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

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0212054CarsonLTREBWe have underway a long-term, large-scale experiment that is addressing one of the primaryhypotheses 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 tropicaltree diversity and abundance. This experiment is essentially a community-level test of the Janzen-ConnellHypothesis that predicts that the removal of ground-dwelling mammals will ultimately cause adecline in tropical tree diversity. Under this scenario, mammals preferentially prey on the juveniles oftree 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 ColoradoIsland (BCI) and the adjacent mainland (Gigante Peninsula). Since1994, we have monitored theconsequences of excluding ground-dwelling mammals in 8 large (1355m2) fenced exclosures andadjacent unfenced control plots. Four pairs of treatment and control plots were established on BCI and4 pairs were located on Gigante in similar mature forest. Prior to initiating any treatments, wecensused, measured, identified, and tagged all woody plants 50 cm in height in all plots. Inaddition, 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, theexclusion of mammals led to a substantial and sustained increase in seedling density.The results of this experiment to date suggest that excluding mammals may significantlyincrease woody species diversity of the seedling layer at least in the short-term (~10 yrs). Thus, ourresults contrast sharply with the community-level predictions of the Janzen-Connell hypothesis. Wepropose a new conceptual model that suggests that the consequences of the short term removal ofmammals (2-25 yrs) from tropical forests, whether by experimental exclusion or more broadly viaover hunting, will cause first an increase in woody species diversity and thereafter potentially adecrease (see Fig. 1). This result will first appear in the seedling layer (1-50 cm in height) and thenmove as a wave through larger size classes. The data to date, however, even after 7-years, remainequivocal. The short term increase in diversity clearly documented inside exclosure plots may be atransient effect of the 1998 El Nino Southern Oscillation (ENSO) event. Specifically, the ENSO eventcaused a severe and extended dry season that increased seedling mortality. Because there was a higherseedling density inside the exclosures, these sites may have been buffered from the drought. Webelieve that it is imperative that we continue this experiment and monitor the next phase as the seedlinglayer moves into the small sapling size class. The conservation implications of this work are clear; theoverexploitation of the mammalian fauna in many tropical forest reserves, parks, and fragments mayhave 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 dynamicsfrom long-term forest trajectories.