Little and large - mammals as monitors of ecological resilience
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Grasslands such as prairies and savannahs support one third of the human communities around the world including those in North America that live and depend on the grassland of the Great Plains. In Texas, grasslands are the basis of agricultural systems for both animal production and dryland crops. Three ecoregions span the grasslands of central Texas across a decreasing precipitation gradient from the humid Post Oak Savannah, through the Blackland Prairies, to the semi-arid Edwards Plateau. Texas grasslands are being altered by expansion of woody shrubs and by conversion to exurban development in response to human population growth. The resilience of this grasslands is difficult to evaluate because there are few measures to assess the ability of the system to absorb disturbances and continue to maintain functions that sustain plant, animal, and human communities.The pace at which animals integrate changes in the plant community increase as body size decreases. Populations of small mammals such as Cotton Rats reflect responses to landscape changes at finer scales than large mammals such as white-tailed deer, which are culturally and economically important. Measures of the number and the proportion of adults and young in an animal population can indicate environmental change, but those measures are best combined with indicators of factors that constrain animal production, such as limiting nutrients. Trace minerals that are stored in the liver, such as copper (Cu), iron (Fe), and zinc (Zn) integrate nutrient supplies on a seasonal scale that span life history events including reproduction and growth. Trace nutrients are used for enzymes in pathways for resistance to disease. Populations are more vulnerable to diseases and other stresses when animal immune responses are suppressed, especially when contact between animals are increased at high density.This project combines measures of animal density (e.g. abundance indices and demographics) and mineral availability (e.g. soil minerals, plant minerals and abundance) with those of food supply (e.g. isotopic indices of diet) and animal condition (e.g. liver mineral stores). We will study three grassland ecoregions across a precipitation gradient from the moist Post Oak Savannah in the East to the dry Edwards Plateau in the West.