A symbiotic relationship that benefits one organism and harms another is called parasitism. A parasite is an organism that gets its nutrition from the tissues of another organism without killing it. The organism on which the parasite lives is called the host. Although parasites usually do not kill their hosts, they often weaken them.
Parasitism can be a limiting factor. In a parasitic relationship, one organism (the parasite) lives in or on another organism (the host), which is usually harmed by the parasite. Almost all organisms have some parasites, but the parasites usually do not cause death because death of the host would result in death of the parasites. However, if a host has too many parasites, the host might die. Thus, the limitation is the number of parasites that the host is carrying.
The limitation is density-dependent. Parasitism usually causes death only when the parasites are in very dense host populations. The possibility that the parasites will be passed from one organism to the next is greater in a dense host population. As the rate of transfer increases, the number of parasites in each host also increases. Too many parasites on one host may cause death by interfering with the hostís nutrition or metabolism. The parasites may also reduce fertility of the host. As a result, the size and density of the host population decreases. Host organisms then have more space available to them, and transfer of parasites is less frequent. Since hosts have fewer parasites, the host population can survive and increase in number again.
There are two kinds of parasites. Those that live outside or on the body of the host is called ectoparasites or external parasites.
Examples of this kind of parasites are:
The parasites that live inside the body of the host are called endoparasites or internal parasites.
Some examples of this kind of parasites are:
a. Ascaris or roundworm
e. Trichine spirale
If we love our children, we must love the earth with tender care and pass it on, diverse and beautiful, so that on a warm spring day 10000 years hence they can feel peace in a sea of grass, can watch a bee visit a flower, can hear a sandpiper call in the sky, and can find joy in being alive.
~Hugh H. Iltis