Submitted to: Florida Entomologist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/21/2006
Publication Date: 12/15/2006
Citation: Liu, H., Stiling, P., Pemberton, R.W., Pena, J. 2006. Insect faunal diversity among invasive, non-invasive and native congeners and implications for the enemy release hypothesis. Florida Entomologist. 89:475-484. Interpretive Summary: Factors contributing to the naturalization of invasive plants are important and are currently poorly understood. The so-called Enemy Release Hypothesis says that plants naturalize and spread may do so because they escape insect herbivores that impact other plants. We tested this hypothesis by examining the number of insect herbivore species on Eugenia species, small trees in the myrtle family. We compared the insect herbivore fauna on native, invasive and noninvasive Eugenia species. We found that invasive Eugenia did not experience release in terms of total number of herbivore species, but did in terms of insects with narrow diets and internal feeders. However, contrary to the study’s prediction, the non-invasive Eugenia experience less, not more, herbivore pressure from insects with narrow diets compared to the invasive Eugenia. This result suggests that escape from natural enemies is unlikely to be the primary reason for the spread of the invasive Eugenia. The study’s three-way comparison of related native, non invasive, and invasive plants is a new and informative approach to better elucidate factors responsible for plant invasion.
Technical Abstract: The enemy release hypothesis (ERH) has been frequently invoked to explain the naturalization and spread of introduced species. One ramification of the ERH is that invasive plants sustain less herbivore pressure than do the native species. Empirical studies testing the ERH have mostly involved two-way comparisons between invasive introduced plants and their native counterparts in the invaded region. Testing the ERH would be more meaningful if such studies also included introduced non-invasive species because introduced plants, regardless of their abundance or impact, may support a reduced insect herbivore fauna and experience less damage. In this study, we employed a three-way comparison, in which we compared herbivore faunas among native, introduced invasive, and introduced non-invasive plants in the genus Eugenia which all co-occur in South Florida. Contradictory to our predictions, we found that invasive introduced Eugenia support a greater total number of herbivore species than both the native and the non-invasive introduced Eugenia. In addition, the numbers and percentages of oligophagous and endophagous insect species were greatest on the native Eugenia, but they were not different between the invasive and non-invasive introduced Eugenia. One oligo- and endophagous herbivore has likely shifted from the native to the invasive, but none to the non-invasive Eugenia. In summary, the invasive Eugenia encountered equal, if not greater biotic resistance than the non-invasive Eugenia, including from oligophagous and endophagous herbivores. Our data only provided limited support to the ERH. We would not have been able to draw this conclusion without inclusion of the non-invasive Eugenia species in the study.