Abstract
To investigate the application of the size-frequency distribution in paleoecology, the size-frequency distributions for a series of assemblages obtained from two cores taken in Copano Bay, Texas are examined from four perspectives. These four perspectives are the size-frequency distribution of each of the dominant species and three measures of the size-frequency distribution for the assemblage as a whole, defined according to the maximum size of the species, the maximum size of the assemblage, and the maximum size of a group of assemblages. In each case, the size-frequency distribution was evaluated in terms of abundance and converted by a hindcasting model to two indices of trophic structure, paleoproduction and paleoingestion. Significant temporal changes in community structure identified in this way included changes in environmental optimality, food supply, persistence of population dynamics, and the division of resources amongst small and large species. Paleoingestion was consistently bimodally distributed when plotted by size class because small and large species contributed substantially to energy flow. Only the larger size classes accounted for significant paleoproduction. Thus the smaller size classes included small, long-lived species that reached near-maximum size. Habitat optimality was identified by examining the degree to which individuals reached a fraction of their species' maximum size. Significant downcore changes in habitat optimality occurred only for the large species in the assemblage; those potentially most affected by changes in food supply. Species replacement in the community included not only guild replacement but also size replacement. The size-frequency distribution for the community was persistent over hundreds of years whereas those for the individual species frequently were not. So, too, was the distribution of paleoproduction and paleoingestion amongst the size classes. Thus, unlike the population dynamics of each species, the population dynamics and the distribution of energy flow through the constituent size classes was a temporally persistent feature of community structure and represents a valuable parameter in the analysis of the fossil record. A change in assemblage maximum size likely records variations in food supply. Variations in assemblage maximum size and the replacement of chemoautotrophic species with filter-feeding ones suggests that a substantial increase in food supply occurred over the last 400 yr at this site.
Published Version
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