Understanding the structure and functioning of polar pelagic ecosystems to predict the impacts of change.

The determinants of the structure, functioning and resilience of pelagic ecosystems across most of the polar regions are not well known. Improved understanding is essential for assessing the value of biodiversity and predicting the effects of change (including in biodiversity) on these ecosystems and the services they maintain. Here we focus on the trophic interactions that underpin ecosystem structure, developing comparative analyses of how polar pelagic food webs vary in relation to the environment. We highlight that there is not a singular, generic Arctic or Antarctic pelagic food web, and, although there are characteristic pathways of energy flow dominated by a small number of species, alternative routes are important for maintaining energy transfer and resilience. These more complex routes cannot, however, provide the same rate of energy flow to highest trophic-level species. Food-web structure may be similar in different regions, but the individual species that dominate mid-trophic levels vary across polar regions. The characteristics (traits) of these species are also different and these differences influence a range of food-web processes. Low functional redundancy at key trophic levels makes these ecosystems particularly sensitive to change. To develop models for projecting responses of polar ecosystems to future environmental change, we propose a conceptual framework that links the life histories of pelagic species and the structure of polar food webs.

Female-Female Pairing in Western Gulls (Larus occidentalis) in Southern California.

Pairs of females that remain together from one year to the next are associated with the presence of supernormal clutches in western gull nests. Intervals between laying of eggs in supernormal clutches are less than those found in normal clutches, a result indicating both females in a pair contribute to the clutch. Most eggs in supernormal clutches are infertile. The pairs of females occupy territories that are not shared with a resident male. In three homosexual pairs one of the females exhibited behaviors normally ascribed only to males.

Econometrically accounting for identities and restrictions in models of interregional migration.

"Interregional demographic identities and other restrictions are explicitly taken into account in a simple time-series model of interregional migration. This goal is accomplished through the imposition of adding-up constraints on the estimated coefficients of a multi-equation linear system and by testing for homogeneity and symmetry. The pitfalls of utilizing the described procedures for migration analysis and forecasting are discussed. Full implementation of the technique is shown to require far more time-series observations on interregional migration than are commonly available."

Econometrically accounting for identities and restrictions in models of interregional migration: further thoughts.

PIP: The authors reply to some comments concerning "specification, estimation, and hypothesis testing in models of interregional migration which are internally consistent." They also attempt to clarify some additional conceptual matters in these three areas.^ieng

Forecasting state and local population growth with limited data: the use of employment-migration relationships and trends in vital rates.

In this study, the rate of growth of state and local populations in the United States is related to the growth rate of national employment, and to demographic and economic features of the regions concerned, in order to make more realistic long-term population projections. Both natural increase and migration are taken into account in the model developed. "The model is estimated for two regions, the El Paso, TX, standard metropolitan statistical area (SMSA) and the State of New Mexico. Several statistical problems arise because of the limited number of available time-series observations. Fully dynamic historical simulations track the population quite closely, in one case never differing from the actual value by more than 1.17% and in the other never differing by more than 1.36%."

Economic effects of immigrants on native and foreign-born workers: complementarity, substitutability, and other channels of influence.

PIP: The authors use Standard Metropolitan Statistical Area (SMSA) data constructed from 1980 census microdata files and other sources to estimate a structural model of native/foreign-born labor demand and labor supply which distinguishes the effects upon real wages of each type of labor and on the employment of natives. The authors specify, econometrically estimate, and simulate the structural model which incorporates not only a production structure channel through which immigrants influence area real wages and employment, but also demand and native labor supply channels. It is noted that while these are not the only channels through which immigrants may affect native workers, the model nonetheless constitutes a step in the direction of a general equilibrium approach. In the production structure channel, immigrants and natives are found to be substitutes in production. Immigration lowers foreign-born wage rates and leads to lower wages for natives. The negative effects of the production channel usually are ameliorated through the demand channel. Further, immigrants add to local demand through their earnings and potentially through non-labor income, while also lowering unit costs and local prices which enhances real incomes and potentially net exports, and thus the demands for local output and area labor. The author discusses findings of interest from the simulation results based upon an analysis of all areas.^ieng

Equilibrium and disequilibrium in migration modelling.

"Equilibrium and disequilibrium approaches to migration modelling are elucidated and distinguishing features are identified. Relevant econometric evidence bearing on these distinctions is reviewed. A summary of current knowledge and priorities for future research is presented." (SUMMARY IN FRE AND GER)

Jobs versus amenities in the analysis of metropolitan migration.

"This paper demonstrates that jobs and wages are considerably more important than location-specific amenities in explaining net metropolitan migration of employed persons [in the United States]. These results, which are derived mainly from a unique set of annual migration data, differ considerably from the earlier findings of P. E. Graves...that show amenities to be powerful contributors to the analysis of net metropolitan migration. Several hypotheses are offered and tested to explain the appreciable difference between Grave's results and those of the present study, but the importance of economic factors as opposed to amenities persists."

Migration and employment change: empirical evidence on the spatial and temporal dimensions of the linkage.

"This paper uses a unique set of [U.S.] data to estimate the magnitude of the linkages between employment changes and net employment migration. A simple simultaneous system is specified and estimated for 17 consecutive years. In an average year two extra jobs attract about one additional net migrant, and one additional net migrant has a direct effect on area employment of almost 1.4 jobs." The data are from the One-Percent Social Security Continuous Work History Sample and are for the years 1958 to 1975. The changes in this relationship over time as the economic situation varies are analyzed.