A roadmap for future research on insularity effects on plant–herbivore interactions

Abstract

State of the art Theory predicts that herbivore pressure should be weaker on islands than on the mainland, owing to lower herbivore abundance and diversity because of dispersal constraints and environmental filtering. As a result, plants on islands should invest less in defences against herbivory. Although early empirical studies supported these predictions, recent systematic island–mainland comparisons have questioned this paradigm, with some studies reporting either no difference between islands and mainland or higher herbivory and plant defences on islands. Current data therefore appear to be unsupportive of predictions on insularity effects on plant–herbivore interactions, calling for more research to reassess predictions and to test underlying mechanisms for observed patterns. Research opportunities To meet this challenge, a renewed research programme based on the accrual of studies with specific features is needed. These should include more robust experimental designs with replication within and across systems, integrative and more nuanced assessments of plant defensive phenotypes and herbivory, a food web approach that considers the multi‐trophic context in which plant–herbivore interactions are embedded, and a consideration of historical factors (e.g., island origin and biogeographical factors, defensive anachronisms). Outlook This new research programme will require integration of evolutionary ecology research on plant–herbivore interactions with island biogeography, palaeoecology and community ecology to understand the influence of factors acting at different scales, from local factors driving herbivory and plant defences to historical processes and regional drivers of species composition determining species traits and their interactions.

Evidence for Reductions in Physical and Chemical Plant Defense Traits in Island Flora

“…We suggest that reduced defense against mammalian herbivores in island flora is likely to be a general phenomenon but that methodological drawbacks often obscure the detection of this pattern. Future research comparing island and mainland plant defense traits would benefit from a number of approaches (also highlighted more broadly in [26]). First, studies that focus on traits with unambiguous roles in plant defense against herbivores (e.g., latex exudation, alkaloids, cardenolides) may be more insightful for understanding selection imposed by herbivores than studies that focus on traits with multiple potential functions (e.g., phenolic compounds and leaf area).…”

“…Thus, the degree to which island plants show reduced defensive traits seems highly variable. Reasons for a lack of reduced defensive trait expression in island plants are numerous (see [26]) but might include (i) recent introduction of non-native mammalian herbivores that favor increased defensive traits, either through phenotypic plasticity or directional selection; (ii) an extended history of coevolution with native large herbivores that are no longer present (e.g., [27]); (iii) selection by invertebrate herbivores on islands; (iv) ongoing gene flow from mainland populations that limits the degree of differentiation in island populations.…”

Endemic island plant–herbivore interactions: Kamehameha butterfly (Nymphalidae) and Hawaiian Urticaceae

“…For new insights into island plant–herbivore interactions, studies that examine native and novel species interactions and that incorporate both putative defense traits and measures of herbivore performance are needed (Monroy & García‐Verdugo, 2019; Moreira & Abdala‐Roberts, 2022; Moreira et al., 2022; Tanentzap et al., 2011; Watts et al., 2011). As the most isolated archipelago in the world, the Hawaiian Islands offer compelling opportunities to examine plant–herbivore interactions in a famously disharmonic biota.…”

Towards a holistic perspective on the development of island syndrome by examining its occurrence patterns in insular plants

“…Additionally, terrestrial plant lineages exhibit diverse responses to distinct island environments due to their different evolutionary adaptations and functional traits. Several widely accepted concepts such as dispersal syndrome, floral syndromes, Baker's rule, and loss of anti-herbivory traits have been questioned and may not universally apply to all taxa (Pannell et al 2015, García-Verdugo et al 2017, Monroy and García-Verdugo 2019, Hetherington-Rauth and Johnson 2020, Moreira et al 2021, Moreira & Abdala Roberts 2022. Gaining a clear understanding of the variation in the occurrence of island syndrome across islands and taxa, while accounting for sampling bias, is crucial for identifying the specific underlying drivers and conditions responsible for its development.…”