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Predators, Parasites and Other Relationships
By Laurence Pringle
Some people think of predators as “bad”, though humans themselves are the greatest predators the world has known. Sometimes individual predators do prey upon farm animals, and these individuals have to be controlled. Too often, however, people try to wipe out entire populations of predators, with the mistaken idea that they are doing good.
People usually believe that predators have an easy time of it, killing defenseless prey. But studies of predators and their prey show that this isn’t so. After observing tigers in Africa, Dr. George Schaller wrote: “The tiger’s seemingly unbeatable array of weapons — its acute senses, great speed (but over short distances only), strength and size, and formidable claws and teeth — have given many naturalists the impression that the tiger can kill at will. ... My experience shows quite the contrary — the tiger has to work quite hard for its meals. ... I estimate that, for every wild prey killed, the tiger makes twenty to thirty unsuccessful attempts.”
Another biologist made the same observation about wolves. After studying North American wolves for twelve years, Dr. L. David Mech concluded that these predators often fail to kill prey that they find. Also, wolves tend to kill animals that are either young, old, sick, weak, injured or diseased. Dr. Mech wrote: “As is true with most predators, the wolf is an opportunist ... The predator takes whatever it can catch.
If the wolf could capture prime, healthy prey, it certainly would. But most of the time it cannot. It happens that all the prey species of the wolf are well equipped with superb detection, defense, and escape systems. As long as these systems are in good working order, a prey animal is usually safe from wolf attack.”
Predators are usually bigger and fewer in number than the animals they prey upon. The reverse is true of parasites. These organisms live on or in other living things — their hosts, often spending an entire lifetime with them. In parasitism the parasite gets food and sometimes shelter, while the host gains nothing and may even suffer in some way from the relationship.
Very few living things are free of parasites, which are usually smaller and more numerous than their hosts. Indeed, many parasites have parasites of their own. Jonathan Swift exaggerated only a little when he wrote:
Some biologists believe that most of the individual organisms now living are parasites, since there are many parasitic fungi, bacteria, flatworms, insects, ticks, and mites. Parasites are an important part of all communities, and like predators, often affect the number of other organisms in a community. Man has tried to use this ecological knowledge by deliberately bringing parasites or predators into an area where they might control the numbers of some pests. Sometimes this works well; often it does not.
In the 1870s, sugar-cane planters in Jamaica were losing about a fifth of their crops to rats, and a planter brought mongooses from India in hopes that they would prey on the rats.
Within a few years the number of rats had dropped dramatically. The rats became harder to find. Then the mongooses began eating native mammals, ground-nesting birds, snakes, lizards, land crabs, and anything else they could find. They even took to eating sugar cane. Some of the creatures they wiped out had been useful controls on insect numbers, and the insect damage to sugar cane increased. The mongooses themselves became pests in need of control.
In another instance, house sparrows were brought to the United States from England in hopes that they would help control elm spanworms in New York City’s Central Park. The birds did not control the insects and have spread across most of the nation, crowding out bluebirds and other native birds with which they compete for food and nesting sites.
People do learn from their mistakes, and experiences with mongooses, house sparrows, and other introduced organisms led to the passage of strict laws controlling the importation of plants and animals to the United States. The idea of using parasites and predators to control pests has not been abandoned; it is just done with much greater care and advance study. This method of limiting the number of pests is called biological control, and there is hope that it will someday eliminate the need for many of the insect poisons used today.
The close association between parasite and host is an example of symbiosis which means “living together”. There are a number of other examples of symbiosis in nature. In some relationships, one organism benefits and the other is not affected at all. This is called commensalism. Fish called remoras attach themselves to sharks. They get a free ride and eat fragments of the sharks’ food.
There are many other commensal relationships in the sea: practically every worm burrow, shellfish, and sponge contains animals that depend on the host for shelter or food scraps. A biologist found 13,500 animals living within the pores of a large sponge collected off the Florida Keys. The animals were mostly small shrimps, but the total included nineteen species, among them a small fish.
In some symbiotic relationships, both organisms benefit. The most common and wide spread example of this mutualism is a team of plants called lichen. You can find lichens clinging to rocks and tree trunks almost anywhere. Part of the lichen is a fungus. Within it are colonies of green algae cells. The fungus provides support and traps water which is used by the algae. The algae make food which is consumed by the fungus. Thus both kinds of plant benefit.
The organisms that make up a lichen couldn’t survive long apart. In other cases of mutualism, the two organisms may be together only part of the time. Birds called egrets often perch on the backs of African mammals such as rhinoceroses. The birds feed on lice and ticks in the mammal’s skin. This benefits both organisms. Also, the rhinoceroses may be warned of danger when an egret flies in fright from its back. But neither species is so dependent on the other that it can’t survive by itself.
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