| Summary: | The top-down effects of consumers and bottom-up effects of resource availability are
important in determining community structure and ecological processes. I experimentally
examined the roles of consumers — both detritivores and predators — and habitat context in
affecting nutrient cycling using the detritus-based insect community in bromeliad leaf wells. I
also investigated the role of multiple resources in limiting plant productivity using meta analyses.
The insect community in bromeliads only increased nitrogen release from leaf detritus in
the presence of a predator trophic level. When only detritivores were present, the flow of stable
isotope-labeled nitrogen from detritus to bromeliads was statistically indistinguishable from that
in bromeliads lacking insects. I suggest that emergence of adult detritivores constitutes a loss of
nitrogen from bromeliad ecosystems, and that predation reduces the rate of this nutrient loss.
Hence, insects facilitate nutrient uptake by the plant, but only if both predators and detritivores
are present. Moreover, predators can affect nutrient cycling by influencing the spatial scale of
prey turnover. This mechanism results in a pattern opposite to that predicted by classic trophic
cascade theory.
Increasing habitat complexity can have implications for nutrient cycling by decreasing
the foraging efficiency of both predators and their prey, and by affecting the vulnerability of
predators to intraguild predation. Along a natural gradient in bromeliad size, I found that,
depending on the relationship between community composition and habitat size, habitat
complexity interacts with the changing biotic community to either complement or counteract the
impact of predators on nutrient uptake by bromeliads.
In contrast to the existing emphasis on single-resource limitation of primary productivity,
meta-analyses of a database of 653 studies revealed widespread limitation by multiple resources,
and frequent interaction between these resources in restricting plant growth. A framework for
analyzing fertilization studies is outlined, with explicit consideration of the possible role of
multiple resources. I also review a range of mechanisms responsible for the various forms of
resource limitation that are observed in fertilization experiments.
These studies emphasize that a wider range of predator and nutrient impacts should be
considered, beyond the paradigm of single resource limitation or classic trophic cascades.
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