Abstract
Population fluctuations are generally attributed to the deterministic consequences of strong non-linear interactions among organisms, or the effects of random stochastic environmental variation superimposed upon the deterministic skeleton describing population change. Analysis of the population dynamics of the mussel Mytilus californianus taken in 16 plots over 18-years found no evidence that these processes explained observed strong fluctuations. Instead, population fluctuations arose because environmental stochasticity varied with abundance, which we term density-linked stochasticity. This phenomenon arises from biologically relevant mechanisms: recruitment variation and transmission of disturbance among neighboring individuals. Density-linked stochasticity is probably present frequently in populations, as it arises naturally from several general ecological processes, including stage structure variation with density, ontogenetic niche shifts, and local transmission of stochastic perturbations. More thoroughly characterizing and interpreting deviations from the mean behavior of a system will lead to better ecological prediction and improved insight into the important processes affecting populations and ecosystems.
Highlights
Ecologists have long been fascinated by the fluctuations exhibited by natural populations, and interested in elucidating the mechanisms by which they arise
We present a detailed analysis of the population dynamics of the mussel Mytilus californianus that probes the roles of deterministic non-linear dynamics, additive effects of environmental stochasticity, and density-linked stochasticity. We find that the former two cannot generate the strong dynamical patterns that we observed in our data, but that DLS can
The seemingly predictable pattern of fluctuations arises because stochastic forces such as wave disturbance have little effect at moderate abundances, leading to predictable recovery times, but very strong effects at high abundance
Summary
Ecologists have long been fascinated by the fluctuations exhibited by natural populations, and interested in elucidating the mechanisms by which they arise. We present a detailed analysis of the population dynamics of the mussel Mytilus californianus that probes the roles of deterministic non-linear dynamics, additive effects of environmental stochasticity, and density-linked stochasticity We find that the former two cannot generate the strong dynamical patterns that we observed in our data, but that DLS can. Analysis and Modeling We analyzed the time series by exploring relationships between mussel abundance at a census point as a function of mussel abundance one or more census points in the past, combined across all plots, using non-linear regression with maximum likelihood, and used simulations of the resulting relationships to determine how well they recreated the dynamical patterns observed in the data. We checked for robustness of our best-fitting density-linked stochastic model using model averaging, in which parameter estimates for each model considered were combined using the Akaike weights of each model as a weighting factor, based on all model variants considered for either first or fourth order datasets
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