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 plants established on primary successional surfaces created during 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 experimental and observational time series (9-12 years) elucidating a third example. Sitka willow (Salix sitchensis) is the first woody 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 height, while wetland individuals often exceed 4m. We investigated whether stem-boring by a non-native weevil, Cryptorrhyncus lapathi (Curculionidae) may account for the small stature and low density of upland willows.
Surveys conducted annually for 11 years across a 10 km2 area indicated that in uplands an average of 63% of basal stem area was damaged 54% killed by borers. Eight years of stem-borer exclusion from upland plants caused a 9x increase in basal stem area (strongly correlated with biomass), while unprotected plants did not grow larger and frequently died.
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 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 herbivore suppression of propagule production in spatially-expanding populations. The distribution of interaction strengths in early successional or novel communities, and how that distribution is likely to change through time, are virtually unstudied. Our results suggest strong interactions relative to more developed communities, with the potential to shift the population dynamics of the interactors and possibly drive successional species replacement. In addition, in some cases, such as willows, these interactions may effectively reduce species’ landscape distribution from their fundamental niche, which they were able to fill early in succession, to their realized niche.