Introduced mangroves exhibit less leaf damage and greater performance than native mangroves

Abstract

The enemy release hypothesis (ERH) predicts that introduced species leave most enemies behind during the invasion process, resulting in less enemy damage and increased performance in their introduced range. In Hawai‘i, introduced red mangroves (Rhizophora mangle) convert open shorelines into dense mangrove forests. While previous studies show introduced mangroves harbor a lower occurrence of damage on various plant structures, it remains unknown if the magnitude of damage to mangrove leaves and metrics of performance differ between introduced and native populations. In this study, we tested some of the predictions of the ERH through leaf damage surveys and a year-long damage experiment replicated in 8–10 sites in the native (Caribbean and Florida) and introduced (Hawai‘i) range of red mangroves. In each site, we (1) compared the percentage of leaf area damaged, (2) measured metrics of performance (leaf loss, leaf, twig, and propagule production), and (3) experimentally tested how rapidly necrosis spreads on artificially-damaged leaves of R. mangle. Our results were largely consistent with the ERH. Native mangroves exhibited orders of magnitude higher leaf damage than introduced mangroves, suffered nearly twice the leaf loss, and produced fewer twigs and propagules than introduced mangroves over one year. Leaf production and the expansion of necrotic area on leaves were similar. Broadly, our study demonstrates that introduced mangroves experience substantially less leaf damage and thus, may be exhibiting evidence of increased growth and fecundity. Our results may help explain why introduced mangroves continue to be so productive and are considered invasive in Hawaiian habitats.