The ability of herbivores to regulate plant communities is generally appreciated but thought to be unimportant in recently-initiated communities. However, herbivory on Lupinus lepidus (alpine lupin), a keystone facilitator on Mount St. Helens volcano, Washington, USA, was previously demonstrated to depress its colonization rate and thereby delay soil and community development in the first 13-20 years of primary succession. We hypothesized that such extreme effects are a consequence of the large scale of disturbance and resulting metacommunity context, in which low diversity communities lacking species that might buffer herbivore impacts. In this study we investigated whether the negative effects of specialist insect herbivores (leaf-miners) on L. lepidus population dynamics observed in the 1990s continued over ensuing decades and whether those effects would recur when primary succession is re-initiated. To do this we used 20 years of transect-based observational surveys (years 18-38 of succession) and conducted a 6-year herbivore exclusion experiment on a 2006 debris flow that returned a portion of the area buried by the 1980 eruption to barren rock surfaces. The exclusions were conducted in 4 10mx10m plots with paired controls, initiated when the first lupins colonized.
Results/Conclusions
As in previous research in this system, leaf-miner damage was severe in low-density areas, exceeding 60% defoliation in the 2006-initiated control plots, and inversely proportional to lupin cover, with damage <1% in high-density areas. Exclusion of herbivores resulted in a 3x increase in lupin cover by year 3. However, in contrast to observations two decades earlier, lupins colonized bare surfaces in 2006-initiated sites more quickly, colonizing all areas in only 5 years compared to >15 following the 1980 eruption. In addition, leaf-miners more rapidly found newly colonizing lupin than 20 years earlier. Finally, a previously unobserved soil pathogen caused wide-spread mortality of adult lupins in 2016 across the entire study area and a consequent collapse of leaf miner populations in 2017. Subsequently, lupin cover has increased faster in exclusion plots. Both observational and experimental data indicate that specialist insect herbivores still have a strong negative effect in newly initiated communities on Mount St. Helens that diminishes as communities develop. Proximity of the 2006-initiated surfaces to a more-developed metacommunity than was present in the 1990s enabled more rapid colonization by both lupins and herbivores and introduced new species interactions that may diminish insect herbivore effects compared to when the plant-herbivore interaction occurred in relative isolation.