Regional variation in insularity effects on acorn herbivory in European oaks challenges predictions of lower herbivory on islands

Ecological theory predicts that herbivore abundance and diversity should
be lower on islands compared to the mainland, resulting in lower
herbivory. However, island-mainland comparisons have focused on herbivory
in vegetative tissues, primarily leaves, while herbivory in reproductive
tissues, such as fruits and seeds, which directly influence seedling
establishment and plant fitness, has lagged behind. In this study, we
compared insect herbivory in oak (Quercus) reproductive structures (i.e.,
acorn damage) across seven species in three regions: Lesbos Island vs.
mainland Greece, the Balearic Islands vs. mainland Spain, and Bornholm
Island vs. mainland Sweden. For each species, we selected three islands
and three mainland populations, and sampled 10 acorns from four trees per
population (N = 150 trees). We also analysed acorn chemical defences
(phenolics) and nutrients (nitrogen and phosphorus) to test their
association with herbivory and their contribution to island-mainland
differences in acorn damage. We obtained climatic data for each population
to assess whether these factors mediated insularity effects on acorn
traits and herbivory. We found no main effect of insularity, meaning acorn
damage did not differ overall between mainland and island populations.
However, we observed region-specific patterns whereby acorn damage was
higher in the Balearic Islands than in mainland Spain, while no
significant mainland-island differences were found in the other regions.
There were no main insularity effects on acorn traits, but a
region-by-insularity interaction affected phosphorus, with higher average
values in mainland Sweden than in Bornholm Island, whereas the reverse was
observed in Greece. A follow-up mechanistic test indicated that insularity
indirectly influenced acorn traits (e.g., phosphorus) via climate, but
these trait differences did not explain acorn damage patterns. Our results
challenge the theory on insularity effects on herbivory and suggest that
local processes are driving contrasting outcomes for acorn herbivory
across the studied regions.