All frugivores are not equal: exploitation competition determines seed survival and germination in a fleshy-fruited forest herb

摘要

Multiple-frugivore systems are common in deciduous forest communities, but multiple dispersal has rarely been examined from the perspective of plant fitness. We compare the consequences of fruit consumption by two co-occurring vertebrate frugivores (Odocoileus virginianus, white-tailed deer, and Procyon lotor, raccoon) for a fleshy-fruited forest herb (Podophyllum peltatum, mayapple). Shoot emergence, flowering, and fruit removal were monitored in natural populations in southeastern Ohio, USA. Survival of ingestion was tested in feeding trials. Germinability of ingested and non-ingested seeds was tested by experimental planting. Only 2.6 % of mayapple shoots flowered. All flowers appeared to initiate fruit, but 66 % aborted after 1-2 weeks. Of the fruits remaining, 82 % were removed unripe, evidently by deer. Only 5.5 % of fruit survived to ripen in mid-July at which time all were quickly removed. In feeding trials, 28 % of seeds survived ingestion by raccoons but only ca. 1 % survived ingestion by deer. Seeds hand harvested from unripe fruits had a lower proportion germinating in the following spring than those taken from ripe fruits as a result of significantly lower over-winter seed survival. Seeds ingested by raccoons showed significantly higher germination than non-ingested seeds and were more likely to be taken by secondary dispersers. It appears raccoons have the capacity to disperse a large proportion of mayapple seeds, whereas deer are primarily seed predators. By removing unripe fruit in early summer, deer are preempting late-summer consumption by raccoons thereby reducing the net fitness benefit to the forest herb. Frugivore service appears to be additive, potentially reducing herb fitness in proportion to consumption by the inferior dispersal agent.