Statistical analysis
All analyses were conducted in R v4.0.2 (http://www.R-project.org/). The
structure of the final linear models and sample sizes break down by the
levels of categorical explanatory variables are summarized in (Table 2).
Non-significant interactions were removed from the models. Significance
of the final models was evaluated using ANOVA tables using Type II and
III sums of squares for models without and with significant interaction,
respectively. Differences among groups were analysed using Tukey’s HSD
post-hoc test. We have accounted for the ontogenetic effects by using
fork length as a proxy of individuals age. Fork length has been used as
explanatory variable in all models. When models contained other
explanatory variables correlated to the fork length, we used residuals
from a linear model between the log transformed variable of interest and
fork length to prevent collinearity (see Table 2).
We tested the variability in content of ALA and EPA, and of total lipids
in macroinvertebrates across all sampling sites (i.e., category
with 6 levels – 3 streams × 2 populations [allopatric, sympatric])
and subsidy types (category with 2 levels – aquatic or terrestrial
macroinvertebrates). Since there was no difference lipid quality of
aquatic and terrestrial subsidies across the six sampling sites (see
results for details), we did not include the stream or sampling site in
the models. We collected samples of dorsal and ventral muscle tissue
because concentration of adipose cells is often higher in ventral than
in dorsal muscles and thus these two tissues can differ in their lipid
content (Fauconneau et al. 1995; Ebm et al. 2021). However, the total
lipid content and the relative content of n-3 LC-PUFA were repeatable
across dorsal and ventral muscle samples (total lipids:
Radj = 0.25, 95% CI [0.06, 0.44]; ALA:
Radj = 0.59, 95% CI [0.44, 0.72]; EPA:
Radj = 0.62, 95% CI [0.48, 0.74]; DHA:
Radj = 0.56, 95% CI [0.41, 0.70]. Therefore, for
further analysis we have used an average value across tissues for each
individual. Total lipids in all models were reported as mass fractions
(mg/g of the dry mass), while all models and figures including n-3
LC-PUFA reported the relative fatty acid contents (% of fatty acid
methyl esters; with exception of Fig. 1). This is a common approach
(e.g., Twining et al. 2019; Ebm et al. 2021) because mass
fractions of fatty acids are often closely correlated to total lipid
mass fractions, while the relative content of fatty acids is independent
of the total lipids and thus more comparable across the sample types and
studies, particularly when only a part of an organism (e.g.,muscle tissues) is analysed.
Table 2 – Summary of the final models as reported in the
results. The following categorical variables have been used: sex (male,
female), sampling site (6 levels, see Table 1), competition (allopatry,
sympatry), prey subsidies (aquatic, terrestrial). Note that model 1 was
run with and without sex as an explanatory variable because sex was only
determined in the subsample of dissected fish, which substantially
reduced the sample size compared to the model without sex. However, the
two models yielded similar results.