Browsing by Subject "stable isotopes"
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Item Biodiversity Conservation in Sub-Saharan Africa: A Case Study of the African Dwarf Crocodiles (Osteolaemus Spp.)(2014-11-20) Smolensky, Nicole LimungaOverexploitation of wildlife is a leading threat to biodiversity in tropical Africa. Effective management requires integrating information on the extent of exploitation, distribution, and status of exploited species. I explore how trade filters affected the final destination of bushmeat for different species involved in the trade. I highlight the trade in reptiles, in particular African Dwarf Crocodiles (Osteolaemus tetraspis) to investigate why they are rare in markets yet ubiquitously hunted. Hunting locations and methods determined the types of species entering bushmeat markets while selling conditions and prices determined whether species were traded locally or in urban markets. To prioritize conservation efforts of over-exploited species, it is important to determine the distribution and status of populations. I conducted detailed sampling of Osteolaemus populations in Cameroon and around the Cameroon Volcanic Line (CVL), to investigate the distributional limits and number of cryptic Osteolaemus species in the country. I found that O. tetraspis extends west beyond the CVL, thus, this mountain chain does not represent the distributional limit of this species. I also found O. osborni in Cameroon. I provided information on the population ecology of O. tetraspis and O. osborni in Cameroon to facilitate independent conservation of these two species. Both species are threatened in Cameroon based upon low encounter rates, young population structures and continued threats of habitat loss and hunting pressure. Crocodilians link nutrients and energy between food webs through their movements across heterogeneous habitats. These connections may differ among habitats and as they undergo ontogenetic shifts in diet. I compared food web associations of Ostoelaemus species inhabiting a large river and small tributary using stable isotope analyses of carbon and nitrogen. Osteolaemus species inhabiting perennial rivers have aquatic food web associations as opposed to the largely terrestrial food web associations detected when they occupy swamp habitats. These species have large dietary overlap between juveniles, adults and, sexes. Through my research, I have provided a working knowledge of the distribution, ecology, and hunting pressure of Osteolaemus species necessary for assessing their conservation status and developing sound management. These widely distributed species should be regionally managed to conserve their evolutionary diversity.Item Ecology of Wintering Black-capped Vireos in Mexico(2013-11-06) Powell, Robert AndrewThe black-capped vireo (Vireo atricapilla) is an endangered Neotropical migratory songbird that has received considerable attention in its breeding range, but relatively little attention in its winter range in Mexico. To address information needs regarding winter ecology of the black-capped vireo, I conducted research in Mexico focused on investigations of the winter distribution, habitat use, and migratory linkages between breeding and wintering sites. Over 3 winter periods in 2002?2004, I identified and described the geographic distribution for the black-capped vireo at study sites across 8 states in western Mexico and determined if differential migration occurs among different classes of individuals. I documented winter occupancy in the 5 most northern Mexican states surveyed (Sinaloa, Durango, Nayarit, Jalisco, and Colima), and identified a strong association between sex/age class and winter latitude with adult males occupying habitat at more northern latitudes relative to females and juveniles. During two winters in 2003-2004, I conducted field research at study sites in Mexico to evaluate patterns of winter habitat use by black-capped vireos and determine which habitat characteristics may influence vireo use of winter patches. Winter habitat use by black-capped vireos was best predicted by increasing values of slope and foliage cover, and by decreasing values of canopy cover and tree diameter. Vireo use plots characterized as thorn forest had greater foliage density, greater shrub density, less canopy cover, and smaller tree diameter than plots classified as tropical deciduous or semi-deciduous forest, suggesting that thorn forests may be most suitable for vireo occupancy during the winter months. I also used stable carbon, hydrogen, and nitrogen isotopes in black-capped vireo feathers obtained across 3 states in the breeding range and 8 states in the wintering range to determine if vireo feathers collected on the wintering grounds could be used to assess breeding origins and to determine if a relationship exists between breeding and wintering latitudes such as chain or leapfrog migration. Feathers collected at both breeding and wintering sites displayed considerable variability in isotopic composition for all 3 isotopes analyzed, and thus did not provide sufficient information to establish migratory linkages between breeding and wintering sites.Item Ecosystem resilience following salinity change in a hypersaline estuaryBreaux, Natasha JohnsonItem Effects of Woody Vegetation Removal on Soil Water Dynamics in a South Texas Shrubland(2013-07-30) Mattox, April MarieEcosystem changes from grassland to shrubland in the Rio Grande Plains are thought to have negative effects on the hydrology of the region. The increase in woody plants, known as woody encroachment, may alter the amount of water moving beyond the root zone of plants. Water moving beyond the root zone is referred to as deep drainage, and has potential to become aquifer recharge. A vegetation manipulation project was designed to understand the effects of woody vegetation removal on soil water dynamics in the recharge zone of the Carrizo-Wilcox aquifer of south Texas. The primary objective of the project was to determine the potential to increase groundwater recharge through woody vegetation removal. To understand the effects of vegetation removal on various soil textures we studied changes in soil water, rooting depth, and the role of water redistribution by woody vegetation. Woody vegetation was removed using common methods of cut-stump and roller chop across three soil types. Soil water contents and changes were measured using neutron moisture meter to a depth of 180 cm. Average rooting depth was determined across three soil types. Soil and stem water stable isotopes were used to understand soil water movement. Rooting depth was determined to between 140 and 160 cm for all soil textures. Soil water content and changes were analyzed at three depth increments: 0-60, 60-120 and 120-180 cm. ANOVA analysis showed that there was no treatment response in average soil profile water in the sandy or sandy loam soils. There was a significant decrease in soil profile water for clay loam soil in response to roller chopping. Changes in soil profile water were the greatest in the sandy roller chopped soils. Below 120 cm, three months had significant differences in change in soil water in the sandy roller chop plot. During dry conditions, Honey mesquite shifts water use to deeper in the soil profile. In clay loam soils under dry conditions there is evidence of water being moved up from below 2 m soil depth to drier shallow soils. Roller chopping in sandy soils is the vegetation removal treatment and soil type most likely to result in water moving beyond the root zone. Although treatments had significant effects on soil moisture dynamics that interacted with soil type, we did not find support for deep drainage effects over the Carrizo-Wilcox aquifer from woody vegetation removal.Item Evaluation of Natural Markers to Assess Cross-Shelf Connectivity of MesoAmerican Reef Fish Populations in Belize(2014-05-06) Wetmore, Lynne S.Quantitative evaluations of early-life connectivity in reef fish populations are critical to the effective identification and management of productive nearshore nurseries. The present study evaluates the use of natural markers in assessing both trophic and population connectivity for three species of snappers (Lutjanidae) utilizing back-reef nurseries in southern Belize: dog snapper (Lutjanus jocu), gray snapper (L. griseus), and schoolmaster (L. apodus). Juvenile snappers were collected from inner- and outer-shelf nurseries across two latitudinal regions in the Belize coastal lagoon in order to: (a) utilize tissue stable isotopes (?^(13)C and ?^(15)N) as dietary tracers to characterize organic matter production across the continental shelf, and (b) evaluate the utility of otolith stable isotopes (?^(13)C and ?^(18)O) as a marker of nursery origin. Isosource models (based on tissue ?^(13)C and ?^(15)N) revealed distinct differences in food web dynamics between pristine nurseries and those influenced by heavily impacted watersheds. Juvenile snappers at pristine sites were supported by organic matter derived from both benthic sources (seagrass/benthic diatoms) and the water column (phytoplankton), while sites impacted by anthropogenic runoff displayed significant decreases in water column-based production during the rainy season, accompanied by significantly decreased juvenile snapper condition (muscle lipid content) for all three species. These results emphasize the high land-sea connectivity in this system and indicate that runoff from impacted watersheds has the potential to disrupt trophic production in nurseries across the continental shelf. Otolith stable isotopes (?^(13)C and ?^(18)O) appeared to be strongly related to salinity gradients within the Belize coastal lagoon and, consequently, showed considerable promise in identifying juvenile snappers to shelf position. Both isotopes were both consistently enriched in snappers from outer-shelf nurseries, where freshwater influence was minimal, and ?^(18)O was enriched in the northern sampling region, which receives lower amounts of freshwater input compared to the south. Although individuals of each species were classified to specific study sites with varying success (58-81%), discrimination to shelf position was consistently high for all species (74-92%), indicating that otolith ?^(13)C and ?^(18)O may be useful in determining relative contribution rates of juvenile snappers produced at inner- and outer-shelf nurseries within tropical back-reef systems.Item Fish Assemblage and Food Web Structure in Whedos (Shallow Floodplain Habitats) of the Oueme River, West Africa(2012-10-19) Jackson, AndrewIn the Oueme River, a lowland river in Benin, Africa, artificial ponds constructed in the floodplain (whedos) are colonized during the high-water period by a presumably random sample of fishes from the river channel. As water slowly recedes from the floodplain, fishes are isolated in whedos until they are harvested near the end of the dry season. I surveyed fishes in whedos and adjacent main-channel and floodplain habitats during two low-water (2008 and 2009) and one falling-water (2010-2011) periods, and measured a suite of physicochemical variables including dissolved oxygen, temperature, specific conductivity, and percent cover of aquatic vegetation in the falling-water period to investigate if fish assemblage structure of whedos resulted from stochastic or deterministic processes. I also investigated food web structure of whedos by analyzing carbon (delta13C) and nitrogen (delta15N) stable isotope ratios of fish and primary producer tissue samples, and samples of net primary production, soluble reactive phosphorus (SRP), NH4+, NO2-, and NO3- collected during the falling-water period. Whedos were covered with dense growth of aquatic vegetation, and dissolved oxygen concentrations were lower in whedos compared to a natural floodplain depression and the main channel. Multivariate analyses revealed that habitat types were distinct with regard to fish assemblage structure and abiotic conditions. Assemblages in whedos and natural floodplain depressions were differentiated from those of the river channel, with the floodplain habitats being dominated by piscivorous fishes that tolerate aquatic hypoxia. These results indicate that fish assemblage structure of whedos was influenced by deterministic processes during the falling- and low-water periods when these water bodies were isolated. Floodplain habitats were more nutrient-rich than the river channel, and whedos were net heterotrophic. Microphytobenthos and C3 macrophytes accounted for a large fraction of fish biomass in whedos, compared with the river channel, which was mainly supported by seston. Whedo food webs had fewer trophic transfers compared to the food web of the river channel.Item Floodplain River Foodwebs in the Lower Mekong Basin(2013-11-15) Ou, ChoulyThe Mekong River is one of the world?s most important rivers in terms of its size, economic importance, cultural significance, productivity, and biodiversity. The Mekong River?s fisheries and biodiversity are threatened by major hydropower development and over-exploitation. Knowledge of river food web ecology is essential for management of the impacts created by anthropogenic activities on plant and animal populations and ecosystems. In the present study, I surveyed four tropical rivers in Cambodia within the Mekong River Basin. I examined the basal production sources supporting fish biomass in the four rivers during the dry and wet seasons and explored the relationship between trophic position and body size of fish at various taxonomic levels, among local species assemblages, and across trophic guilds. I used stable isotopes of carbon and nitrogen to estimate fish trophic levels and the principal primary production sources supporting fishes. My study provides evidence that food web dynamics in tropical rivers undergo significant seasonal shifts and emphasizes that river food webs are altered by dams and flow regulation. Seston and benthic algae were the most important production sources supporting fish biomass during the dry season, and riparian macrophytes appeared to be the most important production source supporting fishes during the wet season. In the river with strong flow regulation from an upstream impoundment, seston and benthic algae were even more important production sources supporting fishes during the dry season. My findings challenge the Eltonian theory of size-based trophic structure in food webs and also contradict the broadly accepted prediction of the fishing-down-the-food-web concept. Eltonian and fishing-down-the-food-web concepts propose that trophic level is strongly correlated with body size, but I found no significant correlation between body size and trophic position for fish assemblages. Results suggest that body size distributions are not useful for prediction the trophic structure of communities with diverse detritivores, omnivores and insectivores, but that it is a good predictor of trophic position among piscivorous fishes.Item Nitrogen nutrition of Alexandrium tamarense : using δ¹⁵N to track nitrogen source used for growth(2009-05) Smith, Christa Belle; Erdner, Deana L.; Pease, Tamara Kaye; McClelland, James W.Alexandrium tamarense is a harmful algal species that can produce saxitoxins, a suite of powerful neurotoxins that bioaccumulate up the food chain and can have severe economic and health impacts. With harmful algal blooms increasing temporally and spatially, it is important for us to understand the relationship between harmful algal blooms and nutrients, particularly nitrogen from anthropogenic sources. To this end, the stable nitrogen isotopic composition (δ¹⁵N) of medium nitrate, algal cells and toxin in both nitrogen-replete and nitrogen-limited batch cultures of A. tamarense were measured in order to assess the potential for using the δ¹⁵N of the toxin as a tracer of the nitrogen source used for growth. A. tamarense cells grown under nitrate-replete conditions were depleted by 1.5‰ relative to the growth medium, and saxitoxin was depleted by 1.5‰ relative to the whole cells. Under nitrate-limiting conditions, the isotopic difference between cells and saxitoxin changed as nitrate in the growth medium was depleted, indicating uncoupling of toxin synthesis and cell growth rates under changing external nutrient conditions. Determination of the absolute magnitude of the isotopic differences between the medium nitrate and either the cells or the saxitoxin was confounded by 1) using two different nitrate sources – one nitrate source was used to grow the inoculum and a different nitrate source was used for the experimental medium - with different ‰ values and 2) the presence of an unidentified, isotopically-light, nitrogen blank in the low-nitrate medium samples. I conclude that STX nitrogen isotope values have the potential to be used as nitrogen source indicators. However, overall fractionation between whole cells and STX is unknown due to the uncoupling between cell growth and STX synthesis observed during my nitrogen-limited experiment. Based on previous research on cell growth and toxin production dynamics under different nutrient regimes, it is also reasonable to assume that the observed results here may differ if a different nitrogen source was utilized by the cells for STX production. Further research could include isotope analysis of cultures grown on different nitrogen sources, such as ammonium and urea; isotopic analysis of additional compounds, such as amino acids; or use of additional stable isotopes, such as C or O.Item Seasonal isotope and trace-metal profiles of serially-sampled Conus gastropods: proxies for paleoenvironmental change(Texas A&M University, 2006-08-16) Gentry, David KeithWe test the fidelity of shallow-water gastropod skeletons as multi-proxy archives of seasonal paleo-environmental change by performing isotopic and trace-metal analyses on specimens of Conus ermineus from the Gulf of Mexico. Four adult specimens were collected from Stetson Bank in the Flower Garden Banks National Marine Sanctuary during the summer of 2002. Shell samples were milled along axes of growth to produce time-series profiles spanning up to eight years. We corrected the profiles for growth rate effects and compared the tuned results with in situ temperature and salinity records at the reef surface and temperature profiles from nearby surface buoys. Examination of sample densities in δ18O cycles shows that shell growth is faster during summers and slower during winters. Tuning the profiles versus time yields δ18O values that co-vary closely with seasonal temperatures to a high degree of coherency (R2 = 0.84). The δ13C profiles show cyclic variation modified by ontogenetic decreases in δ13C. These ontogenetic trends are attributable to decreasing metabolic efficiency, while seasonal cycles reflect hydrographic changes in the gastropods?? habitat. Salinity and δ13C of dissolved inorganic carbon show a strong correlation at Stetson Bank (R2 = 0.80), and early summer shell δ13C minima coincide with local salinity minima during times of peak river discharge. The terminations of these δ13C minima occur during annual upcoast reversals of shelf currents in this area. These effects are augmented by summer stratification and productivity minima that further decrease seawater δ13C. Sr/Ca ratios increase through ontogeny, most likely due to decreasing metabolic efficiency. However, seasonal variations in Sr/Ca profiles show strong similarity with δ18O profiles, confirming the temperature dependence of Sr/Ca and minimal influence of salinity on shell δ18O at Stetson Bank. The results of this study show that tuned δ18O and Sr/Ca profiles can be used to reconstruct seasonal paleotemperatures. Carbon isotope profiles and environmental data also demonstrate the utility of Conus δ13C as a proxy for freshwater flux and shelf circulation.Item Woodland development and soil carbon and nitrogen dynamics and storage in a subtropical savanna ecosystem(Texas A&M University, 2005-02-17) Liao, Julia Den-YueWoody plant invasion of grasslands is prevalent worldwide, but the biogeochemical consequences of this vegetation shift remain largely unquantified. In the Rio Grande Plains, TX, grasslands and savannas dominated by C4 grasses have undergone succession over the past century to subtropical thorn woodlands dominated by C3 trees/shrubs. To elucidate mechanisms of soil organic carbon (SOC) and soil total N (STN) storage and dynamics in this ecosystem, I measured the mass and isotopic composition (δ13C, δ15N) of C and N in whole-soil and soil size/density fractions in chronosequences consisting of remnant grasslands (Time 0) and woody plant stands ranging in age from 10-130 years. Rates of SOC and STN storage averaged 10-30 g C m-2yr-1 and 1-3 g N m-2yr-1, respectively. These accumulation rates increased soil C and N pools 80-200% following woody encroachment. Soil microbial biomass (SMB-C) also increased after woody invasion. Decreasing Cmic/C org and higher qCO2 in woodlands relative to grasslands suggests that woody litter is of poorer quality than grassland litter. Greater SOC and STN following woody invasion may also be due to increased protection of organic matter by stable soil structure. Soil aggregation increased following woody encroachment; however, most of the C and N accumulated in free particulate organic matter (POM) fractions not protected within aggregates. Mean residence times (MRTs) of soil fractions were calculated based on changes in their δ13C with time after woody encroachment. Free POM had the shortest average MRTs (30 years) and silt+clay the longest (360 years). Fine POM had MRTs of about 60 years, reflecting protection by location within aggregates. δ15N values of soil fractions were positively correlated with their MRTs, suggesting that higher δ15N values reflect an increased degree of humification. Increases in SOC and STN are probably being sustained by greater inputs, slower turnover of POM (some biochemical recalcitrance), and protection of organic matter in aggregates and association with silt and clay. Grassland-to-woodland conversion during the past century has been geographically extensive in grassland ecosystems worldwide, suggesting that changes in soil C and N dynamics and storage documented here could have significance for global C and N cycles.