View - Kowalewski, M. - Virginia Tech

BAMBACH—SUPPORTING PREDATORShave always been a minority of the number ofgenera, but their diversity history has followed thesame general outline as all Metazoa combined. But,as can be seen by the relative positions of the twocurves, the proportion of diversity comprised ofpredators clearly increased during the Ordovicianand also in the Cenozoic. This is illustrated directlyin Figure 19b, which shows the calculatedproportion of global marine diversity comprisedof predators through the Phanerozoic. The diversityof predators as a proportion of total metazoandiversity has increased, but in a stepwise fashion,not continuously. The proportionality follows athree phase sequence of transitions and relativelystable intervals, parallel to that recently reportedby Bambach and Knoll for other parsings of theglobal marine fauna (Bambach and Knoll, 1997;Bambach, Knoll, and Sepkoski, 2002). During theCambrian to mid-Ordovician interval, predatorsincreased continuously as a proportion of allmetazoan genera, from less than five percent inthe earliest Cambrian to twenty percent in the mid-Middle Ordovician before their proportion of totaldiversity dropped slightly as the OrdovicianRadiation leveled off. Then, for the 200 millionyears from the start of the Caradoc to the end ofthe Permian, the proportion fluctuated around amean of 0.15 (15%), never rising over 0.19 or fallingbelow 0.10 despite the changes in total diversity andthe turnover and replacement of taxa that occurredduring that time. Selectivity in the end-Permianextinction left the survivors in the earliest Triassicwith an unusually high proportion of predators(0.275). The apparent trend from lower to higherproportion of predators in the Late Permian mayjust be a fluctuation from a low proportion, butwithin the established range (similar to thefluctuation in the Early Carboniferous); but it couldalso include a “Signor-Lipps Effect” influencerelated to apparent early truncation of some rangesin the highly selective end-Permian extinction. Thenew proportion of predators created by theselectivity of the end-Permian extinction did notpersist. During the Early and early Middle Triassicthe proportion of predator taxa decreased, andpossibly by the beginning of the Carnian, butcertainly in the Early Jurassic (after the recoveryfrom the perturbation associated with the end-Triassic extinction), the proportion of predator taxastabilized—but at a new proportionality. From thestart of the Carnian until the end of the Cretaceousthe proportion of metazoan marine generacomprised of predators averages 0.219 with nointerval higher than 0.238 and only two (at the endof the Triassic and the start of the Jurassic) fallingbelow 0.206. In fact, except for the two intervalsat the Triassic-Jurassic boundary, no other intervalsin the entire post-Paleozoic have a proportion ofpredator taxa that overlaps with any in the 200-million-year interval extending from the LateOrdovician through the Permian. Following the end-Cretaceous extinction another transitional intervaloccupied the Paleocene, and from the Eocene to theHolocene the proportion of metazoan diversitycomprised of predators has always exceeded 0.3,with a mean of 0.36 during the Neogene.Predators and Energetics.—The increase froman average of fifteen percent predator taxa in thePaleozoic to more than thirty percent in the laterCenozoic implies there has been a major change inthe structure of the marine ecosystem over time. Ifthe efficiency of energy transfer between levels inthe food pyramid has stayed nearly constant (andthere is no evidence that it has not), then a reasonablehypothesis for the increased proportional diversityof predators is that either the biomass of prey specieshas increased or the populations of prey taxa havebecome more stable (or both), permitting greaterspecialization by predators on more reliable foodsupplies. Thus, predators could increase theirproportional share of total diversity by specializingon fewer prey taxa. In some predator groups thathave increased diversity markedly in the Cenozoic,specialization is clearly documented. Two examplesare the remarkable niche subdivision achieved bythe genus Conus in Hawaii (Kohn, 1959) and therange of highly specialized claw morphologiesevolved in the crabs (Warner, 1977). Vermeij hasdiscussed these and a number of related conceptsat length, emphasizing especially (and importantly)the coevolutionary responses of escalation(Vermeij, 1987, 1994).345