The inflationary effects of environmental fluctuations in source–sink systems

Abstract

Ecological communities are open to the immigration of individuals and are variable through time. In open habitats immigration may permit populations of a species to persist locally even though local biotic and abiotic processes tend to exclude such ''sink'' populations. A general model for a sink population reveals that autocorrelated environmental variation can dramatically inflate local abundance and that such populations display a characteristic ''outbreak'' pattern. An experimental protist microcosm exhibits these predicted effects. Because the many ecological and environmental processes that set the rate of exclusion are typically autocorrelated, these theoretical and empirical results have broad implications for our understanding of community structure and highlight a previously unsuspected potential effect of anthropogenic climate change.A n abiding theme in ecology is that local communities are restricted subsets of regional species pools (1). Because local communities are assembled by immigration and depleted by extinction, community structure may arise from patterns in the success and failure of population growth after immigration, driven either by interspecific interactions with resident community members, abiotic fluctuations in habitat quality through time, or some combination of the two (2). A growing body of theory and evidence suggests that spatial flows of individuals among habitats are essential for maintaining the long-term integrity of populations and communities (3, 4). One consequence of the movement of individuals into habitats that are either intrinsically low in quality or have strong resident competitors or predators is the creation of population ''sinks'' (5, 6). In a sink habitat, in the absence of immigration, populations suffer extinction because local births are insufficient to balance local mortality. The continued presence of such otherwise excluded species within a local assemblage depends on recurrent immigration from sources, a form of the ''rescue effect'' (7). But if habitat quality and hence local recruitment vary because of fluctuations in biotic or abiotic environmental variables, then sink populations may experience periods of net positive local growth. The local abundance of species in such habitats should thus reflect the interplay of immigration and temporal fluctuations within the habitat.One general property of environmental variation of particular importance for ecological dynamics is its autocorrelation structure, which defines the statistical dependency of successive values in a time series. In contrast to random fluctuations where successive values are statistically independent, autocorrelated fluctuations have a serial dependency. This property of environmental variability has been the focus of much recent study because (i) the environmental fluctuations of many physical variables (such as temperature and rainfall) are positively autocorrelated (8-10) and (ii) fluctuations of natural populations, which are often large in magnitude (11), are also often positivel...

Persistence of structured populations in random environments

“…In general, simulations suggest that the magnitude of this inflation increases with ρ. This observation is consistent with recent theoretical and empirical work on the inflationary effects of autocorrelation on sink populations (Gonzalez and Holt 2002).…”

Stochastic environmental fluctuations drive epidemiology in experimental host–parasite metapopulations

“…Previous theory and experimental work have demonstrated a beneficial effect of red noise on population growth and persistence for sink populations [14]. Our results indicate that this effect extends to the more complex case of parasitic organisms interacting with their host.…”

“…The differences between red and white noise treatments would not have been apparent if species' responses to variable environments were an average of their responses to the corresponding constant environments. It is the nonlinear response to stochasticity that underlies the differences in the dynamics between the red and white noise treatments [14].…”

Temporal variation in temperature determines disease spread and maintenance inParameciummicrocosm populations

“…Conversely, population densities declined by 80 per cent during the first week of the experiment when in constant restrictive conditions of 358C, reaching final levels of less than 20 individuals ml 21 (electronic supplementary material, figure S2). This confirms that our high-temperature treatment imposes a high level of stress on our Paramecium populations.…”

“…The effect of fluctuating environments on the behaviour of biological systems can be considered within a sourcesink framework [21], whereby short exposure to permissive conditions can allow population recovery, and enhance persistence, in otherwise restrictive conditions (sinks). In this experiment, we investigated how different variable environments, with mean temperatures ranging across a gradient presenting increasingly sink-like conditions, affected Paramecium and parasite populations.…”