Predicted effects of climate factors on mountain species are not uniform over different spatial scales

Abstract

The selection of relevant factors and appropriate spatial scale(s) is fundamental when modelling species response to climate change. We evaluated whether the effects of climate factors on species distribution/occurrence are consistently modelled over different spatial scales in birds, and used a two‐scale approach to identify species–climate correlations unlikely to represent causal effects. We used passerine birds inhabiting mountain grassland in the Apennines (Italy) as a model. We surveyed four grassland species at 400 sampling points, and built habitat selection models (territory scale) and distribution models (seven algorithms, landscape scale). We compared the effect of climatic predictors on occurrence/distribution highlighted by models over the two spatial scales, and with the effects supposed a priori based on the climatic niche of each species. Models at the territory level included at least one climatic predictor for three species; the observed effect of climatic predictors was seldom consistent with supposed effects. At the broadest scale, distribution models for all species included climatic predictors, with varying consistence with supposed effects and findings at the finer scale. Despite the importance of climate for species distribution, occurrence could be more directly related to other factors, with important implications for understanding/predicting the impacts of climate/environmental changes. Our approach revealed key variables for grassland birds, and highlighted the scale‐dependent perceived importance of climate. At the local scale, climate effects were weak or hard to interpret. We found a general lack of consistence between supposed and observed effects at the territory level, and between landscape and territory models. Our results show the importance of predicting the potential effect of climatic factors prior to the analyses, carefully selecting ecologically meaningful variables and scales, and evaluating the nature and scale of climate–species links. We call for caution when predicting under future climates, especially when mechanistic effects and consistency across scales are lacking.

Disentangling direct and indirect effects of local temperature on abundance of mountain birds and implications for understanding global change impacts

“…Differently from what we expected, our results indeed did not show a direct temperature effect for the water pipit, a mountain-specialist grassland bird that had been reported to be associated with cold climates in other studies focusing on species occurrence ( e.g. , Brambilla et al, 2017 , 2019a ), and to experience variation in breeding success according to nest-site orientation ( Rauter, Reyer & Bollmann, 2002 ). The most likely explanation for such differences is that, in this cold, inner-Alpine area, the climatic conditions of alpine grasslands are close to the thermal optimum of this species.…”

“…Disentangling direct and indirect effects of climate on species occurrence and abundance is crucial to assess distribution and population drivers, and to evaluate the potential impacts of climate change on wild species ( de Chazal & Rounsevell, 2009 ). The correlative link between climate and species (especially for homoeotherm animals) often reflects an indirect effect, which is actually mediated by other factors associated with climate, such as habitat characteristics or the availability of key resources ( Brambilla et al, 2019a ). Here, we investigated the drivers of bird abundance in a mountain context, where climate and habitat effects are intermingled, explicitly distinguishing between direct and vegetation-mediated impacts of local temperatures.…”

“…The most likely explanation for such differences is that, in this cold, inner-Alpine area, the climatic conditions of alpine grasslands are close to the thermal optimum of this species. On the other side, the previous studies reporting an effect of local climate on this species’ occurrence were carried out at a much larger scale, including also low-elevation, warmer massifs ( Brambilla et al, 2017 ), or took place in more southern and definitely warmer mountain chains ( i.e ., Apennines; Brambilla et al, 2019a , Brambilla et al, 2020 ). The large differences in temperature ranges between our and those previously investigated study areas probably explains the differences with our results, which revealed no direct effect of temperature on abundance (besides the different focus on occurrence vs .…”

“…This highlights the importance of spatial scale when studying the influence of climatic factors on species distribution and abundance, as different or even contrasting results may arise from analyses carried out at different scales ( e.g. , Franklin et al, 2013 ; Brambilla et al, 2019a ).…”

Predictive models of distribution and abundance of a threatened mountain species show that impacts of climate change overrule those of land use change

“…All species distribution models within our ensemble had good predictive accuracy and emphasized that ring ouzel occurrence is mostly driven by climate, namely the mean ambient temperature, here integrated over the breeding season. Although it is generally recognized that climate is a weaker predictor of bird occurrence than habitat circumstances at finer scales (Brambilla et al., 2019; Howard et al., 2015; Thuiller, 2004), there are several examples of alpine species for which ambient temperature remains a crucial predictor at territory scale (Brambilla et al., 2019; Chamberlain et al., 2013; Jähnig et al., 2020). The second most important variable was solar radiation during the breeding season, with an optimum towards higher values.…”