Introduced species can become invasive and cause catastrophic alterations to the system they invade. Both zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena bugensis) have caused significant ecosystem alterations wherever they have invaded. These Dreissena species have caused changes in water quality and biodiversity and have disrupted energy pathways which can have cascading effects on other trophic levels. Recently quagga mussels invaded Lake Powell, a reservoir located in the southwestern USA, creating the possibility of a trophic cascade that could alter energy flow in the reservoir and change the trophic niche structure of the fishes in the lake. However, due to Lake Powell’s large size, dynamic nature, and complex hydrological structure, the effects of quagga mussels on fish species is uncertain. To determine impacts of quagga mussels on Lake Powell fishes, we quantified trophic niches of five species of sport fish over three years (2017-2019) using stable isotopes of nitrogen, δ15N, and carbon, δ13C. We test the following hypothesis: quagga mussels will cause a shift in trophic niche in more pelagic fishes such that pelagic fishes decrease in trophic position and shift toward use of more littoral energy. In addition, we compare the trophic niche of these species with a previous study on the trophic niche of fish in Lake Powell prior to full colonization of the lake by quagga mussels (2014-2015). In general, fish in the southern region of the lake exhibited a trend of decreasing δ15N suggesting decreasing trophic position and an enrichment of δ13C indicating a littoral energy shift in some species. Fish in the northern region of the lake exhibited a slight increase in trophic position and a shift towards pelagic energy across the same time period. These shifts support the hypothesis with pelagic fish experiencing a trophic niche shift, in the direction predicted, but only in the southern region of Lake Powell. Additionally, this shift is not exclusive to pelagic fish, but happened in all five species. Sediment laden input from the Colorado River may offset the impact of quagga mussels in the northern region of the lake resulting in observed regional differences.

Increased biodiversity generally enhances terrestrial ecosystem productivity. While niche-use efficiency is thought to drive the biodiversity-productivity relationship, the mechanisms within niche-use efficiency are not well understood. A potential mechanism for niche-use efficiency is nutrient-use efficiency. To measure nutrient-use efficiency, we calculated nitrogen-resorption efficiencies (NRE) because nitrogen is an important growth limiting nutrient for forest productivity. We used a plantation implemented as a full factorial design that included two levels of competition, implemented as different planting densities (one- and two-meter planting densities), and three diversity levels (monocultures, two-, and three-species plantings) that included three hardwood tree species (northern red oak (<i>Quercus rubra</i>), black cherry (<i>Prunus serotin</i><i>a</i>), and American chestnut (<i>Castanea dentata</i>). For our nitrogen-resorption efficiency data, we found that NRE increased as diversity and planting density decreased, but the magnitude of the response varied among species. This outcome suggests that while increased diversity likely provides a release from intra-specific competition, different combinations of species will play a critical role in shaping biodiversity-productivity relationships. Forest nutrient cycling can also be influenced by herbivory. To address the effects of forest diversity on herbivory rates, we monitored rates of foliar damage along with foliar nitrogen content. To measure foliar nitrogen content, we collected spectral data from early, midseason, and late season foliar samples. To assess foliar damage, we collected and imaged leaves from two canopy positions in order to measure late season foliar area and estimate pre damaged foliar area. We found that diversity and foliar nitrogen content have a positive relationship, and diversity does influence canopy damage but the effects vary among species and density. Upon further analysis, we found that foliar nitrogen content and canopy damage are correlated. Meaning individual trees showed a release from intraspecific competition, which lead to an increase in available nutrients and higher canopy quality, showing that stands with higher canopy quality experienced higher levels of damage.<br>

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The Allideghi Wildlife Reserve in the Amibara District of Afar Regional State, Ethiopia, has international significance for harboring endangered Grevy's Zebra and other wildlife dependent on grasslands. The reserve is increasingly used by pastoral people and their herds. Impacts of livestock on native vegetation include direct effects of grazing and indirect effects from livestock-facilitated dispersal of an invasive plant, Prosopis juliflora. The main research objective was to determine effects of pastoralism and vegetation change on prospects for sustaining the Allideghi Wildlife Reserve as grassland habitat for Grevy's Zebra. Methods included use of driving surveys to quantify resource use by herbivores, vegetation analysis, and engagement with local people. Resource-use patterns of livestock across the Allideghi grassland were often positively affected by proximity of water, while that for wild ungulates was often negatively affected by proximity of people. Livestock concentration at a major borehole has created a large piosphere with concomitant reductions in herbaceous standing-crop, productivity, and species richness; plant species have shifted from grasses to forbs in severely grazed sites. Vegetation further from the borehole was resilient in response to moderate grazing pressure in terms of species composition and productivity. Since being introduced at a nearby commercial plantation in the 1970s, P. juliflora has been dispersed to the Allideghi Wildlife Reserve via livestock; cattle, sheep, and goats eat the pods and deposit seeds in manure at settlements and favored foraging areas. Prosopis juliflora greatly reduced species richness and basal cover of native herbaceous vegetation in the Allideghi grassland. Analysis of remotely sensed images from the past 30 years indicated major land-use change in the district due to agricultural expansion as well as land-cover change due to Prosopis encroachment and heavy grazing. Recent efforts have been undertaken by various agencies to control P. juliflora, via harvest in the district, but this has yielded variable and often negative results. Without a concerted effort to limit livestock grazing and control spread of P. juliflora, the future for the grassland and wildlife at the Allideghi Wildlife Reserve is grim. Agencies and policy makers need to promote science- and community-based approaches to help rectify the situation. (311 pages)

The Nile monitor (Varanus niloticus) is a large, carnivorous lizard that has become a notorious invasive species in Florida, USA. Initially released in the 1980s from the pet trade, the species has since established at least three breeding populations and spread throughout much of southern Florida. While current control efforts have failed to eradicate V. niloticus, it is important to attain a better understanding of its invasive dynamics to guide and inform better control strategies. In this study, available georeferenced records of V. niloticus in Florida were compiled and linked to a habitat classification map to evaluate ecotype preferences. Factored with bioclimatic data, the regional spread of V. niloticus was modelled for contemporary and projected (i.e., in the year 2050) presence using Maxent and Mahalanobis Distance models. Study results indicate that V. niloticus maintains a strong presence in eastern Lee County on the southwestern coast. Populations in Palm Beach and Miami-Dade counties on the southeastern coast may be interconnected, contrary to current descriptions that they are separated from each another. Model forecasts of conditions for the year 2050 identify widespread expansion of V. niloticus in Florida, particularly northward with the establishment of a new population center in Pasco County in the western central peninsula. This is the first known modelling study of V. niloticus in Florida and identifies regions at greater risk for future population expansion.

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To understand the flow of water as a factor that influences aquatic vegetation communities and aquatic plant dispersal, custom-made Global Positioning System (GPS) drones were used to monitor the movement of water in Aliceville Lake, Columbus Lake, and Ross Barnett Reservoir, MS. In each reservoir, the drones drifted in the wind-generated surface current. Analysis of wind speeds suggests that a certain wind speed may be necessary to overcome gradient flow. Wind direction and wind speed should be incorporated in future spatial simulation models for aquatic plant dispersal and distribution. An herbicide evaluation on Cuban bulrush (Oxycaryum cubenese) was conducted to determine what herbicides would effectively control the invasive species. Applications made prelowering were more successful than postlowering applications for all herbicides tested with glyphosate, 2,4-D, triclopyr, diquat, imazamox, and imazapyr resulting in 100% mean biomass reduction. For postlowering applications, glyphosate, triclopyr, and diquat are recommended.

Increasing wetland restoration in the Lower Mississippi Alluvial Valley has been identified as a method to reduce nutrient loading in the Gulf of Mexico. Wetlands have historically been used to treat water through processes facilitated by wetland plants, and relatively few species and plant traits have been identified as important in carrying out these processes. This study focuses on some of those species and traits and aims to identify species differences and plant traits that may be important for wetland nutrient mitigation. Chapter I provides background information on nutrient pollution, wetland biogeochemical mechanisms for nutrient sequestration, and the focal species of the study. Chapters II and III cover the design and methods for this mesocosm study and the experimental results, while Chapter IV provides a discussion of these findings and identifies other questions that need to be addressed to better understand wetland nutrient removal dynamics.

No description available.