Scale- and resolution-invariance of suitable geographic range for shorebird metapopulations

The habitat suitability of a shorebird metapopulation is studied as a function of the scale (extent) and resolution (grain-size) of the environmental covariates with a maximum entropy species distribution model (MaxEnt) for integration with climate change simulations. For this study, the species considered is the Snowy Plover (Charadrius a. nivosus), which is a threatened shorebird whose geographic range spans the northwest and southwestern gulf coasts of Florida. The habitat suitability is analyzed at different resolutions by coarsening the classes of the ecogeographical variables with two algorithms: (i) preserving the information of each class at the finer resolution (conservative algorithm); and (ii) considering the most frequent class (majority algorithm). Ultimately, the most suitable habitat is found to be estuarine and ocean beaches made of alkaline medium and fine white sand and silt. The model fit to the observed species distribution decreases with the resolution. Due to the loss of the physical habitat (barrier islands) resulting from the coarsening operation, there is a threshold below which the model fails to predict the species distribution. As a result, the suitable geographic range is resolution-invariant below a resolution-threshold that is determined by the geomorphological features of the landscape rather than by biological constrains. This result holds for both the coarsening algorithms, however the conservative algorithm allows a continuous mapping of the habitat suitability. The suitable geographical range is found to be scale- and resolution-invariant, yet the habitat suitability at-a-point appears scale-dependent due to the strong heterogeneity of the ecogeographical variables. The scale- and resolution-invariance of the suitable geographic range appears a universal result for metapopulations of different species. This is important for reducing the uncertainty of population viability models that are based also on the choice of extent and grain-size of habitat predictions. However, attention must be paid in choosing a resolution that is not too large in order to correctly represent the physical habitat of the species. This implies a potential increase in the effectiveness of conservation campaigns to face the threats of climate change, such as sea-level rise for the Snowy Plover.