Testing the contribution of vertebrate predators and leaf traits to mainland-island differences in insect herbivory on oaks

Ecological theory predicts that herbivory should be weaker on islands
compared to mainland based on the assumption that islands have lower
herbivore abundance and diversity. However, empirical tests of these
predictions are rare, especially for insect herbivores, and those few
tests often fail to address the mechanisms behind island-mainland
divergence in herbivory. In particular, past studies have not addressed
the relative contribution of top-down (i.e., predator-driven) and
bottom-up (i.e., plant-driven) factors in these dynamics. To address this,
we experimentally excluded insectivorous vertebrate predators (e.g.,
birds, bats) and measured leaf traits associated with herbivory in 52
populations of 12 oak (Quercus) species in three island-mainland sites:
The Channel Islands of California vs. mainland California, Balearic
Islands vs. mainland Spain, and the island Bornholm vs. mainland Sweden (N
= 204 trees). In each site, at the end of the growing season, we measured
leaf damage by insect herbivores on control vs. predator-excluded branches
and measured leaf traits, namely: phenolic compounds, specific leaf area,
and nitrogen and phosphorous content. In addition, we obtained climatic
and soil data of island and mainland populations using global databases.
Specifically, we tested for island-mainland differences in herbivory, and
whether differences in vertebrate predator effects or leaf traits between
islands and mainland contributed to explain the observed herbivory
patterns. Supporting predictions, herbivory was lower on islands compared
to mainland, but only in the case of Mediterranean sites (California and
Spain). We found no evidence for vertebrate predator effects on herbivory
on either islands or mainland in any study site. In addition, while
insularity affected leaf traits in some of the study sites
(Sweden-Bornholm and California), these effects were seemingly unrelated
to differences in herbivory. Synthesis: Our results suggest that
vertebrate predation and the studied leaf traits did not contribute to
island-mainland variation patterns in herbivory, calling for more nuanced
and comprehensive investigations of predator and plant trait effects,
including measurements of other plant traits and assessments of predation
by different groups of natural enemies.