Relationships between El Niño Southern Oscillation and Demographic Patterns in a Substitute Food for Collared Peccaries in Panama1

Abstract

Environmental fluctuations can play an important role in shaping demographic processes in plant and animal populations. On Barro Colorado Island (BCI), Panama, El Niño Southern Oscillation (ENSO) events followed by mild dry seasons have been shown to stimulate variation in community-wide fruit production. This variation has been postulated to lead to famine in frugivore and granivore populations, but few if any studies have assessed further implications for other members of the ecological community. This study examined the ways in which variation in fruit production associated with an El Niño event on BCI may influence demographic and evolutionary traits in a plant species that is only consumed by frugivores during periods of fruit scarcity. Specifically, I assessed the effects of herbivory by collared peccaries (Tayassu tajacu) on demographic rates and patterns in the terrestrial bromeliad, Aechmea magdalenae. Aechmea magdalenae usually suffers little or no herbivory, and rain forest collared peccaries, which are primarily frugivorous, have not previously been reported to eat this species. Rates of survival and reproduction of 296 A. magdalenae rosettes in three transects of differing densities were recorded from 1997 through 1999. Mortality rates due to herbivory increased significantly from 1997 to 1999 and showed a close correspondence with enhanced and then decreased rates of community-wide fruit production associated with the 1997 ENSO. The densest portions of the A. magdalenae population showed lower mortality rates and higher rates of reproduction than did the less dense regions. Over the study period, A. magdalenae populations decreased by 45 percent. The implications of periodic herbivory by hungry frugivores are discussed with respect to population regulation, seedling dispersal, and evolution of A. magdalenae's dense growth habit.