Enemy release and plant invasion: patterns of defensive traits and leaf damage in Hawaii

Invasive species may be released from consumption by their native herbivores in novel habitats and thereby experience higher fitness relative to native species. However, few studies have examined release from herbivory as a mechanism of invasion in oceanic island systems, which have experienced particularly high loss of native species due to the invasion of non-native animal and plant species. We surveyed putative defensive traits and leaf damage rates in 19 pairs of taxonomically related invasive and native species in Hawaii, representing a broad taxonomic diversity. Leaf damage by insects and pathogens was monitored in both wet and dry seasons. We found that native species had higher leaf damage rates than invasive species, but only during the dry season. However, damage rates across native and invasive species averaged only 2% of leaf area. Native species generally displayed high levels of structural defense (leaf toughness and leaf thickness, but not leaf trichome density) while native and invasive species displayed similar levels of chemical defenses (total phenolics). A defense index, which integrated all putative defense traits, was significantly higher for native species, suggesting that native species may allocate fewer resources to growth and reproduction than do invasive species. Thus, our data support the idea that invasive species allocate fewer resources to defense traits, allowing them to outperform native species through increased growth and reproduction. While strong impacts of herbivores on invasion are not supported by the low damage rates we observed on mature plants, population-level studies that monitor how herbivores influence recruitment, mortality, and competitive outcomes are needed to accurately address how herbivores influence invasion in Hawaii.