While animal and plant species in temperate latitudes have the option of following their climate zones either towards the poles or to higher elevations, tropical species can only head to the hills.

"Average global temperature increases, geographically, at a linear rate from 60°N or S latitude towards the equator, but levels off between the Tropics of Cancer and Capricorn," Robert Colwell of the University of Connecticut, US, co-author of a recent paper in Science, told environmentalresearchweb. "Our study is the first to point out that the lack of a latitudinal temperature gradient within the tropics has a serious consequence for small-ranged tropical species: many would have to shift thousands of kilometres north or south to maintain their current climatic regime in a warming climate."

Instead, the most likely escape route in the tropics is to follow temperature zone shifts upward in elevation on tropical mountainsides, says Colwell. And, in a second blow, because lowland tropical forests are already the warmest forests on Earth, there are no "replacement" species waiting in the wings to take over from the species that live there, as there are for many places at higher latitudes.

"If a substantial number of lowland tropical species are already living near their upper temperature limit, as evolutionary principles, climate history and recent studies suggest," he added, "then lowland biotas may undergo attrition, as some species shift upwards in elevation, and others suffer decreased fitness in the lowlands."

Colwell and colleagues from Friedrich-Schiller University in Jena, Germany, Colgate University, US, University of California, Los Angeles, US, and Evergreen State College, US, took data for the Barva Transect in Costa Rica. This indicates that about half of the insect and plant species they looked at may be subject to lowland attrition.

"Because tropical species have such narrow temperature tolerances, elevational ranges are very small," said Colwell. "In our study, we predicted that about half of the species on the Barva Transect have such narrow ranges that, with the 3 °C of warming predicted by the IPPC over the current century, their predicted range will no longer include any portion of their current range."

Factors such as habitat fragmentation add to the challenge. Colwell suggests that the most important safeguard for tropical species may be continuous habitat corridors over wide elevational gradients. He hopes that his study will stimulate more experimental and survey work with tropical species, while the data that the team has collected will act as a baseline for follow-up studies.

Further north
Meanwhile at higher latitudes, Craig Moritz of the University of California, Berkeley, US, has also been studying the effect of climate change on animal species, this time in Yosemite National Park, US. Together with colleagues from the University of California, Berkeley and Colorado State University, Moritz has compared the ranges of small mammals in the park today with those of their forebears almost 100 years ago, using data collected by Joseph Grinnell and his team from 1914 to 1920.

Grinnell's study investigated how species distributions change with elevation – as a result of his findings he came up with the concept of the ecological niche. Moritz replicated Grinnell's measurements, spanning a 3000 m elevation gradient. This enabled him to track how species distribution has changed over nearly a century in a region that, as it's protected, hasn't undergone any major land-use changes.

Over this period, the park has experienced a massive 3.7 °C increase in monthly minimum temperature – i.e. the temperature overnight. By comparison, maximum temperatures haven't changed very much. The global average temperature has increased by roughly three-quarters of a degree Celsius since pre-industrial times.

Moritz's team found an upwards shift in the elevational ranges of many of the species. Those that formerly lived at low elevations have moved higher, while high-elevation species have contracted their ranges upwards. The researchers say this, in the main, is a reflection of climate change.

"Some of the higher elevation species that have contracted now have much reduced and more fragmented ranges than they did in the early 20th century," Moritz told environmentalresearchweb. "That in particular seems to be a direct impact of climate change. At lower levels, some of the shifts we're seeing are due to changed fire regimes, and just normal ecological succession."

Overall there's been remarkably little change in terms of species richness – the number of species present – in a particular elevational band, according to the researchers. It appears that the upwards contraction of some high-elevation species has been compensated by lower elevation ones moving up.

But Moritz has been surprised by the magnitude of the changes. "We've had some species move by up to 1 km in elevation," he said. "This is not subtle." Indeed, one species, the pinyon mouse (Peromyscus truei) completely changed its habitat. "It's not that the habitat shifted and the species are following, they're moving way ahead of the vegetation," said Moritz.

This particular finding contradicts the notion that it's possible to predict the effects of climate change on wildlife by modelling vegatation changes and assuming that small mammals, insects, molluscs and so on will track the motions of the vegetation that they're used to.

Strangely enough, the researchers found that some species have shifted their ranges while other, closely related species haven't. "That's totally reorganized the communities, so that ecological interactions are nowadays quite different," said Moritz. "That's neither good nor bad, that's just how it is."

The researchers can't see any clear life-history or ecological explanations for the difference in range shifting between close cousins. But they'd like to see detailed physiological and ecological studies of some of the species to find out more. "That's a little beyond our ken, we're a bunch of rat-trappers," said Moritz. "We've opened up a lot of really interesting problems for the real card-carrying ecologists to follow up on."

In order to give flora and fauna the best chance of responding to climate change by migration, Moritz, like Colwell, is in favour of protecting areas across elevational gradients to maintain ecological integrity. "They can still be multiple-use, grazing lands etc as long as there's sufficient habitat remaining in a continuous fashion," he said.

"That's what the paleontological record tells us too – that many species don't change in form, at least that we can see from fossils, they shift their ranges [under climate change]," he added. "We've made an unholy mess of that by putting Walmarts and freeways and what-have-you everywhere. That's going to limit the capacity of species to move."

Moritz, who reported his work in Science, now plans to repeat the survey across multiple transects in California, again tracking Grinnell's studies in the Sierra Nevada, southern mountains and northern mountains. Some of these areas appear to have experienced less climate change than Yosemite.

With this amount of uncertainty in how species will react and adapt to climate change, it looks like mitigating greenhouse gas emissions as much as possible and protecting landscapes, particularly those with elevation gradients, are the most clear-cut answers to conserving wildlife.