Functional diversity in agricultural landscapes: evidence of long-term clustering and multi-scale effects of land use on avian communities

Functional diversity (FD) is an essential community property connecting biodiversity, ecosystem functioning, and conservation objectives. In agricultural landscapes, avian communities, which play key functional roles, are facing large-scale biodiversity erosion, largely due to land-use changes. However, the long-term spatiotemporal dynamics of FD and its responses to land use remain poorly understood. This study addressed this gap by analysing bird community data collected from lowland agricultural landscapes at a regional scale between 2007 and 2021. We analysed temporal trends in FD metrics (richness, divergence, evenness), their relationships with species richness, changes in functional composition, patterns of clustering/overdispersion, and examined the multi-scale effects of land use across a local-to-landscape gradient. FD metrics were computed using a probabilistic hypervolume approach and standardised against null expectations. Cause–effect relationships were explored using generalized additive models. Results revealed a decline in FD metrics, despite recent recoveries in richness and divergence. Species richness increased linearly and negatively influenced FD metrics. These findings, coupled with evidence of functional clustering, suggested a loss of functions and originality in the functional space, an increase of redundancy, and a shift towards traits associated with forest-dwelling, larger-bodied or omnivorous species. Land use was found to affect FD, likely acting as an environmental filter promoting trait clustering. Built-up areas exerted a detrimental effect on all FD metrics at smaller scales, while arable land increased FD across all scales. Woody features increased FD primarily at the landscape level, while hedgerows showed a reversed effect on evenness. Meadows and shrubs were important to sustain functional divergence at the local scale. Wetlands increased functional richness and divergence without affecting evenness. These findings raise important questions on the relationships between FD and species richness, emphasise the value of long-term data, and highlight the benefit of a multi-scale spatial approach for guiding land-use planning and biodiversity conservation.