What Effect are Humans Having on Biological Diversity?

The presumption is "that we are driving biodiversity to lower levels," said Steven D. Gaines, who is director of  UC Santa Barbara's Marine Science Institute. "Certainly, if you think about it at the global level, this is true because humans have done a lot of things that have driven species extinct."

But Gaines and fellow researcher Dov Sax took it down to the smaller level, by examining island chains all across the planet, and the biological diversity found therein. "These were all oceanic islands," Gaines said, "which means islands that are far enough away from a continent that they're not getting regular exchanges with the mainland."

They also used information gathered by the original explorers, who almost to a one brought with them a naturalist whose passion was cataloguing all the native species of plants and animal. What they found was that, on the islands, diversity is on the rise, and in some instances, to a large degree. In fact, the diversity has skyrocketed so that some might cause to ask, is it not better to have twice the number of species as we once did.

According to Gaines though, it just isn’t that simple. "What Dov and I worked on a few years ago is the fact that the vast majority of introductions (of species) don't have large negative effects," Gaines said. "Indeed, most species that get introduced don't have much effect at all. It doesn't mean that they're not altering the ecosystem, but they're not driving things extinct like some of the big poster-child stories we've been hearing about."

 

Nevertheless, their study did show that human colonization has a large impact on the native ecosystems. One example they used was New Zealand. Prior to human arrival, there was about 2,000 native species of plants on the two islands. However since colonization, while there have been some extinctions the human presence has introduced nearly 2,000 more species."The dramatic increase in the number of species has changed how the system functions," Sax said. "Changing the abundance of natives versus exotics affects all of the other species that used to depend on the natives for food or shelter. So, it's not in any way to say that increasing biodiversity is a good thing."

Given their research, Gaines and Sax have been forced to ask new questions, to which they hope to narrow it down to one explanation:

-          Are the islands undersaturated? Can you still keep throwing species in there, with the result that nothing is going to happen?

-          Are they now oversaturated? Are there limits in how many species an ecosystem can hold?

-          Are we building an extinction debt? "Which means," Gaines said, "that by going in and mucking up the system, we may have already created the setting where too many species have been packed in, and we just haven't waited long enough to see these extinctions start to happen.

"The whole point of this study was to start looking down the path to see which of these wildly different scenarios might be right," Gaines added. "We haven't nailed the answer yet, but we've set the stage for answering whether islands are now saturated or not."

Posted by Josh Hill.

http://www.eurekalert.org/pub_releases/2008-08/uoc--soi082608.php