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Spa­tial mea­sure­ments of pop­u­la­tion den­sity could reveal when threat­ened nat­ural pop­u­la­tions are in dan­ger of crash­ing.

Many fac­tors — includ­ing cli­mate change, over­fish­ing or loss of food sup­ply — can push a wild ani­mal pop­u­la­tion to the brink of col­lapse. Ecol­o­gists have long sought ways to mea­sure the risk of such a col­lapse, which could help wildlife and fish­ery man­agers take steps to pro­tect endan­gered pop­u­la­tions.

Last year, Mass­a­chu­setts Insti­tute of Tech­nol­ogy (MIT) physi­cists demon­strated that they could mea­sure a population’s risk of col­lapse by mon­i­tor­ing how fast it recov­ers from small dis­tur­bances, such as a food short­age or over­crowd­ing. How­ever, this strat­egy would likely require many years of data col­lec­tion — by which time it could be too late to save the pop­u­la­tion.

In a paper pub­lished online in the April 10 edi­tion of Nature, the same research team describes a new way to pre­dict the risk of col­lapse, based on vari­a­tions in pop­u­la­tion den­sity in neigh­bour­ing regions. Such infor­ma­tion is eas­ier to obtain than data on pop­u­la­tion fluc­tu­a­tions over time, mak­ing it poten­tially more use­ful, accord­ing to the researchers.

Lei Dai, lead author, MIT grad­u­ate stu­dent in physics »

Led by Jeff Gore, an assis­tant pro­fes­sor of physics, Dai and Kir­ill Korolev, a Pap­palardo Post­doc­toral Fel­low, grew yeast in test tubes and tracked the pop­u­la­tions as they approached col­lapse. Yeast cells coop­er­ate with other mem­bers of the pop­u­la­tion: Each of the organ­isms secretes an enzyme that breaks down sucrose in the envi­ron­ment into smaller sug­ars that it can use as a food source. All of the yeast ben­e­fit from this process, so a pop­u­la­tion is most suc­cess­ful when it main­tains a cer­tain den­sity — nei­ther too low nor too high.

In last year’s study, the researchers found that in pop­u­la­tions of yeast that are sub­jected to increas­ingly stress­ful con­di­tions, pop­u­la­tions become less and less resilient to new dis­tur­bances until they reach a tip­ping point at which any small dis­rup­tion could wipe out a pop­u­la­tion.

This phe­nom­e­non can be spot­ted quickly in yeast, which pro­duces about 10 new gen­er­a­tions per day, but mea­sur­ing these pop­u­la­tion fluc­tu­a­tions for species such as fish or deer would take much more time. In hopes of find­ing more use­ful sig­nals, the researchers turned their atten­tion to spa­tial infor­ma­tion.

There goes the neigh­bour­hood
In their new study, the researchers the­o­rised a new type of indi­ca­tor that they call “recov­ery length” — the spa­tial coun­ter­part to recov­ery time. This idea is based on the obser­va­tion that pop­u­la­tions liv­ing near the bound­ary of a less hos­pitable habi­tat are affected, because the neigh­bour­ing habi­tats are con­nected by migra­tion. Pop­u­la­tions fur­ther away from the bad region grad­u­ally recover to equi­lib­rium, and the spa­tial scale of this recov­ery can reveal a population’s sus­cep­ti­bil­ity to col­lapse, accord­ing to the researchers.

To test this idea, the researchers first estab­lished sev­eral linked yeast pop­u­la­tions in a state of equi­lib­rium. At the end of each day, a cer­tain per­cent­age of each pop­u­la­tion was trans­ferred to adja­cent test tubes, rep­re­sent­ing migra­tion to adja­cent regions. The researchers then intro­duced a “bad” habi­tat, where only one in every 2,500 yeast sur­vives from one day to the next. This reduc­tion in pop­u­la­tion mim­ics what might hap­pen in a nat­ural pop­u­la­tion plagued by over­fish­ing, or by a dras­tic reduc­tion in its food sup­ply.

The MIT team found that pop­u­la­tions clos­est to the bad habi­tat had the hard­est time main­tain­ing an equi­lib­rium state. Pop­u­la­tions far­ther away main­tained their equi­lib­rium more eas­ily.

“There’s some dis­tance you have to go away from the bad region in order to get recov­ery of the pop­u­la­tion den­sity,” Gore says. “How far you have to go before you reach equi­lib­rium is the recov­ery length, and that tells you how close these pop­u­la­tions are to col­lapse.”

The recov­ery length varies based on how much stress the pop­u­la­tions are already under. To apply this find­ing to a nat­ural pop­u­la­tion, pop­u­la­tion den­sity would need to be mea­sured in a range of adja­cent areas at increas­ing dis­tances from a good/​bad bound­ary. This infor­ma­tion could then be mapped to reveal the recov­ery length. “What’s great about the recov­ery length is you don’t need a long time series. You could just mea­sure it at one moment in time,” Gore says.

The MIT researchers are hop­ing to expand their stud­ies to nat­ural pop­u­la­tions such as hon­ey­bees, fish­eries or forests. They are also study­ing more com­plex exper­i­men­tal ecosys­tems involv­ing sev­eral micro­bial species.


(Source: MIT press release, 10.04.2013)