Directed seed dispersal and metapopulation response to habitat loss and disturbance: application to Eichhornia paniculata

1 Seed dispersal is often directed towards locations with particular characteristics, particularly where seeds are dispersed by animals. The potential importance of directed seed dispersal for the response of plant metapopulations to habitat loss and changes in disturbance rate is assessed, and illustrated with a case study of a metapopulation of an aquatic plant. 2 The Levins model is extended to include preferential dispersal towards suitable habitat and towards unoccupied patches. Both increase patch occupancy, decrease the minimum habitat cover required for persistence and increase the maximum allowable disturbance rate, while preferential dispersal towards unoccupied patches also makes reductions in abundance due to increased disturbance rates more threshold‐like. 3 Applying classical metapopulation approaches, which assume random dispersal, to a species that features directed dispersal, is expected to give systematic errors in prediction. For example, where dispersal is directed towards suitable habitat regardless of occupancy, the Levins model will tend to overestimate the response to habitat loss, and where dispersal is directed towards unoccupied patches, the Levins model will tend to underestimate the response to changes in disturbance rate. 4 Eichhornia paniculata is an aquatic plant restricted to ephemeral pools. Seed dispersal is by waterfowl, and so is directed towards suitable habitat. Data on habitat cover and patch occupancy from Husband and Barrett in 1998 fit well with our model but deviate significantly from the predictions of the Levins model. The parameter estimates imply dispersal very strongly directed towards suitable habitat, and that without this the minimum density of pools required for persistence would be at least 10 times greater than the highest densities observed, implying that the species would go locally extinct. 5 To our knowledge, this is the first quantitative demonstration that the nature and strength of directed dispersal affects the robustness of fragmented plant populations to anthropogenic disturbance. This calls for increased attention to be paid to the behaviour of seed‐dispersing animals, and how this behaviour varies between different plant communities.

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