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ANNUAL VARIATIONS IN GROWTH AND DENSITY IN NATURAL POPULATIONS OF FRESHWATER SNAILS IN THE WEST OF SCOTLAND
Author(s) -
HUNTER W. RUSSELL
Publication year - 1961
Publication title -
proceedings of the zoological society of london
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.915
H-Index - 96
eISSN - 1469-7998
pISSN - 0370-2774
DOI - 10.1111/j.1469-7998.1961.tb06175.x
Subject(s) - biology , ecology , population density , population , productivity , trophic level , demography , sociology , economics , macroeconomics
Surveyed over nine years, stable populations of Ancylus fluviatilis, Physa fontinalis and Lymnaea peregra , all with annual cycles, showed variation from year to year in mean size and in population density at the breeding season. This variation in adult mean size is about twice as great in L. peregra (i.e. approximately ± 17.5 per cent) as in the other two species. Fluctuations in population densities at breeding can be five‐fold in Ancylus and L. peregra and three‐fold in Physa. Annual variations in assessed productivity (as volumes of molluscan tissue per unit habitat) can be over seven‐fold in Ancylus , more than three‐fold in Physa , and over eight‐fold in Lymnaea peregra . With exceptions, there is general agreement between the annual figures for the different species, and they can be correlated with certain of the weather conditions prevailing during the preceding year. Severity of the winter appears the most important climatic cause of these molluscan fluctuations, particularly in Physa and L. peregra. Annual variations in Ancylus seem more closely related to spring sunshine, and a possible trophic basis is suggested for this difference between the species. In relating this to other studies the interaction of external and endogenous factors in controlling life cycles is briefly considered. It is suggested that the plasticity in growth cycles and reproductive behaviour shown by freshwater snails is of fundamental selective value. Measurements of variations in growth and density in molluscan populations are proposed as a means of assessment of annual variations in total organic productivity of fresh waters.