Identifying the appropriate spatial scales to study populations and connectivity has long been recognized as a problem in basic and applied ecology. However, objectively identifying those critical scales remains a challenge. In their new Nature Communications paper, Fletcher et al. 2013 use innovative methods extended from statistical physics and social sciences to apply network modularity to identify critical scales of connectivity for ecological and evolutionary population processes.
Modularity refers to clustering within networks into modules that are tightly linked to themselves and sparsely linked to other modules. In the context of populations in a landscape, modularity refers to habitat patches or local populations that are strongly connected to other patches or populations, through movement or gene flow, and weakly connected to the network of habitat patches or the larger population (i.e., the metapopulation). Fletcher et al. ask: How much modularity do populations exhibit? How much do spatial and non-spatial components contribute to modularity? How does module identification change inferences made about connectivity and metapopulation persistence? They address these questions using four examples that are illustrative of the vastly different scales at which organisms use landscapes. The examples are: cactus bug (Chelinidea vittiger) movements among cactus patches, Everglades snail kite (Rostrhamus sociabilis plumbeus) breeding-season movements among wetlands, bullfrog (Rana catesbeiana) gene flow among wetlands, and Florida black bears (Ursus americanus floridanus) gene flow among core populations.
Three of the four case studies showed strong modularity and so revealed critical spatial scales (only bears did not, which the authors attribute to wide ranging movements within their range). In two of the three species, modularity was not explained by geographic arrangement alone, suggesting that the functional modularity of populations can be non-intuitive. Inclusion of modularity into connectivity and population viability assessments showed higher metapopulation persistence than when modularity was ignored. For cactus bugs, once modularity was identified it became apparent that there were a few key patches for connectivity and metapopulation persistence that would not have been identified had modularity been ignored. Modularity perhaps supports persistence of populations through increased rescue effects due to added linkages or increased asynchrony of populations dynamics among modules.
Applications of this work to the management and study of populations extend not only to identify proper spatial scales, but also to identify habitat patches and linkages that are critical for persistence of imperiled populations. The latter could provide strategic information for management and conservation of land and corridor planning.
Fletcher, R.J., Jr., A. Revell, B. E. Reichert, W.M. Kitchens, J.D. Dixon, and J.D. Austin. 2013. Network modularity reveals critical scales for connectivity in ecology and evolution. Nature Communications 4: 2572.