The additive contributions of density-dependence and density-independence to population dynamics

Background/Question/Methods

The manner in which population sizes change is typically characterized based on the presence of density-dependent versus density-independent factors. However, all populations are fundamentally influenced by some combination of both and it is this interaction that ultimately shapes population dynamics. Because these interactions are complicated and not well resolved, we sought to determine the joint effects of intrinsic and extrinsic factors on the population growth rate of a threatened salmonid, the Lahontan cutthroat trout (Oncorhynchus clarkii henshawi).

Results/Conclusions

The observed temporal variability in population sizes was greater than that observed elsewhere in the subspecies but within the range observed for the genus. While greater population size variability is typically considered demonstrative of density-independence, here population growth rate was both density-dependent and tied to extrinsic factors. The additive contribution of density-dependent and -independent factors explained ~53% of the observed variation in population growth rates, the majority of which (~60%) was explained by the effects of conspecific density. This suggests that density-dependence had a stronger effect than did density-independence but that both substantially contributed to observed population dynamics. These results demonstrate the importance of understanding and quantifying both the density-dependent and independent factors shaping the dynamics of populations.