Are current projections of climate change-impacts on biodiversity misleading?

Why is life on Earth not evenly distributed? Geographic patterns of species diversity and their underlying processes have intrigued scientists for centuries, and continue to spur scientific debate. Studies carried out over the past 20 years have led to the conclusion that species diversity is best predicted by contemporary patterns of energy and water, the so-called “contemporary climate” hypothesis.

Because current climate gradients are correlated with past climate variability, it has also been suggested that current climate acts as a surrogate for evolutionary processes that have been triggered by past climate variability, giving rise to the “historic climate” hypothesis. Now, new high-resolution data on historic climate has allowed Dr Araújo in collaboration with Dr Rahbek and other colleagues to finally directly test the “historic climate” versus “contemporary climate” hypotheses of biological diversity. Their illuminating results are published in a recent paper in Ecography. Contrary to the expectations of many scientists they found that historic climate variability was a better predictor of reptilian and amphibian diversity in Europe than contemporary climate.

“The lack of quantitative spatial data on variation in climate over historical time has prevented more rigorous testing of these diverging hypotheses”, says Dr. Miguel B. Araújo from the National Museum of Natural Sciences (CSIC) in Madrid. As a consequence, “the debate on the causes of diversity gradients has turned to some degree into a discussion of semantics”.

Recent developments in general climate models have finally facilitated high resolution predictions of past climates. In collaboration with leading climatologists working on paleoclimate modeling in the United Kingdom, Drs. Araújo, Rahbek and colleagues provide the first comparative test capable of differentiating between the contribution of contemporary and historical climate drivers of diversity gradients across a complete lineage of species at a continental scale.

“In recent years, analytical attempts to shed light on the role of history in determining today’s patterns of species richness have focused on the strong residual variation of models using contemporary climate”, explains Dr. Carsten Rahbek from the Center of Macroecology at the University of Copenhagen. “It has been argued that these residuals provide information about the role of historical rather than contemporary constraints. However, such an analytical approach assumes that contemporary climate is the main explanatory force. In other words, the contemporary and historical hypotheses are not tested simultaneously in a directly comparable manner, and historical hypotheses are only invoked to explain what is left to elucidate after the implementation of contemporary environmental processes”, says Dr. Rahbek.

“Our results are striking in that they contradict previous studies of large-scale patterns of species richness” affirms Dr. Rahbek. “They provide the first evidence, using a quantitative analytical approach, that historic climate can contribute to current patterns of richness independently of, and at least as much as contemporary climate”. This study has profound implications for the study of diversity on Earth, and challenges the current view that patterns of contemporary climate are sufficient to explain and predict diversity.

Differentiating between contemporary and historical hypotheses is important, not only for theoretical reasons: “an understanding of the mechanisms that generate and maintain diversity provides valuable insights for predicting the impacts of contemporary climate changes on biodiversity”, says Dr. Araújo. “If contemporary climate does drive species richness, then current climate variables could be used to accurately predict the effects of climate change on biodiversity. If, as shown in our study, the mechanisms underlying contemporary patterns of species richness are in fact strongly influenced by the history of climate, then current-climate predictions may be seriously misleading and alternative approaches to predict the effects of climate change on biodiversity must be developed”.