Coherent whole food web responses to outbreaking

14 The world is astoundingly variable, and individuals to whole communities must respond to 15 variability to survive. One potent example of nature’s variability is the massive fluctuations 16 in spruce budworm (Choristoneura fumiferana Clemens) populations that occur over 35 17 years. We examined how the parasitoid community altered its parasitism of budworm and 18 other caterpillars in response to these fluctuations. Budworm and other caterpillar species 19 were sampled from balsam fir in three plots for 13 years in Atlantic Canada. These 20 caterpillars were reared to identify any emerging parasitoids. We found that the parasitoid 21 community showed a simple linear, and indiscriminate, response (i.e., no preference and so 22 densities purely dictated parasitism rates) to changes in budworm densities relative to other 23 caterpillar species on balsam fir. Furthermore, we observed strong changes in topology and 24 distributions of interaction strengths. These observations suggest parasitoid movement 25 between hardwood stands and balsam fir stands is integral to the population dynamics of 26 budworm. Furthermore, our study remarkably shows that species communities coherently 27 alter species interactions in response to variable resources, fundamentally shifting food web 28 pathways in a manner similar to generalist apex predators. 29

were obtained for rearing from this branch, no more branches were selected for collection of caterpillars for rearing. If less than 100 budworm were obtained from the first branch selected, then another branch was selected and all budworm and all other caterpillars from 155 that branch were collected and reared, even if the final total number of budworm exceeded 156 100. When budworm populations were low, obtaining more than 100 budworm individuals 157 became difficult. As a result, all budworm and all other caterpillars that were found on the 158 sampled branches were collected for rearing. Overall, for every sampling day, all budworm 159 and all other caterpillars were reared from the subset of branches of the 20 collected each 160 sampling day. All      four groups (function adonis, R package vegan): three and two relative years before the 245 peak; one relative year before and after the peak, and the peak; two and three relative years 246 after the peak; and four to ten relative years after the peak. In this perMANOVA, we used 247 the Bray-Curtis dissimilarity measure, constrained permutations within each plot, and 248 maintained the temporal order of permutations. 250 Given the potential for changes in total diversity, parasitism rates, and species turnover to   Table   295 1). However, as each most abundant parasitoid taxon was dropped, there was a trend for 296 decreasing slopes and intercepts. The community of parasitoid taxa did not differ between 297 before and during the peak, but the parasitoid community in these two periods did differ   Fig. 3. nMDS of parasitoid community over time. The colour of each point and ellipse corresponds to the four temporal groups: three and two relative years before the peak (before); one relative year before and after the peak, and the peak (during); two and three relative years after the peak (after); and four to ten relative years after the peak (endemic   Table S1.  Table 1).

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The similar observations in host preference and parasitoid diversity between plots combined

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The observed large coherent changes in total diversity, parasitism rates, and species our study, we can see large changes in topology with many parasitoid taxa emerging from budworm and/or other caterpillars in some years and not others. We also found dramatic 413 shifts in the distribution of interaction strengths over the budworm cycle. When budworm 414 were at high densities, the distribution of interaction strengths showed a dichotomy of 415 strong-weak interaction strengths but skewed with a preponderance of weak interactions.

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As budworm densities declined, the distribution of interaction strengths became uniform. 417 We acknowledge that spatial sampling effort (in terms of number of plots) differs between more than the single balsam fir resource compartment that we have examined in this study. 449 We suggest that the separate resource compartment has hardwood trees as the basal drops and yet the parasitoid community must be maintained by some mechanism otherwise 462 the swift parasitoid community response to increased budworm abundance could not occur.

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Our study revealed that the parasitoid community responded to changing densities of The data is currently privately archived in Dryad for journal editors and reviewers. If/when 494 the manuscript is accepted, the data will be publically available.