View - Kowalewski, M. - Virginia Tech

BENGTSON—ORIGINS AND EARLY EVOLUTION OF PREDATIONPREDATION AND DIVERSITYIN THE EARLY BIOSPHEREAn imaginary biosphere without predatorswould be very different from what we areacquainted with. There would be various kinds ofphoto- and chemoautotrophs making use ofavailable energy gradients to reduce carbon forenergy storage and constructional/physiologicalpurposes. There would be organisms scavengingexcess organic matter, but there would be noorganisms directly interrupting the lives of othersby pilfering their tissues.Leaving aside the question of whether such aShangri-La for primary producers anddecomposers is even theoretically possible, theselective pressures would be very different fromthose that affect most organisms today. Survivalrequirements would center around positioningoneself with respect to chemical, temperature, andlight gradients, and the only need to move wouldbe in order to adopt to shifting gradients—forexample, varying light intensities or redoxboundaries. Although competitive interactionswould not be excluded, they would mostly berelated to relative efficiencies of energy conversionsystems. Under such circumstances diversitieswould be low and stable. The most complex benthicecosystems would likely be layers of physiologicallydifferentiated microbes, i.e., microbial mats. In theplankton, diversities would possibly be even lowerbecause of the movement and mixing of watermasses, which reduces spatial heterogeneity.The cropping principle (Stanley, 1973a, 1976b)suggests in its general form that the introductionof predation into a low-diversity ecosystem willcreate a self-propagating feedback system ofdiversification. Stanley specifically discussed theappearance of cell-eating heterotrophy amongplanktic protists, which in his view may have beenthe driving force behind the eventual burgeoningof multicellular organisms and the Cambrianexplosion. The phenomenon of predation-inducedvariability is well established in different kinds ofecosystems (see references in Stanley, 1973a,1976b; as well as Porter, 1977; Kitchell, 1983;Richards et al., 1999), and we may ask the moregeneral question of whether some of the majordiversity changes in the early fossil record werepredator-induced.Theories to that effect abound. At the base ofthe bush of life, the origin of cells has beeninterpreted as a symbiotic or predatory event(Maynard Smith and Szathmáry, 1995; Scudo,1996; Cavalier-Smith, 2001). The origins ofeukaryotic cells, multicellularity, and hard tissuesare commonly interpreted to be primary results ofpredation (see below). These evolutionaryinnovations clearly had a great effect on diversityby introducing new kinds of organisms withunexploited capabilities of diversification.Less dramatic, but perhaps stronger in longtermeffect, are the diversity effects caused by thedynamics of predator–prey interactions atestablished levels of organization. Although thediversity effects of such interactions are commonlydescribed in the ecological literature in terms ofequilibrium models (where the predator–prey ratiois drawn toward a stable value), this may not be agood description of natural systems. Predators maydrive their prey to local extinction (Katz, 1985) ormake them more susceptible to extinctions by otheragents (Schoener et al., 2001). The net effects ondiversity are dependent on a number of factors,such as the existence of refuges, the selectivity andintensity of predation, etc., but as a general rule,selective predation on dominant species increasesdiversity (Kitchell, 1983). Competitive interactionmay also influence diversity, though its effects mayhave been overstated in the past (Gould andCalloway, 1980; Benton, 1983). In the end,diversity may be less dependent on direct effects,such as those of predator–prey interactions, thanon more-or-less complex cascades of indirecteffects of biotic interactions (Menge, 1995).There are of course also environmental (sealevel, temperature, oxygen level, nutrientavailability) and preservational parameters thataffect diversity, and these may or may not beindependently analyzed. Predation itself may biasthe apparent diversity in the fossil record. Preyeaten by predators may be totally destroyed and291