Interactions between different anti-herbivore defenses influence patterns in their evolution. Optimal defense theory predicts that plants will decrease their commitment to defenses for which the cost of production outweighs the benefit. According to this theory, when two different defenses are redundant (i.e. one does not provide additional protection in the presence of the other), plants should not allocate resources to both simultaneously, leading to a trade-off in their expression. This study explores patterns of expression and interactive effects of two anti-herbivore defenses in Conocarpus erectus, a common tropical (and subtropical) mangrove. This long-lived tree generally occurs in two morphotypes – “silver” plants have a dense layer of leaf hairs on the leaf surface, while “green” plants have very few leaf hairs. Silver plants receive less herbivore damage than green plants, suggesting that leaf hairs are an effective defense against herbivores. Conocarpus erectus also produces extrafloral nectaries that attract ants. These ants can protect C. erectus from herbivores, but the effect of ants may not be as strong on plants already defended by leaf hairs. This may lead to a trade-off in the expression of leaf hairs and extrafloral nectaries. I used ant-exclusion experiments and observational studies to explore these hypotheses.
Results/Conclusions
The effect of ant exclusion on herbivory differed for green and silver plants. Ant exclusion led to a 24% increase in herbivory in green plants, but had no effect on herbivory in silver plants. Ant exclusion also decreased growth in green plants by 9%, while growth in silver plants was unaffected by ant exclusion. The fact that ants did not provide a benefit for plants that were defended by leaf hairs is consistent with the results of an observational study showing that silver plants have fewer, smaller extrafloral nectaries than green plants.
