The Functional Diversity of Birds on the East Frisian Islands across Space and Time in the Context of Climate Change

The population trends of birds in the Wadden Sea - one of the most important stopovers for more than 20 million birds along the East Atlantic Flyway - have been the subject of ornithological research for decades. The populations of breeding birds and of resting birds (waterbirds and waders) are, among other factors, the basis for determining the status of the Wadden Sea National Parks and the World Heritage Site. Although trait-based Functional Diversity (FD) can provide a stronger link between ecological functions of species and the environmental factors influencing an ecosystem, mainly organism- and abundance-based studies have so far reported spatial differences and temporal changes of bird populations in the Wadden Sea. Thus, trait-based analyses can now be used to assess impacts within the ecosystem and provide valuable information for nature conservation.

In our study, we use abundance data of breeding and resting birds of the East Frisian Islands from 1996 to 2021 and link them to a matrix of species-specific functional traits to derive different aspects of FD. Using Null Models based on the observed changing structure of species abundance, we calculate the potential range of FD measures to estimate the impact of environmental changes such as climate change on FD. 

Our results show that FD varies between breeding and resting birds and between islands over time. We found that abundance-based diversity measures such as the Shannon index are increasing through time, which affects FD. Measured values are in the lower range of potential FD and are significantly linked to trends in the climate variables (air temperature, precipitation and wind speed). For example, increasing air temperature influences an increase in Functional Dispersion. Given the observed links between the strong differences in temporal dynamics of FD across the East Frisian Islands and their different degrees of environmental change, we conclude that investigations of FD can provide new information on the climatic drivers of assemblage-level changes in species abundances.