Invasion of exotic species is a global threat to native species, biological diversity, and ecological restoration projects. Pomacea maculata is a macrophyte herbivore often misidentified with one of the world’s most invasive and destructive exotic snail, Pomacea canaliculata, but has a broader geographical distribution and climate tolerance, and greater egg production. This research examines whether the exotic P. maculata and native Pomacea paludosa occupy identical vegetation communities, mechanisms of interference competition, grazing impact differences on Vallisneria americana and to develop an exploratory agent based model. This model uses historical and present data to project how differences between species in life history and grazing patterns can potentially impact South Florida ecosystems. This model examined how P. maculate invasion of South Florida could affect two of the Central Everglades Planning Project’s main environmental restoration goals: function of key vegetative communities and conservation of endangered or threatened species. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2014. / FAU Electronic Theses and Dissertations Collection

Copy of author's previously published work inserted. Bibliography: leaves 261-306. The objectives of the project were: i. to determine whether there are competitive interactions between Aporrectodea trapezoides and A. caliginosa and A. rosea.--ii. to investigate compeditive interactions between A. calignosa, Microscolex dubius and A. trapezoides.--iii . to determine the likely impact of A. longa on soil fauna, especially the native earthworm, Gemascolex lateralis, in native ecosystems.

The effects of harvesting Pacific hake (Merluccius productus) were examined with a multi-species numerical model composed of seven logistic growth equations coupled by a food web. The food web was composed of: California sea lions (Zalophus californianus), hake, sablefish (Anaplopoma fimbria), spiny dogfish (Squalus acanthias), a generic rockfish species (Sebastes spp.), Northern anchovy (Engraulis mordax), and a generic euphausiid species (as an example, Pacific krill, Euphausia pacifica). The model was tuned to mimic stock assessment biomass estimates for the years 1980 to 1991, after which a short term experiment and a sensitivity analysis were conducted. The short term experiment used a factorial design, with hake fishing and fishing for rockfish, sablefish, and anchovy as treatments. It was analyzed with scale analysis techniques. The results indicate that hake (from a management viewpoint) is potentially most important in the offshore community as a prey item for sea lions and as a predator on anchovies, but hake fishing mortality had a small effect on either population during the time span of the experiment. Results also suggest that hake has little or no competitive interaction with other species that are trophically similar. Increased harvesting of hake would probably do little to increase the biomass of rockfish or sablefish. The general conclusion of both the sensitivity analysis and the experiment is that species below their carrying capacity are mostly affected by changes in growth and removal processes while species close to their carrying capacity are mostly affected by processes controlling prey availability. A forty year projection from 1991 to 2031 was conducted to examine the effects of hake fishing on sea lion and anchovy biomass. Results indicate that sea lion biomass will vary inversely with hake fishing effort, while anchovy biomass is directly proportional to hake fishing effort. Results also indicate that hake experience environmental conditions not favorable to recruitment. During favorable conditions the hake population builds up a "surplus" that carries it through periods of unfavorable conditions. Increased hake fishing effort reduces the response of the population to favorable conditions. The results of this research constitute a step from the theory toward the practice of proactive multi-species and ecosystem management. / Graduation date: 1995

Thesis (M.S.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 52-60). Also available on the World Wide Web.

Thesis (M.S.)--North Carolina State University, 2003. / Title from PDF t.p. (viewed on Aug. 21, 2004). Includes vita. Includes bibliographical references (p. 88-94).

The role of interspecific competition as a regulating force in natural populations has been controversial, especially for phytophagous insect communities. A series of manipulative experiments using enclosure cages were conducted to evaluate the role of interspecific competition between a weevil and a fly, two seed feeding agents released against spotted knapweed in North America. The fly, an inferior biological control agent, was the superior competitor. Consequences of the antagonistic interaction included reduced seed destruction compared to if just the weevil was released on its own. The role of plant phenology on insect herbivore density was also assessed. The implications of phenologyinduced variation in insect density were evaluated with respect to competition between the fly and the weevil and were found to be important. Hypotheses of four plantmediated mechanisms of interspecific competition were also tested. Results support resource preemption as a competitive mechanism. / vii, 97 leaves ; 29 cm

The structure and composition of savanna vegetation is influenced by resource availability and disturbance. Grasses, a major component of savannas, influence this resource availability by competing directly with trees for light, water and soil nutrient resources. The direct causes of bush encroachment are not always apparent, but are commonly ascribed to overgrazing and consequent decreased grass competition. The interaction, both above and belowground, between tree and grass seedlings and the surrounding grass sward is dependant on many factors, such as soil depth, seedling species and sward composition. These factors, as well as the presence or absence of defoliation, in the form of grazing or fire dictate whether the system will remain in a transition state as savanna or move towards a stable woodland state. The major competitive effects experienced by the tree seedlings were dependant on grass species and nutrient level. A. nilotica was affected by aboveground competition while A. karroo was affected by belowground competition. E. capensis caused the greatest decrease in A. karroo plant biomass. Both E. capensis and H. hirta had large competitive effects on the aboveground biomass of A. nilotica, while S. africanus had the greatest effect on belowground biomass. Increasing nutrient availability resulted in an increase in the competitive effect exerted on A. karroo, while little to no change was seen in the competitive effect exerted on A. nilotica. Soil depth constrained plant size in both tree species. The intensity of belowground interactions on tree biomass was unaffected by soil depth, while aboveground competition had a significant effect on shallow soils. Belowground competition was also of greater importance than aboveground competition in dictating tree seedling height. Grass seedlings growing on all three soil depths differed in mean mass, with E. racemosa having the least mass and T. triandra having the greatest. Simulated grazing by cutting the surrounding sward resulted in biomass increases in all three grass species. Changes in savanna composition and structure are thus likely to be influenced by initial species composition and soil depth and soil nutrient composition. While grazing creates niches for grass seedling establishment, heavy grazing has been observed to increase grass seedling mortality. Encroachment is thus more likely to occur on intensively grazed shallow and deep soils than on medium depth soils. This highlights the importance of ensuring the grass sward remains vigorous by resting and monitoring stocking rates to ensure veld is not over-utilized. It is then possible to maintain some form of tree-grass coexistence at a level where available grazing is not compromised. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.

The long-term economic viability and ecological integrity of savanna rangelands is being undermined by increasingly dense woody thickets at the expense of palatable herbaceous cover. This process is known as shrub- or bush-encroachment. Bush encroachment is a subset of a broader ecological riddle underlying the coexistence of woody and herbaceous vegetation that has been the subject of many ecological models. The ecophysiological mechanisms and interactions between trees and grasses on which most assumptions of ecological models are premised have seldom been tested empirically. This document synthesises the results of greenhouse and field-based investigations of the underlying ecological mechanisms and ecophysiological interactions between encroaching Acacia mellifera trees and grasses in a semi-arid environment. In a greenhouse study, I determined the contribution of N2 fixation to the N-budget of Acacia mellifera under conditions of both varying N availability and competition from grass. Tree seedlings had longer shoots and greater total dry mass in the absence of grass. The leaf δ15N values were lower with grass than without grasses. Thus, trees were more reliant on N2 fixation in the presence of grasses. N2 fixation may enable the tree seedlings to survive competition with grass at critical and vulnerable developmental stages of germination and establishment. In a field removal experiment, I monitored the growth rates, water relations and mortalities of shrubs around which neighbouring woody plants were removed (target) and control shrubs over three years. Results showed target trees to have benefitted from removal of neighbours, which was manifested in significantly faster growth rates, less negative predawn water potential and a relatively small degree of canopy die-back. Nonetheless, neighbouring trees appeared to prevent the whole plant mortalities resulting from severe environmental stress. Growing in close proximity with neighbours could therefore yield positive and negative ecophysiological effects. In another greenhouse experiment, I tested the effects of the separation of moisture uptake with depth between tree seedlings and grasses on two common substrate types. I also examined the influence of repeated grass clipping on the persistence of soil moisture. Results indicated a three-tier rooting pattern with a top layer exclusively exploited by grasses, an intermediate layer occupied by both grass and tree roots and deeper layers exclusively tapped by trees. Tree seedling biomass was negatively affected by grass competition although the biomass of grass was enhanced in the presence of tree seedlings on sandy substrates only. The repeated clipping of grass benefitted tree seedlings on rocky substrate more than it did on sandy substrate. The effects of heavy grazing on soil moisture availability to woody shrubs and thus bush encroachment may be contingent on substrate type, being more acute on rocky terrains. Grass competition suppresses tree seedlings but the removal of grass by grazing weakens this suppressive effect, particularly on rocky substrates. The insufficiency of space and soil resources on rocky substrates may necessitate increased investment in root biomass by plants. It is not known why grasses have lower densities on rocky substrates than on sandy substrates, but the obstruction by rocks disadvantages grasses against tree seedlings, leaving grasses vulnerable to grazing pressure. This may allow the woody plants on rocky substrates to benefit more from grass removal than on sandy substrates. Root restriction by rock barriers and, perhaps, sparse soil volume further lead to small tree sizes on the rocky substrate. Small shrubs are less likely to compete intensively for resources and cause density-dependent mortality. Intraspecific competition may maintain shrub sizes within the threshold that can be supported by available resource pools. I conclude from my results that the two-layer hypothesis of niche separation between savanna vegetation is valid although there is an overlap in the grass and tree rooting depth/moisture uptake. An additional factor that affects the success of A. mellifera is the substrate.Trees are more dense on rocky substrates but grow larger on sand. I have further shown that A. mellifera trees fix nitrogen when competing with grasses but do not do so when grasses are absent. A mechanistic model of savanna dynamics will need to integrate water use patterns, substrate and nutrients to make effective predictions about encroachment patterns. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.

As restoration ecology has matured as a science there has been increased interest in the relationship between species diversity and landscape health. Degraded landscapes tend to be resource poor, which limits species diversity as only species which are capable of growing and reproducing in these resource limiting environments can inhabit the area. Additionally, the established species are strong competitors for resources and will exclude, by way of inter-specific competition, weaker competitor species attempting to invade the degraded area. Several studies have demonstrated that with increased species diversity the overall productivity and functionality of the grassland increases. Seedling development and competitive interactions between grass seedlings has a significant impact on the final community structure and species diversity. It is for this reason that aspects of the ecology of grass seedlings were investigated. The growth and competitiveness of Chloris gayana, Cynodon dactylon, Digitaria eriantha, Eragrostis curvula and E. tef seedlings were determined under three environmental stimuli, namely nitrogen availability, light availability and exposure to plant-derived smoke (in the form of smoke-infused water). The primary conclusion from the competition experiments was that the species can be split into superior and inferior competitors at the seedling stage. Chloris gayana, E. curvula and E. tef were the most competitive seedlings as they had the largest negative effect on the growth of other species (high nitrogen Relative Interactive Index (RII) = -0.449, -0.203 and -0.379 respectively) and they were least affected by competition (high nitrogen RII = -0.251, -0.168 and -0.248 respectively). The calculated RII indicates the strength of the competitive interactions, the more negative the RII the stronger the competitive interaction. Nutrient availability had limited effect on the competitive hierarchy of the tested species. Chloris gayana seedlings, however, increased in competitiveness with an increase in available nutrients. In other words, there was a decreased negative response to competition in a high nutrient environment (high nitrogen RII -0.251, no nitrogen RII -0.605). When D. eriantha was grown under varying shade, nutrient and competition levels it was evident that the primary stress factor was light deficiency (p<0.001), and nutrient availability had no affect on seedling growth (p=0.069). Smoke-infused water had no consistent affect on the germination success or the seedling’s root and shoot vigour for the five grasses. These results indicate that the introduction of a “2-phase” or “multi-phase” restoration plan may be beneficial for the development of species diverse rehabilitated grasslands. Manipulating the time and space that the different species are planted, or the distribution of nutrient concentration over the area, may increase the survivorship of all the species that are introduced to a restoration site. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.

Copy of author's previously published work inserted. / Bibliography: leaves 261-306. / xxix, 308 leaves : ill. (some col.), maps ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / The objectives of the project were: i. to determine whether there are competitive interactions between Aporrectodea trapezoides and A. caliginosa and A. rosea.--ii. to investigate compeditive interactions between A. calignosa, Microscolex dubius and A. trapezoides.--iii . to determine the likely impact of A. longa on soil fauna, especially the native earthworm, Gemascolex lateralis, in native ecosystems. / Thesis (Ph.D.)--University of Adelaide, Dept. of Soil Science, 1996