Unequal seed dispersal by seed-size sensitive rodents: a case study in Chinese white pine (Pinus armandii)
Variation in individual seed production is high in many plant species, and it is believed to have important implications for many ecological and evolutionary dynamics. However, very few studies target the question of how this variation affects seed-dispersing animal mutualists, and in turn the fitness of individual plants. In this study, we first surveyed the intraspecific variation in seed production (n = 134) in a Chinese white pine population. Secondly, we studied the effects of this variation on seed predation and dispersal by scatter-hoarding rodents, which is the primary seed disperser/predator of this pine species. We wanted to test the following prediction: seeds from some individual trees are more likely to be dispersed by scatter-hoarding rodents, while seeds from the other individuals are more likely to be eaten in situ or ignored by rodents with no opportunity of being dispersed.
For 134 individual trees, there was very high variation in seed size, with mean seed mass ranging from 0.038 to 0.361 g among trees. This variation in seed size was correlated with neither tree size nor total seed set per tree. Furthermore, for 30 of the 134 trees that produced seeds two years later, there was a high correlation in seed size between years, implying consistent differences among individuals. For a subset of 67 trees, we monitored the fate of 15,301 seeds: ignored by dispersers (8380), eaten (4381), or cached (2249). There was a large effect of seed size, with trees producing larger seeds having a much greater proportion cached. In contrast, the proportion of seeds cached was independent of total number of seeds produced. The probability that seeds are cached is an important proximate component of fitness, because cached seeds (as opposed to ignored or eaten seeds) are most likely to give rise to successful seedlings and adults. Even though seed size has a large potential to affect the fitness of individuals, small seeds are nonetheless important as food for rodent dispersers. Therefore, the fitness of large-seeded trees could be enhanced by small-seeded neighbors that help maintain populations of mutualistic dispersers.