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.

No description available.

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. / Science, Faculty of / Zoology, Department of / Graduate

Trophic cascades involving large terrestrial mammals are very seldom empirically demonstrated. The high species diversity often associated with terrestrial systems is thought to modulate the strength of trophic cascades. In speciose systems, species often vary in vulnerability to consumption, hence some species are less responsive to top-down pressure. African large mammalian herbivores are highly diverse and vary greatly in body size, a characteristic linked to vulnerability to predation. Moreover, Africa is one of the last places to still support megaherbivores, species that have grown large enough to be practically impervious to non-human predation. Thus an African ecosystem is the ideal setting to explore trophic cascades in speciose terrestrial systems. In this study I explored patterns in trophic interaction amongst carnivores, vulnerable mesoherbivore and predator-invulnerable megaherbivores in an intact African savanna at a variety of spatial scales. I examined the mechanistic links between trophic levels at the patch scale through fine scale empirical manipulation of predation risk. In addition, I assessed the landscape scale biological relevance of these trophic interactions through correlative observations over large spatiotemporal scales. I present the first evidence of community level trophic cascades explicitly involving megaherbivores. I demonstrate how megaherbivores’ disregard of predation risk masks the effects of predator-triggered trophic cascades and weakens their impact on the ecosystem. The risk of predation triggered a spatial response in vulnerable species, driving them into safe areas, yet did not influence the space use of megaherbivores. This species-specific spatial response had contrasting effects on nutrient distribution. Vulnerable herbivores’ fear-induced foraging behaviour led to localised nutrient accumulation whereas the foraging behaviour of predator-invulnerable megaherbivores led to nutrient redistribution across the landscape. In addition, the fear-driven spatial differences in mesoherbivore grazing impact and nutrient deposition led to landscape scale changes in the distribution and persistence of herbivore maintained grass communities, so-called grazing lawns. However, the grazing activity of the in-vulnerable megagrazer, white rhino (Ceratotherium simum) weakened the effect of this trophic cascade by creating and maintaining grazing lawns within the risky habitat avoided by mesograzers. This study contributes to our understanding of what drives the variation among patterns of trophic control and provides the first evidence of the modulating influence that megaherbivores have on predator-triggered trophic cascades.

Predation accounts for the majority of ruffed grouse mortality beyond the brooding period throughout grouse range. Most studies of ruffed grouse ecology have taken place in the central portion of the species range including the Great Lakes states and southern Canada where aspen (Populus tremuloides) is dominant. Populations in the central range of grouse distribution exhibit 10-year cyclic patterns of decline that have been associated with the invasions of boreal predators such as great horned owls and northern goshawks during crashes in small mammal populations. I completed an accuracy assessment of field sign to determine the role of scavenging as it related to diagnosing causes of proximate mortality. Potentially large numbers of grouse are scavenged after death, which can alter or completely falsify the reported cause of death.. My results indicated that stand-level and micro-site habitat variables did not influence scavenger detection of ruffed grouse carcasses (P > 0.05). Scavenging was limited to 24 of 64 carcasses and was entirely attributed to mammalian species. Scavenging behavior was related to the condition of the carcass following death (P = 0.003) and the ambient temperature (P = 0.01). As the temperature increased and as a carcasses entrails and muscle became more exposed, the probability of being scavenged increased. Nineteen percent of whole carcasses placed in the field were scavenged and would have been attributed to mammal predation based on the field sign; 56% of 32 mock avian kills were scavenged would have been attributed to mammal predation. I related indices of predators and weather patterns to trends in ruffed grouse predation. Using data collected at 10 study sites between February, 1997 and December, 2000, I compared predation rates, and animal and weather indices to predation rates, across sites, years, regions, seasons and month combinations pooled across years. Avian predators were the primary predators of ruffed grouse in the Appalachian region (50% of all predation). Predation rates on ruffed grouse were highest in fall (8.3%), due to high predation on juveniles, and spring (7.4%) in association with raptor migration and pre-breeding activity of grouse. Predation patterns and predator abundance did not indicate the occurrence of predator invasions during the years of this study. Predation rates on ruffed grouse were positively related to the presence of rain and negatively related to the average low temperature and number of rabbits and squirrels observed per hour. Observations of owls and Cooper's hawks per hour were correlated with predation rates on grouse while those of red-tailed, red-shouldered and broad-winged hawks were not. / Master of Science

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Much of the world's species diversity is located in tropical and sub-tropical ecosystems, and a better understanding of the ecology of these systems is necessary to stem biodiversity loss and assess community- and ecosystem-level responses to anthropogenic impacts. In this dissertation, I endeavored to broaden our understanding of complex ecosystems through research conducted on the Cinaruco River, a floodplain river in Venezuela, with specific emphasis on how a human-induced perturbation, commercial netting activity, may affect food web structure and function. I employed two approaches in this work: (1) comparative analyses based on descriptive food web characteristics, and (2) experimental manipulations within important food web modules. Methodologies included monthly sampling of fish assemblages using a variety of techniques, large-scale field experiments, extensive stomach content and stable isotope analyses. Two themes unite the information presented: (1) substantial spatial and temporal variability in food web structure, and (2) how body-size can be used to generalize species-interactions across this complexity. Spatial variability occurred at various scales, from among small fish assemblages on seemingly homogeneous sand banks, to differences among landscape scale units (e.g. between lagoons and main river channel). Seasonal variability was apparent in predation patterns, with relative prey availability and body size primarily resulting in decreasing prey sizes with falling water levels. Body size was also related to functional outcomes of species interactions, for example, a size-based response of prey fishes to large-bodied piscivore exclusion. This pattern was further substantiated at the landscape-scale, as differences in assemblage structure among netted and un-netted lagoons were largely size-based. Trophic position of fish and body size was not found to be related, likely due to the diversity of prey available to consumers, and may signify that commercial netting activity will not decrease food chain lengths. In sum, by describing human impacts within a food web context, I endeavor to provide predictive power regarding a specific human-induced environmental problem, yet still allowing for generality that will broaden the theoretical foundations and applications of food web ecology.

Much of the world's species diversity is located in tropical and sub-tropical ecosystems, and a better understanding of the ecology of these systems is necessary to stem biodiversity loss and assess community- and ecosystem-level responses to anthropogenic impacts. In this dissertation, I endeavored to broaden our understanding of complex ecosystems through research conducted on the Cinaruco River, a floodplain river in Venezuela, with specific emphasis on how a human-induced perturbation, commercial netting activity, may affect food web structure and function. I employed two approaches in this work: (1) comparative analyses based on descriptive food web characteristics, and (2) experimental manipulations within important food web modules. Methodologies included monthly sampling of fish assemblages using a variety of techniques, large-scale field experiments, extensive stomach content and stable isotope analyses. Two themes unite the information presented: (1) substantial spatial and temporal variability in food web structure, and (2) how body-size can be used to generalize species-interactions across this complexity. Spatial variability occurred at various scales, from among small fish assemblages on seemingly homogeneous sand banks, to differences among landscape scale units (e.g. between lagoons and main river channel). Seasonal variability was apparent in predation patterns, with relative prey availability and body size primarily resulting in decreasing prey sizes with falling water levels. Body size was also related to functional outcomes of species interactions, for example, a size-based response of prey fishes to large-bodied piscivore exclusion. This pattern was further substantiated at the landscape-scale, as differences in assemblage structure among netted and un-netted lagoons were largely size-based. Trophic position of fish and body size was not found to be related, likely due to the diversity of prey available to consumers, and may signify that commercial netting activity will not decrease food chain lengths. In sum, by describing human impacts within a food web context, I endeavor to provide predictive power regarding a specific human-induced environmental problem, yet still allowing for generality that will broaden the theoretical foundations and applications of food web ecology.