From predators to dispersers: How variation in seed size alters the outcome of the interaction between scatter-hoarding rodents and an Atacama Desert shrub
Myrcianthes coquimbensis is a large-seeded, fleshy-fruited shrub, endemic to the Atacama Desert. This species has no present-day dispersers; its only vectors of seed dispersal are scatter-hoarding rodents that can act both as predators and dispersers, depending partly on fruit size, which is highly correlated with seed size. Rodents are likely to hoard -and potentially disperse- large seeds because of their higher nutritional value, but this preference can be counterbalanced by the higher costs of handling and transporting larger seeds. We designed a laboratory experiment to assess explicitly whether the foraging behavior, and ultimately the role of rodents as predators or dispersers, depends on the size of both seeds and rodents. Additionally, we examined whether seed size affected emergence probabilities of seeds with simulated partial predation. As a model, we used Octogon degus, Phyllotis darwini and Abrothrix olivaceus; species that consume M. coquimbensis seeds and that in natural conditions, face the decision of choosing fruits ranging from 0.5 to 15 g.
Seed size was positively related to the probability of being selected and hoarded for all rodent species. Hoarded seeds were 3x larger than non-hoarded ones; therefore large seeds are more likely to be dispersed. Additionally, smaller seeds had higher probabilities of being consumed in situ. Contrary to expectations, P. darwini and A. olivaceus consistently selected and hoarded larger seeds than the larger O. degus. The latter also consumed more seeds in situ than the smaller species, and was more likely to consume the seeds it hoarded; thus, compared to the other two species, it acts more as a predator than a disperser. Seed size influenced the probability of emergence from seeds with simulated partial predation: larger seeds were more likely to emerge than smaller ones. We conclude that scatter-hoarding rodents are selecting for large seeds. Moreover, large seeds are more likely to recruit after partial consumption than smaller ones, which may explain the maintenance of large seed size of M. coquimbensis in a desert ecosystem, where having large, recalcitrant seeds would otherwise reduce recruitment probabilities.