Climate change reshuffles northern species within their niches

Climate change is a major threat to biodiversity, potentially rivaling land-use change in impact. Predictive efforts often focus on range shifts and community changes due to species colonization and extinction, implicitly relying on the niche concept – species tolerate a limited range of environmental conditions. While climate change has caused shifts in species abundance and distribution, leading to potential community homogenization, the importance of species shifting within their climatic niches remains less explored. The relative importance of different climatic variables in these shifts may also vary across taxa, space, and time, leading to complex and heterogeneous responses. This heterogeneity hinders assessment of climate change consequences for community structure at large spatial scales. This study addresses this gap by analyzing a unique, long-term dataset from Finland to quantify species responses to multiple climatic variables over four decades.

Existing literature extensively documents the impacts of climate change on biodiversity, primarily focusing on range shifts and community changes resulting from species extinctions and colonizations. These studies often rely on the niche concept, which posits that species thrive within a specific range of environmental conditions. Studies have shown significant shifts in species abundance and distribution in response to climate change, often resulting in increased homogenization of community composition. However, research on how species' positions within their climatic niches change over time remains limited. The interplay of multiple climatic variables and their varying relative importance across different taxa, spatial scales, and temporal periods are not fully understood. This makes predicting the precise impacts of climate change on biodiversity at larger scales a challenge. Previous work has explored the impacts of temperature on ectotherms, demonstrating the effects of climate warming across latitudes. However, a more comprehensive, taxonomically diverse examination of biodiversity responses to changing climate across spatial and temporal gradients is needed.

This study leveraged a unique dataset spanning four decades (1978-2017) across a 1200 km latitudinal gradient in Finland. The data included 1,478 species of birds, mammals, small rodents, butterflies, moths, plants, and phytoplankton. The study area was divided into three bioclimatic zones (north, middle, and south boreal). Annual values of mean temperature, total precipitation, snow cover duration, and the North Atlantic Oscillation (NAO) index were used as climatic variables. The analysis focused on shifts in species' relative niche positions (lower end, optimum, upper end) over time. Joint species distribution models (JSDMs) were fitted for each combination of taxonomic group × bioclimatic zone × decade to assess how climatic variables affect species occurrence patterns and their relative importance over time and across latitude. The models used a Bayesian approach, incorporating spatial and temporal autocorrelation through latent variables. The proportion of variation explained by each climatic variable was assessed, and linear regressions were used to examine changes in their relative importance over time.

The study revealed strong climatic changes with uneven rates of change along the latitudinal gradient. The northernmost zone experienced stronger temperature and precipitation increases and stronger snow cover declines. In later decades, average conditions in the north resembled those of earlier decades in the middle and south. Species' relative positions within their niches shifted substantially over time, with a larger proportion of species showing responses to climate change at higher latitudes. While species turnover was limited, species moved to new domains within their environmental niches. Temperature was the strongest predictor of species occurrence variation, particularly in the north, with a growing proportion of species responding positively to increasing temperatures. Responses to precipitation and snow cover varied across decades and taxa. The relative importance of climatic variables in explaining species occurrences changed significantly over time, varying across zones and taxa. Temperature's explanatory power generally increased over time and latitude. While species turnover did occur, the main observed change was due to shifts in the species relative position within its niche. The proportion of species experiencing a 'thermal release', responding positively to increasing temperature, notably increased.

The findings highlight significant reshuffling within the biome along the latitudinal gradient, primarily due to shifts in climatic conditions relative to species' niches. Quantifying these relative shifts is crucial for understanding species responses to climate change, as organisms may be more sensitive at niche margins. The study's complex species responses to climate change, dependent on zone and climatic variable, underscore the spatial and temporal variation in exposure and sensitivity to environmental change. The changing relative importance of climatic variables highlights the difficulty in extrapolating climate change impacts on biodiversity, emphasizing context dependency. The study supports the notion of high-latitude communities as hotspots of climate change, with an increasing number of species responding to changing conditions. Although only a small fraction of species were unique to specific decades, the observed shifts resulted primarily from species repositioning within their niche space, rather than mass colonization or extinction events. These shifts resulted in altered community structure. The increasing proportion of species at their niche optimum or with positive responses to warming suggests a ‘thermal release’, potentially driven by poleward range shifts and newly suitable areas becoming available. These contrasting responses have implications for species interactions and ecosystem functioning.

This study demonstrates that ongoing climate change is causing substantial reshuffling of species within their niches, particularly in high-latitude regions. The changing relative importance of climatic variables underscores the limitations of simple extrapolations when predicting biodiversity impacts. Future research should focus on the dynamics of niche expansion, contraction, and shifts, and how these translate into abundance changes and adaptive capacity. The combined effects of climate change and land-use change also require further investigation.

The study's focus on Finland may limit the generalizability of findings to other regions. The use of presence-only data and the reliance on specific monitoring schemes may introduce biases. The analysis did not explicitly account for potential interactions between climate change and land-use change. While the study addresses spatial autocorrelation through latent variables, this methodology may not fully account for all unmeasured environmental covariates. The relatively limited duration of some datasets might limit the capacity to capture extremely long-term trends.