THE FUNDAMENTAL IMPORTANCE OF PARASITOIDS IN TERRESTRIAL ECOSYSTEMS

Parasitoids are a trophic class of organisms whose earlier life stages live parasitically in or on their hosts, consuming their tissues during development, and eventually killing the host before it emerges from the host carcass. Parasitoids have an intermediate trophic position between parasites (the host is not killed) and predators (the prey is killed outright). The targeted host gradually succumbs while it spends most of its life history being incrementally consumed by the parasitoid. Parasitoids can be microorganisms (entomophthoralean pathogens), plants (mistletoes), fungi (cordycipitaceous hypocrealeans), invertebrates (Sacculina barnacles) or vertebrates (lampreys). However, it is the insect fossil record that is the best archive for parasitoids, which documents the mid-Mesozoic Parasitoid Revolution in detail. This event begins during the mid Early Jurassic and is represented by a proliferation of parasitoid Hymenoptera (wasps) that consisted eventually of 56 families recognized in the fossil record, later supplanted by 10 fossil families of parasitoid Diptera (true flies). Insect parasitoidism began as ectoparasitoidism in wasps and flies, targeting the integument of much larger hosts, mostly caterpillars or large adult insects, later evolving into endoparasitoidism in which internal host organs were consumed from within. During the Cretaceous, a major fungal diversification event occurred among certain fungi that also targeted insects, resulting by early Eocene times, in the perfection of zombification, by which parasitoid fungal hyphae enter the brain of a host adult ant, rearranges its brain tissue, and effect unusual behaviors. The most profound result of the Parasitoid Revolution was the contrast between pre-parasitoid ecosystems characterized by resource-driven, bottom → up regulation of productivity, and post-parasitoid ecosystems that were consumer-driven by top → down regulation. One explicit manifestation of this trophic shift comes from food-web analyses of the early Eocene Lake Messel ecosystem, in which the top consumer of the lacustrine portion of the food web is a crocodile whereas the top consumer of the terrestrial portion of the food web is a parasitoid fly.