Browsing by Subject "ecophysiology"
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Item Dynamics of water use and responses to herbivory in the invasive reed, Arundo donax (L.)(2010-07-14) Watts, David A.The first objective of this study was to investigate the role of an invasive grass species, Arundo donax (L.), on the hydrologic cycle. At a site on the Rio Grande in South Texas, we measured the gas exchange of carbon dioxide and water vapor at the leaf scale and structural characteristics, such as leaf area and shoot density, at the stand scale. In order to assess the effect of water availability, this study was conducted along transects perpendicular to the edge of the river along a potential moisture gradient. The second objective was to quantify the effect of two herbivores, an armored scale, Rhizaspidiotus donacis (Leonardi), and a stem-galling wasp, Tetramesa romana (Walker),on the photosynthetic and transpiration rates of A. donax. Leaf gas exchange measurements were made to determine the direction and magnitude of the effect on physiological processes and by what mechanisms any effects arose. Stands of A. donax used approximately 9.1 ? 1.1 mm of water per day. This rate of water use was at the high end of the spectrum for plants. The major controls on stand scale transpiration were evaporative demand, leaf area index, and water availability. During two summer seasons, stand scale transpiration varied greatly, following the pattern of variability in precipitation, suggesting that recent rainfall constituted a significant proportion of the water taken up by this species. Herbivory by a stem-galling wasp and a sap-feeding scale, both separately and together, reduced the rates of leaf scale physiological processes in A. donax. The efficacy of the wasp was density dependent, and this herbivore reduced the carboxylation rate of Rubisco. The effect of the scale took approximately five months to manifest, which coincided with generation time. Scale reduced photosynthesis by decreasing the maximum rate of electron transport. When the two insects were both present, the effect of their herbivory seemed to be additive. These results will assist the responsible management agencies in evaluating the propriety of using one or both of the insect herbivores as biological control agents.Item Ecological significance of the genetic variation in Bouteloua curtipendula (Michx.)Torr.(Texas A&M University, 2005-02-17) Andrea, Tomas de Pisani, MariaThe two most common varieties of the grass Bouteloua curtipendula (Michx.)Torr. in North America use different strategies of clonal growth and have contrasting continental distributions. Variety caespitosa (phalanx form) ranges from central Texas westward to Arizona and California and var. curtipendula (guerrilla form) occurs in a more mesic region over the North American Great Plains. This study sought to determine whether the strategies had an ecological significance and investigated the possible relationship between changes in environmental factors and characteristics of each clonal growth strategy. Varieties showed to be morphologically variable, but the variability did not follow the pattern of the precipitational gradient. The abundance of var. curtipendula was related to soil depth and parent material (limestone types). Abundance of var. caespitosa could not be explained by any environmental factor separately. The performance of clones of the two growth forms in response to changes in resource availability (light and nutrients) and defoliation suggested similarities between the varieties in photosynthetic rate and only showed differences in water potential under extreme conditions. The major differences were related to the proportion of biomass allocated to structures related with seed production versus propagation by rhizomes. Intra-variety genetic variation for several life history traits was detected even with a very small sample size. The caespitose growth form showed more biomass and rate of tiller recruitment after defoliation on average, but responses between genotypes were dissimilar. Varieties also showed levels of plasticity in the allocation to reproductive structures in response to environmental factors. Responses to fire were compared between varieties by experimental burnings with increasing load of artificial fuel. Plants of the two varieties reached similar maximum temperatures although var. caespitosa suffered temperatures considered to be lethal for longer periods of time. Results from this study suggested that, although characteristic of the pattern of clonal growth were not distinctly associated to resource availability or defoliation, distribution of the varieties may be related to a combination of biotic and abiotic factors beyond the factors studied here.Item Ecophsiology of Growth in the Pacific White Shrimp (Litopenaeus vannamei)(2010-07-14) Walker, Scott J.Ecophysiological responses of Litopenaeus vannamei were evaluated as functions of 1) salinity and animal size, 2) temperature and the animal's nutritive state, and 3) dissolved-oxygen concentration and animal size. Growth rate, routine metabolic rate, limiting oxygen concentration for routine metabolism, and marginal metabolic scope were determined for L. vannamei maintained and tested at salinities of 2, 10, and 28 ppt, all at 28 C. Routine metabolic rate (RMR) was not demonstrably dependent on salinity but decreased with increasing shrimp weight. Limiting oxygen concentration for routine metabolism (LOCr) was independent of shrimp weight up to 9 g; but, for larger shrimp, decreased with increasing weight. Marginal metabolic scope (MMS = RMR/LOCr) also decreased with increasing shrimp weight and was independent of salinity for shrimp weighing up to 9 g; but, like LOCr, MMS was dependent on salinity for larger shrimp. Growth rate was significantly less at 2 ppt than at 10 or 28 ppt, which gave similar growth rates. The effects of four temperatures (20, 24, 28, and 32 C) on growth, RMR, LOCr, and MMS were examined for fed and starved L. vannamei. Routine metabolic rate increased with increased temperature both for fed and starved shrimp. Marginal metabolic scope and growth appeared to be positively related and, at 20 C, seemed to induce a state of metabolic torpor. Data from the study of chronic effects of hypoxia (~2 mg O2 L-1) vs. normoxia (> 5 mg O2 L-1) on ecophysiological responses indicated that although low-DO environments can depress RMR and growth in L. vannamei, animals grown under hypoxic and normoxic conditions did not differ in their metabolic responses upon acute exposure to hypoxia, providing no evidence of acclimation to hypoxia in L. vannamei. Data from the above experiments were used to parameterize Ecophys.Shrimp, a computer simulation model of shrimp growth in time-varying environmental regimes. One unified model was able to simulate all my experiments; and, with only minimal adjustment of the model parameter MMSO, it also adequately simulated studies taken from the literature. Thus, Ecophys.Shrimp seems capable of realistically representing the ecophysiological dynamics of shrimp metabolism and growth in various culture systems.