Long-term exclosure of large terrestrial vertebrates: Implications of defaunation for seedling demographics in the Amazon rainforest

• We conducted a 7 year mammal exclosure study within a pristine Amazon rainforest.
• Annually we marked new seedlings in both exclosure and open control plots.
• We estimated density, seedling survival, recruitment, and vegetation cover.
• All response variables were significantly higher in exclosures than in controls.
• Defaunation could ultimately change the composition of tropical rainforests.

“Empty forests” in which humans have driven large vertebrate species to extinction lack myriad direct and indirect species interactions. This may alter key ecosystem processes, including trophic cascades, disturbance regimes, and nutrient cycling. Past research concerning the effects of tropical forest defaunation mainly compared sites with an intact fauna with sites having degraded faunal communities. However, experimental studies on seedling demographics at individual sites, with the same abiotic and biotic conditions, over a long period of time are still scarce. In this research we conducted a 7-year experiment within a completely intact and pristine forest in Manu National Park, Peru to elucidate the effects of defaunation on tree seedling demographics and densities. We constructed semipermeable exclosures that prevented access of large terrestrial vertebrates, but allowed free passage of small- and medium-sized species. New tree seedlings taller than 5 cm were tagged and followed. Jolly–Seber mark-recapture models were used to estimate density, survival, and recruitment of seedlings. Seedling density in the exclosures continued to increase significantly after the first year of the study. Only during the first 2 years was seedling survival higher in the exclosures. The numbers of new recruits were consistently higher in exclosures when compared to open control plots throughout the study. In accord with increased density, survival, and recruitment in exclosure plots, we also observed increased understory vegetation cover. Our results suggest that defaunation can have long-term effects on seedling demographics and density, which could ultimately influence the composition and diversity of the canopy tree community.