Does density-dependence really stabilize populations?

Posted on October 1, 2014

Attempts to understand how biological populations are regulated have a long history. A lot of attention has been devoted to the relative importance of density-dependent mechanisms (e.g. those involving competition or predation), partly because of the light that this may shed on population resilience and extinction risk. Density-dependent responses are thought to dampen fluctuations in overall population size caused by varying environmental conditions. Therefore, the sizes of populations with strong density-dependence should be relatively stable. This prediction was tested by reference to data on the dynamics of 126 populations of salmonid fish, representing eight species. Measures of density-dependence were quantified by modelling population abundances at time t as a function of the abundance at time t-1. Most of the variation in density-dependence was found to occur within rather than between species, an outcome presumably caused by local environmental differences. This result implies that systems that were previously thought to be driven by density-dependent mechanisms are actually affected by a wider range of factors. Unexpectedly, across salmonid species density-dependence was strongly and positively related to variability in abundance. The explanation for this unexpected finding wasn’t clear, but it didn’t seem to be due to measurement error, and more research is needed to see if it’s true of other aquatic groups.

Reference: Dochtermann, N.A. & Peacock, M.M. 2013. Inter- and intra-specific patterns of density dependence and population size variability in Salmoniformes. Oecologia 171, 153–162.