Animals that change with the climate
RICHARD CONNIFF looks at how animals around the world are adapting to a warmer climate.
AFTER MILLIONS of years of co- evolution, the relationship between the clownfish and the green sea anemone may be coming to end.
BUT IS CLIMATE CHANGE also affecting the size and shape of animals’ bodies, or the way animals function?
I first started wondering about this when I read a 2013 study on ocean acidification published in the Proceedings of the Royal Society B. It’s a subject I had scrupulously avoided until then because the words “ocean acidification” are, let’s face it, sleepinducing. But stay with me a moment. Each year the oceans soak up about a quarter of all the carbon dioxide put into the atmosphere, with the result that marine creatures now live in water that is 30% more acidic than in pre-industrial times.
Think of it this way: if you jump into a swimming pool where the ph has slipped a little outside the accepted range (say, from 7.0 to 6.5, meaning it is more acidic), you may notice that your eyes sting and your skin starts to itch. You can jump out and run to the shower. But sea creatures can’t.
For the 2013 study, Sue-ann Watson at James Cook University in Townsville, Australia, led a collaboration of Australian and European researchers. They looked at how rising acidity might affect an Indo-pacific conch snail living on coral reefs. (The snail has a pretty white shell with some brown stripes, but it goes by the unlovely name Gibberulus gibbosus, or the “humpbacked conch”.) The good news: spending a week in an aquarium tank with the acidity tweaked to nearfuture levels had no effect on the conch’s ability to right itself after being turned upside down, or, according to a follow-up 2015 study, on its respiration. The bad news: it rapidly lost its ability to jump.
Right. I know. You never imagined that a snail could jump in the first place. But to escape predators these conches can jump the equivalent of their own body length. They may leap five times in a row, and with good reason. Their predators are cone snails which use a lance to inject a deadly toxin into the soft muscle at the foot of the shell. Not jumping would be a fatal mistake. But the team found that increasing acidity altered the function of a key neurotransmitter, slowing down or stopping the escape response in many individuals. If some individual conches are more tolerant of higher acidity and warmer seas, and if this difference is based on individual genetic variation, those individuals “may be favoured by natural selection”, Watson and her co-authors write in their latest study. Those animals would become “the winners in a warmer acidified future”.
In another experiment in Watson’s lab, twotoned squid, a tiny Indo-pacific species, also changed its behaviour in ways that could arguably prove either beneficial or fatal. Squids normally assume a defensive posture in response to a threat, but elevated CO2 and temperature triggered a more extreme response. They were twice as likely to emit a cloud of ink, or to jet-propel themselves out of the picture. That might appear to make them safer – but it comes at a higher energy cost and the likelihood of more predator chases.
Other predicted changes seem to portend a rapid and unambiguous sorting out of winners and losers. Acidification may prove to be a huge ecological boon for seagrass, for instance. It seems to thrive in the highly acidic waters around sea vents. This is not because it’s partial to acidified water as such, says Catherine Collier, also of James Cook University. Rather, seagrass tolerates acidified seawater and that allows it to take
IT TOOK ONLY 18 YEARS OF WARMING TO PRODUCE SIGNIFICANT CHANGES IN ARCTIC BUTTERFLIES.
advantage of the extra dissolved CO2 “which means the seagrass photosynthesises quicker”. But for other species, the likely changes are downright alarming.
You may remember clownfish being adorable in the movie Finding Nemo. (Or maybe you found them annoying after your kid insisted on seeing the video for the 30th time.) Clownfish live in coral reefs and enjoy protection from coral reef anemones. These anemones may look like flowers but their tentacles are armed with harpoon-like stinging barbs that are deadly to the many larger fish, eels and sharks that might prey on clownfish. But the clownfish are safe thanks to a thick mucous shield covering their skin. So they hide out among the tentacles and in return for this protection, they dart out to drive off butterfly fish and peck at the anemone with their pointy mouth.
It is a beautiful friendship – and a reason clownfish live for up to 30 years, six times the expected lifespan for a fish that size. But after millions of years of co-evolution, that relationship may be coming to end. When Philip Munday at James Cook University and his co-authors tested how clownfish would adapt to the ocean conditions they will encounter later in this century, they found that more acidic waters cause the clownfish to hang out in the wrong neighbourhoods. The acid seems to affect their ability to sniff out the reefs containing the coral reef anemone. Not only do they become attracted to unfavourable habitats – they also swim directly toward the scent of their predators. Clearly, that’s bad for the clownfish, but what about the anemones? No one has tested yet how the sea anemones will respond to acidified oceans but an anemone abandoned by its clownfish is usually eaten by butterfly fish.
“The point is,” says Watson, “behaviours are changing as we increase CO2 and acidity levels in the oceans. Some of the changes may be good, some bad. If we understand [only] a small part of the interactions, we don’t know what the knock-on effects might be.” Her ambition is to study a few model species and use them to discern broader trends – snail conches perhaps standing in for other mollusks, sea grasses for some marine plants, clownfish for certain fish species.
But it is unlikely that laboratory predictions will keep up with the rapid pace of change. In some cases the shifting climate is already altering how some species look. Once again the changes are unpredictable and defy conventional scientific thinking. German researchers, for instance, recently compared specimens of 11 European bird species over a period of 120 years, from 1889 to 2010, to see if the steadily warming climate had caused them to shrink a little.
A theory, known as Bergmann’s Rule, says species should be bigger in cold climates, and smaller in warm climates. Introduced species have tended to obey the rule. Australian brushtail possums introduced to New Zealand in 1837, for instance, needed less than 50 years – 30 or 35 generations – to evolve larger skulls in colder parts of their new habitat. Moreover, they grew larger even though they occur at far higher densities in New Zealand forests where there are no predators.
But when the German researchers, who published their finding in PLOS ONE in 2014, added up their bird data, they found no change over 120 years, or at least nothing consistent with climate change.
On the other hand, Arctic butterfly species in northwestern Greenland do appear to obey Bergmann’s rule. It took only 18 years of warming conditions to produce significant changes. Scientists at the Zackenberg Research Station measured 4,500 specimens of two butterfly species facing “extremely high climate change risk”. They reported in the journal Biology Letters that the butterflies’ wings had become shorter as the climate warmed between 1996 and 2013.
But Bergmann’s rule can be trumped by Allen’s rule. Yes, there do seem to be a lot of rules for such an unruly topic as wildlife. According to Allen’s rule, animals tend to have bigger limbs in warmer areas – it is a way of shedding body heat.
And Australian parrots, it seems, prefer Allen’s rule. A 2015 study published in the Journal of Biogeography from Matthew Symond’s group at Deakin University in Melbourne looked at four parrot species in Australia from 1871 to 2008. It found that the surface area of the parrots’ bills had increased by as much as 10%, apparently in response to the warming climate.
CAN WE REALLY BE SURE these changes have anything to do with climate? Animal populations commonly get larger or smaller on account of resource availability, competition and other factors. But for conservationists, one of the great hopes in the face of climate change is that many threatened species will be able to shift their range, finding new homes when their old ones become less habitable. That’s already happening, of course, but even highly mobile species such as birds exhibit what’s come to be known as “migration lag”. In Sweden, for instance, suitable habitat for certain bird species has already extended by almost 300 kilometres toward the Arctic. So far, though, the birds have moved only slightly more than 100 kilometres northward, according to a 2012 study published in Ecography by Åke Lindström and colleagues at Lund University, Sweden.
Lag becomes a bigger problem the smaller you are. The authors of the Artic butterfly study, for instance, were concerned about the ability of the shrinking species to fly far enough to reach more favourable habitats.
The most startling physical modification produced by climate change has to do with tongue size. I’m not sure what the rules are here, but tongue size makes a huge difference for birds and insects, and the plants they pollinate. Tongue length and flower size often evolve in lockstep. In one well-known story, Charles Darwin received samples of an astonishing orchid from Madagascar in which the nectar was located at the bottom of a 36-centimetre-long tube called the nectary. “Good Heavens what insect can suck it?” he wondered to a friend. Darwin conjectured there must be a moth with a proboscis long enough to reach the nectar. When just such a moth was discovered 41 years later, biologists named it praedicta (“predicted”) in Darwin’s honour.
Darwin might find it less straightforward to predict what is happening today. In a study published in Science in September 2015, Nicole Miller-struttmann of SUNY College in New York and colleagues looked at plants and bumblebees that have co-evolved at high elevation in the Rocky Mountains west of Denver, Colorado. Comparing current specimens with historical specimens in museums, they found that the tongues in some bumblebee species had lost a quarter of their length over the past 40 to 50 years, a rate of 0.5% per year. Yet their overall body size had not changed. The explanation? Warmer summers had reduced the abundance of flowers with deep nectary tubes – the bees’ co-evolutionary partners. The bumblebees are becoming generalists out of necessity, visiting almost any flowers still available. This is good news in a way. “Evolution is helping wild bees keep pace with climate change,” the authors concluded.
But a large part of the wonder and variety of the natural world has to do with its quirky specialists: its clownfish, its praedicta moths. To survive, one possibility is that those specialists must now relocate or become generalists. If that happens, wildlife in one place could increasingly become like wildlife in another, all around the world.
It promises to be a far more boring world, if it were not also so damned scary.
“EVOLUTION IS HELPING WILD BEES KEEP PACE WITH CLIMATE CHANGE.”