Wild elephants wander across the crowded flatland of India; the forested river banks through fields in Brazil; a ribbon (缎带) of green spreads across Europe where the Iron Curtain used to be. Using such wildlife corridors (走廊) to link up larger but isolated (孤立的) protected areas is the most widely used method for stopping biodiversity decline (生物多样性衰退), with millions of dollars spent creating and protecting them every year. But has enthusiasm for a neat idea got ahead of the science?
As wild habitat is broken into isolated parts by farms, roads and settlements, we need to link them up with corridors of green. Then even if the entire habitat can't be recreated, old migration (迁徙) patterns can be brought back, escape routes created ahead of climate change and — perhaps most importantly — isolated populations can interbreed (杂交), improving their genetic (基因的) diversity and their ability to survive.
Recently, Paul Beier, a biologist from Northern Arizona University, and his colleague Andrew Gregory, warned that "in spite of much research, there is little evidence that protection corridors work as expected." There is, they say, plenty of evidence that wild animals will move through corridors. But supporters of corridors want, and claim, much more than this. They say that animals don't just go for a walk in their protection woods, but that they move in forever and interbreed with neighbouring populations. In this way corridors supposedly linked isolated and endangered populations into an interbreeding — and much more powerful — whole.
Such claims sometimes hold up. In the United Kingdom, the expansion (扩张) of Kielder Forest in the 1960s provided a link between isolated populations of endangered red squirrels. Genes from isolated populations have now "spread through hundreds of forest parts" across 100 kilometers and more. But the Kielder Forest is much wider than an ordinary corridor. Few studies have looked for gene exchange in corridors; even fewer have found it. One study researched the genetic diversity of small marsupials (有袋类动物) in a narrow forest corridor crossing 4.5 kilometers of grassland in Queensland, Australia. It found that genetically distinct populations had kept on staying at either end. Mixing was impossible.
Other studies have shown that protection corridors work. But most have looked at short corridors of 100 meters through largely natural landscape. "That species can travel along short corridors in a natural setting doesn't mean that they will be successful travelling along much longer corridors which are in a landscape greatly affected by human beings," says Gregory, "still less that such movements occur frequently enough to allow enough gene exchange to occur so that the connected habitat blocks function as one population."
Perhaps we shouldn't make the perfect the enemy of the good. Is any corridor surely better than none? But consider this. The edges of wild areas are known danger zones for wildlife, where enemies and diseases may invade (侵略). Linking two existing protected areas with a long narrow corridor may uncover it to greater danger along these edges. Unless the benefit exceeds (超过) the threat, then there is serious possibility to do harm.