This project will test the hypothesis that insect herbivory is an unrecognized major factor influencing recovery of biological systems following the catastrophic 1980 eruption of Mount St. Helens (MSH). The work will quantify impacts of insect herbivores on two key "ecosystem engineering" plants: lupins, which facilitate soil development; and willows, the main source of three dimensional vegetation structure required by many animals. It will also evaluate the effects of herbivores on bird and mammal assemblages. The proposed work will extend the continuous study of insect herbivore impacts at MSH to 18 years, and of mammal community assembly and response to vegetation to 28 years, providing the most comprehensive data set on vertebrate response to catastrophic disturbance in the coniferous forest biome.
How biological systems recover from catastrophic disturbances such as volcanic eruptions is fundamental to a basic understanding of how communities of plants and animals assemble and function, and provides the theoretical basis for environmental restoration. MSH has provided a unique opportunity to test ecological understanding of recovery from disturbance. This work builds on this legacy by elucidating previously unrecognized mechanisms controlling recovery, and by providing long-term records accessible to future scientists. The project also enhances science education through student involvement and work with the National Volcanic Monument and its non-profit partner, the Mount St. Helens Institute.
This LTREB grant supported long term monitoring studies of the development of the ecological communities at Mount St. Helens (MSH), Washington, a volcano that erupted with tremendous force in 1980. Studies focused on characterizing the spatial and temporal variation in the composition of mammal and bird communities at MSH, and the effect of insect herbivores on plant populations and animal communities in areas undergoing ecological recovery. The grant also supported experimental studies investigating the causes and effects of insect herbivory by the stem-boring weevil (Cryptorynchus lapathi) and the stem-boring sesiid moth (Paranthrene robiniae) on Sitka willow (Salix sitchensis). Sitka willow is the dominant woody species that is recolonizing those landscapes most heavily devastated by the eruption; it is the first plant to provide three-dimensional structure and microclimates necessary for bird and small mammal colonists. Stem-boring insects are present at very high densities at MSH and are the primary willow herbivores, causing almost universal damage on willow stems when those stems reach a suitable diameter. We demonstrated that these specialist herbivores curtail establishment of Sitka willow on the Pumice Plain by affecting above and belowground processes. Aboveground, spread of Sitka willow into drier upland habitat on MSH is reduced by persistent stem-borer attack. Upland willow stems may escape mortality through size refuges (being either too big to be killed by attacking insects, or too small to be colonized by the insect), but stem-borers are much more likely to attack flowering stems (Fig. 1). Consequently, Upland Sitka willow devote a substantial amount of resources to so-called compensatory growth, repeatedly trying to regrow to replace stems damaged or destroyed by the insects. This results in a matrix of small shrubs whose dynamics are driven by a plant’s ability to replace lost stems counterbalanced against the stem-borers’ capacity to remove them. In contrast, Sitka willow growing in riparian stands are attacked far less (Fig. 1) and stems tend to survive attack regardless of size. In addition to curtailing aboveground net primary production, stem-borers are reducing the quality and quantity of willow litter inputs. Without herbivory, leaves are shed each fall and stems usually remain intact. However, after being attacked, leaves senesce prematurely and stems often break off and fall to the ground during the growing season. We found that leaves from attacked and un-attacked stems did not differ in percent carbon or nitrogen but leaves from non-attacked stems had twice the amount of phosphorus as un-attacked leaves and were larger (Fig. 2). These interesting results suggest that stem-borers are partitioning annual leaf litter inputs into two pools - higher quality larger leaves from non-attacked stems and lower quality smaller leaves from attacked stems. These herbivore-induced alterations in leaf quality and quantity likely slow nutrient cycling and reduce plant availability in a system that is already severely nutrient impoverished. As a result insect herbivory is influencing the ecological recovery via nutrient pathways as well. We also found that the Pumice Plain population of Sitka willow is strongly female-biased. Through experimental tests of male-female differences in physiological response to drought stress, we demonstrated that female plants can tolerate the dry conditions of upland area better than male plants can, and they do this by reducing carbon fixation to lower levels than do male plants. These results differ from previous studies of willow sexual dimorphism, which typically show female bias in riparian zones and a male bias in drier sites. Our findings suggest that adaptations by female plants to offset their higher costs of reproduction, such as higher levels of compensatory growth, might pre-adapt them to tolerate chronic stem-borer herbivory, which would facilitate successional change over the long term. The bird community at Mount St. Helens is still developing, along with other plant and animal communities. The colonization by birds of the Pumice Plain and the surrounding blowdown zone has been dependent upon the habitats available, and has been slower in areas that were more devastated by the eruption. The bird community of the Pumice Plain is highly diverse, and includes over 45 species. However, almost half of the bird species present are concentrated in small patches of wetland where there is also greater habitat structure and complexity (Fig. 3). These areas also host the highest density of individual birds on the Pumice Plain. Long term patterns of bird colonization on the Pumice Plain indicate that niche-based processes have been important to bird community assembly. Though nineteen species have been observed breeding on the Pumice Plain, immigration from the surrounding region remains important for the persistence of bird populations in this habitat.
