Landcover Modulates the Responses of Bird Communities to a Steep Geoclimatic Gradient

Aim: Environmental gradients are key drivers of geographic variation in biodiversity, but their interactive effects with landcover are unclear. We tested the hypothesis that landcover can modulate the effect of environmental gradients on species abundance, richness, uniqueness, and spatial turnover and assessed if the shape of this modulation is additive, synergistic, or antagonistic. Location: Jordan Rift Valley. Taxon: Birds. Methods: We sampled bird communities in Winter, Spring, and Late Spring along a ~70 km stretch of the Jordan Rift Valley, which spans a strong rainfall gradient. We used GLMs and multivariate analyses to quantify the effect of biotic and abiotic variables on bird abundance, richness, compositional uniqueness, and turnover and to test if a transition from the Jordan Floodplain to the adjacent Jordan Plain can modulate the effect of the rainfall gradient on these metrics. Results: Bird abundance and richness were significantly higher along the Jordan Floodplain compared to the Jordan Plain, though sites in the latter contributed significantly more to regional beta-diversity. The effect of landcover on the response of species abundance, richness, and uniqueness to the rainfall gradient varied across sampling seasons and among community metrics, but we found no evidence of modulation. In contrast, we found moderate to high rates of bird species turnover along the rainfall gradient, with turnover rates exhibiting a consistently monotonic linear trend. Absolute turnover rates were consistently higher along the dry Jordan Plain compared to the mesic Jordan Floodplain, suggesting an additive modulation. Main Conclusions: Local landcover can additively modulate the effect of a climatic gradient on bird species turnover. In water-limited environments, this effect may be driven by a perceived shortening of the climatic gradient along mesic habitats because their microclimatic conditions are more consistent across space, and they provide thermal refugia from the steep rainfall gradient in the surrounding landscape.