Pleistocene molluscan paleoecology and community structure of the Puerto Libertad region, Sonora, Mexico

Abstract

An intertidal Pleistocene sandflat community is analyzed in terms of community structure, survivorship, inferred biomass distributions, predation patterns, and trophic structure. The distributions of the first-level consumers was controlled by substrate grain size. The distributions of the predators mirrored the concentrations of their prey. Because of differing burrowing abilities, the Libertad infauna produced a community stratification. This property concentrated large segments of biomass at different levels within the substrate. This mechanism buffered biospace-related competition. When similar interspecific depth habitation zones overlapped, one or the other taxon was density limited. Survivorship curves constructed from size-frequency data and the function D = S 1n ( T+1) demonstrated that Lucina approximata experienced constant mortality; Chione califonniensis had a well-developed bimodality of age distributions and Tagelus californianus had increasing population mortality. These contrasting mortality patterns were related to depth of habitation and substrate stability. The internal volumes displaced by a growth series of selected taxa were used to estimate biomass distributions. In terms of biomass, the dominant community members had largest volumes in juveniles — a characteristic of a stable population. Other accessory taxa had large volumes in adults — a property of stationary populations. Theodoxus lutteofasciatus had constant mortality, Cerithium stercusmuscarum, Nassarius tiarula and Natica chemnitzii had curves similar to Chione. Predation patterns demonstrated that two trophic subwebs existed among the predators. Muricanthus nigritus, Hexaplex erythrostomus and Melongea patula preyed on Ostrea palmula, Megapitaria squalida and Cerithium. Natica consumed the juveniles of Chione, Lucina, Nassarius and Theodoxus. The annelid predators Conus regularis, Acteocina inculta and Bulla gouldiana composed a third subweb, not attacked by any molluscan carnivore. The highest predation rate occurred in Nassarius, the lowest among the deep infauna. Differing food preferences by predators served as a stabilizing mechanism maintaining community homeostasis by providing alternative food reserves. It was shown that bore-hole loci changed through ontogeny in selected taxa. Likewise, borehole patterns differed slightly on right and left valves of the same prey species. In general predation rates declined rapidly with advancing age. Highly localized borehole sites were found in Theodoxus, Natica and Nassarius — the prey of Natica. Cerithum, the prey of Muricanthus, had less localized borehole loci. This suggests the latter's predator was less “sensitive” to prey- “induced” responses for selective manipulation. The Libertad community declined when the substrate closed to recruitment by the accumulation of the shells from previous generations.