The secondary dispersal of forest herb seeds in raccoon feces

Background/Question/Methods

A high density of seeds in feces may negate some benefits of animal dispersal by attracting seed predators or initiating density dependent seedling mortality. Secondary movement after deposition changes the final microsite in which a seed comes to rest, potentially reducing seed density. We followed mayapple (Podophyllum peltatum) seeds deposited in raccoon (Procyon lotor) feces in long-established deciduous forest to understand the role of secondary seed dispersal in changing dispersal outcomes detrimental to plant fitness. To observe seed movement by water and animals, marked seeds were incorporated into raccoon feces and seed displacement was observed for five weeks. The experiment was deployed in a factorial design to test the influence of slope, litter presence and proximity to a tree, factors previously shown to affect movement of forest herb seeds.  Motion/heat triggered cameras were also trained on feces with incorporated mayapple seeds at six sites to determine identity and frequency of animals foraging in raccoon feces in the local system. 

Results/Conclusions

The feces began to disintegrate and shed seeds within one week after experimental placement. Photos showed mayapple seeds actively removed from the feces within 24 hours by white-footed mice, chipmunks, and birds. At the beginning of winter, up to 50% of seeds remained at the site of feces placement, apparently moved in a diffusive pattern by water, or inadvertently by foraging animals. 30% percent of seeds remained within 20 cm of the initial microsite, 20% percent were found up to 100 cm away. Seeds within 5 cm were frequently incorporated into the soil to an average depth of 1cm by arthropod foraging. Microsite slope and leaf litter presence significantly affected seed distance. Seeds in feces on slopes no litter moved farther than those on level soil with leaf litter. Proximity to a tree had no significant effect. Seed movement was arrested by microtopographic indentations in the soil and duff, or adherence to leaf surfaces.  Although seed dispersal of forest herbs by vertebrates can result in initial high seed density, secondary dispersal mechanisms leave few seeds at the initial microsite of deposition potentially reducing negative density dependent effects.