Effect of passage through duck gut on germination of fennel pondweed seeds

Vertebrates are important seed dispersers for many plants. In addition to transport of seeds, ingestion often affects the proportion or rate of seed germination. We present one of the first studies comparing the effects of different waterbird species on the seeds of a subcosmopolitan pondweed, Potamogeton pectinatus. We also present the first comparison of the effects of digestion by ducks (mallard Anas platyrhynchos, shoveler A. clypeata and wigeon A. penelope) and physical-chemical "simulation of digestion" on pondweed seed germination. In two experiments differing in the length of the preceding stratification period, two to three individuais per duck species were force-fed 150 seeds each. Average retrieval, total germination and germination rate did not differ significantly between duck species. Germination rate was higher for duck ingested seeds, intermediate for scarified seeds (i.e. after mechanical removal of the epicarp+mesocarp) and lowest for the controls and acid treated seeds, independently of the length of the stratification period. Total germination, however, did not differ significantly among duck-ingested, scarified, control and acid treated seeds. Consequently the changes in germination rate after ingestion by ducks seem related to the grinding treatment in the gut and unrelated to exposure to acidic conditions. The coexistence of ingested and uningested seeds within a given seed cohort will increase the diversification of seed germination patterns, which can favour the colonisation of habitats characterised by unpredictable environmental conditions.