No description available.

No description available.

Understanding of mechanisms that limit the abundance and distribution of species is central to ecology. The failure of mechanisms to regulate populations can result in population outbreaks. There have been two outbreaks of house mice in the past decade in central Florida. In my study, I examine the efficacy of landscape management in the form of mowing and plowed soil barriers to limit or prevent outbreaks of house mice in a former agricultural area. House mouse populations were highly variable, but were unaffected by mowing or plowed soil barriers. Red imported fire ants were ubiquitous in the study area regardless of land management treatments. Control of fire ants did not result in more house mice on treated plots.

Conserving avian populations requires understanding how they are limited by density-independent factors and regulated by density-dependent processes. To better understand the relative importance of limiting factors and regulatory processes in wintering American Redstarts (Setophaga ruticilla), I carried out two related studies. First, to determine how food availability affects space use, body composition, and migration timing, I experimentally decreased food availability in high-quality mangrove habitat. Using an insecticide, I reduced food by ~80%, which mimicked natural losses in nearby scrub habitats. I found that food-reduced redstarts deposited fat and lost muscle compared to control birds. Subsequently, food-reduced redstarts experienced on average a one-week delay in departure on spring migration. Previous work has demonstrated that for each day delayed after the first male arrival on the breeding grounds, redstarts experience an 11% decrease in the chance of successfully reproducing. Thus, my results demonstrate experimentally, for the first time, that fluctuations in winter food-availability can lead to fitness costs for migratory birds, and that the mechanism involves a fat-muscle trade-off. Second, to understand how limiting factors and density-dependence interact to drive population dynamics, I used four years of data on redstarts wintering in Jamaican scrub and mangrove forests. In a dry and food-limiting year in scrub, I found that individuals on territories surrounded by a high density of conspecifics experienced large losses in food availability, suggesting a density-dependent depletion of resources. These losses in food were correlated with poor body condition, and individuals on high -density territories delayed departure on spring migration. In two wetter and less food-limiting years in scrub, and in all years in high-quality mangrove habitat, no effects of neighbor density were evident and density-independent factors alone appeared to determine body condition and departure date. Previous research has shown that poor body condition reduces annual survival and that delayed departure has carry-over effects into the breeding season, resulting in lower fecundity. Thus, both food limitation and neighbor density appear to drive population dynamics, but density-independent factors may override the negative effects of density when weather conditions are favorable. / acase@tulane.edu

Most animals grow substantially during their lifetime and change in competitive ability, predatory capacity and their susceptibility to predation as they grow. This thesis addresses the implications of this on regulation and dynamics within populations as well as between population interactions. In size-structured populations either reproduction or maturation may be more limiting. If juveniles are competitively superior, the competitive bottleneck will be in the adults and reproduction will be limiting. Mortality will in this case result in overcompensation in juvenile biomass through increased reproduction. Compensation in biomass was demonstrated in Daphnia pulex populations subjected to size-independent mortality, where juvenile biomass did not decrease when a substantial harvest was imposed due to increase per capita fecundity. This supported that juveniles were superior competitors and that population cycles seen in Daphnia are juvenile-driven. Compensatory responses in biomass may lead to that predators facilitate eachothers existence by feeding on a common prey, a phenomenon coined emergent facilitation. In an experimental test of the mechanism behind emergent facilitation it was demonstrated that the invertebrate predator Bythotrephes longimanus was favoured by thinning of its prey Holopedium gibberum. The thinning mimicked fish predation and targeted large individuals while Bythotrephes preferrs small prey. Size dependent predation also occurs within populations, i.e. cannibalism, were large individuals feed on smaller conspecifics. Two populations of the common guppy (Poecilia reticulata) originating from different environments were demonstrated to differ in cannibalistic degree. Cannibalism was also affected by the presence of refuges and females and juveniles from one population were better adapted to structural complexity than the other. The effects of these differences in cannibalism on population regulation and dynamics were studied in long term population experiments. Both populations were regulated by cannibalism in the absence of refuges, and displayed cannibal-driven cycles with suppression of recruitment and high population variability. The presence of refuges decreased density dependence and population variability and harvesting of large females in the absence of refuges led to population extinctions in the more cannibalistic population. The less cannibalistic population had higher population biomass and stronger density-dependence in the presence of refuges. When refuges were present, cohort competition increased and cycles with short periodicity were seen. Large individuals were not only cannibals, but could successfully prey on other species. Small and large guppies were allowed to invade resident populations of Heterandria formosa. Small invaders failed while large invaders succeeded as predation from large invaders broke up the competitive bottleneck that the resident population imposed on juveniles of the invader.

Orientador: Odair Aparecido Fernandes / Resumo: A espécie Diatraea saccharalis, conhecida popularmente como broca-da-cana, é considerada uma das principais pragas da cultura de cana-de-açúcar. Agentes de controle biológico para ovos e larvas são promissores, no entanto, estas fases são difíceis de serem amostradas e ferramentas precisas para liberação e aplicação desses agentes são necessárias. A temperatura é um dos principais fatores abióticos que influenciam o desenvolvimento dos insetos. Desta forma, este trabalho objetivou: a) determinar as exigências térmicas de D. saccharalis e o modelo matemático mais adequado para expressar a relação entre taxa de desenvolvimento e temperatura; b) aprimorar e avaliar os modelos matemáticos Graus-dia e Equações de diferenças na previsão de picos populacionais da praga e c) validar as previsões de campo por meio de modelo de equações diferenciais com retardo e dados obtidos em laboratório. A temperatura de 18 ºC causou 78,16% de mortalidade. A temperatura ótima para o desenvolvimento foi de 28 ºC e a temperatura de 36 ºC foi letal para todas as fases de D. saccharalis. O ciclo total de vida (ovo-adulto) variou, em média, de 39 a 137 dias. A constante térmica obtida pelo modelo linear Ikemoto e Takai foi o de melhor ajuste, com valores para ovo, larva e pupa, respectivamente de: 68,25, 354,24 e 125,20 graus dia (GD). O modelo Brière-1 foi considerado o mais adequado para todas as fases. Embora os modelos de Harcourt e Equação-16 tenham mostrado, respectivamente, bons ajustes para ovo... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The species Diatraea saccharalis, popularly known as sugarcane borer, is considered one of the main pests of sugarcane crop. Biological control agents to control eggs and larvae are promising, but eggs and small larvae are difficult to scout ant tools for precise release or application are necessary. Temperature is one of the major abiotic factors that influence insect development. Thus, this work aimed to: a) determine the thermal requirements of D. saccharalis and the most appropriate mathematical model to show the relationship between development rate and temperature; b) improve and evaluate Degree-day and Equations of differences mathematical models in the prediction of pest population and c) validate insect outbreak forecast through Delay differential equation model and laboratory data. The temperature of 18 ºC caused 78.16% of mortality. The optimum temperature for development was 28 ºC and the temperature of 36 ºC was lethal for all D. saccharalis stages. The total life cycle (egg-adult) varied, on average, from 39 to 137 days. The thermal constant used by the linear model Ikemoto and Takai presented the best fit, with values for egg, larva and pupa, respectively: 68.25, 354.24 and 125.20 degree days (DD). The Brière-1 model presented the best fit for all stages. Although the Harcourt and Equation-16 models showed, respectively, good adjustments for eggs and pupae, they overestimated Tmax. Thus, the minimum, optimal and higher temperatures were, respectively: 12.8, 31.... (Complete abstract click electronic access below) / Doutor

Wild-trapped and laboratory-reared Peromyscus maniculatus (Wagner) were raised as confined colonies indoors with various stocking densities ranging from 0.026 to 0.100 mice per sq. ft. in pens of 40, 77, and 154 sq. ft. The animals were individually tagged and all pens were censused at weekly or semi-monthly intervals to record animals present, body weights, food consumption, and overt reproductive condition. At the termination of each experiment, all mice were necropsied and organ weights of gonads and adrenals recorded. Histological sections were made of the testes and ovaries. Half the colonies were subjected to density-independent exploitation by removal of 50% of the mice in each litter before weaning. Colonies stocked with the wild-trapped P. m. rufinus failed to establish a regular pattern of successful breeding and were terminated after one year. The succeeding colonies were stocked with the lab-reared P. m. sonoriensis and these did breed regularly, and the young survived to maturity. Reduction of the population growth rate to zero at the equilibrium density was accomplished by a cessation of breeding by the original females after an average of 2.9 litters was produced by each female. This, combined with a failure of the progeny born into the colony ever to produce young, caused the population growth rate to remain at zero for up to the maximum of 52 weeks permitted in this experiment. The exact mechanism appears to be psycho-physiological in nature, in that regression of the germinal tissues of the adults occurs, and these tissues failed to mature in the progeny in all experiments. Reduction of the population growth rate to zero at equilibrium density is not accomplished by mortality of either young or adults, except when wild-trapped mice are used. Density-independent exploitation of the colonies increased the number of litters born, the litter size, and the total number of young born. There was a tendency for higher pre-weaning mortality. The net effect of these changes was a tendency for reduction in the equilibrium density of the exploited colonies, which may indicate that this species under these conditions is not totally self-regulatory; i.e., equilibrium density may be determined by both intrinsic and extrinsic factors. There was no discernible effect of exploitation on the measures of body weight, food consumption, adrenal weight, or adrenal weight per gram of body weight. Many parameters were found to be negatively correlated with stocking density: the length of the reproductive period, numbers of litters born in a colony, total number of young born in a colony, litter size, number of nest boxes occupied by the mice at equilibrium, and the number of mice present in a colony at equilibrium. Some physiological measures proved to be positively correlated with density: progeny weight at three months, adrenal weight, and adrenal weight per gram of body weight.

Nest survival often is lower at habitat edges than in habitat cores because of greater nest predation and parasitism near edges. I studied nest survival of breeding birds in bottomland hardwood forests of the lower Roanoke River, North Carolina. Nesting success was monitored in two forest width classes: narrow bands of levee forest that were dominated by two edge types, and wide, continuous levee forest stands that have edges but most forest is relatively far from edge. Nest success of Acadian Flycatchers and Prothonotary Warblers was similar in narrow and wide levees; nest success of Northern Cardinals was greater in narrow levees. Results of my study indicate that edge effects are not universal, and that amount of contrast at edges may interact with landscape context to alter ecological processes, such as nest predation. Bird populations are remarkably constant over time relative to other taxa, implying strong regulation. Avian population ecologists, however, have not studied regulatory mechanisms as often as seasonal limiting factors. Conversely, avian behavioral ecologists seldom emphasize the population dynamic consequences of habitat selection and reproductive success. This study describes the intersection of individual behavior and population regulation in the context of a new model of population regulation, site dependence, which is based on characteristics of breeding sites and behavior of individuals. I studied habitat distribution, age structure, reproductive output, and breeding site fidelity of Prothonotary Warblers (Protonotaria citrea) in two different bottomland hardwood forest habitats of the lower Roanoke River in North Carolina. Older males (³ 2 yr old) were equally common in cypress-gum swamps and mixed oak hardwood levee forest. Pairing success and success of first nests indicated that older males occupied the most suitable territories available in each habitat. Bird density was three times greater in swamps, and birds nesting in swamps averaged greater clutch sizes and fledged more young per nest than birds in levees. Greater reproductive output was the result of greater fecundity because nest survival and predation pressure appeared equal in the two habitats. Annual return rates for plot immigrants vs. previous residents did not differ in swamps. In levees, newly arriving birds were less likely to return the following year than previous residents. Immigrants most likely occupied low quality sites and dispersed in an attempt to improve breeding site quality. Habitat-specific demography and density patterns of this study indicate ideal preemptive distribution. Variance in site quality, between and within habitats, and preemptive use of sites are consistent with theory of population regulation via site dependence. Foraging behavior often reflects food availability. For example, in habitats where food availability is high, predators should move more slowly and attack prey more often than in habitats where food availability is low. I studied the foraging behavior of breeding Prothonotary Warblers in two habitat types to assess relative food availability and implications for habitat quality. The two habitats, levee and swamp forest, differ in hydrology, forest structure, and tree species composition. I quantified foraging behavior with focal animal sampling and continuous recording during foraging bouts. I measured two aspects of foraging behavior: 1) prey attacks per minute, using four attack types (glean, sally, hover, strike), and 2) number of movements per minute (foraging speed), using three types of movement (hop, short flight [£ 1 m], long flight [>1 m]). Male warblers made significantly more prey attacks per minute in swamp forest than in levee forest; the same trend was evident in females. Foraging speed, however, was not different between habitats for males or females. Results indicate that foraging effort is similar in swamps and levees, but that warblers encounter more prey in swamps. Greater food availability may be related to greater reproductive success of warblers nesting in cypress-gum swamps than in coastal plain levee forest. / Ph. D.

<p>The strong increase in number of Great cormorants <i>Phalacrocorax carbo</i> in Sweden in recent years has led to conflicts - particularly with fishery. This thesis focuses on the possible effects of cormorant predation on fish populations. In total, data from 15 lakes in South Sweden were included in this study while most studies were carried out in Lake Ymsen. The results suggest that the impact of cormorant predation on natural fish populations was small, and I observed no decline in fish mass after cormorants established. Cormorant predation on eel was difficult to evaluate because of several confounding factors.</p><p>Ruffe, roach and perch were the most important prey species to the cormorants and most fish taken were small. Cormorants do not seem to catch species and sizes in proportion to their occurrence in the fish community.</p><p>Total fish removal by cormorants varied considerably among lakes (0.2-15.0 kg/ha) and cormorant population sizes at the different lakes were significantly positively correlated with fishery catches, which in turn was significantly positively correlated with total phosphorous levels. Thus, cormorant densities in lakes, and perhaps elsewhere, seem to be governed chiefly by fish densities. The fact that cormorant predation appears not to reduce fish densities suggest cormorants to be regulated by other means than prey depletion. The mechanism behind population regulation could be a behavioural response of fish, making fish more difficult to catch for the cormorants.</p><p>In recent years, cormorant populations have been subjected to intensive legal and illegal actions with the aim to reduce cormorant numbers. However, the actions currently carried are well below the efforts needed to limit population sizes. To conclude, cormorants appear to compete little with fishery, with regards to free-living fish. The main problem is that cormorants sometimes damage and take away fish in fishing gears.</p>

The strong increase in number of Great cormorants Phalacrocorax carbo in Sweden in recent years has led to conflicts - particularly with fishery. This thesis focuses on the possible effects of cormorant predation on fish populations. In total, data from 15 lakes in South Sweden were included in this study while most studies were carried out in Lake Ymsen. The results suggest that the impact of cormorant predation on natural fish populations was small, and I observed no decline in fish mass after cormorants established. Cormorant predation on eel was difficult to evaluate because of several confounding factors. Ruffe, roach and perch were the most important prey species to the cormorants and most fish taken were small. Cormorants do not seem to catch species and sizes in proportion to their occurrence in the fish community. Total fish removal by cormorants varied considerably among lakes (0.2-15.0 kg/ha) and cormorant population sizes at the different lakes were significantly positively correlated with fishery catches, which in turn was significantly positively correlated with total phosphorous levels. Thus, cormorant densities in lakes, and perhaps elsewhere, seem to be governed chiefly by fish densities. The fact that cormorant predation appears not to reduce fish densities suggest cormorants to be regulated by other means than prey depletion. The mechanism behind population regulation could be a behavioural response of fish, making fish more difficult to catch for the cormorants. In recent years, cormorant populations have been subjected to intensive legal and illegal actions with the aim to reduce cormorant numbers. However, the actions currently carried are well below the efforts needed to limit population sizes. To conclude, cormorants appear to compete little with fishery, with regards to free-living fish. The main problem is that cormorants sometimes damage and take away fish in fishing gears.