This pattern of better performance at higher densities offers no evidence for an inadequate absolute forage supply and the presence of population control by density-dependent factors.
Skoog (1968) concluded that no other factor affects caribou more than the climate, and that weather can hold populations at levels where density-dependent factors are not important.
Kayser, "Influence of environmental and density-dependent factors
on reproduction of Little Egrets," The Auk, vol.
Log models suggest that recruitment begins to quickly asymptote above population levels of 30 [m.sup.-2], and the 2nd-order polynomial model suggests that negative density-dependent factors become important above approximately 5 adult individuals [m.sup.-2] (Table 3).
Whether negative density-dependent factors equivalent to the compensatory effects hypothesized for fish populations exist in natural bivalve populations is unknown, but growth inhibition and implied reductions in fecundity, have been found in high densities of some bivalve species (Powell et al.
In addition, each model detected only one of the four density-dependent factors identified by the comprehensive population model (Table 3).
Each month's density estimate is either above or below this varying equilibrium, and thus, density-dependent factors affect the population in relation to a varying equilibrium.
[ldots] We advocate the employment of the terms "convergence" (Murdoc h 1970) or "statistical density-dependence" (Royama 1977) for the purely mathematical context, and reserve the term "regulation" for that context where equilibrium is enforced by causal, tangible, density-dependent factors
. Thus, one might differentiate between a process of stabilization and a mechanism of regulation.
When information on the dynamics of natural enemies, fluctuations in resource availability, and other density-dependent factors, as well as density-independent factors is not available in a changing environment, this approach has great potential and flexibility for approximating the underlying mechanisms of ecological time series data and for modeling various population dynamics.
For example, if the second-order model is identified as the best model, one may infer strong interaction between population density and density-dependent factors. Ricker and Gompertz models, if identified as the best models, may suggest that the conditional density dependence concept would tend to be supported, in which the influences of the density-dependent factors increase conditionally on the increase of the population density, and overall density-dependent factors are less influenced or not affected by population density (Royama 1992, Zeng 1996).
Delayed density-dependent factors
are important in population regulation.
A further purpose of this study is to determine how infection of toads with the lungworm Rhabdias bufonis interacts with density-dependent factors
operating both prior to infection (during the aquatic phase) and after infection (during the terrestrial phase) in determining patterns of growth and survival.