Relationships between abiotic factors, foliage chemistry and herbivory in a tropical montane ecosystem

Herbivore-plant interactions are fundamental processes shaping ecosystems,
yet their study is challenged by their complex connections within broader
ecosystem processes, requiring a nuanced understanding of ecosystem
dynamics. This study investigated the relationship between nutrient
availability and insect herbivory in the Australian Wet Tropics. Our
objectives were threefold. Firstly, to understand which factors influence
nutrient availability for plants and herbivores across the landscape;
secondly, to investigate how trees of different species respond to
nutrient availability; and thirdly, to unravel how the relationships
between resources and plant chemistry affect herbivory. We established a
network of 25 study sites covering important abiotic gradients, including
temperature, precipitation, and geology. Employing a hierarchical
modelling approach, we assessed the influence of climate and geology on
resource availability for plants, primarily in the form of soil nutrients.
Then, we explored the influence of the above factors on the interaction
between herbivory and foliage chemistry across three widespread rainforest
tree species, comparing how these relationships emerged across genera. Our
findings suggest an overarching influence of climate and geology over soil
chemistry, foliar nitrogen, and insect herbivory, both directly and
indirectly. However, individual constituents of soil fertility showed
equivocal influences on spatial patterns of foliage chemistry once site
geological origin was accounted for, suggesting a questionable
relationship between soil nutrients and foliar composition. We have
demonstrated that herbivore-plant interactions are complex dynamics
regulated by an intricate web of relationships spanning different
biogeochemical processes. While our results provide some support to the
notion that herbivory is affected by resource availability, different
species growing under the same conditions can show differing responses to
the same resources, highlighting the importance of identifying specific
limiting factors rather than simpler proxies of resource availability.