Habitat structure and ornithophilous plants influence the compositional uniqueness of bird communities across slopes in a high Andean biosphere reserve

Tropical mountains possess high biodiversity driven by environmental gradients and habitat heterogeneity. In the southern Andes of Ecuador, páramo habitat structure and composition vary with geographical orientation, potentially affecting bird communities. We evaluated how habitat variation influences the compositional uniqueness of bird communities (using Local Contribution to Beta Diversity, LCBD) across the eastern and western slopes of the Macizo del Cajas Biosphere Reserve. We conducted bird surveys along 42 transects on both slopes (3500–4400 m a.s.l.) and characterised vegetation structure, including the abundance of ornithophilous plants. Generalised linear models revealed that LCBD was not influenced by elevation or a grassland-dominated structural gradient, but by a less-disturbed heterogeneous habitat gradient and the interaction between slope orientation and ornithophilous plant abundance. On the western slope, a greater abundance of these plants was associated with higher compositional uniqueness. This community differentiation is associated with a structural change where reduced open grasslands and increased ornithophilous plants likely favour nectarivorous and generalist birds to the detriment of grassland specialists. In contrast, the less-disturbed heterogeneous habitat gradient maintained communities with lower LCBD, supporting both specialist grassland and semi-open environment species. We conclude that habitat heterogeneity influences bird compositional uniqueness, mediated by slope orientation and ornithophilous plant abundance. Because natural habitat heterogeneity promotes bird assemblages that are highly representative of the landscape (i.e., lower LCBD), conservation actions must maintain the natural associations between open grassland and native shrubs, and monitor the proliferation of fast-growing woody plants to mitigate the potential displacement of specialised grassland species.