We define the negative aspect of a "density-dependent factor
" as one that affects the ability of a caribou population to continue to grow as a direct result of that population's density.
Tat is, they denied the conventional understanding that a natural population can be regulated only by density-dependent factors
; however, Frederick Smith (18) reanalyzed Davidson and Andrewartha's data to test whether changes in the thrip population size also showed signs of density-dependent effects.
However, little unequivocal evidence shows that density-dependent factors
regulate population fluctuations of wading birds, because intraspecific competition for breeding and colony sites appears to be unimportant in determining the reproductive success or survival in most species [5,6].
Recent publications have indicated that, despite high fecundity, invertebrates that are subject to high levels of exploitation such as tridacnid clams (Munro 1989) and some scallops (Caddy 1989) may be limited by density-dependent factors
that are positive.
Ecologists have debated the generality and importance of density-dependent factors
(e.g., malnutrition, disease epidemics) to population dynamics for 70 years.
The effect of so-called density-dependent factors
such as predation, disease and dispersal -- that have a proportionally greater effect on mouse numbers when population densities are high -- is included in the model.
We measured dispersal because it potentially affects survival directly by exposing larvae to increased chances of mortality (Weseloh 1998), and indirectly by spreading individuals over a larger area, reducing the local density and thus reducing mortality from density-dependent factors
Populations can be affected by simple density-dependent factors
(Nicholson 1933, Lack 1954; see references in Dennis and Taper 1994), density-independent factors (Andrewartha and Birch 1954, see references in Martinat 1987), and time-delayed or nonlinear density-dependent forces (sometimes leading to chaotic fluctuations; May 1976, Hastings et al.
Thus, the average Malthusian fitness is almost equivalent to the intrinsic rate under uncrowded conditions, but is modified by extrinsic density-dependent factors
under crowded conditions.
Notably, Sinclair (1989:210) dismissed Murray's (1982) contention that population equilibrium in territorial species could occur without the action of density-dependent factors
When the information on fluctuations in resource availability, dynamics of natural enemies and other density-dependent factors
is unavailable, models with time-varying parameters and high-order lags can be used to describe the influence from these unknown density-dependent factors
on population dynamics (Zeng 1996).
All of these significant differences were in the direction expected if density-dependent factors