PREDATORS AND PARASITES
Lawrence Pringle
Some Biologists believe that most of the 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 of predators into an area where they might control the numbers of some pests. Sometimes this works well; often it does not.
In 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 been dropped dramatically. The rats became harder to find, Then the mongooses began to eat 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 other instance, house sparrows were brought to the United Sates from England in hopes that they would control elm spanworms in the New York City?s Central Park. The birds did not control the insects and have spread across most of the nation, crowding out blew birds and other native birds with which they compete for food and nesting sites.
People do learn from their own mistakes and experiences with mongooses, house sparrows and other introduces organisms led to the passage of strict laws controlling the importation of plants and animals> The idea of using parasites and predators to control pest has not been totally abandoned; it is just done with much greater care and advanced 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 a 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 commensalisms> Fish called remoras attack themselves to sharks. They get a free ride and eat fragments of the shark?s 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 all 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 see lichens clinging to rocks and tree trunks almost anywhere. Part of the lichen is a fungus> within it are the 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 plants benefit.
FROM ECOLOGY: SCIENCE OF SURVIVAL
Extract from srjacfer
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