BISHOP, JOHN, G. AND WILLIAM F. FAGAN.* School of Biological Sciences, Washington State University-Vancouver, Vancouver, WA 98686. - Severe insect herbivory concentrated in low density regions of lupines colonizing Mount St. Helens.
Lupinus lepidus began colonizing the Pumice Plains of Mount St.
Helens in 1981, one year post-eruption, beginning with a single plant.
Population growth in this primary successional site was initially
extremely rapid, yet extensive colonization of available habitat has
been surprisingly slow. Our previous work demonstrated a major impact
of lepidopteran and anthomiid herbivores on the demography and spread
rate of colonizing lupines. Here we document strong spatial
patterning of insect herbivory. We measured demographic parameters
and herbivory in the initial colonizing patches ("core
patches"), and newly-founded patches distant from the core
("edge patches"), for ~12000 plants in ~60 patches, from
1990-95. Stem-boring, leaf-mining, and seed-eating insects increased
through time and severely decreased seed production and survivorship
in low-density edge portions of the expanding population, and had less
effect in the core. For example, in 1994-95 stem-boring tortricid
moth incidence was 77% in edge patches vs. 24% in core patches; in
1993-95 noctuid leaf-miners infested 68% of plants in the youngest
edge patches, vs. 8% at the core. Together, these moths increased edge
mortality to ~90% in 1995, over a baseline of ~30% (1991-93). Damage
data collected in 1996, 1998, and 1999 along transects running from
core areas to edge areas show that the pattern of inverse density
dependent herbivory has persisted and is also present at smaller
spatial scales, i.e. from the center to the margin of core patches.
We are investigating whether predators, parasites, or plant quality
may explain the absence of herbivores in the high density core.
Key words: colonization, density dependence, insect herbivore, Lupinus lepidus, Mount St. Helens, primary succession