Aims Climate change is known to drive both the reshuffling of whole assemblages and range shifts ... more Aims Climate change is known to drive both the reshuffling of whole assemblages and range shifts of individual species. Less is known about how local colonizations and extinctions of individual species contribute to changes at the community level. Our aim was to estimate the contribution of individual species to a change in community composition attributed to climate change and to relate these species-specific contributions to species' commonness, climatic niche characteristics and life history traits most likely to influence species sensitivity to climate change. Location Sweden. Methods Focussing on birds, we analysed changes from 1998 to 2012 in the Community Temperature Index (CTI), a measure of the average climatic niche of a community. Using a jackknife approach we assessed the contribution of individual species to the temporal trend in CTI in four different regions across Sweden, controlling for habitat distribution. We further tested whether species contribution was related to population trends and rarity to identify species most vulnerable to climate change. Results Community Temperature Index had increased over time with the greatest gains occurring in the north of the country, reflecting the larger temperature increases in this area. Changes in the regional CTI were driven both by warm-dwelling species colonizing new sites and by extirpations of colddwelling species. Furthermore, the community changes were influenced by both rare and common species. At the same time, the distribution changes of a large number of species were seemingly unaffected by climate change. Main conclusions Both range expansion and contractions contributed to the relative increase of warm-dwelling species in Swedish bird communities. We successfully identified the climatic impacts on some of Sweden's rarest species, including cold-dwelling species in the mountainous north. Our approach may be an efficient tool to use when characterizing the impacts of climate change on species and communities.
Urbanization is considered as one of the most important land-use and land-cover (LULC) changes wi... more Urbanization is considered as one of the most important land-use and land-cover (LULC) changes with multiple pervasive effects on biodiversity. However, the quantitative assessment of biodiversity responses to urbanization remains challenging because some species can be directly and negatively affected by the spread of human settlements, while others can benefit from this LULC change. Moreover, although species sensitivity to urban settlements (their "synanthropy") can either correspond to the spatial segregation of individuals within urban habitats or to their positive temporal trends in these habitats, these two facets are hardly distinguished explicitly. Here, we confronted the fine-scale spatial distribution of all the buildings in France with the spatial distribution and population trends of the 119 most common French breeding birds monitored in 2124 plots from 2001 to 2012. We developed and tested two indicators of "synanthropy". The first indicator (S1) differentiates species along a continuous gradient from urban "avoiders" (low S1) to urban "dwellers" (high S1). The second indicator measures the beneficial or detrimental effect of building densities on the temporal trends of the populations. It allows the segregation among urban "losers" having lower temporal trends with increasing buildings (low S2) from urban "winners" (high S2) having more positive trends in more urbanized areas. We then tested the relationships between S1 and S2 with a set of species and life history traits. Finally, we transposed these species indicators to communities using community weighted means to test the link between the synanthropy of communities with bird species richness, and the spatial, temporal and spatio-temporal trends of the synanthropy of bird communities. We found that 43% of the species were "urban dwellers", and 18% "urban winners". Both urban dwellers and winners were species widely distributed and locally abundant. Urban dwellers were mainly ground feeders but did not nest on the ground. At the community level, high species richness was associated with medium-values of community synanthropy, following the intermediate-disturbance hypothesis. We found that the average value of community synanthropy and their trend were not randomly distributed in space. These two indicators of synanthropy can be used in different taxonomic groups and areas to assess the proportion of synanthropic species within communities, to monitor their temporal trends and their spatial distribution and represent a straightforward complement to the synthetic indicators of human footprint on biodiversity.
Although how rare species persist in communities is a major ecological question, the critical phe... more Although how rare species persist in communities is a major ecological question, the critical phenotypic dimension of rarity is broadly overlooked. Recent work has shown that evaluating functional distinctiveness, the average trait distance of a species to other species in a community, offers essential insights into biodiversity dynamics, ecosystem functioning, and biological conservation. However, the ecological mechanisms underlying the persistence of functionally distinct species are poorly understood. Here we propose a heterogeneous fitness landscape framework, whereby functional dimensions encompass peaks representing trait combinations that yield positive intrinsic growth rates in a community. We identify four fundamental causes leading to the persistence of functionally distinct species in a community. First, environmental heterogeneity or alternative phenotypic designs can drive positive population growth of functionally distinct species. Second, sink populations with negative growth can deviate from local fitness peaks and be functionally distinct. Third, species found at the margin of the fitness landscape can persist but be functionally distinct. Fourth, biotic interactions (either positive or negative) can dynamically alter the fitness landscape. We offer examples of these four cases and some guidelines to distinguish among them. In addition to these deterministic processes, we also explore how stochastic dispersal limitation can yield functional distinctiveness.
Beyond the local abundance of species, their functional trait distinctiveness is now recognized a... more Beyond the local abundance of species, their functional trait distinctiveness is now recognized as a key driver of community dynamics and ecosystem functioning. Yet, since the functional distinctiveness of a species is always relative to a given species pool, a species distinct at regional scale might not necessarily be distinct at local or community scale, and reciprocally. To assess the importance of scale (i.e. the definition of a species pool) when quantifying the functional distinctiveness of species, and how it might distort the ecological conclusions derived from it, we quantified trait distinctiveness of 1350 plant species at regional, local and community scales over ca 88 000 grassland plots in France. We measured differences in functional distinctiveness of species between regional (mainland France), local (10 × 10 km cell) and community (10 × 10 m plot) scale, and tested the influence of environmental predictors (climate and nitrogen input) and contexts (environmental distinctiveness, frequency and heterogeneity) on these variations. In line with theoretical expectations, we found large variation in functional distinctiveness (in particular between regional and community scales) for many species, with a general tendency of lower distinctiveness at smaller scales. We also showed that nitrogen input – a key aspect of high land use intensity – and environmental frequency partly explained the differences between local and regional scale only. These results suggest the role played by environmental filtering on species distinctiveness at local scale, but the determinant of distinctiveness variations at community scale still need to be elucidated. Our study provides robust empirical evidence that measures of ecological originality are strongly scale‐dependent. We urge ecologists to carefully consider the scale at which they measure distinctiveness, as ignoring scale dependencies could lead to biased (or even entirely wrong) conclusions when not considered at the scale of interest for the respective research question.
Little is known about whether changes in lake ecosystem structure over the past 150 years are unp... more Little is known about whether changes in lake ecosystem structure over the past 150 years are unprecedented when considering longer timescales. Similarly, research linking environmental stressors to lake ecological resilience has traditionally focused on a few sentinel sites, hindering the study of spatially synchronous changes across large areas. Here, we studied signatures of paleolimnological resilience by tracking change in diatom community composition over the last 2000 years in four Ecuadorian Andean lakes with contrasting ecoregions. We focused on climate and anthropogenic change, and the type of biological responses that these changes induced: gradual, elastic, or threshold. We combined multivariate ordination techniques with nonlinear time‐series methods (hierarchical generalized additive models) to characterize trajectories of community responses in each lake, and coherence in such trajectories across lakes. We hypothesized that remote, high‐elevation lakes would exhibit synchronous trends due to their shared climatic constraints, whereas lower elevation lakes would show less synchronous trends as a consequence of human density and land‐cover alteration. We found that gradual and elastic responses dominated. Threshold‐type responses, or regime shifts, were only detected in the less remote lake, after a long period of gradual and elastic changes. Unexpected synchrony was observed in diatom assemblages from geographically distant and human‐impacted lakes, whereas lakes under similar broad‐scale environmental factors (climate and ecoregion) showed asynchronous community trajectories over time. Our results reveal a complex ecological history and indicate that Andean lakes in Ecuador can gradually adapt and recover from a myriad of disturbances, exhibiting resilience over century to millennial timescales.
Proceedings of The Royal Society B: Biological Sciences, Jan 19, 2022
Despite evidence of a positive effect of functional diversity on ecosystem productivity, the impo... more Despite evidence of a positive effect of functional diversity on ecosystem productivity, the importance of functionally distinct species (i.e. species that display an original combination of traits) is poorly understood. To investigate how distinct species affect ecosystem productivity, we used a forest-gap model to simulate realistic temperate forest successions along an environmental gradient and measured ecosystem productivity at the end of the successional trajectories. We performed 10 560 simulations with different sets and numbers of species, bearing either distinct or indistinct functional traits, and compared them to random assemblages, to mimic the consequences of a regional loss of species. Long-term ecosystem productivity dropped when distinct species were lost first from the regional pool of species, under the harshest environmental conditions. On the contrary, productivity was more dependent on ordinary species in milder environments. Our findings show that species functional distinctiveness, integrating multiple trait dimensions, can capture species-specific effects on ecosystem productivity. In a context of an environmentally changing world, they highlight the need to investigate the role of distinct species in sustaining ecosystem processes, particularly in extreme environmental conditions.
(228/300) : Global changes alter the dynamics of biodiversity, and are forecasted to continue or ... more (228/300) : Global changes alter the dynamics of biodiversity, and are forecasted to continue or worsen in the decades to come. Modelling approaches used to anticipate these impacts are mainly based on the equivalence between spatial and temporal response to environmental forcings, generally called space-for-time substitution. However, several processes are known to generate deviations between spatial and temporal responses, potentially undermining the prediction based on space-for-time substitution. We here used high-resolution data from the french breeding bird survey to quantify and map the deviation between spatial and temporal patterns of bird abundances resulting from the dynamics of 124 species monitored in 2,133 sites between 2001 and 2012. Using independent empirical data, we then tested specific predictions linked to the forcings (anthropogenic activities) and processes (asynchronous biotic responses) potentially generating these deviations. We found that deviations between spatial and temporal patterns of abundances were particularly structured in space for bird communities. Following our predictions, space-time deviations were positively correlated with the human influence on ecosystems, and linked with colonization-extinction ratios and community completeness, two markers of ongoing asynchronous responses to environmental forcings. Our results suggest that the discrepancy between space and time dynamics are related to anthropogenic forcings and disequilibrium responses to these forcings. Investigating deviations between spatial and temporal patterns of biodiversity might open promising perspectives for a formal quantification of disequilibrium state of biodiversity at large spatial scale.
Aim: The functional trait composition of plant communities is thought to be determined largely by... more Aim: The functional trait composition of plant communities is thought to be determined largely by climate, but relationships between contemporary trait distributions and climate are often weak. Spatial mismatches between trait and climatic conditions are commonly thought to arise from disequilibrium responses to past environmental changes. We aimed to investigate whether current trait-climate disequilibrium is likely to emerge during plant functional responses to Holocene climate warming. Location: North America. Time period: 14-0 ka. Major taxa studied: Terrestrial plants. Methods: We joined global trait data with palaeoecological time series and climate simulations on 425 sites. We estimated plant community functional composition for three leaf traits involved in resource use. We then quantified disequilibrium in plant trait temporal responses to climate change during two contrasted periods: a period of high climate variability (14-7 ka) and a period of low climate variability (7-0 ka). Results: Functional trait composition showed consistent deviation from climatic equilibrium during both periods. The temporal dynamics of trait composition tends to be positively correlated with climate equilibrium expectations during Holocene climate warming (14-7 ka), but not during a subsequent period of low climate variability (7-0 ka). Main conclusions: Long-term functional responses of plants to climate change showed mixed evidence for both equilibrium and disequilibrium responses. Temporal trait dynamics were closer to the expectations of spatial dynamics under high climate variability, indicating that the relevance of space-for-time substitution might be dependent, in part, on climate variability. Our results also suggest that current mismatches between trait and climatic conditions might arise because of a divergence of factors influencing trait dynamics during periods of low climate variability. These findings provide a counterpoint to the common assumption that contemporary traitclimate mismatches result from lagged responses to past climate warming. Our study also demonstrates the need for a deeper investigation of the potential influence of non-climatic factors on functional plant community dynamics.
Community Weighted Means (CWM) are valuable tools describing community composition with respect t... more Community Weighted Means (CWM) are valuable tools describing community composition with respect to one given trait. They have been widely used as indicators in global change studies to measure biodiversity responses to environmental perturbations. However, how individual species contribute to such community indicators has hardly been investigated. One of the reasons lies in the absence of a methodological framework relating changes in community dynamics to species-specific population variations. Here, we present a comprehensive framework allowing a finer interpretation of changes in CWM, and we propose a way to compute species contributions to these indicators. We present an analytical framework allowing the quantification of species-specific contributions to changes in the mean (CWM) and the variance (Community Weighted Variance, CWV) of trait distributions in species assemblages monitored through time and space. We apply this approach to a case study investigating the impact of climate change on common bird assemblages in the French Mediterranean area between 2001 and 2012. This approach allows us to identify that a small proportion of the species drive the changes observed at the community level indicator, and allows the identification of those species. Moreover, we show that the speciesspecific contributions are not homogeneous between taxonomic groups and that migratory species tend to have a higher impact. This novel decomposition and interpretation of Community Weighted Means and Variances (for which specific software package is provided along with this article) sheds new light on the drivers of community modifications in response to environmental changes across time and space. Moreover, it represents a relevant and simple way to assess particular aspects of species-specific responses to environmental changes and it is straightforward to use for widely used ecological data on any species group.
Agriculture, Ecosystems & Environment, Feb 1, 2020
Rural landscapes of western Europe have considerably changed in the last decades under the combin... more Rural landscapes of western Europe have considerably changed in the last decades under the combined pressure of climate and land use changes, leading to a dramatic decline of farmland biodiversity, including common farmland birds. The respective roles of climate and land use and cover changes in driving bird population trends are primarily assessed at national or continental levels. Yet, it is often challenging to integrate their intertwined effects at such large scales due to the lack of data on fine-scale land cover changes. Here, we used a long-term bird monitoring scheme, combined with a land cover survey, conducted during 30 years (1981-2011) across 780 sites in a 20,000 ha study area in southwestern France, dominated by low-intensity farming systems. We tested the direct effect of temporal changes in climate and land use on the dynamics of two community-level metrics: the bird Community Thermal Index (CTI) and bird Community Generalization Index (CGI). We used a novel method to assess the contribution of species-specific dynamics to CTI and CGI trends. We observed a significant increase in CTI and a significant decrease in CGI between 1981 and 2011, i.e., bird communities now have higher thermal preferences and are more specialized than 30 years ago. Bird CTI and CGI changes were both related to local climate-and land use-related drivers, especially mean temperature increase and hedgerow loss. Trends in CTI and CGI were primarily driven by the loss of cold-dwelling and generalist species, and secondly by a gain in hot-dwelling specialists. Our long-term study brings new empirical evidence that the effects of climate and land cover changes on bird communities are intrinsically intertwined, and need to be considered together to monitor and predict the future of farmland biodiversity. It also suggests that low-input, diversified agriculture combined with the maintenance of semi-natural habitat cover can contribute to the conservation of both specialist and generalist bird communities in agricultural landscapes experiencing rapid climate change.
Aims Climate change is known to drive both the reshuffling of whole assemblages and range shifts ... more Aims Climate change is known to drive both the reshuffling of whole assemblages and range shifts of individual species. Less is known about how local colonizations and extinctions of individual species contribute to changes at the community level. Our aim was to estimate the contribution of individual species to a change in community composition attributed to climate change and to relate these species-specific contributions to species' commonness, climatic niche characteristics and life history traits most likely to influence species sensitivity to climate change. Location Sweden. Methods Focussing on birds, we analysed changes from 1998 to 2012 in the Community Temperature Index (CTI), a measure of the average climatic niche of a community. Using a jackknife approach we assessed the contribution of individual species to the temporal trend in CTI in four different regions across Sweden, controlling for habitat distribution. We further tested whether species contribution was related to population trends and rarity to identify species most vulnerable to climate change. Results Community Temperature Index had increased over time with the greatest gains occurring in the north of the country, reflecting the larger temperature increases in this area. Changes in the regional CTI were driven both by warm-dwelling species colonizing new sites and by extirpations of colddwelling species. Furthermore, the community changes were influenced by both rare and common species. At the same time, the distribution changes of a large number of species were seemingly unaffected by climate change. Main conclusions Both range expansion and contractions contributed to the relative increase of warm-dwelling species in Swedish bird communities. We successfully identified the climatic impacts on some of Sweden's rarest species, including cold-dwelling species in the mountainous north. Our approach may be an efficient tool to use when characterizing the impacts of climate change on species and communities.
Urbanization is considered as one of the most important land-use and land-cover (LULC) changes wi... more Urbanization is considered as one of the most important land-use and land-cover (LULC) changes with multiple pervasive effects on biodiversity. However, the quantitative assessment of biodiversity responses to urbanization remains challenging because some species can be directly and negatively affected by the spread of human settlements, while others can benefit from this LULC change. Moreover, although species sensitivity to urban settlements (their "synanthropy") can either correspond to the spatial segregation of individuals within urban habitats or to their positive temporal trends in these habitats, these two facets are hardly distinguished explicitly. Here, we confronted the fine-scale spatial distribution of all the buildings in France with the spatial distribution and population trends of the 119 most common French breeding birds monitored in 2124 plots from 2001 to 2012. We developed and tested two indicators of "synanthropy". The first indicator (S1) differentiates species along a continuous gradient from urban "avoiders" (low S1) to urban "dwellers" (high S1). The second indicator measures the beneficial or detrimental effect of building densities on the temporal trends of the populations. It allows the segregation among urban "losers" having lower temporal trends with increasing buildings (low S2) from urban "winners" (high S2) having more positive trends in more urbanized areas. We then tested the relationships between S1 and S2 with a set of species and life history traits. Finally, we transposed these species indicators to communities using community weighted means to test the link between the synanthropy of communities with bird species richness, and the spatial, temporal and spatio-temporal trends of the synanthropy of bird communities. We found that 43% of the species were "urban dwellers", and 18% "urban winners". Both urban dwellers and winners were species widely distributed and locally abundant. Urban dwellers were mainly ground feeders but did not nest on the ground. At the community level, high species richness was associated with medium-values of community synanthropy, following the intermediate-disturbance hypothesis. We found that the average value of community synanthropy and their trend were not randomly distributed in space. These two indicators of synanthropy can be used in different taxonomic groups and areas to assess the proportion of synanthropic species within communities, to monitor their temporal trends and their spatial distribution and represent a straightforward complement to the synthetic indicators of human footprint on biodiversity.
Although how rare species persist in communities is a major ecological question, the critical phe... more Although how rare species persist in communities is a major ecological question, the critical phenotypic dimension of rarity is broadly overlooked. Recent work has shown that evaluating functional distinctiveness, the average trait distance of a species to other species in a community, offers essential insights into biodiversity dynamics, ecosystem functioning, and biological conservation. However, the ecological mechanisms underlying the persistence of functionally distinct species are poorly understood. Here we propose a heterogeneous fitness landscape framework, whereby functional dimensions encompass peaks representing trait combinations that yield positive intrinsic growth rates in a community. We identify four fundamental causes leading to the persistence of functionally distinct species in a community. First, environmental heterogeneity or alternative phenotypic designs can drive positive population growth of functionally distinct species. Second, sink populations with negative growth can deviate from local fitness peaks and be functionally distinct. Third, species found at the margin of the fitness landscape can persist but be functionally distinct. Fourth, biotic interactions (either positive or negative) can dynamically alter the fitness landscape. We offer examples of these four cases and some guidelines to distinguish among them. In addition to these deterministic processes, we also explore how stochastic dispersal limitation can yield functional distinctiveness.
Beyond the local abundance of species, their functional trait distinctiveness is now recognized a... more Beyond the local abundance of species, their functional trait distinctiveness is now recognized as a key driver of community dynamics and ecosystem functioning. Yet, since the functional distinctiveness of a species is always relative to a given species pool, a species distinct at regional scale might not necessarily be distinct at local or community scale, and reciprocally. To assess the importance of scale (i.e. the definition of a species pool) when quantifying the functional distinctiveness of species, and how it might distort the ecological conclusions derived from it, we quantified trait distinctiveness of 1350 plant species at regional, local and community scales over ca 88 000 grassland plots in France. We measured differences in functional distinctiveness of species between regional (mainland France), local (10 × 10 km cell) and community (10 × 10 m plot) scale, and tested the influence of environmental predictors (climate and nitrogen input) and contexts (environmental distinctiveness, frequency and heterogeneity) on these variations. In line with theoretical expectations, we found large variation in functional distinctiveness (in particular between regional and community scales) for many species, with a general tendency of lower distinctiveness at smaller scales. We also showed that nitrogen input – a key aspect of high land use intensity – and environmental frequency partly explained the differences between local and regional scale only. These results suggest the role played by environmental filtering on species distinctiveness at local scale, but the determinant of distinctiveness variations at community scale still need to be elucidated. Our study provides robust empirical evidence that measures of ecological originality are strongly scale‐dependent. We urge ecologists to carefully consider the scale at which they measure distinctiveness, as ignoring scale dependencies could lead to biased (or even entirely wrong) conclusions when not considered at the scale of interest for the respective research question.
Little is known about whether changes in lake ecosystem structure over the past 150 years are unp... more Little is known about whether changes in lake ecosystem structure over the past 150 years are unprecedented when considering longer timescales. Similarly, research linking environmental stressors to lake ecological resilience has traditionally focused on a few sentinel sites, hindering the study of spatially synchronous changes across large areas. Here, we studied signatures of paleolimnological resilience by tracking change in diatom community composition over the last 2000 years in four Ecuadorian Andean lakes with contrasting ecoregions. We focused on climate and anthropogenic change, and the type of biological responses that these changes induced: gradual, elastic, or threshold. We combined multivariate ordination techniques with nonlinear time‐series methods (hierarchical generalized additive models) to characterize trajectories of community responses in each lake, and coherence in such trajectories across lakes. We hypothesized that remote, high‐elevation lakes would exhibit synchronous trends due to their shared climatic constraints, whereas lower elevation lakes would show less synchronous trends as a consequence of human density and land‐cover alteration. We found that gradual and elastic responses dominated. Threshold‐type responses, or regime shifts, were only detected in the less remote lake, after a long period of gradual and elastic changes. Unexpected synchrony was observed in diatom assemblages from geographically distant and human‐impacted lakes, whereas lakes under similar broad‐scale environmental factors (climate and ecoregion) showed asynchronous community trajectories over time. Our results reveal a complex ecological history and indicate that Andean lakes in Ecuador can gradually adapt and recover from a myriad of disturbances, exhibiting resilience over century to millennial timescales.
Proceedings of The Royal Society B: Biological Sciences, Jan 19, 2022
Despite evidence of a positive effect of functional diversity on ecosystem productivity, the impo... more Despite evidence of a positive effect of functional diversity on ecosystem productivity, the importance of functionally distinct species (i.e. species that display an original combination of traits) is poorly understood. To investigate how distinct species affect ecosystem productivity, we used a forest-gap model to simulate realistic temperate forest successions along an environmental gradient and measured ecosystem productivity at the end of the successional trajectories. We performed 10 560 simulations with different sets and numbers of species, bearing either distinct or indistinct functional traits, and compared them to random assemblages, to mimic the consequences of a regional loss of species. Long-term ecosystem productivity dropped when distinct species were lost first from the regional pool of species, under the harshest environmental conditions. On the contrary, productivity was more dependent on ordinary species in milder environments. Our findings show that species functional distinctiveness, integrating multiple trait dimensions, can capture species-specific effects on ecosystem productivity. In a context of an environmentally changing world, they highlight the need to investigate the role of distinct species in sustaining ecosystem processes, particularly in extreme environmental conditions.
(228/300) : Global changes alter the dynamics of biodiversity, and are forecasted to continue or ... more (228/300) : Global changes alter the dynamics of biodiversity, and are forecasted to continue or worsen in the decades to come. Modelling approaches used to anticipate these impacts are mainly based on the equivalence between spatial and temporal response to environmental forcings, generally called space-for-time substitution. However, several processes are known to generate deviations between spatial and temporal responses, potentially undermining the prediction based on space-for-time substitution. We here used high-resolution data from the french breeding bird survey to quantify and map the deviation between spatial and temporal patterns of bird abundances resulting from the dynamics of 124 species monitored in 2,133 sites between 2001 and 2012. Using independent empirical data, we then tested specific predictions linked to the forcings (anthropogenic activities) and processes (asynchronous biotic responses) potentially generating these deviations. We found that deviations between spatial and temporal patterns of abundances were particularly structured in space for bird communities. Following our predictions, space-time deviations were positively correlated with the human influence on ecosystems, and linked with colonization-extinction ratios and community completeness, two markers of ongoing asynchronous responses to environmental forcings. Our results suggest that the discrepancy between space and time dynamics are related to anthropogenic forcings and disequilibrium responses to these forcings. Investigating deviations between spatial and temporal patterns of biodiversity might open promising perspectives for a formal quantification of disequilibrium state of biodiversity at large spatial scale.
Aim: The functional trait composition of plant communities is thought to be determined largely by... more Aim: The functional trait composition of plant communities is thought to be determined largely by climate, but relationships between contemporary trait distributions and climate are often weak. Spatial mismatches between trait and climatic conditions are commonly thought to arise from disequilibrium responses to past environmental changes. We aimed to investigate whether current trait-climate disequilibrium is likely to emerge during plant functional responses to Holocene climate warming. Location: North America. Time period: 14-0 ka. Major taxa studied: Terrestrial plants. Methods: We joined global trait data with palaeoecological time series and climate simulations on 425 sites. We estimated plant community functional composition for three leaf traits involved in resource use. We then quantified disequilibrium in plant trait temporal responses to climate change during two contrasted periods: a period of high climate variability (14-7 ka) and a period of low climate variability (7-0 ka). Results: Functional trait composition showed consistent deviation from climatic equilibrium during both periods. The temporal dynamics of trait composition tends to be positively correlated with climate equilibrium expectations during Holocene climate warming (14-7 ka), but not during a subsequent period of low climate variability (7-0 ka). Main conclusions: Long-term functional responses of plants to climate change showed mixed evidence for both equilibrium and disequilibrium responses. Temporal trait dynamics were closer to the expectations of spatial dynamics under high climate variability, indicating that the relevance of space-for-time substitution might be dependent, in part, on climate variability. Our results also suggest that current mismatches between trait and climatic conditions might arise because of a divergence of factors influencing trait dynamics during periods of low climate variability. These findings provide a counterpoint to the common assumption that contemporary traitclimate mismatches result from lagged responses to past climate warming. Our study also demonstrates the need for a deeper investigation of the potential influence of non-climatic factors on functional plant community dynamics.
Community Weighted Means (CWM) are valuable tools describing community composition with respect t... more Community Weighted Means (CWM) are valuable tools describing community composition with respect to one given trait. They have been widely used as indicators in global change studies to measure biodiversity responses to environmental perturbations. However, how individual species contribute to such community indicators has hardly been investigated. One of the reasons lies in the absence of a methodological framework relating changes in community dynamics to species-specific population variations. Here, we present a comprehensive framework allowing a finer interpretation of changes in CWM, and we propose a way to compute species contributions to these indicators. We present an analytical framework allowing the quantification of species-specific contributions to changes in the mean (CWM) and the variance (Community Weighted Variance, CWV) of trait distributions in species assemblages monitored through time and space. We apply this approach to a case study investigating the impact of climate change on common bird assemblages in the French Mediterranean area between 2001 and 2012. This approach allows us to identify that a small proportion of the species drive the changes observed at the community level indicator, and allows the identification of those species. Moreover, we show that the speciesspecific contributions are not homogeneous between taxonomic groups and that migratory species tend to have a higher impact. This novel decomposition and interpretation of Community Weighted Means and Variances (for which specific software package is provided along with this article) sheds new light on the drivers of community modifications in response to environmental changes across time and space. Moreover, it represents a relevant and simple way to assess particular aspects of species-specific responses to environmental changes and it is straightforward to use for widely used ecological data on any species group.
Agriculture, Ecosystems & Environment, Feb 1, 2020
Rural landscapes of western Europe have considerably changed in the last decades under the combin... more Rural landscapes of western Europe have considerably changed in the last decades under the combined pressure of climate and land use changes, leading to a dramatic decline of farmland biodiversity, including common farmland birds. The respective roles of climate and land use and cover changes in driving bird population trends are primarily assessed at national or continental levels. Yet, it is often challenging to integrate their intertwined effects at such large scales due to the lack of data on fine-scale land cover changes. Here, we used a long-term bird monitoring scheme, combined with a land cover survey, conducted during 30 years (1981-2011) across 780 sites in a 20,000 ha study area in southwestern France, dominated by low-intensity farming systems. We tested the direct effect of temporal changes in climate and land use on the dynamics of two community-level metrics: the bird Community Thermal Index (CTI) and bird Community Generalization Index (CGI). We used a novel method to assess the contribution of species-specific dynamics to CTI and CGI trends. We observed a significant increase in CTI and a significant decrease in CGI between 1981 and 2011, i.e., bird communities now have higher thermal preferences and are more specialized than 30 years ago. Bird CTI and CGI changes were both related to local climate-and land use-related drivers, especially mean temperature increase and hedgerow loss. Trends in CTI and CGI were primarily driven by the loss of cold-dwelling and generalist species, and secondly by a gain in hot-dwelling specialists. Our long-term study brings new empirical evidence that the effects of climate and land cover changes on bird communities are intrinsically intertwined, and need to be considered together to monitor and predict the future of farmland biodiversity. It also suggests that low-input, diversified agriculture combined with the maintenance of semi-natural habitat cover can contribute to the conservation of both specialist and generalist bird communities in agricultural landscapes experiencing rapid climate change.
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Papers by Pierre Gauzere