Biogeographical comparison of Lagarosiphon major between native South Africa and invaded New Zealand: A natural enemy release case study?

The Enemy Release Hypothesis (ERH) suggests that the absence of natural enemies (herbivores, parasites, and diseases) in non-native environments allow exotic plants to allocate more resources towards growth and reproduction, resulting in increased invasiveness. This study uses the submerged macrophyte Lagarosiphon major (Ridl.) Moss ex Wager (Hydrocharitaceae), native to Southern Africa, as a case study of this hypothesis. This species is invasive in many parts of the world, including Ireland, the United Kingdom and New Zealand. Here, we investigate the applicability of the ERH in explaining the invasiveness of L. major in New Zealand. This was achieved by comparing L. major biomass and cover, and plant species richness between L. major’s native range in South Africa and its invaded range in New Zealand, where specialist natural enemies were presumed to be lacking. The findings revealed significantly higher L. major biomass (∼100 % increase), cover (∼28 % increase) and shoot production (∼91 % increase) in the invaded range compared to the native range. Additionally, overall species richness and abundance was negatively correlated with L. major abundance in the invaded range. Plants in the native range were associated with more natural enemies and experienced higher levels of damage relative to the invaded sites. Overall, these observations align with the ERH, but should be interpreted with caution as other influential factors such as competition were not measured. Nevertheless, this suggests that L. major could benefit from the integration of biological control measures with the existing methods of control. Two natural enemies, Polypedilum tuburcinatum Andersen (Diptera: Chironomidae) and Hydrellia lagarosiphon Deeming (Diptera: Ephydridae) have already been identified as promising candidates, for biological control of L. major in New Zealand and other regions invaded by this weed.