Doctor of Philosophy, The Ohio State University, 2008, Physics

Ecological systems such as forest and lakes can exhibit multiple stable states, abrupt transitions and self-organization as a control parameter is varied. Understanding the dynamics of these systems and devising easily quantifiable measures with predictive capabilities using the theoretical tools of stochastic dynamics and nonequilibrium statistical physics form the focus of this thesis.We study simple ecological models with no spatial degrees of freedom, that show a catastrophic transition as a control parameter is varied and propose a novel early warning signal that exploits two ubiquitous features of ecological systems: nonlinearity and large external fluctuations. It is shown that changes in asymmetry in the distribution of time series data, quantified by changing skewness, is an early warning signal of impending regime shifts. Using simple analytical calculations, model simulations that mimic field measurements and an analysis of real data from abrupt climate change in the Sahara, we study the feasibility of skewness calculations using data available from routine monitoring. We consider a spatially explicit model of collapse of vegetation in one and two spatial dimensions. An analytical calculation based on the mean-field approximation shows that spatial variance and spatial skewness (with an appropriate sign) increase as one approaches the threshold of vegetation collapse. Our numerical calculations show that an increasing spatial variance in conjunction with a reversal in the initial changing trend of spatial skewness is a superior indicator of an impending spatial ecological regime shift when spatially explicit data are available. These results are shown to hold under several different dispersal kernels such as Gaussian, fat tailed and Cauchy. Vegetation in semi-arid regions exhibits striking spatial patterns. Theoretical models often ignore the strong fluctuations in parameters such as those arising from seasonality. We present a fully seasonal rainfall mod (open full item for complete abstract)

Committee: C Jayaprakash PhD (Committee Chair); David Stroud PhD (Committee Member); Dick Furnstahl PhD (Committee Member); Michael Poirier PhD (Committee Member) Subjects: Ecology; Physics

Doctor of Philosophy, The Ohio State University, 2009, Economics

My research focuses on issues related to housing returns in the U.S. housing market. The first essay, "Explaining the Variance of the House Price Appreciation Rate" is motivated by an interesting geographic pattern in the variance of house price appreciation rate across the United States. Specifically, I find that the variance of the house price appreciation rate (or housing returns) is significantly higher along the coastal areas than inland areas of the United States. This paper investigates the geographic pattern both theoretically and empirically. The theoretical model of this paper is founded on the monocentric urban area model developed by Capozza and Helsley (1988 & 1990). Based on extensions to this model, the relationships between the variance of the house price appreciation rate and exogenous variables related to the urban economy are identified. My empirical results are largely consistent with the model's predictions. In particular, I show that the variance of housing returns is significantly and positively related to the variance of household income, the degree of land regulation (or growth control), land leverage, and transportation costs in each MSA. The second essay, "Skewness of the Housing Returns and Housing Tenure Choice" focuses on the third moment of the housing returns and investigates its influence on a household's tenure choice (the choice between owning and renting). In the finance literature, it is generally accepted that, holding the mean and variance of the returns constant, risk-averse households have a preference for financial investments with positively skewed returns. Homes are a large investment for most American families and thus I examine in this paper whether the preference for a positive skewness also exists in the housing market. Based on the model of housing tenure choice developed by Henderson & Ioannides (1983) and Fu (1995), I derive the relationship between housing tenure choice and the skewness of housing returns. The emp (open full item for complete abstract)

Committee: Donald Haurin (Committee Chair); Lixin Ye (Committee Member); Bruce Weinberg (Committee Member) Subjects: Economics