Foliar herbivory pushes plant individuals towards the periphery of a plant-floral visitor interaction network

Indirect interactions are major forces structuring communities. In
terrestrial ecosystems, changes in plant traits caused by herbivory have
the potential to mediate cascades of indirect interactions that can
profoundly affect community structure. We have begun to understand the
mechanisms behind indirect effects mediated by trait changes in plants,
but we still know little about how these interaction-level mechanisms
affect higher levels of community organization, in particular network
structure, and how this is dependent on community context. In this study,
we aimed to understand how herbivory affects floral traits, flower
visitation and the position of plant individuals in a plant-flower visitor
network. To do this, we assessed the natural incidence of leaf and
flowerhead herbivory in a tropical shrub and tested how it affects
flowerhead density and the interactions between flowers and Lepidoptera,
Hymenoptera, and Diptera at the pairwise and network levels. We also
tested how the local density of conspecific plants adds context dependency
to these interactions by affecting both herbivory and flowerhead
visitation in focal plants. Increased leaf herbivory reduced flowerhead
density per plant and indirectly reduced flowerhead visitation by
Hymenoptera, while for Diptera direct effects of herbivory were important
and Lepidoptera were not affected. At the network level, plants with more
flowerheads were more connected and central. By reducing flowerhead
density, leaf herbivory caused individual plants to lose centrality,
becoming less connected and therefore peripheral in the plant-flower
visitor network. This dynamic is context dependent since increased plant
conspecific local density reduced leaf herbivory in the focal plant and
indirectly benefited its flowerhead density, flowerhead visitation and
network centrality. Synthesis: We demonstrate how the effects of herbivory
on a key plant trait can cascade to shape the interactions between
individual plants and their flower visitors and how the indirect effects
between herbivores and pollinators can modulate the structure of
interaction networks in plant-based ecosystems.