Scientists question models to analyse impacts of climate change on biodiversity: past predicts species richness better than contemporary climate

Are current projections of climate change-impacts on biodiversity misleading? This is the urgent question arising from the study "Quaternary climate changes explain diversity among reptiles and amphibians", published as an open access article in the journal Ecography. A team of evolutionary biologists, ecologists, biogeographers and climate scientists from Spain, Denmark and the UK address the complex question by looking at the way past climate change influences current species richness. By looking at the distribution of amphibians and reptiles in Europe, they found that past climate changes predict species diversity better than contemporary climate change. The surprising findings could have major implications for future studies and possibly for policy strategies on biodiversity in the context of global warming. They may also shed light on how plants will adapt to the changing climate.

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 distribution 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 have allowed the scientists to finally directly test the 'historic climate' versus 'contemporary climate' hypotheses of biological diversity. Their findings offer a new, illuminating perspective on the debate: 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. Araújo from the Deptartment of Biodiversity and Evolutionary Biology at Spain's National Museum of Natural Sciences (CSIC). As a consequence, the debate on the causes of diversity gradients has turned to some degree into a discussion of semantics.

But 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, 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.

Our results are striking in that they contradict previous studies of large-scale patterns of species richness. 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. - Dr. Carsten Rahbek, Center of Macroecology, University of Copenhagen

The findings have 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.

The scientists took species data of all European amphibian and reptile species, and projected them on a 50 km European grid (map, click to enlarge):
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Three measurements of species richness were used: total number of species per grid cell; number of species per grid cell among the top 50% narrower-ranging species and number of species among the 25% wider-ranging species. The 50% threshold for narrow ranging species was selected because of the highly skewed frequency distribution of range sizes (i.e. most species having narrow range sizes and very few having wide range sizes).

Correlates of species richness were examined using two contemporary climate variables (annual mean temperature and annual total precipitation sum), and two variables reflecting long term climate stability (the anomaly between mean annual temperatures and annual total precipitation sum in the Last Glacial Maximum (LGM) and at present). The model incorporated prognostic cloud, water and ice, had a mass-flux convection scheme with stability closure and used mean orography.

The model was integrated for the LGM over 20 simulated years and climatological means were compiled for the final 14 year. Time-series analysis of various climate variables for the entire 20 year simulation shows that disregarding the first 6 year allows the climatology model to reach full equilibrium.

Importance of past changes
Statistical analyses reveiled spatial correlations which yielded data that surprised the scientists.

They found that both mean contemporary annual temperatures and historic temperature stability between the LGM and the present significantly predict species richness of reptiles and amphibians in Europe. Species richness among reptiles is also significantly correlated with contemporary precipitation, whereas species richness among both reptiles and amphibians is significantly related to 'historic' precipitation stability.

Because contemporary temperature values are highly correlated with historic temperature stability, partial regression analysis was used to partition the effects of contemporary climate (both the energy and water-energy variants) and historic climatic stability.

Variation due to historic climate stability was seen to be greater than variation explained due to factors associated with contemporary climate, despite important shared variance between the two components. These results were consistent with the initial prediction that historic climate changes can take precedence over contemporary climate in explaining current gradients of species richness.

Reptiles and amphibians rely on external warmth to raise their body temperature and become active. Their ability to cope with lower temperatures is limited, and many species find it difficult survive in regions where mean annual temperatures are below freezing. However, despite evidence that contemporary temperature and precipitation exert strong effects on the richness and distributions of individual species of reptiles and amphibians in Europe, the scientists found, in concurrence with their second prediction, that the distribution of narrow ranging species is markedly constrained by the mean annual freezing conditions in the LGM, whereas widespread species are more constrained by current mean annual freezing conditions.

These results thus support the prediction that a large number of widespread species are likely to have large range sizes because they have been able to largely adjust to current climate conditions by means of colonization, while narrow-ranging species are at least partly restricted because of their poor ability to track climate changes.

There are certainly other factors causing rarity among species, but if colonization ability was not limiting the post glacial distribution of species, one would expect that several endemics of southern European alpine and temperate environments would now extend their ranges into central and northern Europe.

Given that there are generally more narrow ranging species than there are wide ranging species and that narrow ranging species are less likely to be at equilibrium with current climate conditions, it is likely that the impact of historic climates on current species richness is a more widespread phenomenon than previously acknowledged by proponents of contemporary climate hypotheses.

However, the scientists also predict that the historic signature on contemporary richness gradients is likely to be reduced among organisms with greater colonization abilities, such as birds and some plants. This prediction is supported by two recent studies that analyzed the impact of contemporary LGM climates on the richness of a selected sample of northeastern Australian endemic fauna and European flora; these studies demonstrated that historic climate was the single best explanatory variable of richness among narrow-ranging low dispersing endemic animals in Australian rainforest as well as narrow-ranging plant species in Europe, whereas contemporary climate was best at explaining richness among wide-ranging and good-disperser species.

Looking to the future
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.

"If contemporary climate does drive species richness, then current climate variables could be used to accurately predict the effects of climate change on biodiversity. But if, as shown in the 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", Dr. Araújo concludes.

Map: Species richness among all (a) European reptile (left) and amphibian (right) species; (b) the top 50% narrow-ranging; and (c) the top 25% wide-ranging species. Species richness scores in each map are divided into thirty three equal-frequency color classes, such that maximum scores are shown in red and minimum scores are shown in blue. The horizontal line through the south of Europe represents the 0°C isotherm during the Last Glacial Maximum (LGM; 21 kya), whereas the line through the north represents the current 0°C isotherm. The paleoclimate simulation used to draw the 0°C isotherm in the LGM is based on the HadAM3 General Circulation Model.

References:
Miguel B. Araújo, David Nogués-Bravo, José Alexandre F. Diniz-Filho, Alan M. Haywood, Paul J. Valdes and Carsten Rahbek, "Quaternary climate changes explain diversity among reptiles and amphibians", Ecography (OnlineEarly Articles), 23 Oct 2007, doi:10.1111/j.2007.0906-7590.05318.x

Eurekalert: Are current projections of climate change-impacts on biodiversity misleading? - November 21, 2007.