Complex population dynamics and complex causation: devils, details and demography

Abstract

Population dynamics result from the interplay of density-independent and density-dependent processes. Understanding this interplay is important, especially for being able to predict near-term population trajectories for management. In recent years, the study of model systems-experimental, observational and theoretical-has shed considerable light on the way that the both density-dependent and -independent aspects of the environment affect population dynamics via impacting on the organism's life history and therefore demography. These model-based approaches suggest that (i) individuals in different states differ in their demographic performance, (ii) these differences generate structure that can fluctuate independently of current total population size and so can influence the dynamics in important ways, (iii) individuals are strongly affected by both current and past environments, even when the past environments may be in previous generations and (iv) dynamics are typically complex and transient due to environmental noise perturbing complex population structures. For understanding population dynamics of any given system, we suggest that 'the devil is in the detail'. Experimental dissection of empirical systems is providing important insights into the details of the drivers of demographic responses and therefore dynamics and should also stimulate theory that incorporates relevant biological mechanism.

Figure 1

Context-dependent life-history variation. Factor plots for a factor analysis of six life-history traits for soil mites living in environments characterized by high and low food availability. The angle between vectors reflects the correlations between the traits (less than 90°, positive correlations; greater than 90°, negative correlations; 90°, no correlation). Under low food conditions, the size of the egg an individual hatches from (egg length) is strongly positively correlated with survival to adulthood (recruitment) and age at maturity, negatively with size at maturity but uncorrelated with adult survival (survival) or fecundity. Conversely, under high food conditions, egg size is strongly correlated with fecundity and adult survival and not with juvenile survival to recruitment or size and age at maturity. Figure after fig. 4 in Plaistow et al. (2006).