Can Seed Hardness Be an Ecological Filter in Seed Dispersal by Rodents?

Seed hardness has been acknowledged as one of the most significant physical traits influencing seed consumption and caching by animals. From an evolutionary perspective, a hard seed shell should potentially be regarded as a dispersal and predator escape strategy rather than a maladaptive trait of plants. However, to our knowledge, this prediction has not been adequately tested in the context of seed dispersal. Here, we offered seeds with hard shells to the main seed consumers widely distributed in two distinct forest ecosystems to test our hypothesis that a hard seed shell might act as an ecological filter to deter predators but attract obligatory seed dispersers. Our studies demonstrated that seeds with the hardest shells consistently and directly deterred small-bodied seed consumers that have been proven to be either larder-hoarders or scatter-hoarders in the two forests. Nevertheless, rodents with the largest body size and seed handling capacity seemed to be effective seed dispersers targeting hard seed shells. The deterrence to seed predators and the attraction to an effective seed disperser reflects the evolutionary significance of seed hardness in the seed dispersal syndrome. Our studies in different forest ecosystems strongly suggest that a hard seed shell is not an evolutionary dead end in plant–animal interactions. On the contrary, the outcome of a hard seed shell in the seed dispersal syndrome is of evolutionary importance for plant–animal mutualistic interactions in various forest ecosystems.