IS PARASITISM A MISSING INGREDIENT IN MODEL-ECOSYSTEMS

Parasites are rarely incorporated into model ecosystems. Here, we pres ent simple ecosystems (prey-predator-detritus) and show how parasites can modify the level of debitus by debilitating the prey and thus incr easing ingestion by the predator. Two types of parasites are modelled: simple life cycle parasites (SLC), which infect only prey, and comple x life cycle parasites (CLC), which infect prey at the larval stage an d mature in a predator when the predator ingests infected prey. We sho w that for SLC parasites the production of detritus is less reduced wh en the rate of manipulation increases. Inversely, the production of de tritus may be more reduced when the rate of manipulation by CLC parasi tes increases. Parasites, which manipulate their hosts, could either s tabilize (SLC parasite) or de-stabilize a system (CLC parasite). Decre asing parasite aggregation could also have de-stabilizing effects. (C) 1997 Elsevier Science Science B.V.