PARENT SESSION
Session #18: Mechanisms underlying the maintenance of diversity and mode of regeneration in tropical forests: tests of alternative hypotheses. Organized by: W.P. Carson, C. Gehring, and T. Theimer.
Wednesday, August 8, 2001. Madison Ballroom B
Studies on the alteration of neotropical forest understories due to mammalian defaunation: experimental approaches and a model of the possible underlying mechanisms.
Dirzo, Rodolfo1, Mendoza, Eduardo1, 1
ABSTRACT- Previous studies suggest that contemporary Neotropical defaunation (i.e. the loss of medium/large mammals due to forest fragmentation and hunting) in Mexican forests is correlated with significant modifications of the forest understory. Defaunated understories show a reduced species diversity and high seedling density (seedling carpets with predominance of some particular species). In this paper we present results of recent experimental manipulations consisting in the establishment of exclosures and control plots in the understory of two contrasting forests (defaunated and intact). These experiments support the findings derived from our previous observations. After two years, exclosures in the intact forest show a reduced diversity of plants compared to control plots in the same site. In contrast, comparisons between exclosures and control plots in the defaunated forest do not show significant differences in species diversity. We present a graphical model of the possible underlying mechanisms that lead to reduce diversity in the absence of mammals. This model assumes that in the absence of the medium/large mammals small rodents become the predominant guild of seed predators. Such small rodents may concentrate their predation on a subset of the available seed pool on the forest soil. In particular they concentrate their activities on small and less toxic seeded plants, therefore promoting the establishment of seedling carpets of large seeded species. Such seedlings, in the absence of large mammalian herbivores may then become locally abundant. We present a discussion of the types of experimental manipulations directed to test the model and highlight some preliminary results on selective seed predation in exclosures and control plots.
KEY WORDS: plant-animal interactions, neotropical defaunation