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Keywords:

  • Białowieża Forest;
  • ecosystem services;
  • functional niche;
  • mutualistic networks;
  • optimal foraging;
  • plant–animal interactions;
  • resource specialization;
  • seed dispersal

Summary

  1. Seed dispersal by frugivores is the basis for regeneration of fleshy-fruited plants in forest ecosystems. Previous studies have reported a decrease in forest specialist frugivores due to logging and forest edges. Forest generalists appear less sensitive and may even increase at forest edges. Such changes in the abundance of frugivores may have consequences for consumer/resource ratios and competition in plant–frugivore networks.
  2. Optimal foraging theory predicts an increase in dietary specialization of animals at low consumer/resource ratios due to reduced competition. A decrease in forest specialists in logged forests should cause decreased consumer/resource ratios, increased dietary specialization and reduced redundancy, whereas an increased abundance of forest generalists at edges may compensate for a loss of specialists.
  3. In Europe's last old-growth lowland forest (Białowieża, Eastern Poland), we recorded fruit removal by frugivores from fleshy-fruited plant species in the interior and at edges of logged and old-growth forests for 2 consecutive years.
  4. The abundance of forest generalists increased at forest edges, whereas specialists were unaffected. Conversely, logging resulted in a decrease in abundance of forest specialists but had no effect on the abundance of generalists. Accordingly, consumer/resource ratios increased from interior to edges and were reduced in the interior of logged forests compared with the interior of old-growth forests. As predicted by optimal foraging theory, a decrease in consumer/resource ratios coincided with increased dietary specialization and a loss of redundancy in the interior of logged forests. Despite low dietary specialization, redundancy was reduced at forest edges as forest generalists dominated plant–frugivore interactions.
  5. Synthesis. We show that a shift in frugivore assemblages at forest edges and increased dietary specialization of frugivores in the interior of logged forests involved a loss of redundancy compared with continuous old-growth forests. This suggests that seed dispersal services in secondary forest habitats depend on an impoverished subset of dispersal vectors and may suffer reduced adaptive potential to changing environmental conditions. Thus, our study highlights the value of old-growth forests for the conservation of frugivore-mediated seed dispersal processes.