AboutZoos, Since 2008


Ecol­ogy buys time for evolution

pub­lished 27 April 2013 | mod­i­fied 19 March 2014

Great tit caterpillarCli­mate change dis­rupts songbird’s tim­ing with­out impact­ing pop­u­la­tion size (yet).

Song­bird pop­u­la­tions can han­dle far more dis­rupt­ing cli­mate change than expected. Density-​dependent processes are buy­ing them time for their bat­tle. But with­out (slow) evo­lu­tion­ary res­cue it will not save them in the end, says an inter­na­tional team of sci­en­tists led by the Nether­lands Insti­tute of Ecol­ogy (NIOO-​KNAW) in Sci­ence this week — pub­lished on 26 April.

Yes, spring started late this year in North-​western Europe. But the gen­eral trend of the four last decades is still a rapidly advanc­ing spring. The sea­sonal tim­ing of trees and insects advance too, but song­birds like the great tit (Parus major), lag behind. Yet with­out an accom­pa­ny­ing decline in pop­u­la­tion num­bers, it seems, as the inter­na­tional research team shows for the great tit pop­u­la­tion in the Dutch National Park the Hoge Veluwe.


It’s a real para­dox,” explain Dr Tom Reed and Prof Mar­cel Visser of the Nether­lands Insti­tute of Ecol­ogy. “Due to the chang­ing cli­mate of the past decades the egg lay­ing dates of the great tit have become increas­ingly mis­matched with the tim­ing of the main food source for its chicks: cater­pil­lars. The sea­sonal tim­ing of the food peak has advanced over twice as fast as that of the birds and the repro­duc­tive out­put is reduced. Still, the pop­u­la­tion num­bers do not go down.” On the short term, that is, as Reed, Visser and col­leagues from Nor­way, the USA, and France have now cal­cu­lated using almost 40 years of data from this songbird.

Relaxed com­pe­ti­tion

The den­sity depen­dence is only buy­ing the birds time, hope­fully for evo­lu­tion­ary adap­ta­tion to dig in before pop­u­la­tion num­bers are sub­stan­tially affected
Prof Mar­cel Visser, Nether­lands Insti­tute of Ecology »

The solu­tion to the para­dox is that although fewer off­spring now fledge due to food short­age, each of these chicks has a higher chance of sur­vival until the next breed­ing sea­son. “We call this relaxed com­pe­ti­tion, as there are fewer fledg­lings to com­pete with,” first author Reed points out. Out of 10 eggs laid, 9 chicks are born, 7 fledge and on aver­age only one chick sur­vives win­ter. That last num­ber increases with less com­peti­tors around.

This is the first time that den­sity depen­dence — a wide­spread phe­nom­e­non in nature — and eco­log­i­cal mis­match are linked, and it is a real eye-​opener. Reed: “It all seems so obvi­ous once you’ve cal­cu­lated this, but peo­ple were almost sure that mist­im­ing would lead to a direct pop­u­la­tion decline.”

Lim­ited flexibility

The great tits that lay eggs ear­lier in spring are more suc­cess­ful nowa­days than late birds, which pro­duce rel­a­tively few sur­viv­ing off­spring. This leads to increas­ing selec­tion for birds to repro­duce early. But the total num­ber of birds in the new gen­er­a­tion stays the same. “That is the sec­ond para­dox,” the researchers state. “Why are pop­u­la­tion num­bers hardly affected, despite the stronger selec­tion on tim­ing caused by the mis­match? The answer is that for selec­tion it mat­ters which birds sur­vive, while for pop­u­la­tion size it only mat­ters how many sur­vive. Visser: “The mor­tal­ity in one group can be com­pen­sated for by the suc­cess in another. But this stretch­ing, this flex­i­bil­ity, is not unlimited.”

The mis­match between egg lay­ing period and cater­pil­lar peak in the woods will keep grow­ing, and so will the impact fol­low­ing the tem­po­rary res­cue, as long as spring tem­per­a­tures con­tinue to increase. “The den­sity depen­dence is only buy­ing the birds time, hope­fully for evo­lu­tion­ary adap­ta­tion to dig in before pop­u­la­tion num­bers are sub­stan­tially affected,” accord­ing to Visser. The new find­ings can help to pre­dict the impact of future envi­ron­men­tal change on other wild pop­u­la­tions and to iden­tify rel­e­vant mea­sures to take. Even rub­ber bands stretch only so far before they break.

(Source: Nether­lands Insti­tute of Ecol­ogy press release, 25.04.2013)

UN Biodiversity decade
WWF Stop Wildlife Crime
Fight for Flight campaign
End Ivory-funded Terrorism
Support Rewilding Europe
NASA State of Flux

Goal: 7000 tigers in the wild

Tiger range countries map

Tiger map” (CC BY 2.5) by Sander­son et al., 2006.

about zoos and their mis­sion regard­ing breed­ing endan­gered species, nature con­ser­va­tion, bio­di­ver­sity and edu­ca­tion, which of course relates to the evo­lu­tion of species.
Fol­low me on: