In order to understand how habitat destruction affects different kinds of species and communities, ecologist Pablo Munguia begins simply.
Munguia hops over the side of a small boat into the northern Gulf Coast waters. He takes a dead pen shell—a common seawater bivalve—and anchors it to the seabed with a cable tie, a piece of plastic mesh and a metal bar. Then he anchors another one about a foot away, and another one, and so on, until he has created a grid of pen shells. Then he makes another few grids, of varying sizes, and then he waits.
“Imagine a large cloud of larvae or animals floating around in the water, looking for habitats,” says Munguia, an assistant professor of marine science. “They’re raining down constantly, looking for somewhere to settle. The soft sediment at the sea bottom is unsuitable, but if they find a hard substrate, like a pen shell, they’re going to colonize it.”
Very quickly, says Munguia, the pen shells become worlds unto themselves. Dozens of species of fish, shrimp, sponges, crustaceans, amphipods, barnacles and bryozoans settle in and begin battling it out with each other for space, food, light and life.
The shells also influence each other. One species might become so successful at reproducing within a particular shell that it begins sending out its juveniles to colonize nearby shells. Other species reproduce by releasing clouds of larvae into the sea, where they can float over and attach to nearby shells. This movement among shells generates a network of communities, known as a "metacommunity" in the ecological world.
“What’s inside will reflect what’s migrating from nearby communities as well as what can persist within the shell” says Munguia. “Sometimes there will be a priority effect, where the species that gets there first will have an advantage that persists over time. Sometimes it’s just a matter of competition, where if you have a paramount competitor—a sponge, for instance—they’ll dominate regardless of when they arrive.”
Munguia chose to use pen shells, which haven’t gotten much attention from community ecologists in the past, for a few reasons. They play host to a rich diversity of species. Their structure makes it easy to determine where the boundaries of the community lie. And they’re easy to manipulate. Munguia can do a census of shells at different stages of their development—after a day, a week, a few weeks, a few months. He can anchor shells at different phases of the moon, or in different seasons of the year. In order to collect a shell, all he has to do is drop a zip-loc bag over it, cut the cable tie, and hop back in the boat.
“There’s a gap in studying metacommunities, between theory and empirical data, because manipulating whole communities is hard,” says Munguia. “For example, if one is interested in understanding habitat destruction, you can’t just go around destroying whole forests – it is logistically impossible and ethically wrong. But with pen shells, you can create them and simulate destruction by removing them. Pen shell communities can be moved around, placed either close together or far apart, and anchored at different times of the year. You can’t do that with coral reefs or forests.”
Because of these advantages, Munguia has been able to study how the life-stage at which a species disperses affects its success in metacommunities over time. He’s been able to explore what are known as source-sink dynamics—how high-quality habitats can end up sustaining populations in nearby, low-quality habitats through immigration.
Munguia’s also performed one of the first experiments to actually test the effects of habitat destruction on metacommunities. Simply by removing pen shells from his grids, he’s able to simulate what happens when a metacommunity shrinks as the result of, say, slash and burn farming, modifications to the landscape, or destruction of reefs after hurricanes.
Among his findings has been that, in many cases, it’s the species that can’t move as adults that survive habitat destruction better, because even though the adults can’t move, when they reproduce they release their gametes over a long distance. For many mobile species, by contrast, habitat destruction can be more devastating because although they can move, it may not be far enough to find another suitable habitat.
“We tend to think of habitat destruction as just the reduction of area, but it’s much more complicated than that,” says Munguia. “Different species have different requirements, different life histories, different dispersal modes, so destruction’s not going to be same for all species. For some species, destruction can be catastrophic, but for others it’s not.”
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