Plants reward seed dispersers in proportion to their effort: the relationship between pulp mass and seed mass in vertebrate dispersed plants

In this paper I develop a null model for the expected relationship between seed mass and the mass of dispersal structure (reward) for vertebrate-dispersed plant species. The model is based on the simple assumption that the reward associated with a given seed mass is commensurate with work required to move it, and predicts that reward mass should scale relative to seed mass with an exponent of 4/3 (1.3). I tested this relationship between- and within-species of vertebrate-dispersed plants from four families from tropical rain forest in north Queensland, Australia. At a community-level there was a significant isometric relationship between log mean pulp mass and log mean seed mass across species. When family membership was considered, the estimate for the common slope between families was 1.32, surprisingly similar to the exponent predicted from commensurate reward. In addition, the 95% CI of the common slope did not include unity, providing no support for isometry. There was also no evidence that the relationships between mean log pulp mass and mean log seed mass were significantly different between families. This simple null model may be a common “rule” governing mean allocation to reward in all plant–animal dispersal mutualisms and its confirmation is the first evidence that animal dispersers have shaped the evolution of seed traits. However, I found no evidence that the scaling relationships within-species were consistently predicted by commensurate reward – a “taxon-level effect”. I suggest that the taxon-level effect arises because mean seed and mean reward mass within each species arises due to community-wide, disperser-mediated selection to produce equally attractive fruits, whereas within-species allometries may be determined by selection for fruit traits that enhance either dispersal probabilities, offspring survival or both, and these will be contingent on the environmental context into which seeds are released