Final answer:
The correct answer is d. Parasites only affect their host species.
Step-by-step explanation:
The correct answer is d. Parasites only affect their host species.
Parasites play important roles in ecological systems, but they do not necessarily have all of the effects listed in the other options. For example, parasites can induce a top-down trophic cascade by reducing the abundance of their host species, which can in turn affect other species in the food web. They can also mediate coexistence between two competing species by selectively targeting one of the competitors. Parasites can serve as biological perturbations, affecting the population dynamics and community structure of their hosts. They can also enhance food web connectance by increasing the number of interactions among species. However, parasites do not only affect their host species, as they can also have indirect effects on other species in the ecosystem.