Browsing by Subject "predators"
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Item Survival and mammalian predation of Rio Grande Turkeys on the Edwards Plateau, Texas.(Texas A&M University, 2004-09-30) Willsey, Beau JudsonTrends in Rio Grande wild turkey (Meleagris gallopavo intermedia) abundance on the Edwards Plateau (EP), Texas, have been either stable or in decline since the 1970s. Four study areas, 2 each within stable (Stable Area A, SAA; Stable Area B, SAB) and declining regions (Declining Area A, DAA; Declining Area B, DAB), were delineated to examine (1) both annual and seasonal survival, (2) relative mammalian predator mean abundance (RMA), and (3) potential effects of lunar phase on scent-station visitation. During February 2001-March 2003, 257 turkeys were captured and instrumented with radio transmitters. Survival probabilities were generated using a Kaplan-Meier product limit estimator; a log-rank test tested for differences among sites. Annual survival was statistically different between regions (stable 0.566 ? 0.081; declining 0.737 ? 0.094; X2 = 3.68, P = 0.055) in 2002. Seasonal survival differed between regions (stable 0.812 ? 0.103; declining 0.718 ? 0.130; X2 = 3.88, P = 0.049) in spring 2003. Annual survival results during 2002 were counterintuitive with turkey trend data. Scent-station transects were established on non-paved ranch roads within study regions. Scent-station indices revealed higher (H = 19.653, P ? 0.001) RMA of opossum (Didelphis virginiana) and skunk (eastern spotted [Spilogale putorius], striped [Mephitis mephitis], or western spotted [S. gracilis]) (SAA, x? = 0.0148; SAB, x? = 0.0151; DAA, x? = 0.0042; DAB, x? = 0.0065) on stable areas. Higher RMA of coyotes (Canis latrans) on declining areas (SAA, x? = 0.0067; SAB, x? = 0.0022; DAA x? = 0.0234; DAB x? = 0.0434) suggested a possible causative factor of the decline, but abundance indices were not verified by empirical data though. Lunar phase was not a significant (T = -0.225, P = 0.822) covariate in scent-station visits by raccoons, opossums (new, x? = 0.0111; full, x? = 0.0324), or unidentified tracks (new, x? = 0.0649; full, x? = 0.0375). Nightly precipitation and wind speed probably influence mammalian use of scent stations more so than lunar illumination.Item The role of piscivores in a species-rich tropical river(Texas A&M University, 2004-11-15) Layman, Craig AnthonyMuch 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.