The spatial spread of the legume Lupinus lepidus at Mount St. Helens, Washington, USA, affords intriguing opportunities to study the effect of trophic dynamics on primary succession. Herbivory on this lupine exhibits striking spatial structure, with inverse density-dependent damage patterns occurring over both small (10-100m) and large (1-10km) spatial scales. To identify the distribution of elemental nutrients (nitrogen, phosphorus) across the landscape, we collected samples of soil and lupine tissue (stems and leaves) from sites along both local and regional gradients in lupine density. On both large and small spatial scales, we found lupine leaves from low-density conditions to be significantly more nutrient rich (increased phosphorus and nitrogen, lower carbon to phosphorus and nitrogen to phosphorus ratios). In addition, native lepidopteran herbivores that specialize on lupines (Gelechiidae: Chionodes sp.) performed better when fed leaves from low-density lupines than on diets of lupine leaves from high-density areas nearby. Together, these data suggest, that lupine tissue chemistry may contribute to the herbivory gradients. Coupled with prior data on spatial differences in the natural enemy assemblage, our data suggest that top-down and bottom-up forces, perhaps acting synergistically, combine to influence the lupine's contributions to primary succession.
Species 1: Lepidoptera Gelechiidae Chionodes sp
Species 2: Lepidoptera Tortricidae Hystricophora sp
Keywords: Lupinus lepidus, Inverse density-dependence
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