Global Change Ecology (BIO347)

Extinction debt – Do current red lists highly underestimate extinction risks?

Plants growing on high mountains are often expected to be among the first losers of the global change. When a species’ habitat changes critically the species has three options: migrate, adapt or die. Parallel with the temperature increase of the last decades, we have seen plant species migrating upwards towards colder habitats. But for high-alpine species migration is not an option as they are already sitting at the last stops – on the highest peaks. However, even after decades of climate change there have been surprisingly little extinctions. Fast adaptation maybe? No, unfortunately part of these species is already using overtime. I say overtime because their extinctions are only delayed after the disturbance, in this case climate change and increased competition (1). Extinction debt is what we call the sum of all the species with delayed extinctions (1).

Scientists modelling future distributions for alpine plant species have found out that a species might remain in its old habitat even 50 years after the habitat has changed unsuitable for the species (2). This tendency towards high delay in extinctions in high-alpine regions is mainly due to longevity of these species. Even if new plant individuals are not able to survive anymore because the habitat has gone through too much change, the old individual may remain after decades. But to be able to remain in the old habitat in the long run, the species would also need to adapt to the new environment, and for alpine specialists with long life cycles this seems very unlike.

Figure 1. Left: Chummer Schwarzhorn, a mountain top in South-Eastern Swiss Alps. Right: Glacier buttercup, a typical high alpine species feared to go extinct, growing on the same mountain. Pictures by Aino Kulonen.

Mountain tops are of great interest because changes in diversity have been fast and well reported, but after all, they represent an extreme environment, covering only a small part of earth land area and small part of total biodiversity. Indeed, we should be even more scared of underestimating the extinction risk and losing essential parts of our biodiversity everywhere. Recently it has been shown how entire Europe is carrying a heavy extinction debt on its shoulders (3). And now we are not only talking about some pretty flowers whose beauty mainly mountaineers would be able to enjoy, but hundreds of species including mammals, reptiles, insects and plants.

A practical way to investigate regional extinction risk is to look at red list species. Red lists contain species that are announced to be endangered in national level. In a recent study national proportions of endangered species were compared with the pressure these countries have put on their ecosystems in years 1900, 1950 and 2000 (3). This pressure was measured as socioeconomic activity; population density, per capita GDP and intensity of land use (Fig 2). The result is alarming: for most taxonomic groups current numbers of endangered species are mirroring the pressure for ecosystem from 1900 and 1950. This means that today’s endangeredness and extinctions are results from the damage done by human already a century ago. The extinction risks we measure today are lagging 50-100 years behind human activities.

Figure 2.Left: Proportions of species facing medium-to-high extinction risks in national red lists in 22 European countries. Right: Trends in three socioeconomic indicators in Europe (human population density in people per square kilometer; per capita GDP in 100,000 International Geary–Khamis dollars; and human appropriation of net primary productivity, which is defined here as the proportion of the total net primary productivity harvested by humans to the net primary productivity of the potential vegetation (NPP₀). Figure from reference 3.

The delay in extinction risk is longest for vascular plants, bryophytes, dragonflies and grasshoppers. For long living plants, long lag is not a surprise but one would think that short living insects would correspond to habitat change faster. However, simulations have shown that insect populations could show a delay of over a century even when their habitat network has been reduced close to the extinction threshold. This might be the case in Europe where agricultural landscape is a matrix of intensively used land area and near-natural habitats. Proportions of endangered fish species are the only ones that mirror best the current pressures to the ecosystem. Fish populations might indeed suffer most directly from anthropogenic actions like water pollution and management of water systems. Proportions of endangered mammals correspond well to both 1900 and 1950’s socioeconomic activity. Shorter delay is explained by their bigger habitat sizes. Fragmentation and degradation of big habitats make species meet their extinction thresholds faster than by species with small habitats.

As the pressure for ecosystems has steadily increased after 1900, we are carrying an extinction legacy that might be massive and almost impossible to measure. The study presented here is based on direct human activities, habitat fragmentation and environmental degradation like pollution (3). Novel pressures for environment like climate change, nitrogen and acidic depositions together with invasions of new species are likely to interact with direct effects and even intensify environmental degradation. With all these different direct and indirect drivers, long lag-times blur the picture and makes it harder and harder to see what is actually causing the extinction of certain species. We might easily target completely wrong drivers with our actions.

Red lists in national and global level might be way too optimistic, because they use criteria like size and recent reduction in population and range (3). Even if the current state of a species seems fine to us, the species might be doomed to extinction already decades or even a century ago. And once we notice that something’s wrong, our attempts to improve the situation might come decades too late (3).

Good news is that once we have a broader perception of the extinction debt, it will be easier to direct conservation efforts to regions and species groups with the greatest debt. As long as a species exists there is hope to save it by habitat restoration and landscape management (1). But if the driver of extinction on mountain tops really is climate change, it is harder to act directly and fast enough to save these vulnerable species. Just like shrinking glaciers, endangered mountain plants seem to work as a touchable proof of global change, but how will we save them? Are botanical gardens our only hope to save samples of these species from increasing temperatures and competition?

by Aino Kulonen

References

1. Kuusisaari et al. (2009). Extinction debt: a challenge for biodiversity conservation. Trends in Ecology and Evolution 24: 10, 564-571.
2. Dullinger et al. (2012). Extinction debt of high mountain plants under twenty-first-century climate change. Nature Climate Change 2, 619-622.
3. Dullinger et al. (2013). Europe’s other debt crisis caused by the long legacy of future extinctions. Proceedings of the Natural Academy of Sciences 110:18, 7342-7347.