Why did the Hornbill not cross the river? Upland habitats rather than a physical barrier limit the distribution of the Brown Hornbill

Aim

In continuous environments, niche limits of species often determine their distribution limits. However, when these limits spatially coincide with a perceived dispersal barrier, the determinants of species' ranges may be confounded. We investigate the distribution pattern of the Brown Hornbill (Aves: Bucerotidae), which spans significant riverine barriers, but stops south of the Brahmaputra River. Considering its preference for low-elevation evergreen forests, we posit that the lack of sufficient habitats north of the Brahmaputra prevents dispersal of the Brown Hornbill, and not the river itself.

Location

The Brahmaputra valley and the Indo-Burma hotspot.

Taxa

Anorrhinus austeni, Aceros nipalensis.

Methods

We analysed citizen-science occurrence data on hornbill presence against a suite of climatic, canopy-specific and topographical predictors to model the environmental niche of the Brown Hornbill. We used presence-only maximum entropy modelling in an information theoretic framework, in conjunction with constructing binary logistic regression models using presence and pseudoabsence data. We compared niche models of the Brown Hornbill and the Rufous-necked hornbill, a close relative with a similar distribution, but which has spanned the Brahmaputra River.

Results

Despite the presence of suitable wet-evergreen forests north of the Brahmaputra River, the hilly terrains in the region act as a biogeographic barrier for the Brown Hornbill, which prefers lowland evergreen forests. Further, highly suitable regions for the Brown and the Rufous-necked Hornbills precisely delineate low and high-elevation evergreen forests respectively, indicating that these birds are separated along an elevational axis.

Main Conclusions

The Brahmaputra River lies at the cusp of two subtly different environmental regimes. It may therefore serve as the niche limit for certain organisms and not as a physical obstacle to their dispersal. Our study implicitly predicts how widespread deforestation prevalent in the lowland evergreen forests of this region adversely impacts the distributions of organisms that depend on them, such as the Brown Hornbill. Further, our study proffers an approach to ascertain determinants of species distributions in a hypothesis testing framework.