Ecological Significance of the Shapes of Abundance‐Frequency Distributions for Coexisting Phytoplankton Species

Abstract

Abundance—frequency distributions for 31 phytoplankton species form Lake Lanao, Philippines, are constructed on the basis of 65 weekly samples. All of the distributions have a positive skew. The shapes of the distributions are quantified in terms of the coefficient of variation and a skew statistic. CV (the coefficient of variation) and skew are highly correlated for the 31 species and vary over approximately an order of magnitude between species. High CV and skew are interpreted as evidence of an opportunistic demographic strategy and low CV and skew as evidence of conservative strategy. Relationships are tested between CV or skew and three sets of variables: (1) mean abundance measured as biomass or numbers, (2) morphofunctional properties such as size and surface: volume ratio, and (3) niche descriptors. Niche descriptors are variables which give the average position of a species on an environmental gradient when net growth of the species is positive. The results of the analysis on the 3 sets of variables are as follows. (1) The mean abundance of a species is not statistically related to the CV or skew of its frequency distribution. This indicates that rate species are as likely to have an opportunistic strategy as common species. (2) Three specific morphofunctional characteristics (volume, surface area, and S:V ratio) are not related to CV or skew, but two general morphofunctional characteristics (morphotype and taxonomic affinity) are related to CV and skew. Filamentous morphotypes and diatoms have the most opportunistic patterns (high CV and skew), while coenobial morphotypes and chlorophytes have the most conservative patterns. (3) Among 5 different niche descriptors, CV and skew are related only to those which incorporate some measure of attrition, especially grazing. Periods of high attrition correspond with success of conservative species, and periods of low attrition with succes of opportunisticii species. Attrition due to grazing is the mechanism by which the general morphofunctional variables (morphotype, taxon) are related to shape of the abundance—frequency distribution.