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Do Biological Invasions Decrease Biodiversit ?

By James H. Brown and Dov /. .a

1es! and no

2he fact is! we don t 'now nearly as much a#out invading s&ecies as we need to3des&ite decades of research #y ecologists! &aleo#iologists! and #iogeogra&hers. 4e cant &redict when invading foreign s&ecies will re&lace native animals and &lants! nor can we &redict when or #y how much invading s&ecies will disru&t the structure and function of ecosystems. 4e dont 'now how serious the threats of alien invaders are to our native flora and fauna5 these are scientific $uestions. "nd! as is often the case in science! the answer is less clear than re&orts in the &o&ular &ress a#out the devastating im&acts of gy&sy moths! 6e#ra mussels! and &ur&le loosestrife might im&ly. 4e do 'now that! as the human &o&ulation grows and s&reads! native &lants and animals #ecome e tinct5 humans are introducing s&ecies into new areas! #oth intentionally and unintentionally. "nd we 'now that glo#al #iodiversity is decreasing as a result of humanassisted invasions. But on a local level! things loo' $uite different. "t small scales! the e tinction of native s&ecies has ty&ically #een more than offset #y the
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coloni6ation of invading s&ecies. "lreadya#undant and wides&read s&ecies have e &and(ed their ranges! more than com&ensating in local s&ecies richness for the restricted endemic forms that have disa&&eared. 2his does not mean that e otic s&ecies have not caused e tinctions. It sim&ly means that! on average! there is locally fewer than one e tinction of a native s&ecies for every successful coloni6ation of an alien s&ecies. 2his will come as a sur&rise to many who #elieve that #iodiversity is decreasing everywhere on earth. But it is true! for continents as well as islands. North "merica &resently has more terrestrial #ird and mammal s&ecies than when the first -uro&eans arrived five centuries ago. "lthough the &assenger &igeon! heath hen! and )arolina &ara'eet are e tinct and the )alifornia condor! red wolf! and #lac'(footed ferret are essentially gone from the wild! these losses are more than offset #y the coloni6ation of house s&arrows! -uro&ean tree s&arrows! roc' doves! ringed doves! mon' &ara'eets! ring(nec'ed &heasants! chu'ar &artridges! house mice! Norway rats! -uro&ean hares! wild #oars! feral horses! ory ! and many other s&ecies.

%ut of a total flora of a&&ro imately 7!+++ vascular &lant s&ecies! )alifornia has more than ,!+++ naturali6ed e otics 8,95 yet fewer than :+ natives are 'nown to have #ecome e tinct 8*9. 2he asymmetry holds even on islands and insular ha#itats. 4ithin the last few centuries following -uro&ean coloni6ation! relatively few insular endemic &lant s&ecies have #ecome e tinct! whereas invading s&ecies have a&&ro imately dou#led the si6e of island floras from *!+++ to ;!+++ on New <ealand5 ,!:++ to *!:++ on Hawaii5 **, to ;*, on =ord Howe Island! "ustralia5 >+ to ,,, on -aster Island5 and ;; to ?+ on Pitcairn Island 8:9. How can this #e? 2he #iota of every &lace on earth is &oised on a continually shifting #alance of filling and em&tying. 2he effects of immigration and s&eciation increase diversity! and the effects of emigration and e tinction decrease diversity. But how full is it? 2here are two e treme views. .ome ecologists and #iogeogra&hers have suggested that the #iota is so saturated with &lant and animal s&ecies that adding immigrating aliens causes the e tinction of an e$ual num#er of native s&eciesmuch li'e a game of musical chairs! where every &layer has to com&ete for a s&ace in order to remain in the game. %ther researchers contend that most &laces on earth are far from saturated and hence the world is a#le to a#sor# coloni6ing aliens without losing as many natives. %ur research su&&orts the latter view in some! #ut not all! local areas. /oreign invaders often evo'e strong emotional res&onses! even from normally o#@ective scientists. 4e are not suggesting that naturali6ed s&ecies are desira#le! nor are we arguing against efforts to &revent immigration or to eradicate foreign s&ecies. But we are calling for more rigorous scientific evidence to su&&ort claims that invading alien s&ecies cause ma@or decreases in local #iodiversity and damage to wild ecosystems5 currently! there is little o#@ective evidence to su&&ort swee&ing claims of wholesale death and destruction. 4e also #elieve that we should try to learn as much from alien s&ecies as &ossi#le. 2hey &rovide a rich set of unintentional and uncontrolled yet highly re&licated e &erimentsones that may ultimately hel& us to #etter understand com&le &atterns of #iodiversity. /rom current scientific research e can dra si conclusionsA ,. Many &laces have e &erienced e tinctions of native fish! &lants! and #irds. *. "lien s&ecies have undou#tedly contri#uted to the e tinction of some of these s&ecies. 2he ultimate cause of e tinction is often am#iguous! however! #ecause other human activities have had su#stantial environmental im&acts. :. Des&ite the e tinction of some native s&ecies! the total num#er of s&ecies usually increased or remained the same in local areas. ;. "lthough we often accuse invading s&ecies of damaging the structure and function of ecosystems! there is usually little hard scientific evidence of such negative im&acts. >. 2he fact that islands show such different ca&acities to a#sor# immigrating #irds and &lants suggests that there is no single definitive e &lanation as to how alien s&ecies im&act l cal #iodiversity. 7. 2he net effect! however! is still a loss of glbal #iodiversity. Many of the invading alien s&ecies are common and widely distri#uted. By contrast! many of the native s&ecies that

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have gone e tinct were endemics and have thus #een lost forever. =iterature citedA ,. Be@mane'! M. and J.M. Bandall. ,CC;. Invasive alien &lants in )aliforniaA ,CC: summary and com&arison with other areas in North "merica. Madroo ;,A,7,(,DD. *. 2i#or! D.P. *++,. California Native Plant Society s Inventory of Rare and Endangered Plants of California! 7th edition. )alifornia Native Plant .ociety! .acramento! )alifornia. :. .a ! D./.! ..D. 0aines and J.H. Brown. *++*. .&ecies invasions e ceed e tinctions on islands worldwideA a com&arative study

0iven the sta'es! our mod s operandi in dealing with invasive s&ecies should #e guilty until &roven innocent
Daniel .im#erloff is the director of the Institute for Biological Invasions at the University of 2ennessee! Eno ville. He wor's on many introduced s&ecies! from the small Indian mongoose on islands worldwide to northern hemis&here &ines in Patagonia. His interest in invaders #egan when he used them to test ecological theories. "fter seeing first(hand their myriad im&acts on natural environments! his interest too' a turn toward conservation. He s&ends much of his time convincing the &u#lic and &olicyma'ers that im&acts of #iological invasions! although one of the great glo#al changes! can #e &revented! eradicated! or managed. .im#erloff started his studies in ecology as a student of -.%. 4ilson.

Photo courtesy of University of 2ennessee By Daniel .im#erloff

4arranted

)ritics have argued that the war against invasive s&ecies is more a#out aesthetics than science. .ome have even gone so far as to accuse conservationists of eno&ho#ia or even racism. I #elieve such arguments are a red herring. 2rue! most introduced s&ecies3#y some rough estimates! C+ &ercent3do not cause su#stantial ecological or economic im&acts. But ,+ &ercent of a large num#er is still a large num#er. 2he U... alone has 7!+++ introduced s&ecies 8e cluding micro#es9 that are thriving without human assistance. 2heir im&acts are staggering. Many 0arry oa' woodlands and meadows in the U... Pacific Northwest are now seas of .cotch #room. %a's have largely re&laced the "merican chestnut! a tree that once occu&ied over ,++ million hectares of eastern North "merica. Dead mans fingers! a Pacific alga! car&ets much of the nearshore seafloor of the southern 0ulf of Maine and the Nova .cotia coast where 'el& forests once stood. "nd massive monocultures of the Pacific F'iller algaG smother sea(grass meadows off the coasts of .&ain! /rance! Italy! and )roatia. "nd all of these changes have occurred within the &ast two centuries. "nd then there are s&ecies such as the "merican chestnut. "lthough it is not threatened! it is difficult to find one in nature nowadays. /or ten moths whose host &lant was the chestnut! it is now too late3they are e tinct. Birds are ta'ing a #ig hit! too. %rnithologists estimate that ,!,?7 8,* &ercent9 of the worlds

#ird s&ecies are threatened with imminent e tinction. /or >,+ of these! the threat is wholly or &artly from introduced s&ecies 8&redators! com&etitors! and her#ivores9. .ome o#servers $uestion whether we should &in the #lame on invasive s&ecies! noting that the &rosecutions evidence is often anecdotal! s&eculative! and #ased on limited o#servation. 1et they fail to mention that the same could #e said a#out the evidence for every other claimed cause of e tinction3 ha#itat destruction! &ollution! disease! and so on. 2he last individuals of a s&ecies almost always disa&&ear while no one is watching. 2he
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occasional rea&&earance of a s&ecies long thought e tinct3for e am&le! the ivory(#illed wood&ec'er3ma'es #iologists loath to declare a s&ecies e tinct for decades #eyond its last une$uivocal sighting. Because no clear #oundaries delimit community ty&es! we cannot sim&ly say a ty&e of community has disa&&eared. Nevertheless! I need only loo' around to see that the oa'(chestnut(hard &ine forests of eastern North "merica no longer e ist in the a#sence of chestnut. 4hats worse! many introduced s&ecies now recogni6ed as invasive horrors were innocuous in their new homes for decades #efore they a#ru&tly s&read. /or instance! Bra6ilian &e&&er was introduced to /lorida a century #efore it e &loded across the southern &art of the state. Unfortunately! were not good at &rediction. If we 'new which invasions would #e harmful and which inconse$uential! it would #e straightforward to e clude only the #ad actors among &lanned introductions. However! invasion #iology is a new field that coalesced as a distinct disci&line only in the ,C?+s. 4e have made great &rogress! #ut our &redictive tools still carry wide confidence limits! and after(the(fact eradication is still e tremely difficult. /or at least the ne t few decades! we will have to assume that most esta#lished introductions are irrevoca#le. 0iven these facts! I #elieve our modus operandi in dealing with deli#erate introductions must #e Fguilty until &roven innocentG3even though it contradicts the #asic tenet of U... @uris&rudence. In other words! our #lac'lists should #e su&&lemented #y whitelists. In the U... and almost all other nations! we currently use #lac'lists. 2he relatively few s&ecies that have made it to the #lac'list are the 'nown trou#lema'ers3every other s&ecies can #e admitted. 2he whitelist fli&s that system on its head. 4e would vet every s&ecies! no matter how a&&arently unthreatening. 4ed have to #e fran' a#out our limitations and grant that some s&ecies denied whitelist status would have &roven innocuous if introduced! and a certain num#er granted such status and introduced will &rove to #e harmful mista'es. But surely it is our o#ligation to act on our 'nowledge a#out invasions rather than ac$uiesce to a &olicy of admitting almost

everything and then studying the conse$uences later? Just as with glo#al climate change! it would #e chea&er and easier in the short term to say that we lac' &roof and should delay regulation. "s in other scientific matters with &u#lic&olicy ramifications! &olicyma'ers and &olitical &undits all too often cite the small minority to suggest that su#stantial controversy swirls around the im&ortance of introduced s&ecies. It does not.

" e nonna i e &ecie ha mf l?

Mar' .agoff directs the Institute for Philoso&hy and Pu#lic Policy at the University of Maryland. He has &u#lished widely in law! &hiloso&hy! and environmental issues. His wor' has a&&eared in Orion and tlantic Mont!l . His most recent #oo' is Price" Principle" and t!e Environment 8) am#ridge University Press! *++;9. In ,CC,! .agoff was named a Pew .cholar in ) onservation and the -nvironment! and in ,CC? he was awarded a fellowshi& at the 4oodrow 4ilson International ) enter for .cholars. He #elieves that ecology is a historical! not a theoretical! science.

By Mar' .agoff

2hat de&ends on your &ers&ective

2hat nonnative s&ecies harm the natural environment is a dictum so often re&eated that one may assume it rests on evidence. It does not. Biologists often use &e@orative terms such as F&ollute!G Fmeltdown!G Fharm!G Fdestroy!G Fdisru&t!G and FdegradeG when s&ea'ing a#out nonnative s&ecies. 2hese words! along with meta&hors #orrowed from war and from cancer &ac' &olitical &unch. Insofar as they convey aesthetic! moral! or s&iritual @udgments! they have a &lace in &olitical de#ates and &olicy discussions. 4hat trou#les me as a &hiloso&her is that these value(laden terms and their under(lying conce&ts &ervade the scientific literature of conservation #iology and invasion
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ecology. 2hese conce&ts are not defined5 general(i6ations #ased on them are not tested. Indeed! if you try to &rove that invasive s&ecies harm natural environments! you ll find yourself in a scientific ma6e of dead ends and circular logic. 2hose who call for additional resources to fight nonnative s&ecies ty&ically &oint to e am&les such as the 6e#ra mussel! &ur&le loosestrife! and honeysuc'le! which they say have had costly and disru&tive effects. - am&les can shore u& nearly any thesis5 that is why e am&les are not arguments. Bather than draw general conclusions from &reselected and #iased e am&les! as the literature often does! we should loo' to scientific studies that consider a sam&le of s&ecies or sites selected at random. -cologists have not shown that nonnative s&ecies! once esta#lished! #ehave differently than native ones. /or e am&le! the stri&ed #ass introduced from the )hesa&ea'e is the most a#undant game fish in the .acramento(.an Joa$uin estuary. Is there anything a#out the stri&ed #ass that suggests its &rovenance5 is there anything a#out it that indicates how long it has #een there? )an one tell from ins&ecting these creatures or these systems whether the stri&er went east or west? 2o tell whether a s&ecies is native or alien! ecologists rely on historical and &aleoecological

evidence. No study demonstrates that alien and native s&ecies or FheirloomG and FinvadedG ecosystems are distinguisha#le in the way they loo' or #ehave. In other words! if on the #asis of historical evidence one grou& of ecologists lists those s&ecies esta#lished at a site that are native and those that are alien! a second grou& of ecologists having no 'nowledge of the historical facts could not tell which list was which sim&ly #y o#serving the interaction of the creatures living in the system. Moreover! for even the most villainous sus&ects! there are two sides to the story. -utro&hication has &lagued the 0reat =a'es for decades. But #y filtering &hyto&lan'ton and other sus&ended material from the water column! the nonnative 6e#ra mussel has hel&ed clean u& =a'e -rie and other &arts of the 0reat =a'es! the Hudson Biver! and many more a$uatic environments. 2hese mussels are much more efficient at filtration than their native counter&arts. Many #irds feed on them! and the mussels e crement &rovides ha#itat for a food chain anchoring a great diversity of s&ecies. Biologists credit the 6e#ra mussel with restoring native grasses and fishes. 4ere it native! the 6e#ra mussel would #e hailed as a savior! not reviled as a scourge. )ommentators often refer to the costs of controlling a s&ecies as a measure of the harm it does. )ontrol costs may #e used as a measure of harm when &eo&le s&end their own money. However! the connection #etween #enefits and costs when a governmental agency s&ends other &eo&les money is less a&&arent. 0overnment agencies may see' huge #udgets for invasive s&ecies &rograms and may then cite these as Fcontrol costsG to @ustify the e &ense. 2he U... /orest .ervice s&ent #illions to fight forest fires! to the detriment of the health of forests. - &erience has shown that the costs of government &rograms are not relia#le measures of their #enefits. -stimates of the economic costs of an invasive s&ecies such as the 6e#ra mussel3it clogs water inta'e valves and filtration &lants3differ wildly. " ,CC: U... )ongressional %ffice of 2echnology "ssessment re&ort &egged damage and control costs at a#out U.H:++!+++ &er year 8,9! whereas in *++> #iologist David Pimentel referred to an u&dated estimate of U.H, #illion &er year 8*9. Munici&alities and industries have had to retrofit facilities to ma'e them mussel(com&ati#le. It has #een a costly one(time endeavor. But it may #e a #argain in view of what might have #een s&ent to save affected a$uatic ecosystems from eutro&hication over time. 2hroughout history! anti(immigration activists have su&&orted their eno&ho#ia with e am&les of individual immigrants who de&end on welfare or commit crimes. -cologists who see' &u#lic funds to e clude or eradicate nonnative s&ecies attri#ute to them the same disre&uta#le $ualities that eno&ho#es have associated with immigrants3for e am&le! uncontrolled fecundity and aggressive #ehavior. 2he &e@orative stereoty&ing of newcomers may #e no more a&&ro&riate in the ecological than in the social conte t. 2he 6e#ra mussel has s&read widely! #ut this suggests only that it

found a niche to occu&y! not that it FharmedG the environment in a scientifically defina#le and testa#le way. -cologists worry that FweedyG s&ecies will s&read around the world5 #ut what is wrong with that! as long as the immigrants generally add to local s&ecies richness or diversity? %ne might give aesthetic! moral! and s&iritual reasons for 'ee&ing s&ecies close to
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where Noah dro&&ed them off. /rom a scientific &ers&ective! however! what defines weediness? "re certain s&ecies weeds #ecause they succeed glo#ally! as do .tar#uc's and 2aco Bell? =iterature citedA ,. %ffice of 2echnology "ssessment. ,CC:. Harmful Non(Indigenous .&ecies in the United .tates. 4ashington D.).! U... )ongress %ffice of 2echnology "ssessment. *. Pimentel! D.! B. <uniga and D. Morrison. *++>. U&date on the environmental and economic costs associated with alien(invasive s&ecies in the United .tates. Ecological Economic >*8:9A *D:(*??.