Extinctions in competitive communities forced by coloured environmental variation

Abstract

Understanding the relationships between environmental fluctuations, population dynamics and species interactions in natural communities is of vital theoretical and practical importance. This knowledge is essential in assessing extinction risks in communities that are, for example, pressed by changing environmental conditions and increasing exploitation. We developed a model of density dependent population renewal, in a Lotka–Volterra competitive community context, to explore the significance of interspecific interactions, demographic stochasticity, population growth rate and species abundance on extinction risk in populations under various autocorrelation (colour) regimes of environmental forcing. These factors were evaluated in two cases, where either a single species or the whole community was affected by the external forcing. Species’ susceptibility to environmental noise with different autocorrelation structure depended markedly on population dynamics, species’ position in the abundance hierarchy and how similarly community members responded to external forcing. We also found interactions between demographic stochasticity and environmental noise leading to a reversal in extinction probabilities from under‐ to overcompensatory dynamics. We compare our results with studies of single species populations and contrast possible mechanisms leading to extinctions. Our findings indicate that abundance rank, the form of population dynamics, and the colour of environmental variation interact in affecting species extinction risk. These interactions are further modified by interspecific interactions within competitive communities as the interactions filter and modulate the environmental noise.

Community extinction patterns in coloured environments

“…This was most probably due to a failure of the linearization techniques employed in the analytical derivation to capture population variation under strong environmental forcing. This work generalizes the results from previous reports (Ripa & Ives 2003;Ruokolainen et al 2007) and extends our knowledge of the behaviour of multi-species competitive communities in variable environments.…”

“…One interesting feature is that with overcompensatory population dynamics, increasing noise autocorrelation can lead to a decrease in EP (Petchey et al 1997;Schwager et al 2006;Ruokolainen et al 2007). We have shown here that this result arises due to the way environmental noise is amplified in the community dynamics, under different regimes of environmental forcing.…”

“…This method can be thought of as introducing demographic stochasticity in small populations (e.g. Petchey et al 1997;Ranta et al 2006;Ruokolainen et al 2007). Communities were simulated maximally 200 time steps and simulation was terminated at the first extinction.…”

“…For example, population growth rate may interact with environmental autocorrelation in influencing extinction events. It has been observed, both in single- (Petchey et al 1997; Schwager et al 2006) and multi-species community models (Ruokolainen et al 2007), that while undercompensatory population growth increases extinction probability (EP) in association with increasing environmental autocorrelation, overcompensatory growth results in an opposite trend. In addition, the magnitude of change in extinction patterns is due to the similarity (correlation) in species' responses to environmental variation.…”

Increasing community size and connectance can increase stability in competitive communities

“…Recent evidence from a study of bacterial diversity has highlighted high levels of ecotype diversity 15 within the same bacterial clades (Koeppel et al, 2008), further emphasizing the need to understand functional relationships within a trophic level (Berlow et al, 2004). Previous theoretical work has shown that variation in species intrinsic growth rate in the community may (Ruokolainen et al, 2007) or may not (Fowler and Lindström, 2002) lead to qualitative differences in community responses following different disturbances in simulated 20 competitive communities. These two studies only examined under-and/or damped overcompensatory dynamics in the stable equilibrium range.…”