1. Anthropogenic disturbances are jeopardizing ecosystem functioning globally. Yet we know very little about the effect of human impacts on ecological processes derived from trophic interactions. By focusing on biodiversity components of consumer and resource organisms, such as the diversity of phylogenetic lineages and the diversity of traits that influence species interactions, it is possible to simultaneously address the responses to disturbances and their effects on processes. 2. Here, we evaluate the consequences of forest loss on the ecological process of frugivory between fleshy-fruited plants and frugivorous birds. For two years, and at 14 sites representing a gradient of forest cover in the Cantabrian Range (N Iberian Peninsula), we monitored fruit and bird abundance, and fruit consumption. We compared the response to forest loss of both plants and birds by assessing changes in phylogenetic and trait-based functional diversity in relation to forest cover. We further evaluated how changes in these biodiversity components translate into functional changes by estimating the degree of functional complementarity of plant and bird species. 3. We found different responses of plants and birds to forest loss. The diversity of plant assemblages did not respond to changes in forest cover, whereas bird assemblages markedly lost phylogenetic and trait-based functional diversity at high levels of forest loss. Functional complementarity of birds was well predicted by phylogenetic and trait-based functional diversity, but functional complementarity of plants depended exclusively on the diversity of traits. 4. Forest loss filtered avian phylogenetic lineages and traits, and influenced how birds contributed to the frugivory process. These results show how the diversity decay of one trophic level may compromise ecological processes derived from trophic interactions. Therefore, we suggest that a multitrophic response-effect framework, which includes measures of functional traits, lineages and species functional contributions across trophic levels, may be required to fully understand the ecological consequences of biodiversity decays.

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