ReviewLoss of seed dispersal before the loss of seed dispersers

- Changes in the effectiveness of seed dispersers can be triggered by disturbance.
- Changes in behavior, population demography, and phenotype alter seed dispersal roles.
- Seed dispersal effectiveness is usually negatively impacted by disturbance.
- We have a poor understanding of when seed dispersal roles are affected positively.
- Disturbance alters both the quantity and quality of seed dispersal effectiveness.

Cryptic function loss occurs when the ecological function of an animal population is significantly altered as a result of disturbance, even though the species is still present in the ecosystem. We reviewed the evidence for cryptic function loss to be widespread among seed disperser populations that persist under disturbed conditions. We found overwhelming support for the seed dispersal effectiveness of animals to be negatively impacted by all forms of disturbance (population decline, changes in community assemblages, habitat change, and climate change). However, seed dispersal was positively affected in some examples, particularly when extirpation of an interacting frugivore or predator enhanced fruit consumption. It is essential that we improve our understanding of the mechanisms by which seed dispersal is lost, even though the disperser's population remains viable, as there are many unanswered questions that hinder an objective understanding of this threat. We need to consider how seed dispersal effectiveness is determined by species population size and small changes in phenotypes. In addition, we must increase our understanding of behavioral impacts on seed dispersal and how it might be regulated by disturbance. The complexity and flexibility of seed dispersal interactions could mean that communities are adaptable to many changes; alternatively, ecosystems might be severely threatened by cryptic function loss because of the myriad ways in which multiple frugivore populations can be simultaneously affected and the fact that early signs of function loss can be hidden by stable population sizes.