Species can respond differently to climatic stressors and anthropogenic land-use change. Environmental heterogeneity, for example, may constrain the dispersal of habitat specialists whereas generalists, because of their higher environmental tolerance, may be less constrained. Dry-forest species may also be more tolerant of disturbance because they are better adapted to more extreme variation in environmental conditions intrinsic to the habitat than wet-forest species. As a result, anthropogenic disturbance may have a smaller effect on dry-forest communities but lead to the homogenization of the avifauna through replacement of wet-forest specialists by dry-forest generalists.
My preliminary research shows that precipitation seasonality is a significant factor driving patterns of bird species turnover in Western Ecuador (Montalvo 2014). Even more, disturbance-tolerant species that have most of their distributional range within the dry Tumbesian region drive the patterns of species composition at landscape scale in the northwest of Ecuador (Montalvo 2014). Preliminary analysis of presence-only data shows that abiotic variables (from Worlclim) have a stronger effect on species turnover (Bray-Curtis dissimilarity) of forest-specialists than disturbance-tolerant species (Fig.1). In other words, changes in species composition along the gradient will be higher for forest-specialists than for disturbance-tolerant species. However, a significant species turnover was still observed for disturbance-tolerant species across environmental gradients, suggesting some differentiation of community composition between wet and dry regions.
Fig .1 Generalized dissimilarity models for bird specialized in forest (blue), disturbed areas (green) and all habitat (red) in western Ecuador using presence data (GBIF). The models show species dissimilarity (species turnover) in the y-axis and several environmental variables in the x-axis. Change of the slope show how fast community composition vary across the environmental variable. Forest birds change more and faster across variables.
At the same time, barriers to dispersal are negligible for species turnover as inferred from the lower impact that geographical distance has on dissimilarity when compared to environmental variables along the region (GDM analysis using presence-only data from GBIF and climate variables from WorldClim) (Fig. 2). This pattern suggests that even with limited barriers to dispersal, range expansion and colonization from dry to the wet region do not occur for many groups of birds with increasing land-use. The fact that human activities homogenize habitats and concomitant environments (and consequently fundamental niches) across regions, but communities still show different composition suggests that biotic interactions might play an important role preventing range expansion of many species. In this context, anthropogenic disturbances might have a limited effect on the homogenization of avian communities. This suggests that different disturbance-tolerant communities will persist (limited homogenization) regardless of the land-use change unless environmental factors change in the future. Climate change might induce range shifts resulting in novel interspecific interactions, heightening the importance of predicting the consequences of species interactions, and behavioral interference is a fundamental but neglected part of the equation (Grether et al. 2017).
Fig. 2. Community dissimilarity explained for different environmental variables. Significant and highly significant variables are showed by * and ** respectively. Although geographic distance remains significant for disturbance-tolerant species, its effect is lower compered to environmental variables suggesting limited or no barriers to dispersal.
