Parts a whole do not make

17 02 2012

I’m particularly proud of our latest paper for three main reasons:  (1) Salva Herrando-Pérez, lead author and contributor-extraordinaire to CB, has worked extremely hard to get this one out; (2) it is published in a really good journal; and most importantly, (3) it’s the very first empirical demonstration over hundreds of species that just because you have a density effect on some vital rate (e.g., survival, fertility, dispersal), this in no way means you have any evidence at all for density dependence at the population level. Let us explain.

Quantifying variation in population size is an important element for explaining and predicting population dynamics. In models where a vital (demographic) rate responds to change in population size, those ‘density-dependent’ relationships are ecologically understood as being demographic signals of trophic and social interactions, such as parasitism, predation or competition for shelter, because the intensity of those interactions varies with population size.

In fact, density-dependent effects reflect the theoretical capacity of populations to adjust growth and rebound from low or high numbers – and so this concept has become an important metric in population management and conservation  (Eberhardt et al. 2008). Read the rest of this entry »