Browsing by Subject "Population"
Now showing 1 - 6 of 6
Results Per Page
Sort Options
Item A stochastic age-structured population model(Texas Tech University, 1998-05) Chowdhury, MarufulAge-structured models for biological species have received much study. In this thesis the age-structured population model is extended to a stochastic agestructured model. The model is solved numerically for two age-structured population examples. The calculational results are compared with Monte Carlo calculations. The results indicate that the two calculational methods are in agreement. Persistence times for two age-structured populations are then studied computationally. It is shown that persistence times increase as the number of age groups increases. Also, persistence times are higher for a stohastic age-structured population than for a nonstochastic (or deterministic) age-structured population.Item Characterization of the stress response of red snapper: connecting individual responses to population dynamics(Texas Tech University, 2008-08) Campbell, Matthew Denis; Diamond, Sandra; Patino, Reynaldo; Strauss, Richard E.In the Gulf of Mexico, red snapper (Lutjanus campechanus) are considered to be the most important species for both commercial and recreational fishing. Red snapper were first classified as an overfished species in 1984 and are still considered as such (Schirripa and Legault 1999). Various studies have shown a high degree of uncertainty with the assumption of post-release survival in the fishery. The intent of this dissertation was to evaluate responses of red snapper to stress in the laboratory and the field, and then model those effects at the population level of organization. Both lab and field data demonstrated that with increasing depth and water temperature, the physiological health of red snapper diminished. This decrease in physiological health resulted in elevated immediate mortality rates due to their inability to cope with elevated stress and increased predation. A triage procedure called BtR score was developed and is a synergistic metric that accounts for external features of barotrauma and loss of reflex response capability. The BtR score showed a significant logistic functional relationship to observed mortality in the field and represents a tool that will allow prediction of immediate mortality. Mean BtR score from field data generated 4 different estimates of release mortality (20%, 30%, 33%, and 39%). These release mortality estimates were used to calculate concomitant increases in fishing mortality for this population, and were then applied in a matrix projection model. Results of the model indicate that release mortality rates 35% and greater generate declining population trends. Application of a slot limit to the fishery or reduction of age 1 red snapper bycatch, were both capable of returning the worst case scenario model back to equilibrium. The overall picture is that release mortality is a significant issue for the red snapper fishery. Continued investigation is recommended as more information about density dependent effects for this population become available. Reductions in total catch, release mortality, and bycatch are recommended to achieve sustainability for this population.Item Demographic transitions and social security: the case of Italy(Texas Tech University, 2006-05) Wigmans, Farinaz; Steinmeier, Thomas L.; Sartor, Nicola; McComb, Robert P.; Hein, Scott E.Most developed countries face increasing challenges to the sustainability of their public pension systems, which were created in an era when the life expectancy of retirees was much shorter than today. In Italy, these challenges are exacerbated by additional factors: Very low birth rates, little immigration and a historically generous public pension system. In this dissertation, the factors affecting future sustainability of the Italian Pay-As-You-Go (PAYG) system are systematically analyzed. First, the development of the population over the next 75 years is studied. The effects of (changes in) mortality coefficients, fertility rates and immigration levels on the population’s size and age distribution are determined stochastically, with a computer program that was specifically developed for this purpose. A unique feature of these calculations is that a distinction is made between the fertility of native Italian and first-generation immigrant women. The latter is much higher, which causes substantial differences when extrapolated over 75 years. It turns out that all reasonable models predict a sharp increase in the dependency ratio, with a peak around 2040 followed by a slow decline. A second computer program uses the predicted population composition as input and calculates the consequences for the PAYG system. This program was designed to study the effects of specific changes in the system's rules, changes regarding the official retirement age, contribution levels, the relationship between pension levels and life expectancy, etc. It turns out that, even after the 1995 reforms, which intended to improve long-term sustainability, public pension expenditures would rise from their current level (~14% of GDP, highest among European countries) to well above 20% of GDP by the middle of the century. It is shown that additional modifications, in particular gradual adaptation of pension levels to the retirees’ remaining life expectancy, may restore sustainability once the population has regained a stable composition. However, because of the current distorted age distribution (cohorts born between 1945 and 1980 being on average 48% more populous than cohorts born after 1980), the pension crisis that looms for the period 2030-2050 calls for major increases in the participation of especially women and elderly (>55) workers.Item Demographic transitions and social security: The case of Italy(2006-05) Wigmans, Farinaz; Steinmeier, Thomas L.; Hein, Scott E.; McComb, Robert P.; Sartor, NicolaMost developed countries face increasing challenges to the sustainability of their public pension systems, which were created in an era when the life expectancy of retirees was much shorter than today. In Italy, these challenges are exacerbated by additional factors: Very low birth rates, little immigration and a historically generous public pension system. In this dissertation, the factors affecting future sustainability of the Italian Pay-As-You-Go (PAYG) system are systematically analyzed. First, the development of the population over the next 75 years is studied. The effects of (changes in) mortality coefficients, fertility rates and immigration levels on the population’s size and age distribution are determined stochastically, with a computer program that was specifically developed for this purpose. A unique feature of these calculations is that a distinction is made between the fertility of native Italian and first-generation immigrant women. The latter is much higher, which causes substantial differences when extrapolated over 75 years. It turns out that all reasonable models predict a sharp increase in the dependency ratio, with a peak around 2040 followed by a slow decline. A second computer program uses the predicted population composition as input and calculates the consequences for the PAYG system. This program was designed to study the effects of specific changes in the system's rules, changes regarding the official retirement age, contribution levels, the relationship between pension levels and life expectancy, etc. It turns out that, even after the 1995 reforms, which intended to improve long-term sustainability, public pension expenditures would rise from their current level (~14% of GDP, highest among European countries) to well above 20% of GDP by the middle of the century. It is shown that additional modifications, in particular gradual adaptation of pension levels to the retirees’ remaining life expectancy, may restore sustainability once the population has regained a stable composition. However, because of the current distorted age distribution (cohorts born between 1945 and 1980 being on average 48% more populous than cohorts born after 1980), the pension crisis that looms for the period 2030-2050 calls for major increases in the participation of especially women and elderly (>55) workers.Item Deterministic and stochastic nonlinear age-structured models(Texas Tech University, 1998-12) Block, Garry L.The Leslie age-structured population model is reviewed, as well as its stochastic analogue. Nonlinear adjustments in the form of the Ricker and Beverton-Holt densitydependent factors are made to these models. The deterministic density-dependent nonlinear models are discussed and compared to their stochastic nonlinear analogs.Item The population explosion in underdeveloped countries and United States foreign aid(Texas Tech University, 1964-08) Keltner, KayNot available