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Top-down effects of predators and bottom-up effects of resources are
important drivers of community structure and function in a wide array of
ecosystems. Fertilization experiments impose variation in resource
availability that can mediate the strength of predator impacts, but the
prevalence of such interactions across natural productivity gradients is
less clear. We studied the joint impacts of top-down and bottom-up factors
in a tropical mangrove forest system, leveraging fine-grained patchiness
in resource availability and primary productivity on coastal cays of
Belize. We excluded birds from canopies of red mangrove (Rhizophoraceae:
Rhizophora mangle) for 13 months in zones of phosphorus-limited, stunted
dwarf mangroves, and in adjacent zones of vigorous mangroves that receive
detrital subsidies. Birds decreased total arthropod densities by 62%,
herbivore densities more than fivefold, and reduced rates of leaf and bud
herbivory by 45% and 52%, respectively. Despite similar arthropod
densities across both zones of productivity, leaf and bud damage were 2
and 4.3 times greater in productive stands. Detrital subsidies strongly
impacted a suite of plant traits in productive stands, potentially making
leaves more nutritious and vulnerable to damage. Despite consistently
strong impacts on herbivory, we did not detect top-down forcing that
impacted mangrove growth, which was similar with and without birds. Our
results indicated that both top-down and bottom-up forces drive arthropod
community dynamics, but attenuation at the plant-herbivore interface
weakens top-down control by avian insectivores.
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