Environmentally Induced Differences in Plant Traits: Consequences for Susceptibility to a Leaf-Cutter Ant

The effects of different light and soil nutrient conditions on foliar chemistry and acceptability of Inga oerstediana seedlings to leaf-cutter ants were investigated. I determined differences among (1) leaves that were initiated and matured under identical conditions but subsequently were subjected to different light and fertilization treatments and (2) leaves that were initiated and matured under different soil, light, and fertilization treatments. I also assessed the relationship between environmentally induced differences in plant growth and the production of carbon-based secondary chemicals (specifically, tannins). Finally, I determined whether increases in tannins corresponded to decreases in the acceptability of leaves to leaf-cutter ants. Once a leaf matured the concentrations of tannins did not change as a function of light and soil nutrient conditions despite very large differences in growth rates among the treatments. Leaves that expanded to maturity under different soil, light, and fertilization treatments did differ in tannin chemistry, and treatment differences in plant growth rates appeared to dictate tannin chemistry. When growth was light limited, tannin concentrations were low, but when growth was nutrient limited, tannin concentrations were high. Only leaves that expanded to maturity under different environmental conditions differed in their acceptability to leaf-cutter ants. Decreases in tannins did not result in parallel increases in acceptability. Despite higher concentrations of tannins, leaf-cutter ants preferred the leaves of seedlings that were grown at 20% light over leaves of seedlings grown at 2% light. However, fertilization increased the acceptability of leaves when seedlings were grown at 20% light, a result consistent with the avoidance of very-high-tanning leaves. From these results I suggest that spatial variation in resource availability in lowland tropical rainforests would result in differences in tannin chemistry, and in susceptibility to leaf-cutter ants. However, temporal changes in resource availability would only be important to tannin chemistry and susceptibility if the duration of the change were long enough to allow new leaves to be produced.