View - Kowalewski, M. - Virginia Tech

LABANDEIRA—PREDATORS, PARASITOIDS, AND PARASITESInvertebrates in general, and insects in particular,dominate continental food webs; among insects,approximately 50 percent of all species areherbivorous and 25 percent are predaceous (Hagen,1987; Wilson, 1992). Site-specific studies providesimilar values (Erwin and Scott, 1980).Nevertheless, the comparative scarcity of evidencefor carnivory in fossil assemblages should not beliethe intricate, diverse, and fascinating roles playedby predation, parasitoidism, and parasitism incontinental ecology during the past 420 millionyears. It is for this reason that an understanding ofthese three types of feeding strategies, subsumedunder “carnivory,” is important.The common conception of predation involvesa consumption of prey resulting in the victim’sdeath. Predation is defined as the total or near-totalconsumption of a live organism (Price, 1980), butalso includes suspended zooplankton in aquaticsystems (Merritt and Cummins, 1984), plantorganisms such as seeds (Janzen, 1978), and pollenor spores (Fuchs, 1975). Both seed predation andpalynivory are nutritionally important for manycontinental arthropods (Proctor et al., 1996), andrepresent feeding strategies that are distinct fromtypical herbivory, which is characterized by survivalof the host plant. Conventional continental predatorsare animals that directly and immediately consumeother animals (Hagen, 1987; Sih, 1987), althoughtrophically equivalent modes are seen in fungi(Evans, 1989) and even plants (Juniper, 1986) thatslowly dispatch their animal prey. Organisms thatfeed on invertebrate prey more slowly andincompletely are known as parasitoids (Godfray,1994): in parasitoidism, a relatively long-lived hostis eventually consumed and dies. A third type ofcarnivory is parasitism (Schmid-Hempel, 1998),characterized by multiple hosts that survive andare only partially consumed. Consumption ofvegetative tissues which does not result in the deathof the host plant has also been termed parasitism(Schoonhoven et al., 1998), but is excluded herebecause of its equation with herbivory in theliterature and its status as the contrasting trophicmode to carnivory. Additionally, the consumptionof suspended plankton and benthic films in aquaticsystems by microvorous filter feeders, sievers, andscrapers includes the consumption of minuteanimals, but is often ecologically equated withheribvory and will not be treated here further (seeMerritt and Cummins, 1984).The life histories of parasitoids are ecologicallytransitional between predators—which rapidlyconsume multiple and often unrelated individualsduring a lifetime—and parasites, which feed throughseveral generations on a near-infinite food resourcefrom mostly single, generally surviving individuals(Price, 1980; Vinson and Barbosa, 1987). Thus,parasitoids pursue an ecologically intermediatestrategy of feeding generally on a predictable andfinite food resource consisting of a single host thatis eventually killed; moreover, they are characterizedby a life cycle that includes a free-living reproductivestage whose habitat is separate from that of thedeveloping larva (Yeates and Greathead, 1997).Additional distinctions between predators andparasites help to define the role of parasitoids.Whereas predators are similar in size to their prey,parasites are quite small compared to their hosts.Also, predators typically are generalized feedersthat acquire and dispatch their prey by force; butparasites generally are tissue specialists that areoften coevolved with their hosts. Lastly, predatorsare characterized by fewer and broader ecologicalniches than are specialist parasites, which typicallypartition monospecific hosts into multiplemicrohabitats. The host-accommodation exhibitedby parasites and parasitoids, in contrast to the rapidconsumption of prey by predators, is a major themein the evolutionary history of invertebrate carnivoryin continental ecosystems.TYPES OF EVIDENCEMuch of our understanding of the paleobiologyof carnivory is inferential, relying on imperfectpreservation, circumstantial evidence, and rareinsights gleaned by way of unique and spectacularmodes of preservation. These data are principallyderived from Fossil-Lagerstätten such as silicifiedhot-spring deposits, sideritic nodules withinorganic shales, ambers, and lacustrine oil shalesthat have particular spatiotemporal distributions in213