Nature Notes: They Are What They Eat

Just about everyone has a rough idea of what “food chains” and “food pyramids” are. The ones at the very bottom are the microbes, single-celled diatoms and the like; the ones at the top are the “top” carnivores. In one respect, the human is a top carnivore. In another, a top herbivore. In all respects — strict vegetarians omitted — an omnivore.

We are the topmost carnivores when we eat, say, the flesh of a mako shark, another top carnivore. Occasionally, a top carnivore, say a tiger, lion, or grizzly bear, will eat one of us, making that beast a topmost carnivore, and so on.

Very few terrestrial top carnivores — tigers, jaguars, grizzly bears, leopards, crocodiles, bald eagles, Komodo dragons, and the like — hunt in packs. Most are individual hunters. A few species such as wolves, wild dogs, hyenas, and occasionally lions will hunt in packs, but the packs rarely number more than seven or eight individuals.

It’s different in the seas, however. Sharks often school, especially for reproduction, but sometimes for hunting. Orcas are marine mammals that hunt in packs. Some seal species also hunt in packs. Seals are frequently preyed upon by orcas, which are at the very top of the food pyramid and at the very end of the food chain.

Size is an important consideration in rating carnivore success. Lions and tigers are the largest cats, white sharks the largest carnivorous sharks. Crocodiles can reach 20 feet in length. Boa constrictors and pythons can get almost that long. 

Yet, as big as carnivores can be, they are not as large as the largest herbivores. On land we think of elephants, rhinoceroses, hippopotamuses, Cape buffaloes, water buffaloes, moose, gorillas, and giraffes. These are all herbivores, and they are larger than all carnivores. In the seas the carnivorous sharks, whales, seals, and dolphins are not as large as the whale sharks, basking sharks, and baleen whales, such as the largest of them all, the blue whale, which sieves krill and plankton. Thus size is a very good deterrent to being eaten.

Speed is another deterrent, but even the speediest of antelopes, deer, and their kin can be captured by big carnivores, especially when those carnivores are ones that hunt in packs. Terrestrial herbivores have another defense against predation. They feed and travel in large groups called herds. A few individuals from each herd are enough to sate the appetites of the carnivores that hunt them. A few are sacrificed for the sake of maintaining the bulk of the herd. In numbers there is survival.

Many top carnivores are loners. They pair up for reproduction, but the rest of the time they fend for themselves. Lions, hyenas, and wolves are exceptions. Wolf packs are led by the alpha males and are highly organized. Snow leopards, jaguars, tigers, lynxes, coyotes, bobcats, mountain lions, and cheetahs, except for coming together to mate, are solitary. There is a good reason for females avoiding the company of males, except for the purposes of reproduction: if hungry, the males will feed on the young cubs.

In the ocean, very often a group of whales traveling in a pod will all be female or all male. Fish, however, very rarely travel in same-sex schools.

Herbivores can be exceedingly long-lived and a few are known to be centenarians. Elephants, macaws, and turtles can live that long. The mammoth Galapagos tortoises that can weigh nearly a ton and our own much smaller box turtles can live to be more than 100 years old. There is always an exception to the rule. The longest living animal of all — the Greenland shark, in the sleeper shark family — can live to be more than 300 years old, and perhaps even as old as 500 years. It’s a carnivore that mostly eats fish.

Albert Durer lived in the 16th century. He was a printmaker and artist. One of his well-known works is a large fish pursuing a smaller fish, which in turn is pursuing an even smaller fish — a classic, if old, reproduction of a marine food chain. Food chains can be very long. A one-celled plant, say, a small diatom, is often at the very bottom. It can be ingested by a large unicellular protozoan, which in turn is eaten by a multicellular but very small hydra, which in turn is eaten by a minnow. The minnow is then swallowed by a snapper, which in turn becomes food for the striped bass. The striped bass can end up in the stomach of a very large grouper or mako shark.

On land, a beetle is eaten by a white-footed mouse, which is killed and eaten by a short-tailed shrew, only to be eaten by a black racer, which is subsequently wolfed down by a coyote. Terrestrial food chains are generally shorter than aquatic and marine ones. The food pyramid is different. The carnivore at the peak of the pyramid feeds on one or two organisms, say shrews, on the next level, the shrew feeds on several beetles, the beetle feeds on several ants, the ants feed on several nematodes, and so on, down to the smallest single-celled plant or bacterium. 

As a general rule, 10 percent of the nutritive value from each individual at one level makes it up to the next ascending level and so on. If there are six different trophic levels, one-tenth of onetenth of one-tenth of one-tenth of onetenth of the original protoplasm becomes part of the top-level predator. That is why the food pyramid is widest at the bottom with a single block at the top.

To provide an example of the most important second or third-level organism in North Atlantic waters, we will consider the sand lance, Ammodytes americanus.

It’s no bigger than a silverside or killifish, yet it is the most prolific marine baitfish around. Destroy the annual sand lance crop, with poisons or by tearing up the bottom in which they tunnel or setting off large underwater explosions, and a very bad fishing year will follow, for both commercial and recreational fishers.

Larry Penny can be reached via email at [email protected].