Is parasitism a missing ingredient in model ecosystems?

Parasites are rarely incorporated into model ecosystems. Here, we present simple ecosystems (prey-predator-detritus) and show how parasites can modify the level of detritus by debilitating the prey and thus increasing ingestion by the predator. Two types of parasites are modelled: simple life cycle parasites (SLC), which infect only prey, and complex life cycle parasites (CLC), which infect prey at the larval stage and mature in a predator when the predator ingests infected prey. We show that for SLC parasites the production of detritus is less reduced when the rate of manipulation increases. Inversely, the production of detritus may be more reduced when the rate of manipulation by CLC parasites increases. Parasites, which manipulate their hosts, could either stabilize (SLC parasite) or de-stabilize a system (CLC parasite). Decreasing parasite aggregation could also have de-stabilizing effects.