Herbivory and Defensive Characteristics of Tree Species in a Lowland Tropical Forest

Rates of herbivory and defensive characteristics of young and mature leaves were measured for saplings of 46 canopy tree species in a lowland tropical rain forest (Barro Colorado Island, Panama). Grazing rates were determined in the field for sample periods in the early wet, late wet, and dry seasons. Leaf properties such as pubescence, toughness, water, protein, fiber, and phenolic contents explained over 70% of the variation among plant species in the rates of herbivory on mature leaves. Leaf toughness was most highly correlated with levels of herbivory, followed by fiber content and nutritive value. Phenol content and phenol: protein ratios were not significantly correlated with damage. Mature leaves of gap—colonizing species were grazed six times more rapidly than leaves of shade—tolerant species. Gap—colonizers have less tough leaves, lower concentrations of fiber and phenolics, higher levels of nitrogen and water, shorter leaf lifetimes, and faster growth rates than do shade—tolerant species. Gap—colonizers did not escape discovery by herbivores to any greater extent than shade—tolerant species, as measured by the spatial distribution of plants or by the intraspecific distribution of herbivore damage under natural or experimentally manipulated conditions. In 70% of the species, young leaves suffered higher damage levels than mature leaves. Although young leaves are more nutritious and less tough and fibrous, they have two to three times the concentrations of phenols. The temporal appearance of young leaves was not correlated with the distribution of herbivory among individuals of a species. Interspecific patterns of defense mechanisms are discussed in terms of current theories of plant apparency, and an alternative model for the evolution of plant defenses is presented.