FIRE ANT MOUND DENSITIES IN THE UNITED-STATES AND BRAZIL (HYMENOPTERA, FORMICIDAE)

To compare fire ant populations (Solenopsis) in North and South America, we surveyed 102 preselected roadside sites, half in the southeastern United States and half in the state of Mato Grosso do Sul, Brazil. Fire ants were considerably more abundant in the United States. They occurred at more sites (100 versus 70%), in higher densities (170 versus 30 mounds/ha), in larger mounds (27.0 versus 13.8 liters), and they constituted a larger fraction of the local ant community (97 versus 13% of occupied baits). These data are consistent with the hypothesis that North American populations of S. invicta have escaped natural biological control; however, cultural and climatic factors are also likely explanations.

Nutrient enrichment is related to two facets of beta diversity for stream invertebrates across the united states

Beta diversity, the spatial or temporal variability of species composition, is a key concept in community ecology. However, our ability to predict the relative importance of the main drivers of beta diversity (e. g., environmental heterogeneity, dispersal limitation, and environmental productivity) remains limited. Using a comprehensive data set on stream invertebrate assemblages across the continental United States, we found a hump-shaped relationship between beta diversity and within-ecoregion nutrient concentrations. Within-ecoregion compositional dissimilarity matrices were mainly related to environmental distances in most of the 30 ecoregions analyzed, suggesting a stronger role for species-sorting than for spatial processes. The strength of these relationships varied considerably among ecoregions, but they were unrelated to within-ecoregion environmental heterogeneity or spatial extent. Instead, we detected a negative correlation between the strength of species sorting and nutrient concentrations. We suggest that eutrophication is a major mechanism disassembling invertebrate assemblages in streams at a continental scale.

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