Insect herbivory and leaf defensive traits in native and non-native populations of an endemic plant in a Macaronesian archipelago

Abstract

The spread of island endemic plants to new islands is an emerging topic, yet the mechanisms driving such invasions remain unclear. While herbivory-related explanations are common for widespread species, their relevance to island endemics is uncertain. Enemy release suggests that introduced plants face less herbivory and invest less in defence, unlike at their native sites with specialized herbivores. Conversely, low defences from minimal native herbivory could make them vulnerable to generalist herbivores in new locations. Additionally, abiotic factors—such as soil type or climate—may influence plant traits and herbivore interactions, potentially overriding or complicating expected herbivory-based invasion outcomes. Here we compared in situ levels of insect leaf herbivory and leaf chemical, physical, and nutritional traits for Rumex lunaria across islands of the Canary Archipelago to which this species is endemic. Specifically, we sampled populations found on islands where this species is native (Tenerife, Gran Canaria, La Gomera, La Palma, and El Hierro) and compared them to invasive (non-native) populations found on Lanzarote. Contrary to predictions, R. lunaria found on Lanzarote (i.e., non-native populations) exhibited higher levels of herbivory but also higher defensive traits—specifically, thicker leaves and higher concentrations of phenolic compounds—compared to populations on islands where this species is native to. To explore underlying correlates to these patterns, we conducted a follow-up structural equation model testing whether abiotic variation between native and non-native populations contributes to explain differences in leaf traits and herbivory. While non-native populations were associated with distinct abiotic conditions relative to native ones—including higher temperatures, reduced and more seasonal precipitation, and denser, more alkaline soils—this did not account for observed differences in leaf traits or herbivory between native vs. non-native populations. Accordingly, these findings suggest that the divergence in leaf defences and herbivory between native and non-native R. lunaria populations is not attributable to the abiotic factors analysed.