Herbivory and Arthropod Diversity within Invaded and Native Forest Types on Hawai’i Island

tion and a decline of native species (Cordell et al., 2009). Introduced species now represent approximately 50% of the floral diversity on the Hawaiian archipelago (Wagner, Herbst, & Sohmer, 1999). Some introduced plants have become invasive and are able to outcompete natives because they use strategies to which local species cannot effectively respond. Studies conducted in other tropical forests have shown that certain characteristics make leaves more palatable to herbivores. For example, young leaves are softer, have greater water content, and have greater nutritional value than mature leaves, and herbivory is demonstrably greater in younger leaves (Coley, 1983). Likewise, gap-colonizing or pioneer species, which tend to have softer leaves, are grazed six times more rapidly than shade-tolerant species (Coley, 1983). Many of the invasive species that now dominate Hawaiian lowland wet forests (HLWF) are considered pioneer species (Zimmerman et al., 2008), which means they may face higher herbivory rates than native species in Hawai’i. Because Hawai’i has no native mammalian herbivores and most of the grazing birds are now extinct (James & Burney, 1997), this study focuses on herbivory damage produced by arthropods. Invasive species are well known for their disruptive biotic impact on native ecosystems. As invasive species increase in abundance, native plant diversity and density often measurably decline along with herbivore populations (Zagrobelny, Bak, & Moller, 2008). However, an introduction of species could also foster greater diversity in arthropods (Mascaro, Hughes, & Schnitzer, 2012). Invasive plants often have different chemical defenses than native plants, which can prevent or reduce herbivory by native fauna. This may cause a decline in the herbivore population or allow for adaptations in herbivores to occur (Zagrobelny et al., 2008). The introduction of non-native plants may also lead to an increase in leaf litter, which in turn may result in an increased population of INTRODUCTION Forest ecosystems support astoundingly rich arthropod abundances. Aside from direct nutritional value to herbivores, trees provide arthropods with shelter, as well as sites for foraging, oviposition, sun-basking, sexual display, and overwintering (Halaj, Ross, & Moldenke, 2000). In reciprocity, arthropods take part in a series of ecological processes such as leaf litter decomposition and competition for resources (Olofssn, Hulme, Oksanen, & Suominen, 2004). They also play vital roles in food web dynamics as herbivores, decomposers, predators, and prey. Arthropods are large contributors to herbivory in forests (Basset, 1991), and one of the most important selective forces in the evolution of plants is their susceptibility to herbivory (Coley, 1983). Due to the geographic isolation and geologic origins of the Hawaiian Islands, their floral and faunal biodiversity are considered disharmonic. In other words, these islands evolved and continue to evolve separately from each other and may lack ecological symmetry. This has resulted in a collection of native flora with few fast-growing pioneer species and a low species richness level of native arthropods (Gillespie & Roderick, 2002). However, over the past one hundred years, the accidental and deliberate introduction of non-native plant and animal species has led to forest degradaHerbivory and Arthropod Diversity within Invaded and Native Forest Types on Hawai’i Island