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1. Road rage
(STORY MAP)

2. Pavement or depravement?

3. Spreading like weeds

4. Amphibian perambulations

5. Down the corridor

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Could habitat corridors help to reverse fragmentation of prairies and forests?

  Road to redemption?
butterfly on a starIf roads are barriers to the movement of species, habitat corridors are the opposite: paths between preserves that, ideally, allow native species to recolonize -- move to -- new quarters. Like the concern about roads themselves, the idea of corridors reflects the theory of island biogeography. Species are prone to extinction on small patches of habitat. If those patches cannot be enlarged, why not link them with "corridors" of suitable habitat?

Corridors caught on among conservationists. In Costa Rica for example, conservationists suggested linking a string of large national parks into a mega-park that would, ideally, allow large carnivores like the panther to roam.

The idea grew into the Mesoamerican Biological Corridor, which stretches -- on paper -- through eight countries from Mexico to Panama. And despite difficult politics and economics (see "Bold Conservation Project..." in the bibliography), the corridor does provide an organizing principle for choosing conservation objectives.

brown butterflies (with yellow fringed black spots) on green leaves
Two (endangered) St. Francis Satyr butteflies mate in the North Carolina woods. Photo courtesy Nick Haddad

Nagging question
But here's a question: Do corridors actually work as highways for desired species? The simple question is difficult to answer simply, because creating corridors is a slow, expensive proposition. Nevertheless, new data indicates that species that need specialized habitats do follow corridors, at least on the small scale susceptible to experiment

"Ten years ago there was basically no scientific evidence for corridors," says Nick Haddad, an assistant professor of zoology at North Carolina State University. Now, he says, evidence for "the idea that corridors do support movement and gene flow is accumulating through experiments, and through the observational studies that can be much larger."

A grassy corridor running through the pines.
The entrance to an experimental corridor, used to measure the movement of species through narrow strips of suitable habitat, at the Savannah River Ecology Laboratory. The 10 species studied preferred the regrowth to the pine forests. Nick Haddad.

Haddad helped run an experiment at the Savannah River Ecology Laboratory in South Carolina to test the movement of 10 species through artificial corridors. The lanes were created by sawing pine trees in a plantation forest, leaving an area with weedy, scrubby growth. Although biological corridors are seldom fashioned with chainsaws, Haddad says biological diversity was higher in the clearings. "It's a disturbed ecosystem... that is diverse and productive, in the middle of a pine forest that is not diverse or productive. Like other situations -- the ones that people actually care about -- a suitable habitat is surrounded by unsuitable habitat."

The study (see "Corridor Use..." in the bibliography), compared whether organisms moved more freely to patches of habitat that are connected by corridors. Although, every organism, whether rat, plant seed, or butterfly, moved better along the corridors, the conclusion was statistically significant only for five of the 10 species, because either too few patches were studied, or too few organisms showed up at the destination.

"We always saw that twice as many individuals had moved through the corridor," Haddad explains. "When we had a really low sample size, even if it was a large effect, we did not always see a statistically significant movement."

Still, he says, "We saw responses across plants, insects, and small mammals." The seeds, marked with a fluorescent dye, were found in bird droppings, he adds, indicating "that the birds are moving in the corridors, even though some are migratory birds that are moving hundreds or thousands of miles. You'd think that these corridors, a few hundred meters long, are so short that birds would not respond," but in fact they changed local animal behavior.

Diagram shows edge of stream with buffer area.
Revegetating corridors along streams reduces water pollution caused by farm-field runoff - and provides a way for animals to migrate from place to place. Photo courtesy Chesapeake Bay Program

This river is a highway
Back in the real world, biological corridors are starting to get built. In North Carolina, Haddad says, buffer strips along rivers and streams, primarily designed to stop sediment and fertilizer in surface water from polluting streams and harming the fishing industry, also offer a migration corridor to animals that depend on stream-bank habitat.

Having shown that corridors do help animals and plants move between experimental habitats, the next step is to probe their real-world effectiveness, Haddad says. "To work as intended, they have to promote movement, recolonization, and gene flow. The real question is, when does that have an impact on the sizes of populations, and biodiversity in a community?"

Future studies, he says, will approach these questions. After all, while the argument for corridors is logical, it is not airtight. Corridors may help desired plants -- or weed seeds -- reach new habitat. "We're trying to put together, what's the ultimate impact on plant populations," says Haddad. "Some interactions are positive... and some negative. Do corridors have a net positive effect, or does it cancel out?"

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