Understanding the mechanisms that control the formation of biological communities on new barren surfaces can improve our understanding of natural communities and ecological restoration. Related studies mostly focus on propagule limitation, plant competition and facilitation, and abiotic amelioration. The ability of herbivores to regulate plant communities is generally appreciated but thought to be unimportant in newly-formed communities. This view has been challenged at Mount St. Helens, where we have demonstrated that key plant colonists such as Lupinus lepidus (alpine lupin), willow, and huckleberry, are inhibited by insect herbivores. A debris flow in 2007 returned a portion of the area buried by the 1980 eruption to barren rock surfaces. Lupin, the first major plant colonist of these surfaces, recently begun to colonize these primary substrates. In this study, we repeat an herbivore exclusion experiment conducted 20 years prior (Fagan and Bishop 2000) to ask two questions: (1) do specialist insect herbivores limit colonization by lupin? and (2) do herbivore effects on lupin alter successional development of plant communities? We investigated the effect of specialist insect herbivores on lupin and the cover of the associated plant community. As lupin is a keystone facilitator in primary succession strong herbivory is expected to delay community development. We removed herbivores from treatment plots for 3 consecutive years and quantified the cover of all plant species and herbivore damage to L. lepidus.
There was substantial variation in the relative growth rate (RGR) of lupin between years with a small positive effect of treatment in year 2 and a stronger positive effect in year 3. Factors decreasing RGR include moss cover and the previous year’s RGR. Increased RGR was also correlated with damage from specialist herbivores. This last may suggest that RGRs will be negatively affected in subsequent years. Lupin cover in 2016 was correlated with prior year RGRs, prior year lupin cover, herbivore removal and moss cover. Again, this last may suggest future negative effects for lupin cover as moss cover appears to decrease lupin’s RGR. Moss cover increased with prior year moss cover while vascular plant cover increased with prior year lupin cover. Herbivore removal treatment increased moss RGR. Lupin cover has increased dramatically in the past 2 years and we expect herbivore effects to increase as facilitative effects of lupin in treatment plots build over time. These results suggest that insect herbivores may play a larger role in shaping succession than previously believed.