Competitive exclusion of a worldwide invasive pest by a native: Quantifying competition between two phytophagous insect on two host plant species

High competitive ability is believed to be an important characteristic of invasive species. Many animal studies have compared the competitive ability of invasive species with a native species that is being displaced, but few have looked at systems where an invasive species has failed to establish itself. The thrips species F. occidentalis is a highly invasive crop pest that has spread from its original range (the western states of the US) to a worldwide distribution. Despite this, F. occidentalis is largely absent or in low numbers in the eastern states of the US, where the native F. tritici dominates. It is possible that F. tritici is competitively excluding F. occidentalis from this region. Larval competition between these two thrips species was tested on two known plant hosts, Capsicum annuum (a crop plant), and Raphanus raphanistrum (an invasive weed), using a response surface design with larval survival as the response variable. Maximum likelihood estimation was used to fit a model and generate quantitative values for competition. On both plant hosts, the invasive F. occidentalis experienced significant competition from F. tritici while F. tritici did not experience significant competition from F. occidentalis. The invasive F. occidentalis appears to be competitively excluded by the native F. tritici. This study confirms the importance of competition in the biotic resistance of a community and is one of the few animal studies to not only test for competition in an apparently resistant ecosystem but also to quantify the level of interspecific competition between two animal species.