Early primary succession on Mount St. Helens: impact of insect herbivores on colonizing lupines
Lupinus lepidus var. lobbii, the earliest plant colonist of primary successional habitats at Mount St. Helens, can dramatically influence successional rates and ecosystem development through N fixation and other facilitative effects. However, 15 yr after the eruption, lupine effects remained localized because high rates of population growth in newly founded patches ([lambda] = 11.2, 1981-1985) were short lived ([lambda] = 1.51, 1991-1995), despite widespread habitat availability. To investigate this paradox, I examined 60 colonizing lupine patches, ranging from low-density patches at the edge of the expanding lupine population, to high-density patches in the population core, including 41 patches created in 1992. Survival, reproduction, and herbivory (for a subset of plants and years) were measured on ~12 000 plants for up to 5 yr (1991-1995). Stem-boring, leaf-mining, and seed-eating lepidopterans and anthomyiid flies, feeding in edge patches and in low-density margins of core patches, strongly affected edge patch demography, but not that of central core areas. For example, in 1994-1995, 77% of edge plants were afflicted by tortricid stem borers vs. 24% in the core, and from 1993-1995 gelechiid leafminers infested 68% of plants in the youngest edge patches, vs. only 8% at the core. Associated adult mortality was 88%, compared to ~30% when absent. Seed predators consumed 36% (range 4-92%, 1991-1995) of seeds in both core and edge patches, with seed loss negatively correlated with seedling number the following year. In contrast to edge patches, resource-dependent seedling mortality appears to govern short-term dynamics in high-density core areas. In conclusion, edge region herbivore effects can account for the observed 7.5-fold difference in [lambda] between patches founded 10 yr apart. Locally, increased lupine mortality creates access to high-quality sites, facilitating succession, but at larger spatial scales diminished population growth is likely to retard facilitation and succession.