A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Honor Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Hornor Society and other rightsholders Connecting Habitats to Prevent Species Extinctions Conservation biologists are creating links between forest fragments where the most animals with small ranges live. Stuart L. Pimm and Clinton N. Jenkins y the time Charles Darwin ers of an analogous nature.” Nothing where species are dying. To find out landed in Brazil in 1832 while in his experiences of England’s natural what we can do to prevent extinctions, on his voyage on the HMS history had prepared him for this dis- we must first understand where spe- Beagle, Europeans had known covery. The taxonomist and illustrator cies are, which ones are vulnerable and Bfor centuries that the tropics house John Gould confirmed the distinctness why, and where they live. an extraordinary variety of strange, of the mockingbird species with his de- Small-ranged species are dispropor- exotic species. Darwin wrote gushing scriptions of them in 1837. Just how sin- tionately at risk of extinction. The “Red letters to his family about the birds, gular they were was novel to him, too. List” of the International Union for the butterflies, flowers, and other thrill- Worldwide, only six known species had Conservation of Nature (IUCN) aspires ing organisms he saw. From reading smaller geographical ranges—and none to assess the risks of extinction of all travelers’ accounts, he had expected of them formed a cluster of species in species. It has done so for more than the richness of the flora and fauna. Yet the same genus. It would take decades 90,000 plant and animal species, out he missed something vitally important for scientists to realize that some special of a total of nearly 2 million species that was unknown at the time: Many parts of the tropics had many species that taxonomists have described. Birds, species are unique to the coastal region with small geographical ranges. mammals, and amphibians are the best of Brazil, and some are found only on Alfred Russel Wallace set out for the known. Overwhelmingly, it’s the spe- a few mountaintops. Later, Darwin’s Amazon the next decade. Yet it was not cies with small geographical ranges realization that there are special places there, but in insular southeast Asia that that are at greatest risk. For birds, the where many species with small geo- he made his observations on small- risk of extinction drops dramatically graphical ranges are concentrated ranged species that he published in his as range size increases. About half the would change science. “Sarawak Law” paper of 1855, which species with ranges smaller than 1,000 What Darwin didn’t know is some- set the clock ticking on the famous pair square kilometers are at risk. thing profound about the sizes of the of papers announcing the theory of This pattern is not surprising. Other geographical ranges of species. It seems evolution. Like Darwin, he marveled things being equal, the destruction of such an esoteric fact: He was unaware that islands so close to each other could habitats is more likely to terminate a that many species have tiny geographi- have different, but related, species. species that occurs, for instance, on a cal ranges. So when he visited the Ga- In the century that followed, there few mountaintops in coastal Brazil, lápagos Islands in 1835, the presence would be massive destruction of these than one that occurs across the entire of four clearly different, but related, special forests where small-ranged Amazon basin. The challenge for con- species of mockingbird on such small species were concentrated. Only in the servation science is what we can do to islands came as a profound shock. “The past few decades have scientists under- protect these threatened species. fact, that islands in sight of each other, stood that this destruction, combined We now know that species with should thus possess peculiar species, with the uniqueness of the species in small geographical ranges, which are would be scarcely credible,” he wrote, these areas, poses one of the greatest so often threatened, are concentrated “if it were not supported by some oth- challenges to conservation. Such places in certain places. And in these mostly typify the frontlines of the fight to re- tropical concentrations, human actions duce global rates of species extinction, not only destroy habitats but also leave Stuart L. Pimm is Doris Duke Chair of Conserva- which are now 1,000 times their normal what’s behind in small, isolated frag- tion at the Nicholas School of the Environment at rates, according to a 1995 study in Sci- ments. These patches may be too small Duke University, and Clinton N. Jenkins is a pro- fessor at the IPÊ-Instituto de Pesquisas Ecológicas ence coauthored by one of us (Pimm). to sustain viable populations of spe- in Brazil. Together they work with nonprofits in de- The salient feature of biodiversity in cies. Restoring corridors—habitat con- veloping countries to help purchase degraded land, the 21st century is that the places that nections between fragments— affords reforesting these areas to reconnect isolated natural inspired Darwin and Wallace to think a cost-effective solution. For the past habitats. Email for Pimm: [email protected] about the origins of species are now decade, we have worked with local 162 American Scientist, Volume 107 © 2019 Sigma Xi, The Scientific Research Honor Society. Repro- duction with permission only. Contact [email protected]. The Atlantic Forest in the state of Rio de Janeiro near Búzios, Brazil, has been fragmented by a given place are in the tropical moist deforestation and changing land use. This region is home to unique species, many of which forests of the world. The patterns are have small ranges and specific habitat requirements. Connecting forest fragments can curb broadly similar for birds, mammals, the local extinction rate, which is higher in smaller areas. By mapping regions where the most and amphibians— the taxa we know species are at risk of extinction and connecting habitats there, the authors aim to maximize the best. Less geographically resolved data effects of their conservation efforts. for insects and plants suggest the pat- terns are also broadly similar. Wallace partners and started corridor projects have described half of the known am- went to the Amazon because that’s in Brazil, Colombia, Ecuador, India, phibian and mammalian species with where the most species are! He made and the Indonesian island of Sumatra. small ranges in the past few decades. his living collecting novelties—species Many more are surely awaiting discov- other collectors had not discovered. He Mapping Conservation Priorities ery. As with birds, large-ranged spe- likely thought that more species would Many more species with small geo- cies were discovered earlier than small- mean more novelties—but he was part- graphical ranges are known now ranged ones. ly wrong, as the next map shows. than in Darwin and Wallace’s time. The century and a half of explora- The map at the top left of page 166 By 1850, taxonomists had described tion since the travels of Darwin and shows where the half of all bird species close to 5,000 terrestrial bird species in Wallace has made it possible to pro- that have the smallest ranges occur. It the world. The histogram on page 164 duce three key maps that show the ar- contains fully half of the species as the shows the numbers of bird species in eas that have, respectively, the greatest left-hand map on page 165, yet it is pro- each category of range size. In 1850, numbers of species, the greatest num- foundly different. Small-ranged spe- 14 years after Darwin returned from bers of species with small ranges, and cies in the Americas are concentrated in his voyage, only 200 (4 percent) of the the greatest numbers of species that are Central America, along the Andes, and known species at that time had ranges threatened with extinction. Geography in a strip of forest along the Brazilian smaller than 10,000 square kilometers. is destiny. Understanding the relation- coast. (Exactly why small-ranged spe- Today, that number is 1,290—close to ships between these geographical pat- cies are concentrated in moist, tropical 13 percent of the more than 10,000 spe- terns is the first vital step in determin- mountains is a matter of some debate.) cies now known. ing where to act to save species. Neither Darwin nor Wallace could We’re still finding more such species. Likely, the maps on page 165 would have had any sense of this map, because In a paper we published in 2010 about not have surprised Darwin and Wal- species with small ranges were only just Edward Parker/Alamy Stock Photo Edward Brazil, we showed that taxonomists lace. The greatest numbers of species in coming into European museums when www.americanscientist.org © 2019 Sigma Xi, The Scientific Research Honor Society. Repro- 2019 May–June 163 duction with permission only. Contact [email protected]. the two men were traveling. (And it harmed those places where small- that include suppressing poaching, re- took many more decades to under stand ranged species are concentrated. Maps ducing demand for animal products, their complete geographical ranges.) provide high-resolution improvements and reducing human-wildlife conflict. Wallace didn’t go back to the Amazon; on his groundbreaking ideas.