COS 1-1
Tails of travelling seeds: Effect of vector species and seed charactersitics on patterns of dispersal of aquatic plants
Plant populations in isolated habitats frequently rely on dispersal by highly mobile animals. Freshwater wetlands are one such type of patchy ecosystem and recent evidence has shown that many aquatic plants are readily dispersed by waterbirds. However, not all plants are dispersed equally and we hypothesize that dispersal outcomes will be strongly influenced by both disperser and propagule characteristics. Egyptian Goose (Alopochen aegyptiaca) and Red-billed Teal (Anas erythrorhyncha), two common southern African waterbird species that differ markedly in size but have similar diets, were fed the seeds of seven wetland plants species. Seven replicates of 100 seeds per plant species were fed to each duck species (a total of 9800 individual seeds). Gut passage of ingested seed was monitored for 36 hours by collecting fecal samples at specific time intervals. Intact seeds were counted and set to germinate in wetland conditions. Using Generalized Linear Mixed Models (GLMMs) we examined the effect of disperser species, seed size and seed hardness on three derived dispersal characteristics: 1) retention time of seeds; 2) recovery of intact seeds and 3) germination of recovered seeds. Finally we derive the corresponding dispersal kernels for each seed species to explore differences in eventual dispersal distributions.
Results/Conclusions
30% of seeds fed to the waterbirds were recovered intact and nearly 30% of these recovered seeds germinated. Our models indicate that the dispersal distance of a particular plant species is most strongly affected by disperser species and not seed traits. Egyptian Goose retained all seed species for a longer duration than Red-billed Teal and the resultant dispersal kernels showed seed dispersal by Egyptian Goose occurs at nearly twice the distance. Whilst smaller seeds had significantly longer retention times, likely due to increased surface area to volume ratios, this did not affect dispersal distance as a result of a trade-off between increased retention time and a decrease in germination. Smaller seeds were also recovered in the highest numbers due to lower digestibility, suggesting a possible mechanism whereby smaller seeds can circumvent the negative effects of increased retention times. There was no effect of disperser or seed traits on eventual germination, and differences in germination were best explained by plant taxon. By exploiting different disperser species wetland plants can be moved at different scales with little cost to germination. We discuss the implications for the role that waterbirds play in maintaining aquatic connectivity in ephemeral habitats.