The ambient energy hypothesis and the habitat heterogeneity hypothesis
The ambient energy hypothesis predicts that areas with high productivity can accommodate more species, and species interaction will be more frequent in the areas with high species richness, which could promote species specialization and lead to finer niche separation and smaller range size (Hildrew, Townsend, & Francis, 1984; Currie, 1991). However, to our knowledge, no empirical study found evidence to support the ambient energy hypothesis, whether along the latitude gradient or the elevational gradient. On the contrary, studies have shown positive or irrelevant relationships between primary productivity and species mean range size (e.g., Luo et al., 2011; J.-Y. Kim et al., 2019). In our case, we also detected a positive relationship between species mean range size and primary productivity.
Habitat heterogeneity also plays an important role in determining the species distribution (Whitton, Purvis, Orme, & Olallatarraga, 2012). Different combinations of habitat types can affect the energy allocation of an ecosystem. For example, higher habitat heterogeneity can promote the subdivision of limited resources (Kerr & Packer 1997). Therefore, the habitat heterogeneity hypothesis was proposed as a supplement to the ambient energy hypothesis (J.-Y. Kim et al., 2019). The habitat heterogeneity hypothesis indicates that complex habitats should facilitate the resource allocation in ecosystems, which could lead to greater species specialization and coexistence. Under certain conditions, an increase in available space and shelters, as well as opportunities for isolation and divergent adaptation, is thought to enhance species coexistence, persistence, and diversification (Rohde 1992). However, our results found little evidence to support the habitat heterogeneity hypothesis, and the area with higher habitat heterogeneity does not necessarily mean a larger area of suitable habitat. Our result showed that habitat heterogeneity was the lowest while the area percentage of suitable habitat types was the highest (Figure S3 in Appendix 2). The relationship between species mean range size and habitat heterogeneity in Lebu Valley may be weak in this case.