In accordance with our hypotheses, we found a consistent diversity-thermal variability relationship when measuring the functional diversity of bird communities, although the relationship was not always statistically significant. The breeding season communities tended to be less thermally variable than the non-breeding season communities, but the impact of diversity on thermal variability was clearer in the non-breeding season. That is, more diverse non-breeding communities have had less change over time in their average thermal niche.
As hypothesized, functional diversity had a stronger effect on thermal variability than taxonomic or evolutionary diversity. Such an effect may stem from higher ecosystem functioning and from established species’ interactions in functionally diverse communities. Functional diversity may contribute to lower thermal variability via greater filling of the environmental niche space in the local community, leading to smaller realized niches. Earlier studies have shown that climate change has resulted in both negative and positive trends in species’ populations 66. Thus, declining populations could cause the local community to lose some of its functionality, i.e. in the total niche space. Therefore, functionally diverse communities could be assumed to have fewer changes in their community compositions and less decrease in their functionality because they already contain several species that can occupy the vacant niche space of the declining species. Such vacant niche space could be occupied by functionally redundant species 67 or by species that adjust or expand their realized niches (e.g., interaction strength rewiring) 38. Birds as a taxonomic group have highly varying and complementary functional roles within communities in terms of their trophic position (e.g., herbivore, top predator) and interaction type with other species (e.g., seed dispersal mutualism, scavenger commensalism). Generally, such functional complementarity among species can stabilize communities, because it reduces competition strength via resource and enemy partitioning as well as facilitation 16.
Community diversity can contribute to community’s climate compositional changes under climate change also via species’ interactions 68. This was supported by our finding that thermal variability in the non-breeding season was associated with the functional diversity measure indicating the trophic diversity and the abundance of higher trophic levels in the communities. This finding suggests that communities including top predators (and thus multiple trophic levels and long food chains) may be better able to resist climate-induced changes in the community composition 2. In general, when the communities are less diverse, each species is likely to have fewer and stronger interactions with other species 69. Thus, the community as a whole is less robust to short-term variations in single species’ abundances. Moreover, disturbances can affect focal species via indirect interactions 70, such as facilitation and apparent competition 71,72. Removal of species can therefore lead to cascading effects through the entire community 73,74. To assess the effect of interaction network properties on community’s thermal variability under climate change at large spatial scales, systematically sampled time-series data of interactions are needed. Until such data are available, studies like ours provide the first steps towards understanding how ecological processes related to interaction networks influence community composition changes under climate change.
We did not find seasonal differences in the diversity-thermal variability relationships, but the average level of thermal variability and the variation in the diversity measures differed between seasons. That is, the mean and variation of functional diversity did not largely differ between seasons but the variation of taxonomic and evolutionary diversities did. Both the observed lower thermal variability and the smaller variation in the diversity measures of the breeding season communities may be due to different selection pressures between seasons. For example, breeding season bird communities may have an overall weaker response to climate change (i.e., fewer changes) because they are less plastic in their mobility, foraging, or other behaviours related to demographic processes 62,75. Moreover, the winter cold has so far been a more limiting factor compared to the summer heat as maximum temperatures have not yet reached upper critical temperatures for most temperate-breeding birds 76, translating into a larger number of projected colonizations in the non-breeding season communities 77. Seasonal differences could also arise from differing location fidelities, as birds typically tend to return to the same breeding locations but track food resources in the non-breeding season 9,78. In the future, studying the seasonal variation in diversity-thermal variability relationships is increasingly important as climate extremes and potentially also the seasonal variation are likely to increase due to climate change 79,80.
Future studies could extend our work to study the different processes influencing diversity-thermal variability relationships at finer scales. Firstly, more detailed understanding on the temperature niches underlying thermal variability could be obtained by using not only mean temperature niches, but also the minimum, maximum and range of temperatures experienced by species within their ranges. Secondly, future studies could also evaluate which species in particular are responsible for the low variability in the average thermal niche in the local communities. This could provide information on the species whose protection would lead to most effective conservation under climate change. Thirdly, further details could be added into the thermal variability measure by assessing not only the realized temperature niches of species across their geographic range but also their experimental temperature tolerance. That is, experimental data could be useful for understanding the fundamental temperature niches of species, as species’ realized temperature niches are influenced by various other factors beyond climate, such as anthropogenic pressures, ecological interactions, and dispersal limitations 10,55. Finally, although bird communities tend to respond mainly to temperature changes rather than precipitation changes or extreme events 81,82, it is possible that in some areas climate change drivers beyond temperature change are more important. For example, in drier areas in southern North America, precipitation may be a more limiting environmental determinant of species' ranges and community properties than temperature. Therefore, future studies could also assess the spatial variation in the importance of different climate change drivers on diversity-climatic niche variability relationship.
Diversity-thermal variability relationships could inform conservation decision making with knowledge on which communities are most susceptible to future climate change. When many species disappear and appear or increase and decrease within a community simultaneously, the species composition and consequently the functional composition of the community are changed. Following such compositional changes, drastic changes in ecosystem functioning may occur if a tipping point is passed 74,83,84. Synergistic effects of climate change with other drivers may cause the communities to reach the tipping points even faster. Indeed, beyond additive effects on thermal variability, diversity can affect thermal variability of communities synergistically with anthropogenic disturbances. Anthropogenic disturbances, such as land use intensification, can cause selective extinctions of particular functional groups sensitive to land use changes 85 and increase synchrony in population dynamics 86. However, concrete conservation efforts can greatly counteract negative effects of anthropogenic pressures on bird communities (Michel et al. 2020).
Based on the results using comprehensive abundance data at a continental scale and spanning 50 years, we conclude that diverse communities have fewer changes in their compositions under climate change compared to simple communities. Importantly, diversity likely has an effect on community’s overall stability through the complex interactions among species. Therefore, it is crucial to consider communities as a whole instead of individual species to allow forecasting climate change effects on biodiversity and ecosystem functioning. Moreover, the large-scale perspective is important because local scale processes may not always reflect continental or global patterns. Our results underscore that diversity can buffer the effects of climate change on species communities, which underlines the importance of jointly considering climate and biodiversity crises.