Animals of the future

2021-11-09

IN JUNE OF this year, not long before the midwinter solstice, catastrophic floodwaters draining from the Gippsland Plain, in southeastern Australia, left in their wake an otherworldly phenomenon: Translucent spider silk, extending half a mile in some places, trailed over riverbanks, roadsides, and fields, rising into glistening spires atop highway signs and shrubbery. On once-humdrum stretches of road, drivers pulled over to stare, take pictures. When a breeze ran through the membrane, it rippled with the fluency of a tide surging in a mangrove swamp. Light trembled on the sodden turf beneath. How improbable that something so delicate, sensuous even, might remain after something so destructive.

To spot the creatures responsible, you would have had to draw close. Sheetweb spiders—constellations of them—clustered in a cosmos of their own froth and protein. A mature sheetweb is rarely bigger than a contact lens; the spiderlings are best made out with a magnifying glass. On days of ordinary weather, millions live in the earth, but when threatened by inundation, the spiders abandon their belowground niches. Each fashions a single thread, a streamer, to function as an emergency airlift. Lofted up by atmospheric currents, and possibly by electrostatic crackle too, the spiders sail on the tips of their lines toward higher terrain, alighting, in time, on fence posts or treetops or ascending farther still. In 2011, a pilot reported crossing paths with clumps of spiders at 2,000 feet. In a departure from habit—wingless as they are—the sheetwebs fly. The tracers of their mass decampment, a strand of silk for every spider, settle on a scale so vast, so uniform, the result looks less like the work of animals than like something mythological or architectural: a mysterious Christo at work, festooning the landscape.

The weather in Gippsland is changing, as is true everywhere. The variability of the region’s climatic extremes has become more pronounced since the 1960s: hotter hot spells, fiercer floods. Scientists foresee longer dry periods split by downpours of worsening severity. When sheetwebs appear, we would do well to view them as a premonition of a future we are failing to avert. That which elevates the spiders out of crisis—their ribbons of silk—attests to how deeply they are, in fact, enmeshed in a nature that grows more chaotic. Weaving is the way arachnids make themselves at home in the world, their webs functioning as dormitories, trip wires, and traps for prey. But as the sheetwebs spin silk to flee an inhospitable habitat, their webs are flotsam from an evacuation.

Though evolution has endowed these tiny asterisks of life with a flight instinct, and an ingenious strategy for extricating themselves from disaster, the spiders cannot stay aloft forever. When the sheetwebs drifted back down in Gippsland, that was not the end of their emergency. Not until after the waterlogged soil dried out did they shrink once more into their myriad hideaways. In countryside mauled by storms, with land submerged below rising rivers, we will see more of the spiders, it seems, for they cannot escape us. And should waters recede too slowly, they may be doomed. If we can set aside our anxious wonder, perhaps we may see in the sheetwebs’ adaptive resources, and their limits, the challenges that await in the quest to acclimate to crisis.

HOW TO INHABIT a world in profound transformation? In this era of climate-change anguish, all field biology might be said to be underpinned by that question. The answers emerging from studies of the animal kingdom shed light not only on the capacity of individual species to accommodate less favorable conditions, but on the guide rails that will govern whether, and how, every life form on the planet will be remade as the coming decades unfold. Conservation movements have historically coalesced around vanishing animals (whether African elephants, Bengal tigers, or monarch butterflies), but a significant line of research in the natural sciences has pursued a different concern: What must animals do now to persist?

Two new books on this subject—Hurricane Lizards and Plastic Squid, by Thor Hanson, an independent conservation biologist, and A Natural History of the Future, by Rob Dunn, an ecologist at North Carolina State University—explore the startling ways in which, short of extinction, fauna (and flora) are responding to cascading changes wrought, in varying degrees, by humankind. They direct their attention not to large mammals imperiled by dwindling wilderness, but to an assortment of minifauna: reptiles, fish, birds, insects, and even—particularly in Dunn’s writing—microbes. Synthesizing a wealth of recent findings, both books open trapdoors onto the vivid lives of other beings in hopes of giving humans a close-grained understanding of our role in habitat change and the varieties of adaptation that may be in store for our species too.

Hanson’s subtitle, The Fraught and Fascinating Biology of Climate Change, clues us in to the author’s goal of spotlighting strategies that permit animals to withstand (perhaps even to exploit) environments in transition. Right away he makes clear that vulnerabilities to change are not evenly distributed: Miseries hovering on the horizon for our species have already arrived for creatures that are susceptible to finer fluctuations of conditions, or that have lower thresholds of tolerance. Yet animals are not all equally entrenched in their existing habits and habitats. Some demonstrate surprising plasticity of behavior, geographic range, and even appearance. A remarkable few have evolved resilience in the face of disasters that human communities already experience as ruinous.

Conditions don’t have to be lethal, Hanson notes, to be consequential. To a sun-basking reptile—a “heliotherm” that regulates its internal temperature by scampering in and out of the shade—high heat is an acute stressor. Hotter weather hasn’t killed fence lizards outright, but when these wriggly reptiles are forced to shelter for almost four daylight hours or longer, they hunt fewer insects, consume fewer calories, and cease reproducing, so their populations wink out all the same.

Other lizard species have demonstrated an extraordinary capacity to expand the breadth of the extremes they can inhabit by revamping their bodies. For anole lizards living on the Turks and Caicos Islands, in the Caribbean—an archipelago wracked by ever more severe weather—the solution to enduring hurricanes lay underfoot, literally. Researchers have documented lizards evolving heritably longer front limbs and larger toe pads on their forefeet, the better to grip onto anchor points when buffeted by gales that damage buildings, uproot trees, and fell electricity poles. The lizards have transmogrified—engineers might say “ruggedized”—to hold their ground in a nature that is more capricious today than at any time in their past.

Our mental models of climate change portray the process as ambient and inanimate—manifest in the air and ocean, in melting ice and multiplying dunes. By contrast, the anole lizards’ story is disquieting in its intimacy. It suggests that the legacy of extreme weather is legible in the flesh too, that physical appendages can be recast by hurricanes, over generations. As human actions alter the atmosphere in ways that guarantee more frequent and severe windstorms, in some sense we could be said to have become indirectly responsible for what some animals are, their very shape. Call to mind the Platonic ideal of a lizard—bony, bronze, and flick-tongued, on the sand of a deserted beach. Do we now have a hand, so to speak, in its feet?

The climate has always driven evolution, of course. The surprise, Hanson points out, is how fast some animals are modified by their surroundings—and in pulses of sudden, lasting change, not by increments. Speckled wood butterflies are developing stronger wing muscles as their borderlands in Scotland warm up and move northward, opening territory to those butterflies best able to cover the distance. Male collared flycatchers on Sweden’s Gotland Island are becoming less ornamented as temperatures climb. Fluffy white forehead patches on the birds (a feature of courtship displays) have perhaps become too burdensome: Males with striking plumes get drawn into more confrontations with rivals, and in hotter weather that competition expends reserves of energy to their detriment. Male three-spined sticklebacks (fish) likewise have grown duller. A flush of bright scales, hitherto enticing to female sticklebacks, proves a fruitless adornment in waters clouded by algal blooms.

In the soot-soiled cities of the Industrial Revolution, peppered moths famously evolved to be darker in color, less visible to birds that sought to eat them. When the air grew clearer, lighter moths prevailed again. Likewise today, where once-durable snowpack has become transient and patchy in Finland, a once predominantly gray species of owl trends toward brown plumage, better camouflage in a tawny domain. Air pollution is still at work here, though it is not smog that dims the owls. Carbon emissions beget snowmelt by way of milder winters. Between the natural selection of adaptive traits and the artificial selection of desirable characteristics (that is, animals domesticated by humans), genetic variation in the wild is today subject to the inducements and penalties introduced by manufactured conditions.

theatlantic.com, 9 November 2021
; https://www.theatlantic.com