1) The diversity of traits within animal assemblages has been shown to affect the magnitude of animal-provided ecological functions. However, little is known about how consistent trait diversity effects are across ecological functions and ecosystems. More importantly, the importance of trait diversity in driving ecosystem functioning, relative to other components of biodiversity, has rarely been assessed. It also remains unclear how environmental gradients filter trait diversity and, ultimately, modulate ecological functions.  

2) Here we test how different biodiversity components (i.e., trait diversity, phylogenetic diversity and abundance) affect the magnitude of avian seed dispersal and insect predation along large environmental gradients. We sampled frugivorous and insectivorous birds and their ecological functions across gradients of forest cover and fruit and insect abundances in woodland pastures and apple orchards in Northern Spain. We measured 6 morphological traits and compiled phylogenetic information on 43 bird species. We used Structural Equation Models to disentangle the effects of environmental gradients and biodiversity components on ecological functions.

3) We found that different avian functions in the same agroecosystem were controlled by different biodiversity components. While seed dispersal was positively driven by bird abundance in woodland pastures, insect predation responded positively to trait and phylogenetic diversity. The positive effects of trait diversity on insect predation were, on the other hand, consistent across woodland pastures and apple orchards.

4) Our results also pinpointed forest cover and resource availability as filters of the different components of avian diversity, suggesting that environmental gradients condition the effects of biodiversity on avian ecological functions.

5) Our findings reveal variable effects of trait diversity on two different avian ecological functions, but consistent effects on the same function across agroecosystems. Consolidating the generalities of trait diversity effects will require further multi-function studies, as well as a unifying framework for animal-driven functions that integrates the causal links between environmental gradients, the different biodiversity components, and ecological functions.

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