Protection Status and Density-Dependent Effects Mediate the Abundance-Suitability Relationship of a Threatened Species

Niche theory predicts that species abundance should increase with environmental suitability, but this relationship is highly variable across species. Understanding the causes of this variation is key to guide conservation actions, especially for threatened species. Here we test whether two potentially important but rarely considered factors, protection status and density-dependence, mediate the abundance-suitability relationship in the threatened and overexploited palm Euterpe edulis. We obtained population density data of E. edulis from 50 sites across the Atlantic Forest and estimated environmental suitability from niche modelling. We classified palm populations as low- or high-density, and as protected or unprotected based on the location of the populations (inside or outside protected areas, respectively). The overall abundance-suitability relationship was positive (R2 = 0.32). Inclusion of protection status increased the explanatory power of the models (R2 = 0.46). The abundance-suitability relationship was positive in low-density and in unprotected populations, but neutral in high-density and in protected populations. These patterns suggest that density-dependent factors mediate the abundance-suitability relationship in E. edulis. Our findings show, for the first time, that protection status is an important factor regulating the abundance-suitability relationship, and suggest that unprotected populations may be more vulnerable to future changes in environmental suitability due to climate change. Explicit inclusion of density-dependence and protection status in explanatory models in future studies may increase understanding of abundance variation across species’ geographical ranges, particularly for overexploited species.