What are parasites primarily known for?

Parasites are primarily known for their relationship with a host in which they take resources from the host without providing any benefits in return. This often results in harm to the host, as the parasites rely on the host for nourishment and survival. This relationship is characterized by the parasite living on or within the host organism, feeding off its tissues or bodily fluids, which can lead to various negative health effects for the host.

In contrast, the other options describe relationships or functions that do not accurately represent the nature of parasitism. For instance, nutrients provided to a host are characteristic of mutualistic relationships, where both the host and the organism benefit. Contributing to the ecosystem more accurately describes non-parasitic organisms, such as pollinators or herbivores that play beneficial roles. Decomposers break down dead organic matter and are critical for nutrient cycling in an ecosystem, but they do not engage in a parasitic relationship with a living host. Therefore, the correct understanding of parasites lies in their exploitative relationship with hosts, as indicated by the correct answer.