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Tem­per­a­ture dri­ves biodiversity

pub­lished 28 Decem­ber 2016 | mod­i­fied 28 Decem­ber 2016

Mt KilimanjaroWhy is the diver­sity of ani­mals and plants so unevenly dis­trib­uted on our planet? Recently, new data has been pro­vided on this core issue of ecol­ogy. They show bio­di­ver­sity to be dri­ven by temperature.

The diver­sity of plants and ani­mals in Earth’s arc­tic regions is mod­er­ate. Trop­i­cal lat­i­tudes in con­trast are teem­ing with dif­fer­ent species where new organ­isms are being dis­cov­ered all the time.

What is the cause of this uneven dis­tri­b­u­tion? Why are the trop­ics home to more species than higher lat­i­tudes? “This ques­tion has intrigued ecol­o­gists for some time,” says Pro­fes­sor Ingolf Steffan-​Dewenter from the Uni­ver­sity of Würzburg’s Bio­cen­ter. “Already about ten years ago, the pub­lish­ers of Sci­ence declared this to be one of the 25 most impor­tant ques­tions of sci­ence to be answered yet.”

Sev­eral hypothe­ses cir­cu­lat­ing
To date, this core ques­tion has been sub­ject to con­tro­versy. One hypoth­e­sis, for instance, is that the pri­mary pro­duc­tiv­ity of a habi­tat is ulti­mately deci­sive for the num­ber of species liv­ing there. Sim­ply put: “A larger cake can sus­tain more species than a small one,” explains Würzburg ecol­o­gist Dr. Mar­cell Peters. Another hypoth­e­sis assumes that the rate of evo­lu­tion and spe­ci­a­tion depend on tem­per­a­ture. Accord­ing to this assump­tion, more species thrive in a warmer cli­mate than in a cold one.

The more groups of ani­mals and plants you inves­ti­gate in par­al­lel, the greater the sig­nif­i­cance of tem­per­a­ture for explain­ing bio­di­ver­sity, whereas the impor­tance of all other vari­ables decreases accordingly
Mar­cell K. Peters, Depart­ment of Ani­mal Ecol­ogy and Trop­i­cal Biol­ogy, Bio­cen­ter, Uni­ver­sity of Würzburg, Germany »

So far, these hypothe­ses have been exam­ined usu­ally by focus­ing on selected groups of species: For exam­ple, the stud­ies observed only birds, bees, ants or ferns and analysed their diver­sity in dif­fer­ent regions of the world, e.g. in North Amer­ica, Europe or along ele­va­tional gra­di­ents in the Alps. “Some stud­ies sup­ported one hypoth­e­sis, whereas oth­ers backed another assump­tion,” Peters says and states that it is still a long way from estab­lish­ing a “gen­eral rule” which ecol­o­gists are aim­ing for.

Unique study con­ducted at Mount Kil­i­man­jaro
In 22 Decem­ber online issue of the jour­nal Nature Com­mu­ni­ca­tions Peters and the team of the Research Unit “FOR1246” funded by the Ger­man Research Foun­da­tion (Deutsche Forschungs­ge­mein­schaft, DFG) now present a new and unique study that is the syn­the­sis of four years work: “On Mount Kil­i­man­jaro, one of Earth’s largest cli­matic gra­di­ents, we observed so many ani­mal and plant groups in par­al­lel as never before,” the researcher says.

Over­all, the team exam­ined eight groups of plants and 17 groups of ani­mals, from bees to bats. 38 sci­en­tists from Ger­many, Tan­za­nia and other coun­tries par­tic­i­pated in the large-​scale study. They were sup­ported by around 50 local dri­vers, car­ri­ers and other assis­tants. “We had to climb in moun­tain­ous areas for sev­eral days to reach the high­est study sites,” Peters says.

The area of study stretched from the savan­nahs at the foot of the moun­tain to the habi­tats at an alti­tude of 4,550 metres that barely sus­tain plants. The data across all groups were col­lected over the same areas and in the same period of time, respec­tively. “This approach allowed us to not only analyse the bio­di­ver­sity of each indi­vid­ual group, but also that of whole communities.”

Diver­sity increases with tem­per­a­ture
The study revealed that bio­di­ver­sity in com­mu­ni­ties is mainly deter­mined by tem­per­a­ture. The warmer it is, the greater the diver­sity. “The more groups of ani­mals and plants you inves­ti­gate in par­al­lel, the greater the sig­nif­i­cance of tem­per­a­ture for explain­ing bio­di­ver­sity, whereas the impor­tance of all other vari­ables decreases accordingly.”

Species richness at different elevations of Mt. KilimanjaroSpecies rich­ness of vas­cu­lar plants (a) and ani­mals © along the ele­va­tional gra­di­ent of Mt. Kil­i­man­jaro. Lower pan­els show trend lines for the num­ber of species, fam­i­lies and orders of vas­cu­lar plants (b) and ani­mals (d) along the ele­va­tional gra­di­ent. All trend lines were cal­cu­lated using gen­er­al­ized addi­tive mod­els (N=30 and N=29 in plants and ani­mals, respec­tively; all trends were sig­nif­i­cant at P<0.001).
The plant and ani­mal images used in the fig­ure are licensed for use in the Pub­lic Domain with­out copy­right, except for the images used for mono­cots (F. & K. Starr, mod­i­fied), bats (O. Peles and Y. Wong), Collem­bola, ground-​dwelling bee­tles (B. Lang), Orthoptera, other aculeate wasps, bees (M. Menchetti), gas­tropods (G. Mon­ger) that are licenced under a Cre­ative Com­mons Attri­bu­tion 3.0 Unported licence. Oliver Niehuis kindly pro­vided the per­mis­sion to use the image of the par­a­sitoid wasp.
Source: Peters et al., 2016. Pre­dic­tors of ele­va­tional bio­di­ver­sity gra­di­ents change from sin­gle taxa to the multi-​taxa com­mu­nity level in Nature Communications.

The sci­en­tists believe that this is strong evi­dence sup­port­ing the assump­tion that tem­per­a­ture is actu­ally more deci­sive for dis­tri­b­u­tion pat­terns of over­all bio­di­ver­sity than pro­duc­tiv­ity or size of habitats.

(Source: Uni­ver­sität Würzburg press release, 22.12.2016)

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