A predator affects the population of its prey, but a parasite may not have the same effect on its host..?

Question

...host population,why?

Answer 1

Parasites harm individuals. The way this acts on populations varies with the density of both populations and with the mode of parasitism. The host may develop immune responses or other mechanisms to compensate that reduce the impact on the population. But the greatest impact is that parasites lessen their hosts fitness. This makes the host more likely to fall as prey. This means it is very difficult to differentiate the effect of a predator on the host population from that of the parasite.
Sublethal parasite effects can disrupt normal predator/prey dynamics. Prey species exhibit regular cycles of abundance or population increase and decrease. Prey like snowshoe hares, with internal parasites, like nematode infections, are more likely to fall prey to predators. Predators and parasites act multiplicatively on the predation rate if both populations are dense.
Parasites show adaptations to their impact on the predator prey dynamic. Helminth parasites have adapted by parasitising the predator. Consumed with the prey they infect the predator as their next host. This indicates adaptation to avoid adding to the cyclic loss of their prey host by infecting the predator thus neutralizing their impact on the predator prey dynamics.

In the wild host individuals are often infected by more than one parasite simultaneously. This increases the impact on body mass & fecundity. In very dense populations heavy parasite infestation can rapidly reduce host density.

Extinction Effect of Parasites on Host Populations
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?indexed=google&rid=daph.chapter.ch9
“In summary, parasites in experimental Daphnia populations have been shown to reduce host density and population survival. With strong effects on host fecundity [parasites] are powerful agents for host population regulation..”

Answer 2

A parasite lives in perpetuity off of the enviornment provided by it's host. Therefore, it is not in the best interest of the parasite for the host to expire. The parasite depends on a healthy host for the parasite to survive. The parasite may cause some discomfort to the host, but generally does not effect the host's life expectancy (just perhaps the quality of life.)

A predator consumes it's prey (along with the parsites that live on/in the prey) and moves on to the next meal.

Answer 3

Overall it really depends on the parasite. There are different parasites with different goals, some with the goal to use the host in life and death where others need the host to stay alive.

Clearly if you have a parasite that needs its host to remain alive to stay alive itself... well that host will remain alive. Thus the population would not suffer.

You have to remember that not all parasites look at their host as "prey". Many times it is more of a partenership.

Answer 4

A predator by definition preys and usually kills and eats things - think of lions and zebras. Parasites often live on the host and not kill it e.g. tics or lice on humans.

So a predator would reduce the population of its prey, but a parasite wouldn't.