Some fishing boats run streamers to buoys that trail 150 feet or so from the boat to deter birds. What is this designed to do? They prevent high concentrations of sea birds from trailing boats and defecating in the water, reducing water quality. They prevent sea birds from being tangled or caught by fishing line. They prevent sea birds from eating the fishing catch. They prevent sea birds from being too close, reducing the potential that they will be harmed when fish are thrown back.

parasites can detrimentally affect their host, but also must not allow their host to die before they can complete their life cycle. some parasites have complex life cycles with many different hosts, and have had to find ways of persisting in many different environments during the change from one host to another. ectoparasites live outside the body of the host, and can more easily  disperse, but are  more exposed to predators, parasites, parasitoids. endoparasites live inside the body of the host, and are therefore protected from the outside environment. but they can be  attacked by the host's immune system. they have therefore developed  mechanisms for dispersal such as  complex life cycles.

digestion

it excretes wastes