Appendix S2. Additional details on materials and methods.
Since the material used in generating 82 food webs is necessarily complex (see Fig. 1 in the paper proper), we were faced with a wealth of methodological decision. This appendix outlines specific details of and justification for particular solutions reached. Nonetheless, the Materials and
Methods section of the article itself should offer the reader a sufficient description of the methods used for him/her to judge the validity of the overall results.
Methods used in generating the insect material
To assess the identity of associated parasitoids, live herbivores were collected and reared.
As many leaf-miners will exhibit a first sap-feeding stage which cannot be reared successfully in laboratory conditions (Hering 1957, Rott & Godfray 2000), the smallest individuals were discarded. This procedure explains the discrepancy between the total number of insects collected and the number of insects reared (Fig 1 in paper proper). For details of the rearing procedure, see
Kaartinen et al. (2010).
Species identifications were based on morphology of emerged individuals and, for the host species, on gall and leaf mine morphology. For parasitoids and for inquiline species, we took advantage of the additional resolution offered by a previous study based on DNA markers
(Kaartinen et al. 2010; see below). Since different generations of the gall wasps do not overlap temporally, vary widely in abundance and are in most cases attacked by different species of parasitic and inquiline wasps, they form effectively separate nodes in the net. Hence, we treated them as separate taxa.
Since the sampling effort per big tree was strictly standardized (30 half-meter branches per tree sampled during each of three sampling events; see paper proper), we assumed that each host species was detected with an equal probability at each site. Hence, no ad hoc corrections for variation in “sample size” were later applied. On small trees, our searching method (complete survey of full foliage; see paper proper) will reveal next to all individuals (A. Tack, personal communication), and extrapolations beyond realised sample sizes would hence lack biological meaning.
That material on small and large trees was collected in different years and by slightly different methods (see section on Material and methods , Insect samples in paper proper) will not bias analyses of landscape context versus local community structure, since here we focus on patterns detectable across larger sets of internally homogenous trees. What the methodological discrepancies may affect are mean estimates for respective material, causing us to explicitly refrain from basing biological inferences on differences detected between large and small trees.
Methods used for dealing with cryptic taxa
In identifying parasitoids and for inquiline species, we took advantage of the additional resolution offered by a previous study applying DNA markers to the current material. More specifically, in the four taxa found to encompass cryptic species by Kaartinen et al. (2010), individuals were assigned to specific taxa in proportion to empirically established frequencies: In
C. eurynota (Hymenoptera: Eulophidae) individuals from the Island of Nötö were found to form a cryptic taxon, and hence treated as a separate species in the food web. In C. trifasciatus
(Hymenoptera: Eulophidae), individuals were split in proportion to the abundance of two cryptic taxa uncovered: 85% and 15%. In E. urozonus (Hymenoptera: Eupelmidae), individuals reared from gall wasps and from the leaf petiole miner H. sericiella (Lepidoptera: Heliozelidae) were treated as separate taxa as supported by DNA markers. In T. flavipes (Hymenoptera: Torymidae), the cryptic clades were delimited according to host phenology: the more common clade attacks spring galls, but the cryptic clade mostly attacks gallers in the autumn. Thus, all parasitoids reared from autumn galls were assigned to a taxon separate from that encountered in the spring. In the inquiline genus Synergus (Hymenoptera: Cynipidae), the species designations based on
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morphological and molecular criteria differed markedly (see also Ács et al. 2010). Hence, individuals were assigned to taxa in proportion to the frequencies established in the sequenced material. For details, see Kaartinen et al. (2010).
Statistical methods
In individual analyses, specific data sets were excluded on the basis of the following considerations.
First, species with very few occurrences will clearly add little information to analyses of species-specific distribution patterns. Hence, taxa observed on fewer than five (among 74) trees were excluded from the analysis summarised in Table 1 and Fig. 3 of the paper proper.
Second, for analyses of parasitoid species diversity, we omitted all data from the nonfragmented zone, since rearing success from these sites proved disproportionately variable and generally low. Nonetheless, we retained all data on host species, since the species identification of these taxa was based on gall and mine characters (see above), and hence independent of rearing.
In analysing parasitoid species diversity on the landscape scale, we only included data from the
BIG TREE LANDSCAPE SCALE MATERIAL , since tree-specific numbers of host individuals in the
SMALL TREE LANDSCAPE SCALE MATERIAL
were too low to yield adequate data on parasitoids.
Based on the same rationale, data on trees in the non-fragmented zone were omitted from analyses of quantitative food web metrics.
References
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Nieves-Aldrey, J.-L., Schönrogge, K. & Stone, G.N. (2010) Phylogeny and DNA barcoding of inquiline oak gall wasps (Hymenoptera: Cynipidae) of the Western
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