AboutZoos, Since 2008


Trop­ics are main source of global mam­mal diversity

pub­lished 01 Feb­ru­ary 2014 | mod­i­fied 25 Decem­ber 2014

Ever since the nine­teenth cen­tury sci­en­tists have recog­nised that some regions con­tain more species than oth­ers, and that the trop­ics are richer in bio­di­ver­sity than tem­per­ate regions. But why are there more species in the tropics?

Species diversity tropicsA new study pub­lished on 28 Jan­u­ary in the jour­nal PLOS Biol­ogy scru­ti­nises most of the liv­ing mam­malian species and reveals a two-​fold mech­a­nism. The researchers show that in the trop­ics the rate at which mam­mals arose was higher, while at the same time the rate at which they became extinct was lower. They also pro­pose that the trop­ics have been a con­tin­u­ous source of diver­sity that allowed repeated coloni­sa­tion of the tem­per­ate regions.

French researchers Jonathan Rol­land, Fabien Con­damine, Frédéric Jiguet and Hélèné Mor­lon (École Poly­tech­nique, CNRS and the MNHN), applied math­e­mat­i­cal mod­els to world­wide mam­malian datasets to address a ques­tion that has fas­ci­nated ecol­o­gists and evo­lu­tion­ary biol­o­gists for decades, gen­er­at­ing scores of hypotheses.

One of the main hypothe­ses argues that species have diver­si­fied more in the trop­ics than in tem­per­ate regions — diver­si­fi­ca­tion is the dif­fer­ence between the rates at which new species emerge and go extinct. How­ever, recent pub­li­ca­tions have shown no link between diver­si­fi­ca­tion rate and lat­i­tude, sug­gest­ing that diver­si­fi­ca­tion may not dif­fer between the trop­ics and tem­per­ate regions. Indeed, because the Earth was largely trop­i­cal 80 mil­lion years ago, the trop­ics may be richer merely because trop­i­cal lin­eages have had more time to diver­sify than tem­per­ate ones.

Com­bin­ing the tree of the rela­tion­ships between the 5,000 mam­mal species with lat­i­tude data, the researchers esti­mated spe­ci­a­tion (the rate at which new species emerge), extinc­tion, and species migra­tion asso­ci­ated with mam­mals liv­ing in trop­i­cal and tem­per­ate regions. Con­trary to what has been sug­gested before, they found that diver­si­fi­ca­tion rates are strik­ingly con­sis­tent with cur­rent diver­sity pat­terns. Lat­i­tu­di­nal peaks in species rich­ness are asso­ci­ated with high spe­ci­a­tion rates, low extinc­tion rates, or both, depend­ing on which mam­malian order you look at, such as rodents, bats and primates.

They also found evi­dence that the migra­tion of species through the ages has been asym­met­ri­cal, with more expan­sion ”out of” the trop­ics than ”into” them. Taken together, these results sug­gest that trop­i­cal regions are not only a reser­voir of bio­di­ver­sity, but also the main place where bio­di­ver­sity has been, and pre­sum­ably is being, generated.

This study shows that math­e­mat­i­cal mod­els can now detect the imprint of trop­i­cal ver­sus tem­per­ate spe­ci­a­tion and extinc­tion on the tree of life, open­ing new per­spec­tives in evo­lu­tion­ary research. It also allows us to assess old hypothe­ses and put diver­si­fi­ca­tion back in the spot­light as a major con­trib­u­tor to the well-​known trop­i­cal abun­dance of mam­mal species. Fur­ther research should now focus on the direct causes of these dif­fer­ences in diver­si­fi­ca­tion, such as tem­per­a­ture or pre­cip­i­ta­tion, that may also impact mam­mal diversification.

(Source: École Poly­tech­nique press release, 28.01.2014)

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