Recovery in frequently disturbed landscapes: Recolonization consequences for source areas

Background/Question/Methods

Populations in frequently disturbed landscapes, such as agroecosystems, may often experience significant losses. In such systems, many species carry important roles as providing units of ecosystem services, those ecosystem functions that are essential for sustaining human populations. To ensure the provision of such services, we need to therefore ensure the population persistence of key species in ecosystems. Recolonization of stressed habitats is one of the major and initial mechanisms of population recovery. The stressed and adjacent, non-stressed areas, can be compared to sinks and sources in classical metapopulation theory which predicts that movement between sinks and sources will not adversely affect the sources, as, by definition, the sinks are suboptimal habitats to which organisms are not generally adapted. When the sink habitats are those that are in essence the same as source habitats, only frequently disturbed species will not be adapted to avoiding them and significant losses from source areas may be assumed. This has been shown in theoretical and some field studies. Furthermore, this phenomenon may be enhanced in riverine networks where habitats are fully connected with a directional movement of organisms (through current velocities and drift).

In our study, we tested such a riverine system and looked for consequences of colonization for the source habitats and the population in the total landscape. We simulated the dynamics of two species often found in European freshwater networks, Asellus aquaticus and Gammarus pulex, both significant contributors to the process of leaf litter decomposition. Our simulated landscapes span from simple linear water bodies to more complex ditch systems found in agricultural landscapes. We looked into how the severity and frequency of stress affected the population sizes in the sink and source habitats. Furthermore, we explored the consequences of recolonization on survival in sink habitats, as a possible compensatory mechanism for the population in the whole landscape.

Results/Conclusions

Our results show that the adverse effects of colonization in source areas depend on the complexity of the specific water network, and are in general spatially constrained. Expectedly, they increase with the frequency and severity of stress and are also dependent on the organism mobility. We discuss these findings in relation to real systems, with a special emphasis on the level of spatial scale at which such effects may be both measured and relevant.