Advective movement by two invasive snails: A comparison in homogeneous and heterogeneous landscapes

Background/Question/Methods

Habitat heterogeneity is an inherent characteristic of natural landscapes that affects many ecological processes, including movement, at multiple spatial scales. Using an empirical model system that consists of two coexisting invasive species (Melanoides tuberculata and Tarebia granifera, both freshwater gastropods), in which advective movement plays an important role, we compared the effects of homogeneous and heterogeneous environments on movement behaviors. We then asked if estimates of habitat-specific movement rates in homogeneous landscapes can be averaged across a heterogeneous landscape comprising a mixture of those habitat types. We used an advection-diffusion framework and model selection to assess the effects of habitat type and heterogeneity using empirical movement data.
Results/Conclusions

Habitat type induced behavioral responses in both study species and, in an advection-diffusion framework, this response was manifested via novel advection rates through each habitat type. However, we also found that these habitat-dependent estimates for advection were not simply proportional to the amount of each habitat in a spatially heterogeneous landscape. Instead, there was a change in the behaviors (of both species) in the heterogeneous landscape that could be attributed to changes in the amount of time spent in a particular patch and/or time spent at habitat boundaries. Models that included a term depicting a movement penalty due to ‘edginess’ of the habitat more accurately represented movement through heterogeneous landscapes. Although the penalty we developed for these models was, for the most part, phenomenological, quantification of the penalty motivates future studies addressing mechanistic understanding of edge effects on movement and mechanistic models that incorporate the behavior of individuals at an edge.