Abstract
Climate change has strong effects on traits such as phenology and physiology. Studies typically assume that climate‐induced trait changes will have consequences for population dynamics, but explicit tests are rare. Body condition reflects energy storage and may directly affect how much can be invested in reproduction and survival. However, the causal pathway by which decreased body condition impacts population dynamics has never been quantified across multiple populations and species. Therefore, we lack a general understanding of the consequences of changes in condition for variables more relevant for conservation, such as population size. Using structural equation modeling, we investigate how temperature‐induced changes in body condition affect reproduction, and the subsequent impact on population growth rates of 19 bird species across 80 Dutch sites over a 21‐year period. Warmer temperatures were associated with decreased body condition, which led to both decreased and increased reproduction at different sites, cancelling out any overall effect. The indirect effect of temperature on population growth (via body condition and reproduction) only explained within‐species variation in the total effects of temperature on population growth. Instead, the direct effect of temperature on population growth (unrelated to condition and reproduction) was the most important pathway underlying the total effects of temperature on population growth, suggesting that unknown variables are mediating this effect. About half of the species are expected to increase under global warming, but this variation was not associated with any species characteristic. Overall, body condition responses to global warming are common, but their consequences on reproduction and subsequently population growth contribute relatively little to the total temperature impacts on population dynamics. Given that warming temperatures have strong effects on population dynamics, understanding the pathways via which temperature impacts population dynamics will be crucial for our ability to predict climate change effects in the future and improve conservation efforts.
Highlights
Much of climate change ecology research has focused on the direct effects of climate on trait responses, such as phenology, morphology and physiology
The non-focal reproduction pathway which was unrelated to body condition, explained substantially more variation among sites in the total effects of temperature on population growth compared to the focal pathway (81% variance explained; Fig 3c)
We found a positive association between sensitivity in body condition to temperature and total sensitivity in population growth to temperature across sites and species
Summary
Much of climate change ecology research has focused on the direct effects of climate on trait responses, such as phenology, morphology and physiology. There would be no change in population dynamics if: (1) the change in trait has little or no impact on any demographic rates, or (2) the change in trait does affect demographic rates but this has little or no impact on population dynamics, or (3) multiple pathways counteract one another and weaken any net population level effects (e.g. decreased survival is compensated for by increased reproduction), and (4) multiple climate/environmental variables have counteracting effects that weaken population level effects These mechanisms have only been explicitly tested in a handful of single species or theoretical studies (Wilson and Arcese 2003, Ådahl et al 2006, Ozgul et al 2010), and large-scale comparative studies are very rare (but see McLean et al 2016). We ask: (1) how much the focal pathway (i.e. temperature > body condition -> reproduction -> population growth) contributes to the total change in reproduction or population growth with temperature, (2) can trait sensitivity to temperature predict demographic or population sensitivity to temperature, and (3) which types of species or habitats are most at risk of flow-on effects from temperature-induced changes in body condition?
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