Monday, August 4, 2008 - 2:10 PM

COS 8-3: Spatial variance in abundance of two lemur species along environmental and disturbance gradients in two tropical dry forest landscapes

Anne C. Axel and Brian A. Maurer. Michigan State University


Patterns of spatial variation in local population density have important implications for basic ecology, biogeography, and the conservation of biological diversity.  Examining these patterns can help identify environmental factors and ecological processes that limit species’ abundance and distribution. Previous data suggested that two sympatric lemurs, Lemur catta and Propithecus verreauxi, vary in abundance along environmental and disturbance gradients, yet until now, there has been no study to examine this variation.  

Objectives included testing for the presence of spatial autocorrelation in lemur abundance, describing any spatial structure, and examining the spatial structure of  the underlying environmental variable, MSAVI2, an index of greenness similar to the well-known Normalized Difference Vegetation Index (NDVI). We conducted lemur and vegetation sampling along gradients within two ecologically similar landscapes (approximately 1500 ha) having different land use histories. Using distance sampling, we estimated lemur density and then performed spatial modeling of animal density  to enable display of density as a continuous surface across the study sites. 


The environmental gradient observed in situ was well-characterized using the modified soil adjusted vegetation index (MSAVI2), an index of greenness related to soil moisture. Moran’s I correlograms were constructed and then evaluated to examine the spatial structure of lemur abundance across the landscapes. Correlograms indicate the presence of spatial structure in both species within just one of the landscapes.  There is, however, an absence of species’ spatial structure within the landscape containing a thriving ecotourist site.  We conclude that exogenous forces (such as water provisioning, inadvertent food provisioning, and introduced species) may be altering the otherwise natural spatial structure in the population.