With extinction continuously altering the fates of plants and animals, the researchers say it may be extremely difficult to predict which organisms will be the next to cease existing and that the wisest conservation plan is one that reaches beyond a particular species to protect entire communities.
A pair of researchers from the University of Wisconsin-Madison, interested in understanding what happens when species go extinct, developed mathematical models looking at changes in a community's tolerance to a particular environmental condition, such as global warming or acid rain.
They found that, as individual species start to disappear, two forces begin to act upon a community, making it either more or less tolerant to the environmental condition.
One of these forces occurs when species disappear in order of their sensitivity to a particular environmental factor, with the least tolerant ones going extinct first. "We know that some species are more sensitive to environmental stressors," says Anthony Ives, a UW-Madison zoology professor and co-author of the Nature paper. "And they often go extinct in order of their sensitivity."
With the disappearance of organisms most vulnerable to a certain condition, such as the increase of nutrients in lake water, the species best suited for that condition are left behind. This ordered extinction, notes Ives, makes the community as a whole more resistant to that environmental pressure and, in a sense, protects it from future degradation.
"One important message is that if we're going to understand the consequences of extinction, we need to pay attention to order," says Bradley Cardinale, a UW-Madison postdoctoral fellow and also co-author of the recent paper. "If species go extinct in a particular order, it is possible for the surviving community to become more resistant overall."
While this finding may sound like good news, there is a downside: the researchers say that a community's resistance to an environmental condition can shift over time due to yet another force - changes in food-web interactions resulting from the extinction of individual species.
All species are part of a food web, whether they are predators, prey or even competition. And, when a member of the food web goes extinct, it indirectly alters the livelihood of the survivors, note the researchers.
Ives explains, "Now free from the species that fed on it or competed with it for food, a species may increase in abundance." By increasing in abundance, he adds, the species makes the entire community more tolerant to the environmental pressure.
However, according to the models, the continuous extinction of organisms from a community ultimately dampens the ability of surviving species to compensate, or increase in population size, and, consequently, makes the community less resistant to changes in the environment.
"The loss of species tends to deplete a community's ability to withstand environmental degradation," says Ives.
Cardinale says that these changes in the food web and their indirect effects on organisms within a community can "change the order of extinction," basically foreshadowing new fates for species. He explains, "A species that seems insignificant now may become important later on once it's released from predation or competition."
Because of the dynamics of the food web, the researchers say it becomes challenging to determine what species may vanish next due to the forces of extinction. This leads them to suggest a more holistic approach to conservation.
"We can't just go out and conserve one species," explains Cardinale. "Because we have no idea what species may make the community resistant in the future, we would be most prudent to conserve as many as we can right now."
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Bradley Cardinale (608) 262-9226, firstname.lastname@example.org