spatial patterning of mortality which can deter competitive extinction.

A deterministic model to study the importance of density-dependence for regulation and the outcome of intra-specific competition in populations of sparse plants

“…Seeds and safe-sites can be limiting at the same time. This is in agreement with Bullock (1976) who stated: 'Stable population size, at any life stage, is not environmentally fixed, e.g. by the number ofsafe-sites, but is also a function of reproductive capacity and dispersal'.…”

Dispersal of annual plants in hierarchically structured landscapes

“…The aggregation of safe sites has been commonly observed in the field (e.g., Fowler 1988). A number of models (e.g., Levin 1974, Bullock 1976, Horvitz and Schemske 1986, Cohen and Levin 1991 have predicted selection for relatively short range of dispersal distances and evolutionary stable values have been shown to be even smaller than what may be otherwise regarded as optimum (Hamilton and May 1977, Comins et al 1980, Olivieri and Gouyon 1985. Experimental measures of seed shadows indicate that mean dispersal distances tend indeed to be small (e.g., Sheldon and Burrows 1973, Levin and Kerster 1974, Rabinowitz and Rapp 1981, Verkaar et aL 1983.…”