COMPARATIVE DEMOGRAPHY AND POPULATION-DYNAMICS OF 2 COEXISTING COPEPODS IN A VENEZUELAN FLOODPLAIN LAKE

I used an inverse matrix projection model to estimate vital rates (growth, survival, and reproduction) for two coexisting copepod species in a Venezuelan floodplain lake as a first step in identifying the ultimate environmental factors that determine distribution and abundance of a species. Zooplankton were sampled four times a week from June through December 1984, and the resulting abundance data were used together with the estimation technique to extract time series of vital rates over a 6-month period. There were marked temporal differences in population maxima of each species. Abundance of Diaptomus negrensis fluctuated with changes in recruitment and mortality of stages NV, NVI, CIII, and CIV. Peak abundance of D. negrensis coincided with a peak in the abundance of the invertebrate predator Chaoborus sp. In contrast, population size of Oithona amazonica was unrelated to recruitment but directly related to survival of the youngest copepodite stages, which was poor until October when abundance of Chaoborus declined. These demographic analyses target the stages most susceptible to environmental (selection) pressures, providing a basis for understanding the evolution of life-history strategies, and suggest the hypothesis (as yet untested) that prolonged coexistence is facilitated between species whose abundance is determined by different demographic (and thus environmental) factors.