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201314Aug20:53

Ecosys­tems change long before species are lost

Infor­ma­tion
pub­lished 14 August 2013 | mod­i­fied 28 June 2014
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Com­mu­ni­ties in nature are likely to be a lot more sen­si­tive to change than pre­vi­ously thought, accord­ing to a new study at Rice University.

DragonflyThe study, which is pub­lished on 12 August in Nature Com­mu­ni­ca­tions, shows that sci­en­tists con­cerned about human influ­ence on the bios­phere need to take a deeper look at how alter­ing the dynam­ics of a pop­u­la­tion — for exam­ple, by remov­ing large mem­bers of a species through over­fish­ing — can have mea­sur­able con­se­quences, said Rice ecol­o­gist Volker Rudolf.

“Nat­ural com­mu­ni­ties are increas­ingly altered through human impact, and ecol­o­gists have long strived to deter­mine how these changes influ­ence com­mu­ni­ties,” Rudolf said. He noted the dis­ap­pear­ance of a species is the most extreme but not the only cause of bio­di­ver­sity loss.

That’s the last thing that hap­pens after you mess up the entire ecosys­tem for a long period of time
Volker Rudolf, lead author, ecol­o­gist, Rice University »

By then, changes forced upon the struc­ture of a pop­u­la­tion — such as the ratio of young to old in a species — have already been felt up and down the food chain. Rudolf sus­pected species play var­i­ous roles and their effects on the envi­ron­ment change as they progress through their life cycles, to the degree that alter­ing these life “stages” within a species could have a sig­nif­i­cant impact. He and Rice grad­u­ate stu­dent Nick Ras­mussen made a con­sid­er­able effort to prove it.

For the painstak­ing exper­i­ments that started in 2009, Rudolf, Ras­mussen and their col­leagues chose drag­on­flies and water-​diving bee­tles to rep­re­sent species that have major impact on their respec­tive com­mu­ni­ties — in this case, fish­less ponds — and then cre­ated dozens of minia­ture envi­ron­ments to ana­lyze that impact. Manip­u­lat­ing the pres­ence of dif­fer­ent devel­op­men­tal stages within a preda­tor species in each pond helped the researchers deter­mine that such changes did alter the dynam­ics of com­plex ecosys­tems in a mea­sur­able way.

“Other than being the largest and most vora­cious preda­tors in these com­mu­ni­ties, they’re totally dif­fer­ent,” Rudolf said of the top preda­tors. “We fig­ured if we saw any gen­er­al­i­ties across these two species, then there’s some­thing to our theory.”

They found that alter­ing which classes of size were present in a pop­u­la­tion also altered the struc­ture of the entire com­mu­nity and ulti­mately how the whole ecosys­tem func­tioned. Also impor­tant, Rudolf said, was that chang­ing the struc­ture of pop­u­la­tions some­times had big­ger effects on the ecosys­tem than chang­ing the preda­tor species. The results, he said, “chal­lenge clas­si­cal assump­tions and stud­ies that say we can make pre­dic­tions by assum­ing that all indi­vid­u­als of a species are the same. You don’t expect a tod­dler to do the same thing as a grownup, and the same is the case for animals.”

The study could also explain why such human activ­i­ties as size-​selective har­vest­ing can alter the struc­ture of entire food webs in some ocean sys­tems, even when no species had gone extinct and the total bio­mass of the tar­geted fish remained the same, he said.

“While these changes would be hard to pre­dict by the clas­si­cal approach, our results sug­gests such changes are expected when human activ­i­ties alter the pop­u­la­tion struc­ture of key­stone species in an ecosys­tem,” Rudolf said. “Thus, nat­ural ecosys­tems are likely to be much more frag­ile then we pre­vi­ously thought.”

(Source: Rice Uni­ver­sity news release, 12.08.2013)

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