LTREB: On the Consequences of Removing Ground-dwelling Mammals for Tropical Forest Diversity: Towards a New Conceptual Framework

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

0212054 Carson LTREB We have underway a long-term, large-scale experiment that is addressing one of the primary hypotheses proposed to explain the maintenance of tree species diversity in tropical forests, namely, that mammals acting as herbivores, seed predators, and dispersers, are critical determinants of tropical tree diversity and abundance. This experiment is essentially a community-level test of the Janzen-Connell Hypothesis that predicts that the removal of ground-dwelling mammals will ultimately cause a decline in tropical tree diversity. Under this scenario, mammals preferentially prey on the juveniles of tree species that are abundant and ultimately keep these tree species in check. The study system is mature-phase tropical moist forest in central Panama on Barro Colorado Island (BCI) and the adjacent mainland (Gigante Peninsula). Since1994, we have monitored the consequences of excluding ground-dwelling mammals in 8 large (1355m 2 ) fenced exclosures and adjacent unfenced control plots. Four pairs of treatment and control plots were established on BCI and 4 pairs were located on Gigante in similar mature forest. Prior to initiating any treatments, we censused, measured, identified, and tagged all woody plants > 50 cm in height in all plots. In addition, we quantified newly recruited seedlings in 28 smaller subplots in each large plot. In total, we have now followed the fate of more than 60,000 individual seedlings and saplings. Overall, the exclusion of mammals led to a substantial and sustained increase in seedling density. The results of this experiment to date suggest that excluding mammals may significantly increase woody species diversity of the seedling layer at least in the short-term (~10 yrs). Thus, our results contrast sharply with the community-level predictions of the Janzen-Connell hypothesis. We propose a new conceptual model that suggests that the consequences of the short term removal of mammals (2-25 yrs) from tropical forests, whether by experimental exclusion or more broadly via over hunting, will cause first an increase in woody species diversity and thereafter potentially a decrease (see Fig. 1). This result will first appear in the seedling layer (1-50 cm in height) and then move as a wave through larger size classes. The data to date, however, even after 7-years, remain equivocal. The short term increase in diversity clearly documented inside exclosure plots may be a transient effect of the 1998 El Nino Southern Oscillation (ENSO) event. Specifically, the ENSO event caused a severe and extended dry season that increased seedling mortality. Because there was a higher seedling density inside the exclosures, these sites may have been buffered from the drought. We believe that it is imperative that we continue this experiment and monitor the next phase as the seedling layer moves into the small sapling size class. The conservation implications of this work are clear; the overexploitation of the mammalian fauna in many tropical forest reserves, parks, and fragments may have long-term negative consequences for forest diversity that may not become apparent for decades. It is only through long-term experiments that we can unequivocally separate out short-term dynamics from long-term forest trajectories.