Charles Darwin once mused on the impacts that predators could have on the landscapes around them. In particular, he wondered - in On the Origin of Species - how neighbourhood cats might affect the abundance of flowers in the fields near his house at Downe in Kent. He concluded the animals' potential to change local flora was considerable.
A robust cat population, he argued, would mean that local mouse numbers would be low and that, in turn, would mean there would high numbers of bumble bees - because mice destroy bee combs and nests. And as bees pollinate clover, Darwin argued that this cascade of oscillating species numbers would result in there being more clover in fields in areas where there are lots of feline pets. Cats mean clover, in short.
It was an idea that took the fancy of Darwin's chief disciple, the biologist Thomas Huxley who extended this cat-clover cascade in 1892 to include old maids. They kept cats, Huxley argued, and those pets would ensure neighbouring fields would be low in mice, high in bees and rich in clover.
And that in turn would have powerful consequences for the British Empire, Huxley added. Cattle graze on clover and cattle means beef. Thus old maids would provide the perfect setting for ensuring plenty of clover and therefore healthy cattle and good roast beef to feed our troops and thus ensure the prosperity of the British Empire. Old maids mean military might, in short.
Around islands that lacked sea otters, urchins had increased in size and in numbers with devastating consequences
Huxley was almost certainly being facetious in outlining his maids‑to-empire chain. Nevertheless, the concept of trophic cascades - as these ladders of interacting predator and prey populations are now known - is recognised today as being a powerful and important force in shaping the natural history of our planet. More to the point, as human activities impact more and more on wildlife, we are changing trophic cascades with profound and unexpected consequences.
This view of nature - looking down from the top - contrasts with previous attempts to understand food chains from changes that affect their bottom rungs to see how animals and predators at higher levels are affected. An example of this approach is provided by scientists who study how reductions in Arctic sea ice might reduce levels of algae (which forms on the underside of sea ice) and which might then affect the creatures that consume alga: the plankton, fish and seals further up the food chain.
Top-down forcing - or trophic cascades - looks at the problem in the reverse direction, with a perfect example being provided by the work of James Este, an American marine biologist who has studied wildlife in the north Pacific Ocean for the past 45 years and has revealed the astonishing manner in which terrestrial and sea predators can change land and marine environments. This top-down picture - with predators influencing the health of plants - is depicted in enthralling detail in his newly published Serendipity: An Ecologist's Quest to Understand Nature (University of California Press).
Estes has spent most of his working life in the Aleutian Islands, which stretch across the North Pacific Ocean from Alaska to the coast of Kamchatka in eastern Russia. "No place in the modern world is much wilder or more remote than the Aleutians," he says.
This isolation has not put the islands beyond the harmful influence of humans, however. As explorers opened up the far north two centuries ago, hunters pushed deeper into the Aleutians in pursuit of the pelts of the sea otters that thrived there in their hundreds of thousands. A member of the weasel family, the sea otter (Enhydra lutris) keeps warm in the water because it possesses the densest fur in the animal kingdom - about 850,000 to a million hairs per square inch. This insulates it from the cold.
However, the sea otter's thick, rich pelt also made it a major target for hunters who, by the 1900s, had brought the animal close to extinction. "Only a dozen or so small colonies survived," Estes tells us. In the end, an international ban on sea otter hunting was imposed, saving the animal from complete eradication.
Since then, the sea otter has become an important poster species for the ecology movement: a lovable, cuddly sea mammal that was saved from extinction thanks to international action. They are the teddy bears of the oceans, as one commentator has described them. Filmed as they float on their backs, cracking open sea urchins, crabs, abalones and other shellfish with flat stones before eating them, the animal is certainly an endearing sight.
But this constant culinary activity masks a serious issue for the sea otter. An adult animal needs to consume vast amounts of food to survive, about a quarter of its own body weight - up to 11kg - every day, hence all that shell crunching and munching.
The question that intrigued Estes when he began his marine studies in the Aleutians in the 1970s was straightforward: given its voracious appetite for urchins, crabs and the like, what was the ecological consequence of that calamitous drop in sea otters numbers last century? To find an answer, he began surveying sea floors around islands where sea otters had survived and others where they had disappeared and had yet to be reintroduced.
What Estes found was striking: around islands that now lacked sea otters, sea urchins - their main prey - had increased in size and in numbers with devastating consequences. The forests of kelp that once grew there in profusion had disappeared. Instead huge urchins littered the barren sea floor, having consumed every kelp plant in sight.
By contrast, near islands where sea otters survived or had been reintroduced, kelp flourished. The discovery was important given the nourishment kelp's underwater forests provide for fish and other sea animals. "Kelp forests, with their high biomass and extreme productivity are key controlling elements of coast ecosystems," says Estes.
Everywhere Estes looked he found the same picture. Islands with sea otters had healthy kelp forests while otter-less islands had barren sea floors littered with sea urchins but no kelp. In nearly eradicating sea otters, humans had disrupted a critical trophic cascade: high sea otter numbers that mean low sea urchin populations that mean healthy kelp forests. As Estes puts it: "Sea otters are clearly more than 'just another brick in the wall'."
In fact, they are now recognised as being a keystone species, whose position in food chains is crucial in maintaining the ecological health of an area. They not only ensure the health of kelp forests but affect many other local species, as Estes's investigations have since revealed. Fish thrive in kelp forests, as do mussel beds, for example.
But most of all, those rich kelp forests - enriched by sea otter activity - play a key role in maintaining global environmental health. Levels of carbon dioxide in the atmosphere are rising in the sea as well as in the atmosphere, and it is increasingly being absorbed by the sea as well, making it more acidic and harmful to many species. But Estes has calculated that healthy kelp forests have the capacity to absorb billions of kilograms of carbon.
"Our results were eye-opening," he states. "The difference in annual absorption of atmospheric carbon from kelp photosynthesis between a world with and a world without sea otters is somewhere between 13 and 43 billion kg (13 and 43 teragrams) of carbon."
Just about every strand of ocean life that Estes looked at was touched in some way.
"Every species in the coastal zone is influenced in one way or another by the ecological effects of sea otters," he concludes. Thus, in removing sea otters from the north Pacific, humanity - in its pursuit of fur for hats, gloves and coats - had not only grievously endangered the species, it had disrupted a large chunk of the marine environment in the Pacific and - for good measure - damaged our ability to deal with the impact of rising carbon dioxide levels on our planet.
Fortunately, the species had been saved from extinction in the nick of time. At least that was how it seemed in the 80s and 90s. Then Estes made a second - disquieting - discovery. When he returned to the Aleutian islands of Adak and Amchitka - where sea otter numbers had been steadily rising to the general good of the islands' kelp forests - he found their populations were now dwindling. "Something had changed, but I didn't know what," he says.
Estes looked elsewhere in the archipelago and found that some sites - such as Clam Lagoon on Adak - still possessed healthy populations. Most others showed population declines, however. Overall, about 40,000 sea otters had disappeared over the course of a few years, he calculated. And just as sea otter numbers dropped, so urchins reappeared on the sea floor and kelp forests began to disappear again.
But why were sea otters disappearing? Could some toxin or disease be responsible? It seemed unlikely, given that otter losses were affecting islands that were thousands of miles apart and that some sites were completely unaffected by otter losses.
The answer was eventually supplied by one of Estes's colleagues, Tim Tinker, who realised two otters that he had recently tagged for future study had gone missing just after a group of killer whales had passed Adak island. Then he recalled that previous losses of tagged otters had also occurred after killer whales had been spotted in the region. Further observations confirmed the idea: killer whales had suddenly taken a shine to sea otter flesh. Only in a few well-protected shallow bays such as Clam Lagoon could the otters escape the attentions of their newfound predators.
Given that, prior to 1991, there had been no confirmed attacks by killer whales on sea otters, the discovery was puzzling. So Estes looked at the history of other related species in the region and uncovered a startling picture. Just as sea otter populations started to plunge in the 90s, so those of harbour and fur seals and then sea lions had started to plummet in the 70s and 80s, all targeted by killer whales. But why?
The answer, Estes concluded, lay with commercial whaling that was carried out in the region after the second world war. "Before industrial whaling, killer whales were sustained by feeding on the immense biomass of great whales in the North Pacific and southern Bering Sea," says Estes. By the time commercial whaling was halted, there were virtually no great whales left for killer whales to eat, and so they expanded their diet first to seals then to sea lions and finally to sea otters. At least that is Estes's argument. Not every scientist accepts it.
Nevertheless the theory is intriguing. The involvement of killer whales means that a new apex predator seems to have appeared at the top of the otter-urchin-kelp trophic cascades and reveals, if nothing else, how this view of the food chain, from the top to the bottom, provides us with an illuminating new way at looking at nature and its tightly interwoven components. As the naturalist John Muir once remarked: "When we try to pick out anything by itself, we find it hitched to everything else in the universe."
Other keystone species
Jaguar (keystone predator)
The jaguar is a dominant predator, sitting on top of the food chain and feeding on a variety of large herbivores like deer, capybara and tapirs. If left unchecked, these prey species could devour most of the plants in a habitat, causing disastrous knock-on effects and population crashes in birds, insects and mammals. It is therefore vital for the health of many tropical ecosystems that jaguars control herbivore numbers.
Prairie dog (keystone modifier)
Prairie dogs are burrowing ground squirrels native to North American grasslands. They dig tunnels to avoid predators and the scorching summer heat, but their burrows also serve as prime nesting areas for many endangered bird species, and act as a natural irrigation system, allowing rainwater deep into the disturbed soil to provide fertile growing areas for new plant life.
Hummingbird (keystone mutualist)
Hummingbird beaks have evolved alongside certain flower groups, altering and mimicking each other's shape until no other animal can drink nectar from or pollinate the plants. The birds are such voracious pollinators that a small rise in their numbers can lead to a population boom in their linked flower species. However, these species rely solely on hummingbirds for survival; if the birds perish, the flowers will follow.
Beaver (keystone modifier)
During the night beavers work using mud, stone and timber to build spectacular dams. These create two distinct types of habitat: ponds upstream of the construction and downstream marshland. Beavers use the ponds as protection from wolves, bears and coyotes, but hundreds of North American species (many of which are endangered) owe their continued survival to the way beavers modify their environment.
Saguaro cactus (keystone host)
The saguaro is a giant cactus native to the Sonoran desert, bridging Central and North America. The cacti stand up to 60 feet tall, 2 feet wide and contain vast amounts of water in an otherwise arid environment. This makes them an attractive home for many insects and birds species, all of which burrow inside or build nests on the cacti's branches and would be unable to survive in the desert without them.
Snowshoe hare (keystone prey)
Snowshoe hares get their name because of their large hind feet, which help them remain on top of deep snow. Being one of the few small mammals that live above ground in arctic environments, they have many predators and must therefore breed at a rapid rate to survive. These hares are a main food source for several species, supporting endangered populations of lynxes, bobcats, coyotes and mountain lions, among others.
- The Observer