Survey
I began a long-term survey of 15 populations of goldenrod in Spring 2015. I demarcated populations at the Koffler Scientific Reserve (KSR), Ontario, Canada with the criteria that (i) goldenrod (and thusEurosta ) were in distinct patches separated from one another by a matrix of grass and other forbs, (ii) populations were separated by at least 50m to ensure relative population isolation (Cronin et al. 2001, Start and Gilbert 2016), and (iii) populations were broadly distributed across the entire study site (see map of locations in Fig. S1).
Beginning in 2015 I collected up to 50 galls per population annually (Table S1). All galls were collected in mid-May within 5m of the centre marker of each population. For each gall collected I began by measuring horizontal diameter. I then scored each gall containing Eurostaas a survivor, those with large holes as having been killed by a bird, and those containing other larvae as having been killed by the corresponding enemy (e.g. Eurytoma gigantea ). I scored empty galls with no sign of habitation as early larval death (Abrahamson et al. 1989). Note that mortality is often high (Weis and Kapelinski 1994, Start 2018b), suggesting that survival is a key fitness component. Further note that gall size is unrelated to larval mass or size of the adult fly (Hess et al. 1996), suggesting that gall size is unlikely to be correlated with other components of fitness like fecundity or mating success.
In addition to measuring individual gall size and species interactions, I also collected key environmental information. I recorded the number of stems out of 100 randomly selected goldenrod that were knocked down and the distance of the population from trees. All three metrics have previously been shown to influence bird attack, and in some cases selection. Furthermore, I estimated density by counting the number of galls on 100 randomly selected goldenrod stems—density should increase following years with high survival but decline following years when enemy attack was high.
In late June 2019, I measured the density (galls per goldenrod stem) of galls in each patch, and measured the horizontal diameter of up to 20 galls per population. Note that these measurements were taken on the 2019/2020 generation, and were taken to measure the ecological (demographic) and evolutionary (trait change) consequences of the previous year’s extreme event (i.e. that occurred in the 2018/19 generation). Together these data represent demographic information (survival and density), species interactions (attack rates), and the fodder to measure natural selection and evolutionary change (trait values).