Rebecca L. Powell

Estimation of Mexico’s Informal Economy and Remittances Using Nighttime Imagery

Abstract: Accurate estimates of the magnitude and spatial distribution of both formal and informal economic activity have many useful applications. Developing alternative methods for making estimates of these economic activities may prove to be useful when other measures are of suspect accuracy or unavailable. This research explores the potential for estimating the formal and informal economy for Mexico using known relationships between the spatial patterns of nighttime satellite imagery and economic activity in the United States (U.S.). Regression models have been developed between spatial patterns of nighttime imagery and Adjusted Official Gross State Product ( AGSP ) for the U.S. states. These regression parameters derived from the regression models of the U.S. were ‘blindly’ applied to Mexico to estimate the Estimated Gross State Income ( EGSI ) at the sub-national level and the Estimated Gross Domestic Income ( EGDI ) at the national level. Comparison of the EGDI estimate of Mexico against the official Gross National Income (

Continental-Scale Distributions of Vegetation Stable Carbon Isotope Ratios

The stable carbon isotope composition (δ13C) of terrestrial vegetation is important for a variety of applications in fields ranging from biogeochemistry to zoology to paleoclimatology. To a large degree, spatial patterns in plant δ13C are imparted by variations in the photosynthetic pathway (C3/C4) composition of vegetation in topical and subtropical regions. Thus, the fractional coverage of each vegetation type must be known in order to predict the spatial distribution of plant δ13C values. Our approach for predicting the C3/C4 composition relies on the strong ecological sorting of C4 plants along temperature gradients, as well as the near-universal restriction of C4 photosynthesis to herbaceous growth forms. We build upon a previous approach to predict C3/C4 vegetation fractions using finer spatial resolution (500 m) MODIS datasets of vegetation growth form (i.e., percent cover of herbaceous, woody, and bare) and crop type coverage, along with precipitation and temperature climatologies. By combining these products, we predict the C3/C4 vegetation fraction at continental-to-global scales. We present a distribution of C3 and C4 vegetation in Africa. The area of land in sub-Saharan Africa covered by C4 vegetation is 6.3 million square kilometre, or approximately 31% of the land surface. The δ13C of vegetation in Africa is estimated from the C3/C4 composition, assuming constant values of −27‰ and −12‰ for C3 and C4 biomass. Strong precipitation gradients in Africa drive the C3/C4 spatial gradients, producing correspondingly strong gradients in vegetation δ13C. These isotopic gradients can be used to infer such information as the migratory connectivity of birds.

A multi‐isotope (δ13C, δ15N, δ2H) feather isoscape to assign Afrotropical migrant birds to origins

A universal challenge in methodology used to study the ecology, conservation and evolutionary biology of migratory species is the quantification of connectivity among breeding, wintering and stopover sites. For the avian Eurasian-Afrotropical migratory system, knowledge of geographical wintering areas used by migrants that breed in Europe remains deficient, despite the advent of satellite transmitters and geolocators. Here we explored the use of theoretical plant δ13C and δ15N landscape distributions coupled with δ2H hydrologic models to construct multi-isotopic avian foodweb clusters for Africa. The cluster analysis identified four distinct regions of Africa based on all three isotopes (13C, 2H, 15N), and five regions based only on 13C and 15N. We applied known isotopic diet-tissue discrimination factors to map equivalent feather isotopic clusters for Africa. The validity of these feather isotopic clusters was tested by examining how well known- and unknown-origin species were placed in regions of Africa using previously published feather isotope data. The success of this multi-isotopic cluster model depended upon the species of interest and additionally on how well potential winter molt origins in Africa were constrained by prior information. Ground-truthing data suggested this approach will be useful for first-order approximation of overwintering regions for Afrotropical migrants and will be improved as our understanding of the nature of isoscapes for Africa is refined.

Characterizing relationships between population density and nighttime imagery for Denver, Colorado: issues of scale and representation

This paper maps and characterizes the correlation between population density and nighttime imagery over Denver, Colorado. Photographs taken at night from the International Space Station (ISS) have finer spatial and spectral resolution than existing nocturnal observing satellites such as the Defense Meteorological Satellite Programs Operational Linescan System (DMSP OLS). We determined the correlation between the city lights of Denver, Colorado, and several representations of population and population density derived from census data. The DMSP OLS proved to have a stronger correlation than any of the finer resolution ISS photograph bands. This study suggests that exclusive use of nighttime images with finer spatial and spectral resolution will not necessarily improve our ability to use nighttime imagery for modelling traditional representations of population. However, analysis of the spatial patterns of error indicates that finer resolution imagery may be a good proxy of conceptualizations of population density that account for human spatial behaviour. Future research may demonstrate that imagery such as the ISS photographs may prove to be uniquely capable of informing more sophisticated representations of complex phenomena such as ambient population density, land-use intensity and impervious surface.

Characterizing Variability of the Urban Physical Environment for a Suite of Cities in Rondônia, Brazil

Urban environments are characterized by high spectral and spatial heterogeneity and, as a consequence, most urban pixels in moderate- resolution imagery contain multiple land-cover materials. Despite these com- plexities, virtually all urban land cover can be generalized as a combination of vegetation, impervious surfaces, and soil (V-I-S components), in addition to water. Previous work has demonstrated the potential of multiple endmember spectral mixture analysis (MESMA) to model the subpixel abundance of V-I-S components. Here, the authors test whether the technique is sufficiently robust to map V-I-S components for a diverse set of cities, selecting 10 urban centers in the state of Rondonia, Brazil, to represent a range of populations, develop- ment histories, and economic activities. For each urban sample, a 20 km × 20 km region centered over the built-up area was subset from Landsat Enhanced Thematic Mapper Plus (ETM+) imagery. MESMA was applied to all subscenes using the same spectral library, model constraints, and selection rules. Accu-

Four Decades of Andean Timberline Migration and Implications for Biodiversity Loss with Climate Change

Rapid 21st-century climate change may lead to large population decreases and extinction in tropical montane cloud forest species in the Andes. While prior research has focused on species migrations per se, ecotones may respond to different environmental factors than species. Even if species can migrate in response to climate change, if ecotones do not they can function as hard barriers to species migrations, making ecotone migrations central to understanding species persistence under scenarios of climate change. We examined a 42-year span of aerial photographs and high resolution satellite imagery to calculate migration rates of timberline–the grassland-forest ecotone–inside and outside of protected areas in the high Peruvian Andes. We found that timberline in protected areas was more likely to migrate upward in elevation than in areas with frequent cattle grazing and fire. However, rates in both protected (0.24 m yr−1) and unprotected (0.05 m yr−1) areas are only 0.5–2.3% of the rates needed to stay in equilibrium with projected climate by 2100. These ecotone migration rates are 12.5 to 110 times slower than the observed species migration rates within the same forest, suggesting a barrier to migration for mid- and high-elevation species. We anticipate that the ecotone will be a hard barrier to migration under future climate change, leading to drastic population and biodiversity losses in the region unless intensive management steps are taken.

Vegetation and soil carbon-13 isoscapes for South America: integrating remote sensing and ecosystem isotope measurements

The carbon isotope composition (δ13C) of terrestrial vegetation and soils is required for a diverse set of research, including carbon cycle studies that utilize global atmospheric CO2 and δ13C data, as well as studies of animal migration and food web dynamics where the δ13C of plants and soils is imparted to animal tissues. We present δ13C maps for South America that correspond roughly to the year 2000, based upon predictions of the abundance and distribution of C3 and C4 vegetation, along with empirical measures of the δ13C of plant leaf and soil endmembers. Our approach relies upon the near-universal restriction of C4 photosynthesis to the herbaceous growth form and the differing performance of C3 and C4 grasses in various climates, along with land-cover and crop-type distributions. Specifically, we predict the percentage cover of C3 and C4 vegetation in each 5-minute grid cell (∼10 km) based on input gridded layers of vegetation growth form fractional cover, crop-area/crop-type distributions, and a high spatial resolution climate data. We develop a consistent set of rules to harmonize the different data layers. The δ13C of vegetation in South America is then estimated based on the C3/C4 composition in each land grid cell, assuming constant mean values for closed C3 tropical forest (−32.3‰), open C3 forest ecosystems (−29.0‰), C3 herbaceous cover (−26.7‰) and C4 herbaceous cover (−12.5‰). In addition to using the mean isotope values, we also incorporate the measured standard deviation for each category. Soil δ13C is estimated for the C4-favored climate regions of South America using two, largely independent approaches: one that is derived from our vegetation δ13C prediction and one that is based on a previously published relationship between fractional woody cover and the δ13C of soil organic carbon. Finally, we present preliminary maps of relative uncertainty in the estimates of vegetation growth form, generated by integrating global measures of accuracy with local measures of neighborhood variability. These maps demonstrate that the highest uncertainty is found in savanna ecosystems, which contain the most heterogeneous vegetation cover and structure along with a high percentage of C4 grass cover.

Characterizing Urban Land-Cover Change in Rondônia, Brazil: 1985 to 2000

Few studies in the Amazon have explored the evolution of urban form and composition in the regions emerging urban hierarchy. This paper presents a methodology to characterize the physical properties of urban land cover in Rondônia across multiple dates using remote sensing imagery. Three descriptive metrics are explored: (a) urban extent in terms of the area and density of impervious surfaces, (b) the relationship between urban area and urban population, and (c) trajectories of urban land-cover composition. Such datasets can contribute to longitudinal studies of urban areas and provide a first step in linking urbanization to regional land-cover change.

Switching Hemispheres: A New Migration Strategy for the Disjunct Argentinean Breeding Population of Barn Swallow (Hirundo rustica)

Background Barn Swallows (Hirundo rustica) breed almost exclusively in the Northern Hemisphere. However, since the early 1980s, a small disjunct breeding population has become established in eastern Argentina, presumably by birds previously derived from those breeding in North America. Currently, it is unknown where these individuals go following breeding and how they have adjusted to a reversal in phenology. Their austral wintering period corresponds to the breeding period of the northern ancestral population and so they can potentially return to these more traditional breeding sites or they may occupy other South American wintering regions left vacant by conspecifics returning to the Northern Hemisphere. Principal Findings We used a three-isotope (δ 13C, δ 15N, δ 2H) approach to investigate potential wintering areas in Central and South America of individuals breeding in Argentina. Feather isotope values differed from those expected and measured at local breeding sites in Argentina indicating molt after the austral breeding period and away from the breeding grounds. Potential molting origins were identified applying likelihood-based assignment methods to a δ 2H isoscape for South America and dichotomous prior information on the distribution of C3 and C4 vegetation types based on modeled vegetation-δ 13C values. Barn Swallows now breeding in Argentina have changed their migratory behavior but presumably use the same cues as those used by the ancestral population, molting their feathers during the austral winter, likely in north-eastern South America.

Modeling land use and land cover change in an Amazonian frontier settlement: strategies for addressing population change and panel attrition.

Research on tropical deforestation has been prolific, yet few studies have assessed the long-term dynamics of frontier migration and the resulting impacts on deforestation. These lacunae arise from the difficulty of obtaining the panel data required to evaluate the dynamic socioeconomic and land use processes of the advancing and aging frontier. Furthermore, the quality and design of household surveys reported in the land use literature are often not transparent, limiting possibilities for comparing results. This article first describes a three-round spatial panel survey of households in a settled and heavily deforested Amazon frontier region. We detail several methods that are employed to ensure and assess data quality. Second, we estimate forest clearing at the agent (household) level, using several sets of explanatory variables and sub-samples that would be generated by applying different field methodologies. We find the definition of the panel agent and the sampling frame to influence our estimations.