The Next Species: The Future of Evolution in the Aftermath of Man (26 page)

In 2011, Estes—along with John Terborgh, professor of biology at Duke University, and twenty-two other biologists from around the world—published an article in
Science
about how large apex predators had been a critical player in the global environment for millions of years, but that their loss might be man’s most extensive and appalling legacy on earth.

The loss of top or apex predators alters the intensity of plant eaters, and this has enormous effects on the abundance and composition of plants. As wolves have returned to Yellowstone National Park and started to prey on elk, willow and aspen trees, which the elk formerly grazed in excess, have also returned.

The opposite picture has occurred on the small islands of Lake Guri, Venezuela, but it proves a point about how predators can change the color of the forest. A hydroelectric dam created Lake Guri, and as the waters rose, it formed an isolated group of islands within the lake. The area surrounding the islands had once been
dense green tropical forest dominated by top predators like jaguars and harpy eagles. But as the water rose, the predators fled the islands and the forest on those islands began to change.

The Duke professor John Terborgh, a lanky, rugged ecologist who ran the school’s tropical research center in the jungles of Peru for more than twenty years, noticed how,
in the absence of predators, prey populations exploded. On one island, iguanas were living at ten times their normal densities. On another, howler monkeys were fifty times denser than on the mainland. On a different island, leaf-cutting ants were living at one hundred times their normal numbers. Only the toughest plants with thorns and lethal chemicals could survive the resultant assault. The island forests were sparse and brown compared to the mainland forest, which was lush and green. Terborgh suggested that
predators played a major part in making the world green by controlling the plant eaters. Uncontrolled, the plant eaters turned the forest brown.

A different sort of example was the Scottish island of Rum, where wolves have been absent now for 250 to 500 years. Rum provides a peek at the consequences of predator loss, which can result in elevated browsing by deer and other herbivores. Though designated a National Nature Reserve in 1957, during its long period of predator absence, the Isle of Rum has transitioned from a forested environment to a treeless landscape.

Sharks are another vulnerable predator, but their problems are directly linked to man, not killer whales. Boris Worm, professor at Dalhousie University, in Halifax, Nova Scotia, Canada, claims in a recent paper that man kills 100 million or more sharks every year. Sharks have persisted for at least 400 million years and are one of the oldest vertebrate groups on the planet, but their populations are disappearing rapidly.

The problem is a global boom in shark fishing largely generated by an increasing demand for shark fins, used in shark fin soup in Asia.
Formerly consumed by Chinese emperors, shark fin soup is similar to champagne: something offered to celebrate good fortunes at weddings, graduations, and business lunches. However, it’s a ritual that is threatening the existence of an animal that has been with us in various forms since the Devonian period. An estimated 38 million sharks are taken to feed the worldwide fin trade each year.

Fisheries kill one in fifteen sharks every year. Sharks are similar to whales and humans in that they mature late in life and have few offspring, which makes their populations uniquely vulnerable.

Peter Klimley, a marine biologist at the University of California at Davis, has been
studying hammerhead shark populations on the Espíritu Santo seamount off Baja California for several decades. Klimley believes that hammerheads use Espíritu Santo as their mating grounds. Hammerhead sharks circle around the top of the underwater mount in a large school as females compete for dominant positions in the center.

Espíritu Santo’s sharks don’t feed while schooling there but travel to nearby feeding areas at night to gorge on squid. Klimley thinks they follow cracks in the seafloor filled with magnetic lava that radiate like spokes from the seamount. Sharks use special sense organs, the ampullae of Lorenzini, which are electroreceptors that can read magnetism like a compass. The hammerhead shark is currently on the International Union for Conservation of Nature (IUCN) endangered species list, and Klimley sees fewer and fewer sharks in the Gulf of California each year. He is currently tracking juvenile hammerhead sharks to find out if they frequent other similar seamounts, with the idea that some of those open-ocean areas might warrant protected status.

Still, hammerhead sharks don’t warrant the greatest fishing pressure. That priority is reserved for great white, bull, and tiger sharks, all known for making serious, unprovoked attacks on humans. Fishermen pursue them for their meat and their fins as well as for vengeance for occasional attacks on humans.

All three sharks inhabit their own unique environments and generally don’t cross paths with one another or with killer whales, though
fishermen like to contemplate such encounters. Tiger sharks are common throughout the tropical world. According to George Burgess, director of the Florida Program for Shark Research and curator of the International Shark Attack File, “Tigers are apex predators throughout most of their range. Killer whales do overlap a bit on the edges of their ranges; however, I am unaware of any recorded killer-tiger interactions. In my opinion, tigers (and whites) are co-apex predators with killers in the areas they coinhabit. All three consume fishes and cetaceans and don’t eat each other.”

Large tiger sharks can grow to 20 to 25 feet (6 to 7.5 meters) in length and weigh more than 1,900 pounds (900 kilograms). Many of the prey that tiger sharks go after are defensive animals like puffer fish, stingrays, and triggerfish. These fish have adapted spines, teeth, and even poisons to ward off predators like tiger sharks. But according to Kim Holland, an associate researcher at the Hawai’i Institute of Marine Biology, “The tiger shark has apparently decided, ‘Heck, we’ll just eat them anyway.’ I can’t tell you how many tiger sharks’ mouths I’ve investigated that were filled with stingray barbs.”

The only other true man-eater is the bull shark. Though not as well-known as the tiger and the great white, bull sharks can be nearly as dangerous. Florida is the shark attack capital of the United States, and bull sharks attack more Floridians than any other shark species. Bull sharks swim in tropical and subtropical waters around the world.

Bull sharks get their name from their short, blunt snouts and pugnacious dispositions as well as their tendency to head-butt their prey before attacking. They are medium-size sharks that can grow up to 11.5 feet (3.4 meters) and weigh up to 500 pounds (230 kilograms).

Bull sharks are the only large sharks that can survive in freshwater. Female bulls enter estuaries, bays, harbors, lagoons, and river mouths to bear their young, which spend their early years in these habitats. Bull sharks have been sighted 2,220 miles up the Amazon River near Iquitos, Peru. They have also been reported up the Mississippi River as far north as the state of Illinois.

Humans kill about 45 million sharks a year, whereas sharks kill fewer than 4.5 of us each year, though that doesn’t include fatal attacks reported as drownings. Burgess thinks shark attacks on humans are highly exaggerated, but that the damage humans do to sharks is very real. Says Burgess, “Their numbers are down by 90 percent or more in some populations.”

Still, sharks are one of nature’s great success stories. Some 2,000 to 3,000 species of fossil sharks have been described as compared to 650 to 800 species of dinosaurs. We still have 1,100 species of them today. Not all sharks are in danger—mainly the big ones, particularly those that eat man.

The great white shark ranges worldwide but most commonly inhabits the coastal waters of North America, South Africa, and Australia. It can measure up to 20 feet (6 meters) in length and 5,000 pounds (2,268 kilograms) in weight. As strange as it may seem, great whites have become an ecotourist attraction for caged divers off the coast of Australia.

Their role as such could provide a better future for the great white shark. A study by University of British Columbia researchers states that
shark ecotourism currently generates more than $314 million a year around the world and that figure is projected to double in the next twenty years. This compares to $780 million for landed sharks, a business that is shrinking. University of British Columbia researchers examined shark fisheries and shark ecotourism data from seventy sites in forty-five countries. Chris Lowe, director of the Sharklab at California State University, Long Beach, says that the ecotourism business in Australia is an effective deterrent to shark fin fishermen. “They report anyone going after white sharks,” Lowe observed. “No one wants to see those tourist dollars disappear.”

The great white shark is listed on the IUCN Red List as “vulnerable.” Relatively little is known about its biology. It is fairly uncommon, most frequently reported off South Africa, Australia, and California. World catches of great white sharks from all causes are
difficult to estimate. The animal matures late and has few offspring, so that if populations were to suffer, it would be slow to rebound.

Off California, great white sharks are ambush predators concealing themselves in the rocky bottoms of offshore islands. These islands are home to a number of seals and sea lions, but the favorite of the great white shark appears to be the elephant seal, the largest of California’s seals.

By
attaching tracking devices to elephant seals on the Channel Islands off California, biologists have recorded that females travel nearly twelve thousand miles annually, and males more than thirteen thousand miles—the longest migration of any mammal on earth. But these animals do not just swim straight to feeding grounds; they dive continuously throughout their journeys, going to extreme depths to feed on deep-sea fish and squid and to avoid great white sharks. These dives add an average of five thousand vertical miles to their lengthy horizontal journeys. “They are basically on the move the whole time,” says Robert DeLong, a biologist with the National Marine Mammal Laboratory in Seattle.

The great white shark is the primary predator of elephant seals in the North Pacific. They circle the Channel Islands during the mating and calving season in the winter, looking for stray seals that leave the safety of the beach to start on their long migrations.

If great white sharks were to go extinct, what could take their place as predators of elephant seals? What could assume the position of top predator off the California coastline? Mako sharks might fill the bill. They are legendary swimmers, reaching sustained speeds of 22 miles per hour (35 kilometers per hour) with bursts to over 50 miles per hour (80 kilometers per hour). Still, they are smaller than great whites. Their maximum length is 13 feet (4 meters). One of the largest mako sharks ever taken was 1,323 pounds (600 kilograms), caught
off California on June 4, 2013. Mako sharks might have to feed in packs to take an elephant seal, and cooperative hunting is not typical of sharks.

Our triumphant Humboldt squid might be picking up that trait. William Gilly at Stanford University put cameras on these squid and
recorded their hunting in tightly coordinated groups in the Pacific, a behavior that is usually associated with fish rather than squid. At present, it is mako sharks that are feeding on Humboldt squid, not the other way around. Still, mako sharks are frequently covered with scars from Humboldt squid. They appear as a ring of small incisions or a series of parallel scars suggestive of squid suckers, which have teeth, being dragged along the skin. Linear scars often begin with a circular mark on a shark’s midsection and lead forward toward the mouth of the fish. At least Humboldt squid are putting up a fight.

Author and professor Callum Roberts reports that all large shallow and midwater predators are disappearing. Could giant and colossal squid move up from the deep sea and establish themselves in shallower waters? The National Evolutionary Synthesis Center’s Craig McClain worked in the Bahamas in a place called the “Tongue of the Ocean,” a deep trough in the waters offshore. He’s also worked in Monterey, where a deep canyon occurs in the continental shelf. Areas like these and the deep ocean off Newfoundland are where giant squid and colossal squid live. Strandings of giant squid on Newfoundland shores are thought to be the result of warm-water incursions into deeper-water canyons, where scientists think giant squid reside. Still, if the loss of polar ice leads to a shutdown of the deep ocean currents that bring oxygen to the deep sea, then there could be strong evolutionary pressure for giant and colossal squid to migrate to shallower depths in the water column.

Giant squid are the biggest invertebrates (animals with no backbone) on earth. The largest of these elusive giants measure fifty-nine feet (eighteen meters) in length and weigh nearly one ton (nine hundred kilograms). In 2004, researchers in Japan took the first images ever of a live giant squid. And in late 2006, scientists with Japan’s
National Science Museum caught and brought to the surface a live twenty-four-foot (seven-meter) female giant squid.

Giant squid, along with their cousins, the colossal squid, have the largest eyes in the animal kingdom, measuring some ten inches (twenty-five centimeters) in diameter. These massive organs allow them to detect objects in the lightless depths where most other animals are blind.

Like other squid species, they have eight arms and two longer feeding tentacles that help them bring food to their beaked mouths. Their diet likely consists of fish, shrimp, and other squid, though some suggest they might even attack and eat small whales. Scientists don’t know enough about these beasts to say for sure what their range is, but giant squid carcasses have been found in all of the world’s oceans.

The range of colossal squid is similarly mysterious, but early whalers found colossal squid beaks in the stomachs of sperm whales, so at least we know what eats colossal squid. The colossal squid is the largest squid in terms of mass. Their cousins, the giant squid, have suckers lined with small teeth, but colossal squid do their kin one better by having their
limbs equipped with sharp claws or hooks.