Extinction risk in terrestrial vertebrates is associated with niche limits that reflect climatic vulnerability
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eLife Assessment
This important study examines the relationship between climatic niche limits and extinction risk across more than 23,000 terrestrial vertebrate species. The large comparative dataset and phylogenetic analyses indicate solid evidence that species with lower upper thermal niche limits are more likely to be classified as threatened, whereas associations with aridity limits are less consistent. However, the treatment of data-deficient species and unclear definitions of niche metrics warrant further scrutiny, and the analyses do not establish that climate or land-use change drives the observed patterns. The findings will interest researchers in macroecology, global change biology, and conservation.
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Abstract
Anthropogenic climate and land-use change are driving an emerging extinction crisis that is expected to intensify in the future. Species’ climatic niche limits shape their sensitivity to these pressures, potentially leading to disproportionate extinction risk among more climatically vulnerable species. We test whether realized climatic niche limits are associated with current and projected extinction risk across >23,000 terrestrial vertebrate species. We assessed the phylogenetic structure of thermal and aridity niche limits and related them to IUCN threat status and simulated future extinctions. We show that realized niche limits are phylogenetically conserved, indicating evolutionary clustering of climatic tolerances. Species with colder upper thermal limits were more likely to be classified as threatened across taxa. Aridity niche limits show weaker and less consistent relationships with current threat status. Simulated extinction scenarios reveal taxon-specific patterns of climatic niche loss compared to random species extinctions. We also show significant reductions in phylogenetic diversity relative to randomized expectations based on simulated species extinctions. We find that extinction risk is systematically associated with species’ climatic niche limits, reflecting evolutionary constraints on environmental tolerance. These results indicate that future extinctions will disproportionately affect climatically vulnerable lineages, with cascading consequences for phylogenetic diversity and ecosystem functioning.
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eLife Assessment
This important study examines the relationship between climatic niche limits and extinction risk across more than 23,000 terrestrial vertebrate species. The large comparative dataset and phylogenetic analyses indicate solid evidence that species with lower upper thermal niche limits are more likely to be classified as threatened, whereas associations with aridity limits are less consistent. However, the treatment of data-deficient species and unclear definitions of niche metrics warrant further scrutiny, and the analyses do not establish that climate or land-use change drives the observed patterns. The findings will interest researchers in macroecology, global change biology, and conservation.
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Reviewer #1 (Public review):
Summary and strengths:
This manuscript examined whether realized thermal and aridity niche limits were associated with extinction risk across more than 23,000 terrestrial vertebrate species. The authors found that climatic niche limits were phylogenetically conserved and that species with lower upper thermal limits were more likely to be threatened, while relationships with aridity niche limits were weaker and less consistent. They then used simulations based on current extinction-risk patterns and found taxon-specific changes in the distribution of climatic niche limits and loss of phylogenetic diversity under simulated future extinction scenarios.
The study addresses an important question at an impressive taxonomic scale, and the underlying dataset and analyses have the potential to provide valuable …
Reviewer #1 (Public review):
Summary and strengths:
This manuscript examined whether realized thermal and aridity niche limits were associated with extinction risk across more than 23,000 terrestrial vertebrate species. The authors found that climatic niche limits were phylogenetically conserved and that species with lower upper thermal limits were more likely to be threatened, while relationships with aridity niche limits were weaker and less consistent. They then used simulations based on current extinction-risk patterns and found taxon-specific changes in the distribution of climatic niche limits and loss of phylogenetic diversity under simulated future extinction scenarios.
The study addresses an important question at an impressive taxonomic scale, and the underlying dataset and analyses have the potential to provide valuable insights into how climatic niche limits relate to patterns of extinction risks.
Weaknesses:
(1) One main concern is a disconnect between the central framing of the study and what the analyses can establish. The manuscript proposes that climatic niche limits increase species' vulnerability to climate and land-use change, yet extinction risk is measured using overall IUCN threat status, which integrates pressures from many different causes, including exploitation (e.g. hunting; pet trade), disease, and invasive species. For several of these threats, there is no clear mechanistic basis for why climatic niche limits should influence extinction risk. Consequently, the observed association does not establish that climate or land-use change is the underlying driver of the relationship between climatic niche limits and extinction risk. Analysing whether climatic niche limits are associated with extinction risk from climate- and habitat-related threats would provide a more direct test of the proposed mechanism.
(2) The treatment of Data Deficient species is another important concern. The authors classified Data Deficient species as Not Threatened in their analysis. However, previous studies have shown that many may in fact be threatened (Borgelt et al., 2022, Communications Biology; Jetz and Freckleton, 2015, Philosophical Transactions of the Royal Society B). This issue is particularly important here given the large number of Data Deficient species, especially among reptiles and amphibians, for which extinction risk may itself be underestimated (Caetano et al., 2022, PLoS Biology; González-del-Pliego et al., 2019, Current Biology). Treating these species as Not Threatened could therefore influence the reported relationships and should be evaluated through additional analyses.
(3) The role of the phylogenetic diversity analysis in addressing the central question is also unclear. The primary question concerns the relationship between climatic niche limits and extinction risk, whereas the subsequent analysis examines phylogenetic diversity loss following simulated extinctions. Disproportionate losses of phylogenetic diversity following non-random extinction are already well studied (e.g. Hughes et al., 2022, Current Biology; Oliveira et al., 2020, Global Ecology and Biogeography; Davis et al., 2018, PNAS). The authors should therefore clarify what this analysis adds beyond existing work and how it provides a meaningful extension of the climatic niche-extinction framework.
(4) The interpretation of the future extinction scenarios is also stronger than warranted by the simulations. The authors appropriately acknowledge that these scenarios do not explicitly incorporate climate or land-use change, but subsequently interpret the results as suggesting that climatic niche constraints shape future extinction pathways and that climatically vulnerable species will be disproportionately affected. Identifying species with lower climatic niche limits as climatically vulnerable is reasonable within the framework of the study. However, because the simulations are based on present-day extinction-risk transition probabilities rather than future climate- or land-use-driven processes, they do not establish that future climate or land-use change will preferentially drive the extinction of these species.
Overall, the manuscript addresses an important question and provides potentially valuable data and analyses. However, several of the central conclusions currently extend beyond what the analyses can directly support. Addressing the issues above would substantially strengthen the manuscript and clarify the scope of its conclusions.
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Reviewer #2 (Public review):
Summary:
The manuscript presents a comprehensive and useful evaluation of the association between extinction risk and niche limits. If these results were clearly articulated, then the manuscript would make a valuable contribution to the literature. However, I struggled to understand the manuscript, as the terms and methodology are not clearly defined. I realize that methods do not belong in the main text, but as written, it is often very difficult to understand what was calculated.
To give an example. In the abstract, the main result is: "Species with colder upper thermal (niche) limits are more likely to be classified as threatened across taxa." In the 1st paragraph of the discussion, we read: "We show that... species with narrower and colder thermal niches face greater threats to extinction...". These are …
Reviewer #2 (Public review):
Summary:
The manuscript presents a comprehensive and useful evaluation of the association between extinction risk and niche limits. If these results were clearly articulated, then the manuscript would make a valuable contribution to the literature. However, I struggled to understand the manuscript, as the terms and methodology are not clearly defined. I realize that methods do not belong in the main text, but as written, it is often very difficult to understand what was calculated.
To give an example. In the abstract, the main result is: "Species with colder upper thermal (niche) limits are more likely to be classified as threatened across taxa." In the 1st paragraph of the discussion, we read: "We show that... species with narrower and colder thermal niches face greater threats to extinction...". These are three different variables: colder upper thermal limits, narrower thermal niches and colder thermal niches. I really did not see where these were all defined in the manuscript, why so many different variables are considered and why the results are not consistent. I think this is likely a case of insufficient care in term definition and usage, but I found it rather difficult to follow the manuscript as a result. Clearer short explanations of terms (including a sense of how they were calculated/what they actually mean) and consistent use of terms would make the manuscript much more understandable.
Next, there are extra bits of information - for example, much of a paragraph in the introduction devoted to dispersal, many references to land-use (though I did not see that land-use was considered in this manuscript), and overly long discussion paragraphs that are highly speculative (or at least lack citations).
Finally, while I am a big proponent of identifying macro-ecological patterns, I would appreciate a critical discussion of what assumptions are made (i.e., everything about a microclimate; no consideration of extremes, etc.) and what the approach may be missing. This is not to invalidate the work, but to give the reader greater confidence that such broad-scale analyses are informative and are not glossing over too much species-specific ecology or finer spatial or temporal scale heterogeneity.
In summary, I think the paper has great potential but would benefit from a very conscious and careful revision.
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