The enemy release hypothesis and beyond: Lagarosiphon major invasion dynamics and management options for New Zealand using native natural enemies from South Africa
Authors:   , Baso Nompumelelo Catherine 
Published: June 2024

Numerous scientific investigations have demonstrated the destructive impact that exotic species can have on ecosystem services beyond a specific threshold. There are many explanations for why introduced plants are likely to be more successful outside their native range. One such explanation is offered by the Enemy Release Hypothesis (ERH), which states that plants automatically become superior competitors outside of their natural range due to release from top-down stressors (herbivory, parasites, and diseases) that is evident in the absence of their natural enemies. The underlying assumption of the ERH is that natural enemies are important regulators of plant species populations, and that the pressures from these natural enemies are felt more readily by native species compared to alien plants. Consequently, in the absence of such pressures, the ERH assumes that exotic plants can allocate more resources towards growth and reproduction, while effectively maintaining accumulated biomass. Classical biological control has previously been cited as evidence for the enemy release hypothesis. Therefore, the overarching aim and theme of this thesis was to investigate the role of ERH on the invasiveness of Lagarosiphon major (Ridl.) Moss ex Wager (Hydrocharitaceae) in New Zealand. Firstly, a literature search and a meta-analysis was used to synthesize existing studies in order to test for general applicability of this hypothesis to aquatic plant invasions. Furthermore, an empirical investigation was conducted in order to directly quantify enemy release in L. major populations invaded areas of New Zealand. To achieve this, various plant parameters of this plant, overall macrophyte and invertebrate diversity were measured and compared between sites in the native range in South Africa and the invaded areas in New Zealand. Although the meta-analysis showed variable evidence for this hypothesis depending on various modulating factors such as study type, plant growth form and measured parameters, for L. major, there was strong evidence of enemy release. The biogeographical comparisons showed that L. major exhibited increased fitness in most of the invaded sites, marked by elevated biomass accumulation, significantly higher shoot production, and the displacement of native plant species. The observed fitness advantages were directly correlated to a decrease in herbivory diversity and pressure upon the plant's introduction to New Zealand. Unlike the native populations, which contend with the presence of at least four co-occurring herbivores, including specialist herbivores, the invaded range had a substantially lower herbivore diversity, with only Hygraula nitens Butler (Lepidoptera: Crambidae) syn. Nymphula nitens, significantly damaging L. major. These findings emphasize the importance of understanding invasion ecology and theories such as ERH in order to advance aquatic plant management and also present valuable insights for developing effective strategies to mitigate the impact of invasive alien species on aquatic ecosystems. Specifically, results from the empirical investigation provide evidence in support of the ERH and highlight the suitability of implementing biological control strategies to manage the L. major invasion in New Zealand. Previous studies have shown the suitability of two specialist herbivores, Hydrellia lagarosiphon Deeming (Diptera: Ephydridae), and Polypedilum tuburcinatum Andersen (Diptera: Chironomidae), as potential biological control agents. This control strategy presents a sustainable and ecologically responsible approach, promoting coexistence between exotic plants and native species rather than displacement through competitive exclusion. With the apparent dominance of L. major at various New Zealand localities, the subsequent objective of this thesis was to investigate the competitive interactions between L. major and another invasive Hydrocharitaceae, Egeria densa Planchon, as driven by herbivory. Combinations of two host specific Ephydrid flies, H. lagarosiphon and H. egeriae, were used at eight different factorial combination of planting densities. The analysis of plant parameters and the application of inverse linear models revealed that L. major often exhibits relatively higher fitness, especially in low monoculture treatments when the two insects were isolated. However, multiple inverse linear models revealed that actual competitive outcomes are dependent on factors such as initial plant density and herbivory regime, with competitive interactions generally being mild. Nevertheless, the presence of H. lagarosiphon resulted in facilitation of E. densa growth. Thus, even at lower densities, these insects still had an impact on the observed interactions, further emphasizing suitability as damaging biological control agents. Lastly, focusing on the abiotic component of L. major invasion, Species Distribution Models (SDMs) were employed to map potential suitable habitat for this species, as well as predict the consequences of climate change on this. Correlative and mechanistic modelling was also used to simulate suitable habitat for potential biological control agents, thus addressing the potential for mismatches between host plant distribution and insect suitable range. The Maximum Entropy Species Distribution Modelling (MaxEnt) algorithm revealed that more than 90% of all freshwater ecosystems in New Zealand are susceptible to L. major invasion, with suitability projected to expand further under future climate scenarios. Moreover, correlative modelling using this method suggests limited suitable habitat for both herbivores. However, degree-day modelling, which also takes into account the physiological requirements, showed that H. lagarosiphon has the potential to produce viable populations in several parts of New Zealand. Overall, this thesis explored the intricate web of biotic and abiotic factors influencing the success of L. major outside its native range. The results emphasize the potential impacts of climate change on the invasion potential and management strategies for L. major. The findings also advocate for the implementation of sustainable and ecologically sound management solutions, such as biological control, to manage this species.