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A third type of species interaction is parasitism, which occurs when one organism (the parasite) feeds on the body of, or otherwise makes use of the energy of, another organism (the host).

The parasite is typically much smaller than its host and rarely kills its host.

Ex: Tapeworms- live inside their hosts.
Blood-sucking sea lampreys- attach themselves to the outsides of their hosts.
Some parasites move from one host to another, as fleas and ticks do.

Parasitism can benefit ecosystems by helping to control the populations of host species.

In a fourth type of species interaction, called mutualism, both species benefit by providing one another with food, shelter, or some other resource.

Ex: Clownfish- usually live with sea anemones, whose tentacles sting and paralyze most fish that touch them except for the clownfish. The clownfish also feed on the leftovers of the anemones' meals. In turn, the clownfish fight off or eat some of the predators and parasites that threaten the anemones.

armies of bacteria that live in our digestive tracts and help us digest our food. They in turn get a safe habitat and a food supply from us, their hosts.

A fifth type of species interaction, called commensalism, benefits one species but has little or no effect on the other.

Ex: Plants called epiphytes attach themselves to the trunks or branches of large trees in tropical and subtropical forests.
They benefit by having a solid base from which to receive sunlight, rainwater, moisture from the humid air, and nutrients falling from the tree's upper leaves and limbs. Their growth apparently does not harm the tree.
About 70% of the earth's known bird species are declining in numbers, and roughly 12% are threatened with extinction, mostly because of the loss, degradation, and fragmentation of their habitats.

About 75% of the word's threatened bird species live in forests. Many of these forests, especially in the tropical areas in Asia and Latin America, are rapidly being cleared or fragmented by roads and development.

In North America, populations of many forest songbirds, including tanagers, orioles, and thrushes, which spend their winters in these tropical areas, have also declined sharply.

Among the world's aquatic, or water-based bird species, about 40% are in danger, many because of the draining and degradation of their wetland habitats.
Other threats to these birds' habitats are oil spills, runoff of pesticides and fertilizers from farm fields, and other pollutants such as sediments and factory emissions.

Another threat to a number of migrating bird species are electrical power lines, cell phone and radio towers, wind turbines, and tall buildings. Every year, hundreds of millions of migrating birds are killed when they collide with such structures wherever they have been erected within bird migration routes.


The extinction of bird species can have a devastating ripple effect on the ecosystems of which they are a part.

Birds help to control populations of rodents and insects that would otherwise devour many plants.

They also help to sustain plant populations through pollination and dispersal of seeds.

Without these free ecosystem services, many plants would become extinct, especially in tropical areas, and this would be followed by the loss of the specialized animal species that feed on those plants.
A sixth major cause for rising extinction rates is over-exploitation—the O in HIPPCO.

With the rapid growth of the human population, hunters and fishers trying to help feed growing populations have depleted populations of many wild species.

However, some protected species are still illegally killed or captured for sale—a practice called poaching. Poachers sell the live animals and the hides, horns, and other body parts that are highly prized, to collectors in the largely illegal wildlife trade. Few of the poachers are caught or punished, and many of the live wild animals transported between countries die in transit.

This threatened white rhinoceros in South Africa was killed by a poacher for its horn. The horn can be worth many thousands of dollars because it is used to make dagger handles and is ground into powders that are used to make medicines.

Tigers, for example, are highly valued for their furs, and for body parts that are used for medicinal purposes in China and other parts of Asia.

A coat made from the fur of the highly endangered Bengal or Indian tiger can sell for $100,000.

2. Wildlife biologists estimate that for every wild animal captured and sold in the pet trade, an estimated 50 others are killed or die in transit.

Tropical birds are popular in this market, and at least 80% of these birds are imported legally or illegally from tropical forests, according to the IUCN. As a result of the pet trade, more than 60 bird species—most of them parrots—are classified as endangered or threatened

The pet trade also affects coral reef ecosystems, especially in Indonesia and the Philippines. Divers go to the reefs and catch tropical fish by squirting poisonous cyanide into the water to stun the fish. This means that many fish die for each fish that is caught and sold. The cyanide solution also kills the coral animals that build and maintain the reefs. Other wild species that are being exploited and depleted because of the pet trade include amphibians and reptiles.

3. Several plant species, too, are in danger of being depleted, largely because of the buying and selling of these plants for use in decorating houses, offices, and lawns. Several species of orchids and cacti are endangered for this reason.