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Abstract
Data on agricultural and natural resource management typically have spatial patterns related to the landscapes from which
they came. Consequently, econometric models designed to explain the determinants of humans' natural resource management
practices or their outcomes often have spatial structure that can bring bias or inefficiency to parameter estimates.
Although econometric tools are available to correct for spatial structure, such tools are largely lacking for use with discrete
dependent variable models. While one obvious solution would be to develop the necessary tools, an alternative is to identify
conditions under which spatial dependency can be managed effectively without formal spatial autoregressive models.
This study examines conditions under which spatial structure corresponds closely to defined agro-ecological zones, making
it possible to model spatial effects by random effects regression. Using household survey data sampled along agro-ecological
zone strata, this article develops two models of links between farmer assets and agricultural natural resource degradation in
southern Peru. The first stage model looks at determinants of crop yield loss over time (an index of soil productivity), while the
second stage model looks at determinants of the extent of fallow cycles in crop rotation, a key agricultural practice reducing
crop yield loss.
Diagnostic statistics for spatial dependency reveal spatial structure, particularly in the fallow model. This spatial dependency
is eliminated in the ordinary least squares (OLS) models by inclusion of the agro-ecological zone random effects. In the spatially
dependent fallow model, comparison of coefficient estimates between OLS and the spatial autoregressive maximum likelihood
models showed OLS with random effects to give virtually identical results to the spatial autoregressive models, making the
latter unnecessary.
These results show that spatial structure in natural resource management models can sometimes be captured by zonal
variables. When this occurs, random effects regression can largely eliminate spatial dependency. A necessary precondition for
this approach with household survey data is prior sample stratification according to landscape characteristics. Where random
effects models can effectively capture spatial structure, they may also offer analysts greater flexibility in analyzing models
with limited dependent variables.
© 2002 Elsevier Science B.V. All rights reserved.