Climate change was meant to mean big trouble for Bicknell’s thrush. One of North America’s rarest songbirds, the species was already struggling with mercury poisoning and habitat loss. As the bird’s mountainous home warmed, ecologists worried that a new threat would move in: the American red squirrel, a mammal that will supplement its seed-rich diet with songbird eggs and nestlings. The squirrels are found downslope of the thrushes and were expected to move up the mountainside as temperatures rose.
But that didn’t happen. Toni Lyn Morelli, an ecologist at the University of Massachusetts Amherst, discovered that the squirrels have actually begun moving downhill. It is good news for songbird conservationists, but puzzling news for ecologists more generally. The squirrels seem to be heading the wrong way.
They aren’t alone. In some locations, as many as half the species studied are defying the prediction that they would shift to cooler places. The discovery is making it difficult for conservationists and land managers to protect vulnerable species. Working on the assumption that the species would move away from the heat, they have been conserving or buying up cooler areas they assumed would become climate refuges. But what if they are protecting land in the wrong places?
“I am left with more questions than answers about what is the best pathway to safeguard things in the face of climate change,” says Inna Osmolovsky, an ecologist at the University of New South Wales in Australia.
Researchers are now going back to basics in order to understand what is going on, and to devise conservation strategies that are a better fit for our warming world.
For most of the past 30 years, ecologists have assumed that climate change-driven range shifts would be largely one way: into colder places. The first good evidence for this came in 1996 when Camille Parmesan, then at the University of California, Santa Barbara, demonstrated that the Edith’s checkerspot butterfly in western North America was quitting the increasing warmth of its southern, low-elevation habitats for the cool of higher and more northerly climes.
By 2003, Parmesan, along with Gary Yohe at Wesleyan University in Connecticut, was even quantifying the rate of the shift. Using data from hundreds of animal and plant species, they calculated that, every decade, species were moving an average of about 6 metres uphill or 6 kilometres towards the poles.
That rate was later revised upwards. In 2011, with more data to work with, ecologist Chris Thomas and his colleagues at the University of York, UK, calculated that species were, on average, actually moving 11 metres further uphill or 17 kilometres poleward each decade. The scientists also found a link between the degree of warming particular species experienced and the size of their shift – the first evidence, they said, of a causal link.
The study caused a stir, but as Thomas pointed out at the time, within the data lay a more complex story. Although species from alpine chipmunks to British butterflies and from red foxes in the Arctic to boreal spruce and pine trees were moving to cooler places, a quarter of species tracked in the analysis were heading downhill, not uphill, and more than a fifth were shifting towards the equator rather than the poles.
With additional data gathered since then, Morelli says it now seems that “less than half of the species” have moved into cooler environments. The big question is: why are so many species apparently travelling the wrong way?
Embracing the heat
The answer is, in a word, complicated. A particularly baffling observation is that seemingly similar animals living in the same habitat are responding to climate change in completely different ways. For instance, about 20 years ago, Steve Beissinger, an ecologist at the University of California, Berkeley, and his colleagues climbed up and down 3000 metres of elevation in Yosemite National Park, California. They wanted to discover how the ranges of 28 small mammal species have shifted since being recorded in the early 20th century – when the national park was about 3°C cooler – by a far-sighted biologist named Joseph Grinnell.
“Half the species showed movement upslope, but the other half didn’t move at all,” says Beissinger. “Two shrews had expanded their ranges downslope.”
Beissinger then moved on to studying birds in the Sierra Nevada mountains in the western US. “And then it got a lot more puzzling,” he says. He and his colleagues discovered that discrete populations of the same bird species were responding differently to climate change. The white-headed woodpecker, for instance, was moving uphill in the north of the Sierra Nevada, downhill in the middle and nowhere in the south of the mountain range.
“We scratched our heads about this for a long time,” says Beissinger. They eventually concluded that temperature change alone is too crude a predictor of animal behaviour.
They now realise that changes in snow and rainfall are important, too. For instance, although shifting downhill may involve moving into an area that is uncomfortably warm, heading uphill might mean moving into one that is uncomfortably wet. If the cost of all that precipitation is deemed too high, a population might move downhill as the least bad option.
It might seem obvious that you need to take precipitation into account when predicting how species will move, admits Morelli. Ecologists are aware that “multifarious complications” affect where a species chooses to live, says Thomas, but they were trying to keep their forecast models simple – and factoring in rain and snow makes them much more complicated. Precipitation changes are “all over the place”, says Morelli. “It doesn’t have the same clear direction [as temperature].”
This isn’t the only factor that can complicate predictions, as Beissinger found in his next study. The Mojave desert, in the south-western US, is home to a riot of small mammals – voles, chipmunks, deer mice, kangaroo rats and the like – as well as 200 species of bird. It has warmed by 2°C over the past century and can sometimes top 45°C (113°F). Beissinger found that, as expected, bird populations there were “collapsing”.
But the mammals were doing just fine.
The reason, he and his colleagues discovered, was that the mammals shelter from the fierce heat of the day in their burrows, while the birds air-fry overhead. “The lesson we took away from that was that even at the same location, species could be responding differently to the same level of climate change,” he says. It is all to do with their actual exposure. The models were going to have to get a lot more complicated if they were to accurately predict how ranges are shifting.
So, biologists have begun throwing everything they can at the problem. They have built massive databases of traits, in which they have collected all kinds of details about how organisms behave, from mobility to dietary preferences. They have studied range shifts all over the globe. The holy grail, says Monte Neate-Clegg, an ecologist at the University of California, Davis, is that somewhere in the data will be clues to a formula that can successfully predict where any particular species or community will move, and so indicate where it would be best for humans to intervene.
The approach seemed promising, says Beissinger. In particular, there was an expectation that organisms that live fast and die young would be among the most successful at shifting upwards and polewards. Such species produce a lot of offspring and tend to tolerate a wide range of conditions, such as temperature and wetness, he says. They are “better at pushing at the margins of their ranges”.
To test this expectation, Beissinger and his colleague Eric Riddell at the University of North Carolina at Chapel Hill analysed all the studies that have tried to explain range shifts using behavioural traits. But they concluded that no reliable predictions can be made using such data.
“They don’t really work super well,” says Lise Comte, an ecologist at the California-based non-profit organisation Conservation Science Partners. “And so, the community overall is a little disappointed.”
To put it another way, researchers are still no nearer to identifying that holy grail formula. In fact, there are so many complicating factors influencing an individual species’ behaviour that there may be no simple way to predict how it will respond to climate change – even though, across all species, you can detect that upwards and polewards signal.
Osmolovsky, who studies the movement of alpine plants in Australia, thinks more attention should be paid to how climate change alters interactions between species: “We’re thinking [about] just this one plant that is doing its own thing in response to climate change,” she says. “But, actually, whole communities are responding and maybe the interactions are also impacted.”
For example, research from last year showed that microbes that are typically harmful to plants can become mutualistic in more stressful environments. Osmolovsky says that “unpredictable friendships” like these can make life easier for a species even as its climate warms.
Beissinger reels off the list that scientists now need to consider when trying to predict how a species will respond to climate change. “Competition, disease, predation and human impacts like land-use change,” he says. Moreover, he adds, species may reduce or even eliminate the need to move by changing their behaviour – breeding earlier in the year, for instance.
Planning for an unpredictable future
Although recognition of this complexity makes it hard to predict how a species will respond to climate change, Neate-Clegg says there are two practical things that can still help conservationists.
The first is to connect habitats together, so that organisms can move up, down, north, south or even east or west, according to how they can best cope with climate change. “The worst thing you can do is fragment these landscapes that species are trying to shift through,” he says.
The other is to enhance the complexity in a habitat, so that species have options for adjusting their behaviour as the climate changes. This means “not just having a forest, but having a complex forest that might provide more options for buffering”, says Neate-Clegg. Lots of different species and ages of tree, for example, mean all sorts of options for shade.
Beyond this, recognising that there is no universal model that can predict each species’ response to climate change also reminds ecologists that successful conservation efforts often require a detailed study of the species in its local habitat.
“Conservation has always been this multi-pronged approach of ecosystem protection by protected areas and then species-level conservation. And I think both avenues could work,” says Neate-Clegg.
Taking this approach can even help explain why the American red squirrels didn’t move into the territory occupied by Bicknell’s thrush. It turns out that a fabulous new habitat has emerged further downhill. For decades, logging and acid rain had wiped out the spruce trees that once grew there and that produce the squirrels’ main food source, cones. Today, both of those pressures are much reduced, and spruce trees are growing at lower elevations again.
In the end, says Morelli, “they are just this little mammal trying to make do in feet and feet of snow”. Climate change may seem to offer incentives for the squirrels to head uphill, but the best answer for them is to move in the opposite direction, and follow the cones.