Spatial and temporal variation in population size of Eichhornia paniculatain ephemeral habitats: implications for metapopulation dynamics

1 The annual plant Eichhornia paniculata occurs in discrete, ephemeral habi tats formed by pools, wet ditches and flooded pastures in arid north-easter n Brazil. We conducted a large-scale geographical survey of populations fou r times over a 7-year period (1982-89) and measured population size, popula tion persistence and patch occupancy. In total, 167 populations were census ed. 2 To investigate the importance of local and regional influences on populat ion size, we posed the following specific questions. Are fluctuations in po pulation size independent of their initial size and independent among years ? Is persistence uniform among populations of different size and age? Are t he proportion of patches occupied related to the density of habitat patches in a region? What are the relations between the size, persistence and dens ity of populations? 3 Population size averaged 86 over the 4-year period with 52% of population s containing less than 100 individuals. Sixty-four per cent of populations persisted from one year to the next, but the rate at which populations beca me absent from a patch was independent of initial population size and time since the last census. For populations that persisted, there was a signific ant positive correlation between the initial census number and their size i n subsequent years. 4 In 29 transects through different regions of north-eastern Brazil, an ave rage 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 correla ted 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 h abitat threshold for species persistence within a region. There was no corr elation between patch occupancy and either the average number of individual s per population or the probability of persistence in a region. Hence, even when E. paniculata is regionally common, it is not necessarily locally abu ndant. 5 We conclude that the distribution of E. paniculata populations in north-e astern Brazil is governed, in part, by metapopulation dynamics.