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Amphib­ian Study shows how bio­di­ver­sity can pro­tect against disease

pub­lished 20 Feb­ru­ary 2013 | mod­i­fied 20 Feb­ru­ary 2013
Pacific treefrog deformitiesThe richer the assort­ment of amphib­ian species liv­ing in a pond, the more pro­tec­tion that com­mu­nity of frogs, toads and sala­man­ders has against a par­a­sitic infec­tion that can cause severe defor­mi­ties, includ­ing the growth of extra legs, accord­ing to a new study by the Uni­ver­sity of Col­orado Boul­der (CU-​Boulder).

The find­ings, pub­lished Feb­ru­ary 14 in the jour­nal Nature, sup­port the idea that greater bio­di­ver­sity in larger-​scale ecosys­tems, such as forests or grass­lands, may also pro­vide greater pro­tec­tion against dis­eases, includ­ing those that attack humans. For exam­ple, a larger num­ber of mam­mal species in an area may curb cases of Lyme dis­ease, while a larger num­ber of bird species may slow the spread of West Nile virus.

How bio­di­ver­sity affects the risk of infec­tious dis­eases, includ­ing those of humans and wildlife, has become an increas­ingly impor­tant ques­tion. But as it turns out, solidly test­ing these link­ages with real­is­tic exper­i­ments has proven very chal­leng­ing in most systems.
(Pieter John­son, lead author, assis­tant pro­fes­sor in the Depart­ment of Ecol­ogy and Evo­lu­tion­ary Biol­ogy)

Researchers have strug­gled to design com­pre­hen­sive stud­ies that could illu­mi­nate the pos­si­ble con­nec­tion between dis­ease trans­mis­sion and the num­ber of species liv­ing in com­plex ecosys­tems. Part of the prob­lem is sim­ply the enor­mous num­ber of organ­isms that may need to be sam­pled and the vast areas over which those organ­isms may roam.

The new CU-​Boulder study over­comes that prob­lem by study­ing smaller, easier-​to-​sample ecosys­tems. John­son and his team vis­ited hun­dreds of ponds in Cal­i­for­nia, record­ing the types of amphib­ians liv­ing there as well as the num­ber of snails infected by the pathogen
Ribeiroia onda­trae. Snails are an inter­me­di­ate host used by the par­a­site dur­ing part of its life cycle.

“One of the great chal­lenges in study­ing the diversity-​disease link has been col­lect­ing data from enough repli­cate sys­tems to dif­fer­en­ti­ate the influ­ence of diver­sity from back­ground ‘noise,’ ” John­son said. “By col­lect­ing data from hun­dreds of ponds and thou­sands of amphib­ian hosts, our group was able to pro­vide a rig­or­ous test of this hypoth­e­sis, which has rel­e­vance to a wide range of dis­ease sys­tems.”

Johnson’s team but­tressed its field obser­va­tions both with lab­o­ra­tory tests designed to mea­sure how prone to infec­tion each amphib­ian species is and by cre­at­ing pond repli­cas out­side using large plas­tic tubs stocked with tad­poles that were exposed to a known num­ber of par­a­sites. All of the exper­i­ments told the same story, John­son said. Greater bio­di­ver­sity reduced the num­ber of suc­cess­ful amphib­ian infec­tions and the num­ber of deformed frogs.

In all, the CU-​Boulder researchers spent three years sam­pling 345 wet­lands and record­ing mal­for­ma­tions — which include miss­ing, mis­shapen or extra sets of hind legs — caused by par­a­sitic infec­tions in 24,215 amphib­ians. They also cat­a­logued 17,516 snails. The results showed that ponds with half a dozen amphib­ian species had a 78 per­cent reduc­tion in par­a­site trans­mis­sion com­pared to ponds with just one amphib­ian species. The research team also set up exper­i­ments in the lab and out­doors using 40 arti­fi­cial ponds, each stocked with 60 amphib­ians and 5,000 par­a­sites.

The rea­son for the decline in par­a­sitic infec­tions as bio­di­ver­sity increases is likely related to the fact that ponds add amphib­ian species in a pre­dictable pat­tern, with the first species to appear being the most prone to infec­tion and the later species to appear being the least prone. For exam­ple, the research team found that in a pond with just one type of amphib­ian, that amphib­ian was almost always the Pacific cho­rus frog, a crea­ture that is able to rapidly repro­duce and quickly colonise wet­land habi­tats, but which is also espe­cially vul­ner­a­ble to infec­tion and parasite-​induced defor­mi­ties.

On the other hand, the Cal­i­for­nia tiger sala­man­der was typ­i­cally one of the last species to be added to a pond com­mu­nity and also one of the most resis­tant to par­a­sitic infec­tion. There­fore, in a pond with greater bio­di­ver­sity, par­a­sites have a higher chance of encoun­ter­ing an amphib­ian that is resis­tant to infec­tion, low­er­ing the over­all suc­cess rate of trans­mis­sion between infected snails and amphib­ians.

This same pat­tern — of less diverse com­mu­ni­ties being made up of species that are more sus­cep­ti­ble to dis­ease infec­tion — may well play out in more com­plex ecosys­tems as well, John­son said. That’s because species that dis­perse quickly across ecosys­tems appear to trade off the abil­ity to quickly repro­duce with the abil­ity to develop dis­ease resistance.

This research reaches the sur­pris­ing con­clu­sion that the entire set of species in a com­mu­nity affects the sus­cep­ti­bil­ity to dis­ease. Bio­di­ver­sity matters.
Doug Levey, pro­gram direc­tor in the National Sci­ence Foundation’s Divi­sion of Envi­ron­men­tal Biol­ogy »

The sheer mag­ni­tude of the recent study also rein­forces the con­nec­tion between deformed frogs and par­a­sitic infec­tion, John­son said. Begin­ning in the mid-​1990s reports of frogs with extra, miss­ing or mis­shapen legs sky­rock­eted, attract­ing wide­spread atten­tion in the media and moti­vat­ing sci­en­tists to try to fig­ure out the cause. John­son was among the researchers who found evi­dence of a link between infec­tion with
Ribeiroia and frog defor­mi­ties, though the appar­ent rise in reports of defor­ma­tions, and its under­ly­ing cause, remains con­tro­ver­sial.

While the new study has impli­ca­tions beyond par­a­sitic infec­tions in amphib­ians, it does not mean that an increase in bio­di­ver­sity always results in a decrease in dis­ease, John­son cau­tioned. Other fac­tors also affect rates of dis­ease trans­mis­sion. For exam­ple, a large num­ber of mos­qui­toes hatch­ing in a par­tic­u­lar year will increase the risk of con­tract­ing West Nile virus, even if there has been an increase in the bio­di­ver­sity of the bird pop­u­la­tion. Birds act as “reser­voir hosts” for West Nile virus, har­bour­ing the pathogen indef­i­nitely with no ill effects and pass­ing the pathogen onto mos­qui­toes.

“Our results indi­cate that higher diver­sity reduces the suc­cess of pathogens in mov­ing between hosts,” John­son said. “Nonethe­less, if infec­tion pres­sure is high, for instance in a year with high abun­dance of vec­tors, there will still be a sig­nif­i­cant risk of dis­ease; bio­di­ver­sity will sim­ply func­tion to dampen trans­mis­sion suc­cess.”

(Source: Uni­ver­sity of Col­orado Boul­der news release, 13.02.2013)
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