The relative and absolute shifts in abundance of five species of freshwater gastropods appear to reflect widespread changes in the drainage basin around Lago de Peten. Fluctuations in the number of shells deposited in 3.7 m of lake sediments are not directly correlated with changes in preserved organic matter or major cations (calcium, magnesium, and sodium), but some long term trends are documented. Two types of changes in molluscan species composition occur: abrupt oscillations in the numbers of individuals in two species (Tryonia exigua and Cochliopina infundibulum) and gradual increases in three others (Pyrgophorus coronatus, Biomphalaria havanensis, and Stenophysa spiculata). Mechanisms producing these changes appear to be major fluctuations in nutrient inflow and sedimentation rates. These factors probably influence competitive interactions among the gastropods; indirect effects may have altered vulnerability of the mollusca to predators. The controlling variable for nutrient inflow and rates of siltation seems to be extensive deforestation by ancient and modern Mayan agriculturalists.
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Alan P. Covich (1976) studied this question.