Do novel community associations confer instability? Insights from plant-insect interactions during primary succession at Mount St. Helens

Background/Question/Methods The effect of consumers on plant colonists is considered relatively unimportant in terrestrial primary succession.  However, several studies have revealed extraordinary impacts of insect consumers on keystone and foundational plant colonists of volcanic deposits from Mount St. Helens’ 1980 eruption. For example, several guilds of lepidopteran herbivores cause extreme population fluctuation and reduce the spread of N-fixing Lupinus lepidus, thereby delaying soil development and altering succession. Herbivores also affect colonization dynamics of Huckleberry (Vaccinium membranaceum), a key potential food resource for animals and the first animal-dispersed shrub to colonize.  Here I present a third example. Sitka willow (Salix sitchensis) is the first colonist to provide three-dimensional physical structure on these sites, thereby forming habitat for several mammalian and avian guilds, but upland plants rarely exceed 1.5m in height, while wetland and secondary successional individuals often exceed 4m.  We investigated whether stemboring by a non-native weevil, Cryptorrhyncus lapathi (Curculionidae) may account for the small stature and low density of upland willows. 

Results/Conclusions Willow damage and density surveys conducted annually for 6 years at the landscape scale indicated that on average 63% of basal stem area was damaged by borers. Salix % cover increased only slightly from its initial 2.5% during the 6 years, except in wetlands, where damage and stem mortality were lower. Three years of stemborer exclusion from upland plants caused a 3.4-fold increase in basal stem area, while unprotected plants did not increase in size.  Soils beneath upland willows had lower bulk density, and borer exclusion created a strong positive relationship between plant size and soil %C and %N.

These examples may represent an expected consequence of novel community associations that occur following intense, spatially extensive disturbances.  During early succession the effects of mutualists and consumers on colonizing plants may be greatly amplified relative to their effect in more mature systems, owing to the temporary absence of secondary consumers or competitors that might weaken the interaction, and to the increased demographic importance of propagules in spatially expanding populations. The distribution of interaction strengths in early successional or novel communities, and how that distribution is likely to change through succession, are virtually unstudied.  Our results suggest an excess of strong interactions, and the potential for radical shifts in the population dynamics of the interactors, which may drive successional species replacement. Similar considerations may apply to novel community associations associated with range expansion.