Many large species have declined worldwide due to habitat fragmentation and poaching. The defaunation of large frugivores and the consequent impairment of seed dispersal may have immediate effects on plant populations, such as the reduction of fruit removal, shorter seed dispersal distances and lower plant recruitment. Size-biased defaunation of seed dispersers may also lead to directional selective pressures on plants because small-bodied animals can only ingest smaller seeds, triggering rapid evolutionary changes in seed size. Although smaller seeds can be dispersed by a high diversity of frugivores; larger seeds produce larger seedlings with higher competitive capacity. Plants with larger seeds may increase the survival probability particularly under future climate change scenarios. Therefore, it is important that advantageous phenotypes are rescued in already impacted populations. We evaluate the potential of phenotypic rescue of a bird-dispersed palm (Euterpe edulis) that present seed size reduction due to defaunation of large frugivores. We used simple theoretical models to simulate the success of two restoration strategies: seed addition - restoration of populations' phenotypic variation by adding missing phenotypes that were originally found in the species or restoration of populations' phenotypic variation by the reintroduction of large-bodied frugivores.
We found that by adding missing phenotypes only, mean seed size values of the palm population decreased back to original in only two generations. Thus, this strategy would probably be unsuccessful over time due to low probability of large seeds being dispersed in defaunated scenarios. Conversely, large seed sizes could be rescued in approximately five generations after seed dispersal processes generated by large seed dispersers were reestablished. Thus, to rescue extreme phenotypes such as large seeds in palm populations is necessary to restore the seed dispersal processes by large frugivores. The most effective strategy to restore phenotypic vatiation in seed size is to rewild defaunated forests with large frugivores. Most tree species in tropical forest are animal dispersed and defaunation of large frugivores is pervasive across fragmented and also non-fragmented forests. Thus, restoring the seed dispersal processes by large frugivores will benefit several bird-dispersed species and may be crucial to face ongoing global change scenarios.