What happens to the tree of life when climate changes?

Climate change is currently among the most prominent scientific topics, both within the scientific community and beyond.

Because humanity is altering global climate in a way that has no precedent in Earth history, we are worried – and rightly so – about the effects of this change, both on our own quality of life and on the living conditions for the world's plants, animals, fungi and microbes. When climate changes, species have to move or adapt to survive; the stronger and faster the climatic change, the less likely the species are to keep up with it. High casualties from man-made climate change have been predicted, but just how many species will go extinct is unknown.

The second big uncertainty is which species will go extinct. Different species have different functions, and what a species does for the ecosystem depends on its place in the tree of life. Thus, if certain parts of the tree of life are lost entirely, the consequences are much more dramatic than if extinction strikes at random. Let me illustrate this with an exaggerated example: if all land plants went extinct, most other terrestrial life would lose its food source, the face of the earth would be changed dramatically and evolution would be thrown back by 500 million years. However, if the same number of species was lost evenly across all branches of the tree of life, the gaps would be filled much more easily by surviving, functionally similar organisms.

Are some plant groups more vulnerable to extinctions from climate change than others?

Why would we expect extinction to have a preference for certain branches of the tree of life? Closely related species usually resemble their common ancestor, and thus each other. If this is true for their vulnerability to climate change, whole groups of close relatives are similarly threatened. During past episodes of mass extinction, there have been repeated instances of whole evolutionary lineages being lost (dinosaurs come to mind). The problem is that we do not know for sure if any of those losses were caused by climate. It may be theoretically expected that related species are similarly vulnerable, but not all biological characters behave like that: some of them change so quickly that species no longer resemble their common ancestors. Despite that caveat, climatic tolerances have been tested in multiple groups of organisms and have often been found to reflect shared evolutionary history – so there is reason to be worried. The next step is to ask if and how those climatic tolerances translate into actual extinction.

Finding the most appropriate data to study

At this point extinction studies are faced with a big challenge. If we study past extinctions, we are dealing with organisms that are long gone, and we only have very indirect evidence of their climate tolerances - our knowledge of their position in the tree of life is often also limited. If we study organisms that are still around, we have that kind of information, but the one thing we do not know is if they are going to go extinct due to climate change. In a recent study together with a team at Aarhus University, Denmark, I used a neat workaround for this dilemma (Eiserhardt et al., 2015). The global extinction of species is never instantaneous – it is the end point of multiple successive local and regional extinctions. By studying the past regional extinction of species (or in our case: genera) that are still around elsewhere, we can get good data on their ecology and evolutionary history while knowing their (regional) extinction as a fact.

Our study focused on the temperate forests of Europe, Eastern Asia, and Western and Eastern North America. The floras of those forests have a turbulent history, shaped by global cooling since the Palaeocene-Eocene Thermal Maximum (~55 million years ago) and the strong temperature oscillations of the past 2.6 million years (ice ages). From fossils, we know that all four regions had a richer tree flora in the past than they have now. It has long been thought that this drop in diversity was caused by climate change (Latham & Ricklefs, 1994; Svenning, 2003), so we chose this system to test how climate-driven extinction prunes the tree of life.

Survivors of cold climate tend to be closely related

First, we showed that most regional extinctions happened in tree genera that are today restricted to relatively warm climates, confirming that their regional extinction was likely driven by cold temperatures. We then showed that extinctions were non-randomly distributed across the tree of life: close relatives were similarly likely to survive. For example, tree genera belonging to the magnoliid clade were hit particularly hard. This group disappeared completely from European temperate forests and almost completely from Western North America. Other groups, like the orders Fagales (oak, beech and relatives), had high survival even in extinction-ridden Europe. Because extinction was concentrated in particular groups, some forest regions lost substantially more of the tree of life than if extinction had been random: in fact, up to 23% more. Strikingly, losses were distributed relatively evenly as long as they were few, but with an increasing number of extinct genera, the losses were increasingly concentrated. Thus, it seems like the more species we are losing, the more we need to worry about the integrity of the tree of life.

Making predictions

Can any of this help us predict the impact of man-made climate change on the tree of life? Unfortunately, reliable quantitative predictions are still beyond reach. The ongoing changes are different from our study system for several reasons: First, earth is currently warming, not cooling – this may have a very different effect on biodiversity than the cooling trends and cold extremes we studied. There are indications that the warm edges of species ranges are less directly determined by species physiology, and more by competition with other species. How species interactions such as competition change when the climate gets warmer is still insufficiently known. Second, climate is not the only aspect of the environment that is currently changing, and the interaction of multiple drivers may change the picture substantially. Third, different groups of organisms may respond differently, so maybe temperate trees are not representative.

Still, our study shows that climate change can, in certain situations, have larger consequences for biodiversity and ecosystems than suggested by the mere number of lost species. It also illustrates that although Earth history does not offer a precise analogue of current climate change, much can be learned about the effect of climate change by looking into the past. Carefully compiled datasets on the present and past distribution of organisms are one of the most valuable assets for understanding how biodiversity responds to climate – or any other environmental factor for that matter. They are the only hard evidence we have for changes happening at timescales that exceed the practically feasible duration of experiments.

References

Eiserhardt W. L., Borchsenius F., Plum C. M., Ordonez A. & Svenning J.-C. (2015). Climate-driven extinctions shape the phylogenetic structure of temperate tree floras. Ecology Letters 18, 263-272. Available online

Latham R. E. & Ricklefs R. E. (1993). Continental comparisons of temperate-zone tree species diversity. In: Species Diversity in Ecological Communities (eds. R.E. Ricklefs & D. Schluter). University of Chicago Press, Chicago, pp. 294-314.

Svenning J. C. (2003). Deterministic Plio-Pleistocene extinctions in the European cool-temperate tree flora. Ecology Letters 6, 646-653. Available online