On the Abundance of Plants Along an Environmental Gradient

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Abstract
The relative importance of density-dependent and density-independent processes in determining population size is discussed. There may be a curvilinear relationship between population size and the net rate of increase of a population along an environmental gradient as a result of the non-linear relationship between growth rate and population density. A consequence of such a curvilinear relationship is that a slight increase in density-independent mortality or a slight reduction in density-independent fecundity can produce a relatively sharp distribution limit along an environment gradient. The data of Keddy (1981, 1982) are re-examined in order to evaluate the relative importance of various density-dependent and density-independent processes in determining the abundance of Cakile edentula along a complex environmental gradient on the sand dunes at Martinique Beach, Nova Scotia [Canada]. Its population density was greatest in the middle of the dunes and declined to both the landward and seaward ends. The abundance of plants on the dunes as a whole is regulated through the density-dependent control of fecundity on the seaward end of the gradient. The abundance of individuals along the gradient is dependent upon the interaction between various density-dependent and density-independent processes. Among the latter, the landward migration of seeds is of prime importance. Without migration, populations would not persist in those areas where the species is most abundant in the field.

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