The challenges that aquatic animals face are different to those faced by land-living animals. Many creatures are neutrally buoyant in water, which means they neither rise nor sink. The major constraint on their movements is not gravity but the frictional drag of water. Other things being equal, big animals can swim faster than smaller ones, and use relatively less energy to do so. But they face a problem extracting oxygen from water. Oxygen demand scales with body volume, which is the cube of fish length (think of the volume of a box—length times breadth times height). On the other hand, the surface area of the gills over which oxygen can be exchanged increases less fast as body size increases, as the square of fish length (the area of a box—length times breadth).32 While being large-bodied is advantageous hydrodynamically, it is a disadvantage when it comes to breathing. This simple fact explains why predatory fish like tuna don’t simply follow schools of their prey, snacking on them when the mood overtakes them until all the prey are gone.33 In a long-distance race it is the bigger predators that run out of puff first. And when a fish is left gasping for breath it can easily fall prey to something else, like a shark. So marine animals must be very careful not to fall into oxygen deficit.
The huge Humboldt squid is one of the only large predators able to withstand low oxygen for extended periods.34 These beasts can reach more than ten feet long with their tentacles extended, and they weigh more than a man. Within its crown of muscular tentacles is a parrotlike beak made of two interlocking blades sharp as butchers’ knives. They swim by jet propulsion, squirting water through a tube, but most of the time they probably just wait for prey to come to them.
Low oxygen zones will expand in a warmer world as surface warming reduces mixing across the thermocline. The effect will compound as heat absorbed by the oceans penetrates deeper, which means they will hold less oxygen. In combination, the volume of the sea capable of supporting life will shrink. The upper limit of low oxygen waters has already moved up to three hundred feet closer to the surface off North America’s west coast.35
The West Coast of the United States has seen repeated mass kills of ocean life on the shallow continental shelf since 2002, events unprecedented in the previous fifty years.36 It turns out that they are due to the strong winds that blow parallel to the coast and drive upwelling, a harbinger of the dark side of strengthening upwelling I just described. As these winds push surface waters offshore, a tongue of deep, oxygen-starved water creeps up over the shelf to replace them and snuff out life. Jane Lubchenco, head of the U.S. National Oceanic and Atmospheric Administration, described the scenes she viewed through video cameras towed through a dead zone off the coast of Oregon in 2002: “We saw a crab graveyard and no fish the entire day. Thousands and thousands of dead crabs were littering the ocean floor, many sea stars were dead, and the fish have either left the area or have died and been washed away.… [S]eeing so much carnage was shocking and depressing.” One of her colleagues, Francis Chan, said, “One of the areas sampled is a rocky reef not far from Yachats, Oregon. Ordinarily it’s… swarming with black rockfish, ling cod, kelp greenling, and canary rockfish… [but] the fish are gone, and worms that ordinarily burrow into the soft sediments have died and are floating on the bottom.”37 Such mass die-offs of marine life will become a regular feature of future oceans unless climate change can be halted.
The Humboldt squid seems to have benefited from the expansion of its low oxygen habitat (and loss of large predatory sharks due to overfishing).38 It used to be common only in the seas off Baja California but in recent years it has expanded north to the Gulf of Alaska. Divers increasingly come into contact with them, as the oxygen minimum layer reaches closer to the surface. To meet a squid in the wild is one of life’s great pleasures. They follow your movements with intelligent, saucer eyes. Their emotions are written on their skin in quick-fire color changes that pulse and ripple in incandescent waves across their bodies. The Humboldts’ uncertain temper adds a little frisson to any encounter. Divers have been roughed up by squid, or had their masks and gear tugged. After the adrenalin rush of initial fear and excitement has passed, most divers feel the squid were more curious than aggressive. If they had really wanted to do harm, their huge strength would have enabled them to do far worse.
The trade-off between body size and oxygen demand explains a pattern that has long been known but never well understood: Fish tend to grow larger in cooler water. This is because they have lower metabolic rates, so they can afford to grow bigger. This suggests that we will have more problems ahead, as the seas warm from climate change. Fish under temperature and oxygen stress will reach smaller sizes, live less long, and have to devote a bigger fraction of their energy to survival, at the cost of growth and reproduction. From our own rather self-centered perspective, their response to the new conditions will be bad because it will reduce production of body mass, which will limit catches. Of course, some exploited species have already seen their body size and growth compromised by overfishing. Thus we see increasing temperature, decreasing oxygen, and high fishing intensities combine to reduce the productivity of fish stocks and undermine the long-term resilience they so desperately need to cope with the changing world around them.
The deep past affords us a bleak warning of what might happen if we fail to control greenhouse gas emissions. The mass extinction at the end of the Permian period 251 million years ago, which I described in Chapter 1, was caused by a planet in the grip of runaway global warming. Life in the sea suffered the one-two punch of anoxia and high carbon dioxide. It took five million years to recover. However, one group of animals that lived within the open waters of the sea, the ceratid ammonoids, which are creatures similar to the beautiful nautilus of our day, rebounded more quickly, returning within a million years of the catastrophe. It is possible that they were adapted to low oxygen conditions. Although the entire group is now extinct, their nearest living relatives are squid, cuttlefish, octopus, and nautilus. Indeed, one of the oldest living forms of squid is Vampyroteuthis infernalis, the Latin name of the “vampire squid from hell.” It is a blood-red beast that hunts deep within the low oxygen zones of today’s oceans. Its success as a predator in these extreme conditions may be a modern legacy of a past catastrophe. Several other mass extinction events in geological history hacked off large branches of the evolutionary tree. The toxic alliance of high carbon dioxide and low oxygen appears to have wielded the ax on more than one occasion.
The medieval Scandinavian and English king Cnut the Great is reputed to have had his throne set down by the sea, where he commanded the tide to halt. The tide, of course, paid no heed, proving that the sea was moved by powers that were beyond the influence of people, even kings. A thousand years later humanity has inadvertently gained the power to move ocean currents. But our powers remain circumscribed, and we must now live with the consequences or find a way to keep the forces we have unleashed in check.
CHAPTER 5
Life on the Move
Jean-Baptiste de Lamarck is famous today, or perhaps infamous, for his wrongheaded theory of evolution. He believed that organisms could pass on to their offspring characteristics that they had acquired during their lifetimes. Giraffes grew long necks, by his account, because generations of ancestors had stretched further and further to reach leaves high on trees. While his evolutionary ideas were discredited by Darwin, Lamarck was in his day a keen observer of life and a distinguished and influential French scientist. One of his other equally mistaken ideas has taken considerably longer to dismantle.
By the end of the eighteenth century it had become clear that people could deplete some species of wildlife to the point of extinction. The aurochs, a huge European wild cow, had been killed off by 1627 and the dodo by 1662. Many animals had been erased from large parts of their ranges, like beavers, wolves, and bears in Europe, and panthers in North America. Aquatic animals seemed more resilient, and Lamarck was convinced that they were in effect impervious to extinction because no natural barriers could contain them. A
s he wrote in 1809 in his landmark work Philosophie zoologique:
Animals living in the waters, especially the sea waters, are protected from the destruction of their species by Man. Their multiplication is so rapid and their means of evading pursuit or traps are so great that there is no likelihood of his being able to destroy the entire species of any of these animals.1
The limit to how far a species will spread from its place of origin depends on its physiology and habitat, and on predators and competitors. A few species do well over wide geographic ranges—tigers, for instance, can survive torrid jungles as well as frozen mountain passes—but most species do well in just one or a few habitats. As they move away from their comfort zone to places where environmental conditions are suboptimal, interactions with better adapted competitors and predators keep them in check.
On land a species’ range will often be restricted by physical barriers such as mountains, deserts, or large bodies of water. For some time now we have generally accepted Lamarck’s contention that few such barriers exist in the oceans. Most marine species release eggs or larvae into the open sea, where they drift for days or even months before finding a new home, and many of those species swim hundreds of thousands of miles in their lifetimes. This received wisdom had scarcely been challenged until my wife, Julie Hawkins, and I started to plot the ranges of species living on coral reefs. Julie spent years amassing lists of species from all over the world, among them a set of exquisitely illustrated fish lists collected by the naturalist Sir Peter Scott on his travels around the world in the 1970s and 1980s.
It was painstaking work, and after ten years and several thousand species, Julie’s mounting grumbles called a halt to our mapping. We found that many coral reef species were not in fact widespread, and some were limited to remarkably small fragments of habitat. The dark indigo Limbaugh’s angelfish, to take one example, is known only from Clipperton Reef, a pinnacle of coral in the eastern Pacific that is isolated from neighboring reefs by nearly six hundred miles of blue water. Or there is the pale dottyback, a shy and ghostly fish found only in the Gulf of Aqaba, a narrow finger of water in the northern Red Sea. Or the splendid toadfish, whose only known haunts are the reefs of the resort island Cozumel in Mexico. In recent years a remarkable catalog of coral reef species with small ranges has been built up.2
A similar picture is forming for marine species in other habitats and regions. Submerged seamounts, rocky oases amid the vast mud plains of the deep sea, sustain extraordinary numbers of species found nowhere else. The great trenches carved into the ocean floor, where tectonic plates grind back into the Earth’s mantle, provide other unique habitats, with their own distinctive plants and animals that are isolated by thousands of kilometers of inhospitable terrain. Brachiopods—they look like bivalve molluscs but are unrelated—have knocked about on this planet since the Cambrian period five hundred million years ago3 and are mostly scarce and solitary today, but in the fjords of Patagonia they build mounds that are an echo of the reefs they made in their heyday hundreds of millions of years ago, before the mass extinction at the end of the Permian.
What will happen to all these creatures as their environment alters under the influence of global climate change? Unless they can adapt to the new conditions, or move, extinction beckons. Adaptation proceeds via natural selection, as Darwin taught us, since those best suited to prevailing conditions leave more offspring, who carry their genes forward. Genetic mutations take place over generations, and the species imperceptibly shape-shifts into something different. The problem is that these evolutionary changes take time; they require many generations to take hold. If conditions change quickly, the only option may be to move. If you can’t move, you die. The history of our planet is punctuated with periods of mass extinction, when geologic or climatic upheavals pushed species beyond their endurance. The comings and goings of life can be read in seaside cliffs, where different fossils are found only in certain layers of rock. While a handful of species seem immortal, most last just one to five million years.4
Polar bears and penguins leap off the pages of every circular from an environmental organization that comes through the letter box, and television correspondents reporting on climate change are packed off to icy latitudes to be filmed with them. They are the most telegenic sufferers of global warming, but there are countless other less familiar species whose existence is also in flux. The North Sea has warmed by 2.25°F in the last twenty-five years, and life there is on the move. Fishermen on the coast near where I live in Yorkshire have seen growing numbers of new species pop into their catches over the last decade, like gray gurnard and cuttlefish. Annual surveys made since 1977 using bottom-trawl nets towed across the seabed show how geographic ranges are shifting under the influence of warming.5 The john dory, for instance, an odd-looking narrow-bodied fish with a long thin jaw, used to be common only in southwest England, but they have moved north, near Scotland, and colonized the North Sea.
Fifteen of thirty-six species surveyed in the North Sea over the last thirty-five years shifted latitudes. The average shift was about 190 miles north. (The temperature difference between the equator and 80oN, roughly the limit of sea ice, is about 54oF; if you divide this difference by the distance, temperature falls by about 1oF for every one hundred miles traveled north.6) An interesting difference emerged when shifters were compared to those who had stayed put. Species that responded quickly to climate change tended to be small-bodied animals that reproduce early in life, while those that did not move were larger-bodied and late to mature. This difference echoes one common throughout nature: Disturbed, fluctuating, or rapidly changing environments are dominated by weedy, opportunistic species. The rats and cockroaches of the sea may cope well with climate shifts, while the tigers and elephants are left floundering as their habitats shrink around them.
One reason why animals must move as the climate changes is that their physiology is tuned to temperature. Enzymes that regulate bodily processes work best within a relatively narrow range of temperatures, and species suffer when those limits are breached. But there are other reasons. Species need food and habitat, and these are undergoing their own moves. In the North Sea, for example, many kinds of microscopic plankton that drift in open water have shifted north over the last forty years.7 Northern zooplankton communities are dominated by a copepod called Calanus finmarchicus. This diminutive relative of crab and shrimp is fat and juicy by copepod standards, and reaches a tenth to a sixth of an inch long. Southern zooplankton are dominated by a more puny species, Calanus helgolandicus, that measures just a thirteenth to a tenth of an inch. This difference may not seem like much, but to larval cod it is crucial. They thrive on fat finmarchicus but starve on scrawny helgolandicus. The northward march of zooplankton has been linked to poor replenishment of North Sea cod, a fact that has compounded the decline from overfishing.
It might seem a simple matter to shift with one’s food sources, but most species are driven by another urge—the need to find mates and reproduce. Over thousands of years many have developed intricate life cycles in which they migrate between predictable feeding and spawning areas, tracking patterns in food availability to maximize survival. Scientists playing with equations have discovered that group phenomena like gathering to spawn appear to respond very slowly to shifts in prey availability.8 Instead of following their food, many fish continue to gather in the same places in order to fulfill their primordial compulsion. A gap thus opens between what they feel compelled to do by their instincts and what they should do to meet their needs. This underscores a point that applies to many of the changes underway in the oceans today: While species can track creeping shifts in conditions, rapid change could overwhelm them.
There is another way for species to stay within their comfort zone: They can go deeper. Water cools with depth, and at the present rate of warming they will soon have to go down as much as eleven feet per year.9 Some North Sea fish in the survey were found not only to head north but to go deeper. But light
falters quickly as you go down. Herbivores will find little to eat beneath the sunlit surface layer, and those that prefer hard bottoms will struggle to find a home in the muddy expanses of the deep.
Warming seas will change marine habitats as the species that build them are killed off or compelled to move. Creatures like oysters and mussels, mangrove trees and salt-marsh grasses, seaweeds and corals are ecological engineers. They develop slowly and take a long time to establish themselves in new terrain, and they are likely to lag far behind the needs of their inhabitants to find more agreeable places to live. The warming seas have already had a profound impact on one such habitat: coral reefs. Shallow-water coral reefs, confined to tropical seas, are the richest of all marine habitats. To drift with a gentle current over a coral reef is one of life’s great thrills. The world that unfolds below is crowded with bright dappling colors in constant motion. It feels like gazing at a Monet painting in which every brushstroke has come to life and is swimming around the canvas.
Warm water corals live on a knife edge; wherever they occur, they adapt to local temperatures. If temperatures rise more than a degree or two above the typical maximum, they bleach. Bleaching happens when there is a breakdown in the relationship between corals and the microscopic plants that live within their tissues and provide them with much of their food. At higher temperatures these microscopic plants, or zooxanthellae, as they are called, begin to harm their hosts, at which point the corals kick them out. Since it is the zooxanthellae that give corals their color, the tissue becomes transparent and colonies turn deathly white, as the chalky skeleton beneath is revealed. Bleached corals are starving; they die within weeks unless temperatures revert to normal.
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