2.6. Species composition and phenologies

We used two approaches to infer differences in species composition between organic and conventional farming sites. First, we used a random sampling model and calculated for all taxa having at least 15 OTUs the probability of having k joint OTUs, given that the organic farming sites had l and the conventional farming sites m OTUs, while the total number (the local pool size) was assumed as n = l + m − k. This probability is given by (Connor & Simberloff, 1978):

and has the random expectation of k _exp = lm/n OTUs. Significant differences ∆k = k _exp − k point to differences in community composition. We note that the observed probabilities p strongly depend on the pool size n and cannot be compared among taxa directly. Therefore, we also estimated from Equation (1) the required number of OTUs, n _req necessary to obtain the observed k at the 5% error level. From n _req, we also obtained the (minimal) degree of undersampling = 100 (1 − n/n _req).

We calculated Spearman's rank‐order correlations (r_S) between all species, which jointly occur at both farming sites. Significantly, negative r_S values indicate structural differences between the two communities in terms of relative abundances. We used one‐way ANOVA to infer the difference in extinction probabilities between both farming types using average OTU abundance at both sites as the dependent variable.

To infer whether organic and conventional farming influence the phenology of arthropods, we first analyzed the combined phenologies of six major arthropod taxa (Araneae, Coleoptera, Diptera, Hemiptera, Hymenoptera, and Lepidoptera). We then assessed for each species whether the peak of emergence was identical in time (within the same sample period) between and within each farming type (OG˄IG, OF˄IF, OG˄OF, and IG˄IF). Counts of the numbers of these joint emergences in comparison with the total numbers of joint occurrences indicate similar or divergence (habitat specific) phenology.