Habitat but not topographic heterogeneity constrains the range sizes of African mammals

Background/Question/Methods

A species’ range size is a central determinant of its extinction risk, as small-ranged species are less likely to persist if threatened. To prevent extinctions, we must identify the factors that constrain species’ ranges. Theoretically, heterogeneous landscapes harbor smaller-ranged species. Landscapes comprising diverse habitat types (high habitat heterogeneity) contain specialist taxa adapted to surviving only in particular areas. Further, landscapes composed of rapidly changing elevations (high topographic heterogeneity) create a barrier to dispersal. To date, the comparative power of habitat versus topographic heterogeneity in constraining ranges has seldom been explored. Here, we disentangle the relative influences of each upon mammalian range sizes in Africa. For each of ~1000 species, we quantified its habitat heterogeneity as the variation in primary productivity between neighboring 1-km pixels across its range, and topographic heterogeneity as the corresponding variation in elevation. We then ran a phylogenetic generalized least squares model, incorporating species’ functional traits, environmental conditions, and heterogeneity measures to predict their range sizes while accounting for their evolutionary non-independence. We assessed each heterogeneity measure’s model coefficient and influence upon the AICc and likelihood of the model. Finally, we repeated our analyses at 50-km resolution to evaluate the sensitivity of our results to spatial scale.

Results/Conclusions

Relative to a model with neither heterogeneity measure included, adding habitat heterogeneity into the model significantly decreased its AICc and increased its likelihood (p<0.0001). Habitat heterogeneity also produced a significant model coefficient of -0.90 (p<0.0001). Conversely, topographic heterogeneity had no significant effect upon the model’s AICc and likelihood (p=0.55) and produced a non-significant coefficient of -0.21 (p=0.11). The results obtained at 50-km resolution differed minimally. We find that after accounting for the functional traits, environmental conditions, and evolutionary non-independence of African mammals, habitat but not topographic heterogeneity relates inversely to their range sizes. Our results confirm the theory that landscapes with heterogeneous habitat types constrain mammalian ranges, as they facilitate niche partitioning and the coexistence of specialist species that are each adapted to particular environments as opposed to vast regions. However, our results run contrary to the idea that landscapes with rapidly changing elevations create a barrier to species’ dispersal. Future work should examine the reasons behind this lack of influence of topographic heterogeneity upon mammalian range sizes in Africa. Our study highlights the importance of conserving regions with high habitat heterogeneity to prevent the extinction of small-ranged species.