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2011

Climate Relicts: Seeking Clues On How Some Species Survive
Yale Environment 360, August 29, 2011
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A two-hour’s drive north of Madrid is an extraordinary sight: forests of beech trees. It’s not the European beech itself that’s extraordinary. After all, Fagus sylvatica grows across a wide swath of the continent. It’s beech trees in central Spain that are strange. To grow, beeches require a moist, relatively cool climate -- a climate that’s almost impossible to find in central Spain. “They’re limited to cool moist valleys in a hot, dry mountain range,” explains Alistair Jump, an ecologist at the University of Stirling who studies the trees.

The trees, which lie about 200 miles south of the edge of the main range of European beeches, did not get to central Spain recently, their seeds carried on the shoe of some German tourist. Evidence from both fossils and genes shows that beeches have lived in some parts of Spain since the last Ice Age. At the time, most of Europe was either buried under ice or too harsh a climate for the beeches to survive. But after the Ice Age, suitable habitats opened up, and the beeches expanded from their southern refuges into the north.

At the same time, Spain became so hot and dry that most of its beeches began to disappear. As the northern edge of the tree’s range expanded, the southern edge retracted. But in a few moist, cool mountain valleys in central Spain, pockets of beeches have survived, from the age of wooly rhinos and Neanderthals to today.

The beeches of central Spain are known as climate relicts. For decades climate relicts have suffered from a benign scientific neglect, considered little more than biological oddities. But Jump argues that it’s urgent that we understand how they have managed to hold on. In a paper to be published in the Annual Review of Ecology, Evolution, and Systematics, Jump and Arndt Hampe of the Donana Biological Station in Spain point out that most species mainly found in temperate regions have left behind climate relicts. They also observe that global warming is starting to push species into new ranges. As some of those species retreat, they may leave behind climate relicts of their own. And today’s climate relicts may hold clues about how to protect species from global warming-driven extinction.

“Understanding how they exist is of fundamental importance,” says Jump.
Climate relicts are often found in places that are several degrees cooler than their surroundings. Mountains offer lots of chilly real estate, but they don’t have a monopoly on climate relicts. Bogs have them, for example, as does the rocky debris that surrounds ice-filled caves. “You have a flow of cold air coming out of ice caves,” Jump explains. “It’s like if you leave the fridge door open.” In northeast Iowa, the ground around ice-filled caves turns out to be home to plants, snails, and mites ordinarily found in the boreal forests of Canada.

It’s not enough for climate relicts to be sheltered from warmer climates. They also need to be sheltered from the species that move in when temperatures rise. Small plants can be shaded out by trees that move in with a climate shift. The grey jay, a bird that lives in Canada, is suffering from an invasion of microbes. Normally, the grey jay survives through the winter on berries, bugs, and other foods it stores away in caches. At the southern edge of its range, the climate has warmed so much that microbes can infest the caches in the fall. The food rots, and the birds have nothing to eat.

To find refuge from these competitors, climate relicts often end up where their rivals can’t get to them. It’s no accident that many climate relict plants grow on cliffs and other rugged places that trees can’t colonize easily. These refuges can also protect climate relicts from diseases that might otherwise endanger them. Butterflies are typically attacked by parasitic wasps, which lay eggs in their caterpillars. Camille Turlure of the University of Louvain in Belgium and her colleagues recently examined relict populations of cranberry fritillary butterflies that survive in bogs. They couldn’t find a single wasp egg in the insects.

Even in a hospitable microclimate, supported by friends and protected from enemies, climate relicts still face a tenuous existence. They face droughts and heat waves. Trees may lose the services of insects that pollinate their flowers. Jump suspects that climate relicts survive on this razor’s edge with some life-saving strategies. Rather than relying only on pollination, for example, many climate relict plants can clone themselves. To weather droughts, they store extra carbon in their roots. Climate relict animals can also survive by adjusting their behavior. Insects can lay their eggs on cool north-facing leaves, for example, instead of the hot south-facing ones.

The mere existence of climate relicts can give ecologists headaches. A number of researchers have been developing computer models that can predict where species can be found today based on the kind of climate they can survive in. Part of the motivation for these models comes from the desire to understand why species shifted their ranges in particular patterns in the past. But many scientists also hope these models will help predict what happens to the world’s biodiversity as the planet warms over the next century. They’re already documenting hundreds of species either moving toward the poles or heading up the slopes of mountains, tracking the climate they prefer.

These models can potentially allow researchers to look into the future. They may be able to predict which forests will become most prone to fires, for example. Many researchers are also concerned that many species won’t be able to shift to ranges of equal size. Instead, their ranges will shrink, putting them at greater risk of extinction. Climate models could potentially allow scientists to identify the species that are most vulnerable.

Climate models are far from perfect, though. For one thing, they can’t predict the existence of today’s climate relicts. They are too coarse to include details about the small habitats where species can find refuges. “We’re cutting climate relicts out of the picture,” says Jump.

That omission won’t matter in large-scale projects of how climate change will affect biodiversity, Jump says. But it will be a problem when ecologists try to determine whether a species is going to entirely disappear from a region. Climate relicts may hang on when computer models say they can’t. “It goes from being common to being rare, but it doesn’t go extinct entirely,” says Jump.

Today’s climate relicts are also important for managing species, Jump argues. Because these populations managed to survive in unforgiving places for thousands of years, it’s possible that during that time, they’ve evolved, picking up valuable adaptations. It might be possible to identify the genes that make climate relict trees drought resistant, for example, and use them to breed trees that farmers could plant in arid regions. Climate relict genes “could increase the resilience of a species across its range,” says Jump.

But Jump is worried that time is running out to study many climate relicts. Some may not be able to withstand the coming climate change, which is expected to go rapidly beyond anything these relicts have withstood over the past 11,000 years. Jump is even more concerned about the pressure that farming, tourism, and other human activities are already putting on climate relicts. Relict stands of beech trees in the Montseny Mountains in eastern Spain, for example, stop abruptly at a high-elevation plain where sheep and goats are grazed. “That plain would be perfect to support more trees,” Jump says wistfully.

Copyright 2011 Carl Zimmer
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