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1 The annual plant Eichhornia paniculata occurs in discrete, ephemeral habitats formed by pools, wet ditches and flooded pastures in arid north-eastern Brazil. We conducted a large-scale geographical survey of populations four times over a 7-year period (1982-89) and measured population size, population persistence and patch occupancy. In total, 167 populations were censused.

2 To investigate the importance of local and regional influences on population size, we posed the following specific questions. Are fluctuations in population size independent of their initial size and independent among years? Is persistence uniform among populations of different size and age? Are the proportion of patches occupied related to the density of habitat patches in a region? What are the relations between the size, persistence and density of populations?

3 Population size averaged 86 over the 4-year period with 52% of populations containing less than 100 individuals. Sixty-four per cent of populations persisted from one year to the next, but the rate at which populations became absent from a patch was independent of initial population size and time since the last census. For populations that persisted, there was a significant positive correlation between the initial census number and their size in subsequent years.

4 In 29 transects through different regions of north-eastern Brazil, an average of 21.6% (range 3.8;1 7.2%) of suitable habitat patches were occupied by E, paniculata. The proportion of patches occupied was positively correlated with the density of patches in a region. No populations were found when the density of patches fell below 0.23 patches km(-1) or 0.18 patches km(-1) in 1988 and 1989, respectively, indicating the probable existence of a habitat threshold for species persistence within a region. There was no correlation between patch occupancy and either the average number of individuals per population or the probability of persistence in a region. Hence, even when E. paniculata is regionally common, it is not necessarily locally abundant.

5 We conclude that the distribution of E. paniculata populations in north-eastern Brazil is governed, in part, by metapopulation dynamics.