A work published in Science on Friday looks at two conflicting ideas on whether species can adapt to climate change. Are ideas about extinction too catastrophic, or does more need to be done to protect biodiversity?
Picture a polar bear, perched precariously on a small iceberg somewhere in the diminishing Arctic icecap. This iconic image is often used to portray the fate species will suffer as human-driven climate change accelerates. Yes, the forecasts are dire. Using various modelling approaches, researchers predict major reductions in species distributions and increased rates of extinction, especially in the tropics and globally across mountains.
But the past tells a different story. There seems little evidence in the fossil record for elevated species extinction during periods of rapid warming, such as the transition from the last ice age into the current Holocene period. Rather, species such as north American trees and mammals shifted geographically, albeit idiosyncratically, or in some cases appear to have adapted without moving.
One obvious result is that, as individual species respond more or less to climate change, local communities change in composition. These results are borne out by comparative genetic studies. They show that the population size of many species fluctuated, but that fluctuation occurred differently across the same landscape.
So which of these two perspectives is closer to the truth? Either the record of past responses is somehow an unreliable guide to the future, or the dire predictions for the future overstate vulnerability. This invited Science review, highlights this problem and, as a bridge, considers evidence on species’ responses to the 20th century climate change.
For those species to persist, they have just two options – adapt or move. But theory tells us that only species with short generation times and high rates of potential population growth will be able to adapt, without moving, in the face of rapid climate change. Species may also be less vulnerable than coarse-scale models predict if they are able to adjust how they use local habitats (for example, by being active at different times or concentrating in cooler places).
The 20th century record reveals a middle ground. Yes, species ranges are shifting – often towards higher latitude and upwards. In some mountain species, this is resulting in severe loss of geographic range and measureable decline in genetic diversity. There are also documented changes in the timing of migration or reproduction and shifts in body size or leaf width, though whether these changes are heritable remains an open question. But, consistent with the fossil record, even closely related species vary in their response – some stay put, while others shift, resulting in changes in the make-up of local groups of species. But why do species vary so much in response? The simple answer is that we don’t yet know, making it all the more difficult to predict vulnerability species by species.
Returning to the disparity between (observed) past and (predicted) future response, it may be that the fossil record underestimates future species vulnerability because of limited resolution – often fossils can only be classified to genus rather than species. Or it may be because species in the distant past had more options to respond as they didn’t have to cope with human-altered ecological systems. In particular, reduction and fragmentation of natural habitats, compounded by introduced predators and herbivores, add additional, potentially fatal constraints to the ability of species to respond to future climate change.
Whatever the true magnitude of impact on species under future climate change, there is no room for complacency. Reducing emission increases as soon as possible will help conservation policy-makers and practitioners increase the resilience of natural systems. We need to take actions now to give species as many opportunities as possible to remain viable – even if not within their current geographic range.
Despite the above “known unknowns”, we do know enough to inform conservation policy. Reducing other ecological stressors – such as invasive species and inappropriate fire regimes – is the right thing to do. Managing already threatened species to maintain large population sizes and ecological breadth remains important. Identifying landscapes that can function as places of refuge from climate change and managing large landscapes to enable dispersal to these refuges is crucial. Inevitably this needs a multi-sectoral approach. National parks are the keystone, but will not be enough.
Craig Moritz receives funding from The Australian Research Council, the National Science Foundation (USA) and the Australian Biological Resources Study
Rosa Agudo is funded by the Spanish private foundation Ramon Areces that covers her salary as a postdoc at the Australian National University.