ECOLOGY AND GENETICS OF EPHEMERAL PLANT-POPULATIONS - EICHHORNIA-PANICULATA (PONTEDERIACEAE) IN NORTHEAST BRAZIL

Organisms adapted to ephemeral habitats often display striking variati on in population size owing to environmental stochasticity, Here we in vestigate the genetic consequences of demographic variation in Eichhor nia paniculata (Pontederiaceae), an annual tristylous aquatic that inh abits ephemeral pools, ditches, and low-lying pastures in the seasonal ly arid caatinga region of northeast Brazil, Annual censuses of popula tions demonstrated striking temporal variation in census number, There was no association between population size and heterozygosity at allo zyme loci, but a significant positive relation was evident with the ha rmonic mean of population size, Both population-size measures were pos itively associated with style-morph diversity, Large populations were more likely to contain the three style morphs at similar frequencies t han small, The amount of heterozygosity within populations was positiv ely correlated with the local density of populations in a region, pres umably reflecting greater opportunities for gene flow, Bottlenecks in population size were not severe enough to reduce genetic variation, an d population persistence versus local extinction was unrelated to the amount of allozyme variation within populations. The demographic and g enetic characteristics of ephemeral populations may be largely irrelev ant to survival where catastrophic changes to local environments throu gh drought, flooding, and human disturbance often occur.