AboutZoos, Since 2008


Com­pe­ti­tion for eco­log­i­cal niches lim­its the for­ma­tion of new species

pub­lished 15 May 2014 | mod­i­fied 15 May 2014

The rate at which new species evolve is lim­ited by com­pe­ti­tion for eco­log­i­cal niches, report sci­en­tists from the Uni­ver­sity of Chicago in Nature. The study, which analy­ses the evo­lu­tion­ary and genetic rela­tion­ships between all 461 song­bird species that live in the Himalayan moun­tains, sug­gests that as eco­log­i­cal niches within an envi­ron­ment are filled, the for­ma­tion of new species slows or even stops.

Liocichla bugunorumTo study what con­trols the process of spe­ci­a­tion, Trevor Price, PhD, pro­fes­sor of ecol­ogy and evo­lu­tion at the Uni­ver­sity of Chicago, Dhanan­jai Mohan of the Indian For­est ser­vice and their col­leagues looked at song­birds in the east­ern Himalayas, a region which con­tains the great­est diver­sity of song­birds in the world. Thought to have orig­i­nated from a sin­gle species around 50 mil­lion years ago, the song­bird sub­or­der – which includes swal­lows, war­blers, finches and crows – con­tains more than 5,000 species, occu­pies a wide range of cli­mates and pos­sesses extreme vari­a­tions in body mass, shape and feed­ing adaptations.

The team col­lected and sequenced DNA from all 461 species of Himalayan song­birds – includ­ing ultra-​rare species such as the Bugun lio­ci­chla (Lio­ci­chla buguno­rum) listed as Vul­ner­a­ble by the IUCN Red List of Threat­ened Species. Of these species, 358 breed within a 10,000 square kilo­me­ter area, offer­ing the abil­ity to com­pare the dif­fer­ence between species in the area and those else­where in the Himalayas. The team then cre­ated a molecular-​based phy­lo­ge­netic tree that detailed the evo­lu­tion­ary rela­tion­ships between all the species. Their find­ings have been pub­lished online on 30 April in the sci­en­tific jour­nal Nature.

Based on genetic infor­ma­tion, the researchers dis­cov­ered that east­ern Himalayan song­birds are, on aver­age, sep­a­rated from each other by six to seven mil­lion years – roughly the same amount of time that humans and chim­panzees have been separated.

There is lit­tle room for more species because niches are increas­ingly occupied
Trevor Price, pro­fes­sor of ecol­ogy and evo­lu­tion, the Uni­ver­sity of Chicago »

“Despite the great diver­sity of envi­ron­ments and abil­ity for species to move between areas, evo­lu­tion in east­ern Himalayas appears to have slowed to a basic halt,” Price said. “Other species have formed else­where, such as in China and Siberia, but most have been unable to spread into this region.”

The researchers attribute this slow­ing of evo­lu­tion to the fill­ing of eco­log­i­cal niches, or exploitable habi­tats or resources for new species to adapt to. The for­ma­tion of new species is usu­ally thought to involve three steps. First, a species expands across an envi­ron­men­tal range. Then a bar­rier, such as cli­mate change or a geo­graphic event, causes the species to sep­a­rate into dis­tinct pop­u­la­tions. Lastly, the devel­op­ment of repro­duc­tive iso­la­tion – the inabil­ity to inter­breed – finalises the spe­ci­a­tion process. This cycle then repeats, cre­at­ing the breadth of diver­sity seen in nature.

Price and his col­leagues argue that the expan­sion of a range can­not occur if there are no eco­log­i­cal niches for a species to expand into. Despite the abil­ity of birds to fly and cross geo­graphic bar­ri­ers, they can­not per­sist in regions where they are out­com­peted by exist­ing species who occupy avail­able niches. In the east­ern Himalayas, the researchers found evi­dence of this in numer­ous dif­fer­ences in feed­ing method and body size that appeared early in the evo­lu­tion­ary his­tory of song­birds. Less dra­matic eco­log­i­cal dif­fer­ences, such as liv­ing at dif­fer­ing ele­va­tions, appeared to form later as the ini­tial adap­tive radi­a­tion slowed.

“Our argu­ment is that niche fill­ing has stopped species from get­ting big ranges,” Price said. “In the east­ern Himalayas, it has become harder and harder for new species to get into that sys­tem, and we are quite close to the max­i­mum num­ber of species that can be accom­mo­dated. There is lit­tle room for more species because niches are increas­ingly occupied.”

This model for diver­si­fi­ca­tion stands in stark con­trast to pre­vi­ous hypothe­ses, many of which have focused on the slow devel­op­ment of repro­duc­tive iso­la­tion as the lim­it­ing factor.

In addi­tion, the researchers dis­cov­ered that the great­est diver­sity of song­bird species were located at around a 2,000 meter ele­va­tion – a more tem­per­ate region com­pared to the trop­i­cal for­est below. They plan to fur­ther study this phe­nom­e­non on return expeditions.

“It’s impor­tant to realise just how old and how much incred­i­ble genetic diver­sity are in these moun­tain­ous forests,” Price adds. “All these species man­aged to deal with warm­ing, glacia­tion and cool­ing with­out chang­ing very much. It’s quite amaz­ing to me to think that in the next 100 years a lot of these may be gone, when they man­aged to get through the last 6 mil­lion years.”

(Source: The Uni­ver­sity of Chicago Med­i­cine press release, 30.04.2014)

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