3.5 tril­lion mi­grants with wings

Each year, in­sects weigh­ing a com­bined 3,200 tons of biomass fly over Eng­land in a great mi­gra­tion.

Los Angeles Times - - THE WORLD - By Deb­o­rah Net­burn deb­o­rah.net­burn @la­times.com

It’s one of the big­gest mass mi­gra­tions on Earth, and you prob­a­bly didn’t even know it was hap­pen­ing.

Each year, from May to Septem­ber, nearly 3.5 tril­lion in­sects tra­verse the skies above south­ern Eng­land, ac­cord­ing to a new study in the jour­nal Sci­ence.

To­gether these winged trav­el­ers make up 3,200 tons of biomass. That’s the mass equiv­a­lent of a herd of 800 fairly large ele­phants march­ing past the clouds.

And if you live in sun­nier climes, like South­ern Cal­i­for­nia, the num­ber and mass of in­sects fly­ing over your head each year are likely to be even greater.

“I be­lieve the num­bers in the south­ern U.K. are close to min­i­mum val­ues for the rest of the world,” said study au­thor Ja­son Chap­man, an en­to­mol­o­gist at the Univer­sity of Ex­eter in Corn­wall. “Al­most any­where I can think of will likely have much higher val­ues, es­pe­cially in the hot­ter parts of the world.”

Chap­man and his col­leagues say that if the den­si­ties they ob­served over south­ern Eng­land are in­deed sim­i­lar or larger over other parts of the planet, this in­sect mi­gra­tion could rep­re­sent the most im­por­tant an­nual an­i­mal move­ments on land.

Chap­man has been study­ing var­i­ous as­pects of high-al­ti­tude wind-borne in­sect mi­gra­tion for 15 years, but for most of that time his fo­cus has been on large noc­tur­nal species — specif­i­cally, large moths.

In this study, how­ever, he and his coau­thors tal­lied mi­grat­ing in­sects of all sizes, both day and night. All the in­sects in the study were fly­ing at least 500 feet off the ground (a bit higher than a 30-story build­ing).

To col­lect the data, the team used a spe­cial en­to­mo­log­i­cal radar that al­lowed them to de­ter­mine body mass, flight al­ti­tude and a host of other in­for­ma­tion about in­di­vid­ual in­sects with a mass of 10 mil­ligrams or more (about the size of a house­fly and big­ger).

The radar can­not ac­cu­rately mea­sure the num­ber of smaller in­sects, which are much more nu­mer­ous than the larger trav­el­ing bugs. To solve that prob­lem, the re­searchers used aerial net­ting to mon­i­tor the tini­est mi­gra­tors.

The au­thors found that 99% of the in­di­vid­ual in­sects that mi­grated through the study area were in the small­est group of in­sects (those smaller than a house­fly). These lit­tle crea­tures made up 80% of the to­tal mi­grat­ing biomass. The 15 bil­lion medium-sized in­sects and 1.5 bil­lion large in­sects were re­spon­si­ble for 12% and 7% of the mass, re­spec­tively.

In ad­di­tion, the team found that 70% of that biomass move­ment from mi­gra­tion oc­curred dur­ing the day­time and that the mi­gra­tion was gen­er­ally north­ward in the spring and south­ward in the fall.

Although it’s neat to think about great swarms of bugs mov­ing from one place to an­other high over our heads, the au­thors add that the study has real-world im­pli­ca­tions as well.

They note that in­sect bod­ies are typ­i­cally com­posed of 10% ni­tro­gen and 1% phos­pho­rus by dry weight and there­fore rep­re­sent rich sources of nu­tri­ents that may be es­sen­tial for plant pro­duc­tiv­ity.

“The 3,200 tons of biomass mov­ing an­nu­ally above our study re­gion con­tains 100,000 kg of [ni­tro­gen] and 10,000 kg of [phos­pho­rus],” they write.

They also found that over the 10 years of the study, the net north­ward move­ment of mi­grat­ing in­sects in the spring al­most ex­actly can­celed out the net south­ward move­ment in the fall.

But on an an­nual ba­sis, the net flux could be up to 200 tons or greater in ei­ther di­rec­tion, they said.

“Such in­sect move­ments rep­re­sent an un­der­ap­pre­ci­ated mech­a­nism for re­dis­tribut­ing nu­tri­ents and en­ergy,” they con­cluded.

Chap­man has al­ready started to in­ves­ti­gate whether other ar­eas of the planet ex­pe­ri­ence sim­i­lar or greater in­sect mi­gra­tions. So far it seems they do.

“We are just stat­ing sim­i­lar work in Texas, and we are swamped by mi­grants there, so num­bers and biomass will be much higher in parts of North Amer­ica,” he said.

Ian Woi­wod

TO AC­CU­RATELY mea­sure the in­sects too small to be cap­tured by radar, re­searchers used aerial net­ting.

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