Inter-American comparison of the genetic erosion of key species in overgrazed semiarid rangelands.
Huber-Sannwald, Elisabeth*,1, Arredondo , Tulio 1, Alpuche Solis, Angel1, Aguiar, Martin2, Smith, Steve3, 1 Instituto Potosino de Investigacion Cientifico y Tecnologico, San Luis Potosi, S.L.P., Mexico2 Universidad de Buenos Aires, Buenos Aires, Argentina3 University of Arizona, Tuscon, Arizona, U.S.A.
ABSTRACT- In the Americas, over 80% of the rangelands are degraded and impoverished in biological diversity through overgrazing. Most rangelands are naturally species−poor; however biological diversity may still occur at the genotype level. Apparent differences in inherent tissue C and N concentrations among genotypes suggest that loss of genotypes may influence biogeochemical processes. We studied the effect of decade−long overgrazing in climatically similar regions of North and South America. We hypothesized that heavy grazing has contributed to the genetic erosion of key species and to the genetic differentiation between heavily grazed and moderately grazed populations of these species. We compared pairs of populations of Bouteloua curtipendula from Arizona, B. gracilis from Central Mexico, and Poa ligularis from Patagonia. From each geographic region, ca. 200 plants were collected and DNA profiles analyzed with AFLPs. The moderately grazed populations were genetically more diverse than heavily grazed populations based on Nei′s index of genetic diversity (B. gracilis: 0.36 vs. 0.35; B. curtipendula: 0.32 vs. 0.30; P. ligularis: 0.28 vs. 0.23). Analysis of molecular variance indicated that only B. curtipendula (P<0.03) and P. ligularis (P<0.028) exhibited genetic differentiation in respect to grazing history. However, most of the total variation was attributed to within population differences (>98%). Our study shows that genetic erosion may result from long−term heavy grazing; however, this vulnerability is species−specific. Bouteloua gracilis was the only species not showing a decline in genetic diversity in response to heavy grazing. Our results are not conclusive as to what factors or mechanisms confer this resistance in some key grass species of rangeland ecosystems; however, our findings support the urgent need for more detailed studies on the effect of land use change on decline in genetic diversity and the relationship between genetic diversity and ecosystem functioning.
Key words: genetic diversity, rangelands, grazing, Bouteloua gracilis
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