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Species numbers found stable—but maybe thanks to invaders

May 13, 2014
Courtesy of University of Vermont
and World Science staff

De­spite the many man-made threats that are kill­ing off an­i­mals and plants glo­bal­ly, the num­ber of diff­erent spe­cies is­n’t drop­ping in many places, a study finds. 

But sci­en­tists warn that might be be­cause some spe­cies are suc­cess­ful in­vaders of ter­ri­to­ry sur­ren­dered by oth­er­s—rais­ing the pros­pect of a situa­t­ion re­sem­bling a so-called “planet of weeds.”

In the study published in the jour­nal Sci­ence, Uni­vers­ity of Ver­mont bi­ol­o­gist Nick Gotelli and col­leagues re­viewed 100 long-term mon­i­tor­ing stud­ies around the world and found the num­ber of spe­cies in many places has­n’t changed much. In 59 cases there was ac­tu­ally a small in­crease; in the rest a small de­crease. That was a sur­prise, be­cause bi­ol­o­gists have spo­ken of a glob­al ex­tinc­tion cri­sis.

But the re­search­ers did dis­cov­er some­thing chang­ing rap­id­ly: which spe­cies were liv­ing in the places be­ing stud­ied. Al­most 80 pe­r­cent of the com­mun­i­ties the team ex­am­ined showed sizeable changes in spe­cies com­po­si­tion, av­er­ag­ing about 10 pe­r­cent change per dec­ade.

“Right un­der our noses, in the same place that a team might have looked a dec­ade ear­li­er, or even just a year ear­li­er, a new as­sem­blage of plants and an­i­mals may be tak­ing hold,” Gotelli said.

Yet “the num­ber of spe­cies in a place may not be our best score­card for en­vi­ron­men­tal change.”

For ex­am­ple, the sci­en­tists write that dis­turbed cor­al reefs can be re­placed by a group of spe­cies dom­i­nat­ed by al­gae. This re­place­ment might keep the spe­cies count the same, but not nec­es­sarily pro­vide the fish­er­ies, tour­ism (“al­gae div­ing” does­n’t have quite the same ap­peal as “reef div­ing”) or coast­al pro­tec­tions that the orig­i­nal cor­al reef did.

“In the oceans we no long­er have many an­chovies, but we seem to have an aw­ful lot of jel­ly­fish,” said Gotelli. “Those kinds of changes are not go­ing to be seen by just count­ing the num­ber of spe­cies.”

The rea­sons this is occurring could be many, the sci­en­tists said, but one is re­lat­ed to what sci­ence writ­er Da­vid Quam­men termed our “planet of weeds.” In oth­er words, in­va­sive spe­cies or suc­cess­ful colonists, such as rats, may be spread­ing in­to new places, keep­ing the lo­cal spe­cies tally up, even as the plan­et’s over­all biodi­vers­ity is de­grad­ed. “We move spe­cies around,” Gotelli said. For ex­am­ple, “there is a huge ant di­vers­ity in Flor­i­da, and about 30 pe­r­cent of the ant spe­cies are non-natives.”


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Despite the many man-made threats that are assaulting the world’s animals and plants, their diversity isn’t dropping in many places, a study finds. But scientists warn that could be because some species are successful invaders of territory surrendered by others—raising the prospect of a situation resembling a so-called “planet of weeds.” In a new study in the journal Science, University of Vermont biologist Nick Gotelli and colleagues reviewed 100 long-term monitoring studies around the world and found the number of species in many of these places hasn’t changed much. In 59 cases there was actually a small increase, and in rest a small decrease. That was a surprise, because biologists have spoken of a global extinction crisis. But the researchers did discover something changing rapidly: which species were living in the places being studied. Almost 80 percent of the communities the team examined showed substantial changes in species composition, averaging about 10 percent change per decade. “Right under our noses, in the same place that a team might have looked a decade earlier, or even just a year earlier, a new assemblage of plants and animals may be taking hold,” Gotelli said. Yet “the number of species in a place may not be our best scorecard for environmental change.” For example, the scientists write that disturbed coral reefs can be replaced by a group of species dominated by algae. This replacement might keep the species count the same, but not necessarily provide the fisheries, tourism (“algae diving” doesn’t have quite the same appeal as “reef diving”) or coastal protections that the original coral reef did. “In the oceans we no longer have many anchovies, but we seem to have an awful lot of jellyfish,” said Gotelli. “Those kinds of changes are not going to be seen by just counting the number of species that are present.” The reasons could be many, the scientists said, including possibilities related to what science writer David Quammen termed our “planet of weeds.” In other words, invasive species or successful colonists or weedy generalists — think kudzu and rats — may be spreading into new places, keeping the local species tally up, even as the planet’s overall biodiversity is degraded. “We move species around,” Gotelli said. For example, “there is a huge ant diversity in Florida, and about 30 percent of the ant species are non-natives.”