The effects of frequency specificity of environmental noise on ecological synchrony
Ecological synchrony is an important topic for ecology, conservation biology, and natural resource management. Many ecological communities exist as geographically fragmented populations which are connected by low rates of dispersal, referred to as metapopulations. If extinction occurs in one of the populations that comprise a metapopulation, dispersal from a neighboring population can recolonize the area of extinction and allow for population recovery. However, if all the populations in a metapopulation become extinct simultaneously, the metapopulation (or possibly the entire species) is lost. The probability of metapopulation extinction is enhanced if the fluctuations in the populations’ densities are correlated through time (i.e. synchronous). There are two main mechanisms of ecological synchrony: correlations among environmental factors (Moran Effect) and dispersal. In previous studies, measurements of ecological synchrony have relied on variations of the correlation coefficient. However, these methods do not take into account how synchrony may vary over different time scales. Our approach considers the correlation of population densities over different time scales by using cospectra to examine metapopulation synchrony in the frequency domain. We conducted simulations of metapopulation models to investigate how different environmental co-spectra, dispersal rates, and population dynamics interact to determine ecological synchrony and probabilities of metapopulation extinction.
Several inferences can be made based on our results. Environmental fluctuations which are positively correlated over long time scales (red-shifted) enhance the probability of metapopulation extinction. This is true even when the total correlation among the environmental factors is zero (low-frequency positive covariance is canceled by high-frequency negative covariance). Dispersal rates can interact with the color of the environmental synchrony to influence probabilities of metapopulation extinction. These interactions also depend on the nature of population dynamics; equilibrium, cyclical and chaotic dynamics respond differently to the factors that synchronize metapopulations. The cospectrum of metapopulation synchrony may also hint at the mechanisms causing ecological synchrony. Metapopulations in which dispersal is the main driver of synchrony have cospectra, which are blue-shifted, whereas metapopulations synchronized by environmental variability have spectra which mirror the cospectra of the environmental factors. We quantified the cospectral interactions between dispersal and the environment fluctuations. Understanding the factors that determine ecological synchrony and their implications is an important research goal, especially given that human activities often result in habitat fragmentation and barriers to dispersal.