Big­ger Pic­ture

A new UBC study says ris­ing tem­per­a­tures are lead­ing to smaller in­sects. What is the im­pact when bugs — 75 per cent of the planet’s an­i­mal species — shrink?

Canadian Wildlife - - CONTENTS - By Alanna Mitchell Il­lus­tra­tion by Pete Ryan

A new UBC study says ris­ing tem­per­a­tures are lead­ing to smaller in­sects. What hap­pens when 75 per cent of the planet’s an­i­mal species shrink?

BACK BE­FORE DI­NOSAURS ROAMED THE EARTH, in­sects were gi­ants that at­tacked from the air. Some had wing­spans of more than a cou­ple of feet across — more like ea­gles than bees, if you will. Bi­ol­o­gists were long puz­zled about how some in this class of in­ver­te­brates got so big. And not only that, but how those colos­sal crea­tures were even­tu­ally wiped out, leav­ing us with the small ver­sions we have on Earth to­day.

It turns out it was oxy­gen. The at­mos­phere of 300 mil­lion years ago had far more oxy­gen than it does to­day. In­sects got big­ger and big­ger as oxy­gen lev­els rose. Un­til, that is, fly­ing ver­te­brates got into the act. Those same high oxy­gen lev­els dur­ing the Car­bonif­er­ous and Per­mian pe­ri­ods even­tu­ally fos­tered the growth of big, rav­en­ous birds in the Juras­sic pe­riod. Snap! The ti­tanic in­sects van­ished into stom­achs. So too did the fos­sil record.

Now, there seems to be an­other at­mo­spheric push af­fect­ing the size of in­sects. Bi­ol­o­gists have long known that high con­cen­tra­tions of car­bon diox­ide in the at­mos­phere, which trap heat against the sur­face of the planet, are caus­ing big changes in what lives where — and when they do things. The ex­tra warmth is prompt­ing some species to move to dif­fer­ent parts of the land or ocean, seek­ing cooler tem­per­a­tures. It is caus­ing some to al­ter age-old habits: when they bloom or mate or mi­grate to nest­ing grounds.

Now, sci­en­tists are won­der­ing whether the ex­tra heat is also af­fect­ing some­thing even more fun­da­men­tal: body size.

For a cen­tury, lab ex­per­i­ments had sug­gested that tem­per­a­ture af­fects how big a crea­ture gets: those grown in hot­ter tem­per­a­tures are smaller. It is a maxim known as the TSR or “tem­per­a­ture-size rule,” and, in the lab, it holds for an­i­mals, plants, pro­tists and bac­te­ria.

But was it also true in the wild, where so many other fac­tors can also in­flu­ence size? And if it were true, would it mean that crea­tures liv­ing in this high-car­bon, warmer world will shrink?

Michelle Tseng, a zo­ol­ogy and botany pro­fes­sor at the Univer­sity of British Columbia, de­cided to find out, along with some stu­dents in a fourth-year class she was teach­ing. The re­sults came out in Jan­uary in the Jour­nal of An­i­mal Ecol­ogy.

Tseng’s stu­dents started by look­ing to the past. They parsed the bee­tle col­lec­tion in the Beaty Bio­di­ver­sity Mu­seum at UBC. Why bee­tles? Be­cause they’re one of the most di­verse and wide­spread orders of the an­i­mal king­dom. In other words, if bee­tles are shrink­ing, it’s a good bet that other crea­tures are, too. And the mu­seum’s col­lec­tion fea­tures 7,000 bee­tles from two dif­fer­ent re­gions of B.C., spread across eight species, col­lected from 1915 to 2015.

Then the stu­dents looked at a cli­mate data­base for the re­gions where the bee­tles had lived. It turns out that au­tumn tem­per­a­tures in the Lower Main­land of B.C. had risen 1.6 C in 45 years, and those in the Okana­gan, 2.25 C, for starters.

Once they crunched the num­bers, the stu­dents found a clear pat­tern. Larger bee­tles be­came smaller as the tem­per­a­tures rose, but smaller bee­tles didn’t get smaller. The four largest species of bee­tles shrank 20 per cent over 45 years. Tseng’s team con­cluded that oth­ers in the or­der Coleoptera — or bee­tles, 350,000 species strong — could be re­act­ing the same way.

Does size mat­ter? Tseng told me it does. In­sect body size is linked to how many off­spring they have and how their ac­tions af­fect the ecosys­tem they live in. For ex­am­ple, she said, big­ger mos­qui­toes suck more blood and larger dung bee­tles roll more dung.

Now the task is to fig­ure out more pre­cisely how smaller bee­tles will af­fect other eco­log­i­cal pro­cesses, she said.

But Tseng’s find­ings lead me in a cou­ple of other direc­tions, too. The first goes back to oxy­gen. Sci­en­tists have al­ready dis­cov­ered, in the lab, that the size of mod­ern aquatic arthro­pods (in­sects, spi­ders, moths, crus­taceans) is af­fected by oxy­gen lev­els. Maybe oxy­gen is hav­ing an ef­fect on the size of land in­sects as well, Tseng posits. And, it turns out that lev­els of oxy­gen in the at­mos­phere have dropped steadily, rel­a­tive to lev­els of ni­tro­gen, since at least 1989, her pa­per notes.

And then there’s the state of in­sects as a whole. Lit­tle no­ticed and less val­ued, in­sects are three-quar­ters of the planet’s an­i­mal species and prop up the web of life. They are in dras­tic de­cline right across the world.

Fewer in­di­vid­ual in­sects, and many shrink­ing from heat and maybe di­min­ish­ing oxy­gen lev­els. And they are so crit­i­cal to life. This is the sort of pic­ture that keeps me up at night.a


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