The increasing expansion of the urban environment has been considered one of the causes of biodiversity loss. Due to the decrease of the quantity and quality of habitats, this process causes the restriction of organisms to sometimes-specific microhabitats. The patterns of variation of the biodiversity along the environmental gradients, however, are still poorly understood. This study evaluated how urban gradients affect the diversity, specific composition and frequency of guild of the community of butterflies. Butterflies were collected in three transects continuously traversing the urban, rural and preserved habitats using baited traps and by active sampling. We found that the urban gradient acts as an environmental filter in the butterfly communities, reducing the richness and frequency of guilds of frugivorous butterflies, structuring the community through turnover, but without effects in the abundance. This study shows the importance of the urban gradients approach in the butterfly community and its applicability in the management of urban areas in order to include native vegetation along the gradients, aiming at increasing environmental heterogeneity, to preserve butterflies and maintain the ecosystem services and network interactions in altered landscapes.
ABSTRACT: We compare changes in the effect of child support payments on the educational attainment of children age 16 to 19 before and after passage of the Child Support Enforcement Amendments of 1984, which strengthened enforcement of child support obligations in the United States. Using 1979, 1984, and 1988 Current Population Survey data, we hypothesize and find a decline in the positive effect of child support between 1984 and 1988 that exceeds any earlier decline. Instrumental variables estimates provide additional evidence consistent with the idea that child support serves as an indicator of unobservable characteristics of noncustodial fathers who pay support. 相似文献
This paper studies the estimation of dynamic discrete games of incomplete information. Two main econometric issues appear in the estimation of these models: the indeterminacy problem associated with the existence of multiple equilibria and the computational burden in the solution of the game. We propose a class of pseudo maximum likelihood (PML) estimators that deals with these problems, and we study the asymptotic and finite sample properties of several estimators in this class. We first focus on two‐step PML estimators, which, although they are attractive for their computational simplicity, have some important limitations: they are seriously biased in small samples; they require consistent nonparametric estimators of players' choice probabilities in the first step, which are not always available; and they are asymptotically inefficient. Second, we show that a recursive extension of the two‐step PML, which we call nested pseudo likelihood (NPL), addresses those drawbacks at a relatively small additional computational cost. The NPL estimator is particularly useful in applications where consistent nonparametric estimates of choice probabilities either are not available or are very imprecise, e.g., models with permanent unobserved heterogeneity. Finally, we illustrate these methods in Monte Carlo experiments and in an empirical application to a model of firm entry and exit in oligopoly markets using Chilean data from several retail industries. 相似文献
In this paper we look at a panel of OECD aggregate fertility and labor market data between 1970 and 1995 and we report some
striking recent developments. Total Fertility Rates (TFR) were falling and Female Participation Rates (FPR) were increasing,
conforming to a well known long-run trend. Along the cross-sectional dimension, the correlation between TFR and FPR was negative
and significant during the 1970's and up to the early 1980's. This seemed consistent with secular comovements. However, by
the late 1980's the correlation had become positive and equally significant. We discuss our findings within the framework
of standard neoclassical models of fertility and labor supply adapted to macro data, as in Butz and Ward (1979).
Received: 14 April 2000/Accepted: 29 December 2000
All correspondence to Pedro Mira. Namkee Ahn is grateful for financial support received from the Bank of Spain and from Spain's
Ministerio de Educación y Cultura, grant SEC97-1249. We benefited from comments by two anonymous referees and by seminar participants
at FEDEA, CEMFI and ESPE-98. All remaining errors are our own. Responsible editor: John F. Ermisch. 相似文献
Statistical methods of dimension reduction and classification are used to obtain homogeneous local-area clustering with regard
to the most relevant demographic parameters. The dimension reduction is conducted in two stages using Principal Component
Analysis and a modified k-mean procedure is proposed to determine the final clusters. This clustering will be useful in future
demographic studies at a local level, in particular to obtain forecasts of demographic rates and population projections. The
region of Castile and León in Spain is used to illustrate the method. A Poisson model is used to explore the advantages of
the new clustering over the more conventional classification based on provinces. 相似文献
Social Indicators Research - United Nations Development Program presented the Human Development Index for ranking the countries with regard to three dimensions, namely being knowledgeable, a long... 相似文献
In this paper we consider the optimal decomposition of Bayesian networks. More concretely, we examine empirically the applicability of genetic algorithms to the problem of the triangulation of moral graphs. This problem constitutes the only difficult step in the evidence propagation algorithm of Lauritzen and Spiegelhalter (1988) and is known to be NP-hard (Wen, 1991). We carry out experiments with distinct crossover and mutation operators and with different population sizes, mutation rates and selection biasses. The results are analysed statistically. They turn out to improve the results obtained with most other known triangulation methods (Kjærulff, 1990) and are comparable to results obtained with simulated annealing (Kjærulff, 1990; Kjærulff, 1992). 相似文献
Evidence is offered that meaningful patterns of mother-child interaction could be detected in families more than 10 years after a family-centered, early age, preventive intervention. The interactions of mother-child dyads in a situational test during the child's early adolescence were examined for three experimental conditions: a lower socioeconomic experimental group that participated in the intervention for approximately 2 years, an untreated lower socioeconomic, and an untreated upper middle socioeconomic control group.Factor analyses of family interaction measures revealed three factors, which were called Democratic-Participatory Interaction, Mixed Dominance Interaction, and Mother Controlled Interaction. Separate ANOVAS based on factor scores were performed for each factor. Both the intervention group and the middle socioeconomic comparison group were significantly higher than in the lower socioeconomic comparison group in the first factor. 相似文献
Self-Exciting Threshold Autoregressive (SETAR) models are a non-linear variant of conventional linear Autoregressive (AR) models. One advantage of SETAR models over conventional AR models lies in its flexible nature in dealing with possible asymmetric behaviour of economic variables. The concept of threshold cointegration implies that the Error Correction Mechanism (ECM) at a particular interval is inactive as a result of adjustment costs, and active when deviations from equilibrium exceed certain thresholds. For instance, the presence of adjustment costs can, in many circumstances, justify the fact that economic agents intervene to recalibrate back to a tolerable limit, as in the case when the benefits of adjustment are superior to its costs. We introduce an approach that accounts for potential asymmetry and we investigate the presence of the relative version of the purchasing power parity (PPP) hypothesis for 14 countries. Based on a threshold cointegration adaptation of the unit root test procedure suggested by Caner & Hansen (2001), we find evidence of an asymmetric adjustment for the relative version of PPP for eight pairs of countries. 相似文献