In this study, we found that two thirds of the European breeding bird communities experienced spatiotemporal shifts in their composition. Although the directions of community composition shift varied substantially, the shifts were largely directed to north, east, and west, rather than south, corroborating most research findings on directional climate-driven distribution shifts of single species in the northern hemisphere5. Following our hypotheses, both geo-physical and bio-climatic barriers consistently, and predictably, influenced community composition shifts, such that bird communities generally shifted in distances and directions to avoid ecological barriers.
Breeding bird community compositions have likely shifted due to the influence of global change drivers on single species distributions1. Whilst our findings highlight spatiotemporal shifts of communities, ecological communities are undergoing compositional and functional homogenization globally28–30. As biotic homogenization proceeds, the dissimilarity decreases among different communities across space31,32 but increases between observations of the same community over time28. Indeed, we found cases of high dissimilarities when comparing bird communities in some grid cells in the 1980’s to all grid cells in the 2010’s, suggesting that those community compositions observed in the 1980’s had no corresponding compositions in the 2010’s. Such ‘community composition extinctions’ can result from losing and/or gaining bird species in the local community, both of which have been observed across Europe33 and globally34 as a result of environmental change. Importantly, if many communities consistently shift away from certain areas, there may be substantial impacts on interaction network configurations, metacommunity dynamics, and ecosystem functioning over large spatial scales23,35. The lack of compositional matches between bird communities in the 1980’s and the 2010’s may also reflect the appearance of novel community compositions in the latter that did not occur in the former period, potentially leading to unpredictable alterations in ecosystem functioning36.
Out of the four ecological barriers considered in our study, elevation and coastlines do not change within an ecologically relevant time span, posing constant, geo-physical limitations to community composition shifts. On the other hand, biome boundaries and temperature change are more gradual across space and likely to change within an ecologically relevant time span, thereby posing weaker and potentially adaptable bio-climatic limits to community composition shifts. Together the observed effects of ecological barriers on community composition shifts indicate that species’ shifts, and thereby communities that consist of these species, may be mediated through least geo-physical resistance delineated by coastlines and elevation and through most optimal bio-climatic conditions delineated by temperature gradients and biomes. Coastlines was the most important determinant of community composition shift distances, such that the farther away the community was located from the coastline, the farther it shifted. In parallel, bird communities largely moved along directions with the least change in elevation relative to their original location, implying that communities are tightly associated with certain abiotic and biotic conditions along the elevational gradient37. Contrary to community composition shift direction, elevation resistance did not strongly influence community composition shift distance, which may be caused by the fact that in areas of high topographic variation, species may not need to shift far to track their original abiotic and biotic niche1,35. Temperature barriers tended to be associated with longer shifts (and directional shifts towards north) of the bird community compositions when the most similar temperature conditions were also located farther away (and towards north). This aligns with earlier findings that show the importance of the climatic niche in shaping single species’ ranges38. Biome boundaries showed differential effects on community composition shift distances and directions, such that the communities located closer to biome boundaries shifted farther, while the communities shifted directionally away from close biome boundaries. Although we did not test differences in edge-contrast of different biomes, it is unlikely that species shift across biome boundaries39 even when the biomes have relatively similar abiotic conditions (e.g., across cold and warm deserts)40, as the biotic conditions likely differ considerably among biomes.
Understanding how ecological barriers influence biodiversity shifts at large spatial scales can inform both ecological research and conservation management23. The environmental drivers influencing ecological communities potentially follow a spatial hierarchy, such that the ecological barriers – including those arising from climate change – likely govern species and community shifts at broader, macroecological scales, while other processes, such as e.g., species interactions, land use change, and habitat fragmentation, may limit the shift potential at smaller spatial scales35. For example, climate change may mediate shifts of many species through certain areas, but the realized shifts would be limited by anthropogenic landscape modifications35. In addition, different global change pressures, such as climate change, habitat loss and fragmentation, acting separately or in synergy, can favor community shifts in different directions18. Therefore, including ecological barriers in global change studies could lead to more realistic predictions of species and community shifts by setting the outer bounds of potential shifts41. These predictions can be further applied to large scale conservation decision making. So far, species with a primarily high latitude or high elevation distribution have been a major concern in the climate change context because their poleward or uphill shifts have natural limits42,43. Our results suggest that similar limitations apply in relation to other ecological barriers, and at the level of entire communities, as the ecological barriers guide the distance and direction of their shifts. In terms of conservation planning and climate change mitigation, understanding the effects of ecological barriers on species’ shifts can help identify major shift barriers and inform placement of key corridors that facilitate movement of species between climatically suitable areas.
Our study highlights the role of ecological barriers for species and communities under global change. Although our results on the ecological barrier effects on biodiversity shifts are general, it is likely that the relevance of a particular barrier depends on the ecosystem and taxonomic group in question. For example, precipitation change may be a relevant bio-climatic barrier for plant community composition shifts in arid ecosystems44. Here, we provide the first evidence of observed rather than predicted shifts of entire communities across large spatiotemporal scales as a function of constant geo-physical and more dynamic bio-climatic ecological barriers, which substantially advances previous research exploring the impacts of ecological barriers as determinants of single species’ range shifts23,35. Our approach in studying spatiotemporal shifts under global change is especially useful as it accounts for the composition of entire communities instead of summarizing the communities into richness and beta diversity measures that may remain unchanged over time even when the community composition changes entirely10,28. We suggest that ecological barrier data combined with species’ traits can provide complementary information for why species and communities shift slower than expected, for example, based on climate predictors alone6,33,45. Moreover, we suggest that by quantification of community composition shifts with abundance data, it is possible to observe community composition changes that influence the community’s functionality via ecological interactions before any species goes locally extinct. Finally, we suggest that time series of ecological barriers and community composition shifts could be combined to study more realistic velocities of community reshuffling and biodiversity shifts in general14,46. Understanding biodiversity shifts under global change is key to predict future ecosystem functioning and integrity, and design effective management and adaptation strategies. Our study adds to the current knowledge by unveiling, for the first time at a continental scale, that shifting communities are strongly, and predictably, affected by a range of ecological barriers. This underscores the importance of moving beyond simple climate change measures when studying community shifts.