Spatially structured herbivory and primary succession at Mount St Helens: Field surveys and experimental growth studies suggest a role for nutrients

Abstract

1. The 1980 eruption of Mount St Helens (Washington, U.S.A.) created a 60-km2 region of primary successional habitat. Since colonising in 1981, the spatial spread of the legume Lupinus lepidus at Mount St Helens, Washington, U.S.A., has afforded intriguing opportunities to study the effect of trophic dynamics on primary succession. 2. Insect herbivory on this lupine has exhibited striking spatial structure for over a decade, with inverse density-dependent damage patterns occurring over both small (10-100 m) and large (1-10 km) spatial scales. 3. Hypothesising that lupine nutritional chemistry might underlie the spatial patterns in herbivory, the distribution of elemental macronutrients (nitrogen, phosphorus) across the landscape was characterised. 4. Samples of soil and lupine tissue (roots and leaves) were collected from sites along both local and regional gradients in lupine density. On both large and small spatial scales, lupine leaves from low-density conditions were significantly more nutrient rich. 5. In addition, in a laboratory growth study native lepidopteran herbivores that specialise on lupines (Gelechiidae: Filatima sp.) performed better when fed leaves from low-density, high-nutrient lupines than on diets of low-nutrient lupine leaves from high-density areas a few metres away. 6. These data suggest that spatial heterogeneity in lupine nutrient chemistry may underlie the remarkable herbivory gradients witnessed at Mount St Helens.