Spatial resource distribution patterns play an important role in mediating density-dependent phase change (gregarization) in locusts. The degree of contagion or aggregation of resources in a habitat can increase the probability of locust gregarization by increasing the frequency of contact among individual locusts. The spatial distribution patterns of two resources upon which gregarization can occur, the tussock grasses Aristida pungens and Panicum turgidum (Poaceae), were examined in two adjacent regions of the desert locust (Schistocerca gregaria Forskål) plague recession area in Mauritania that differ in their frequencies of locust gregarization. The hypothesis that the distribution of grass tufts should be more aggregated and thus more likely to promote locust phase change in the high frequency gregarization area was tested. Tufts were more abundant, and both species were larger in the high frequency gregarization area. The spatial distribution patterns of tufts in both areas were largely aggregated at the 200- to 2000-m2 scale corresponding to the population-level scale of locust resources. As predicted, the degree of aggregation was more extreme across the high frequency gregarization area. This study provides support across a large area for the predicted association between local resource distribution and locust gregarization. The observed differences in grass abundance and size between the high and low frequency gregarization areas suggest that factors such as topography or hydrology may underlie differences in plant distribution and contribute to locust gregarization in the high frequency area.
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