3.2 | Core species niche overlap and dispersion
For species present before and after river regulation (core species), we
calculated species pairwise trophic overlap (i.e., niche overlap) to
quantify potential changes in resource competition. We used the R
statistical package nicheROVER which employs a Bayesian framework
to estimate the directional probability that a randomly drawn individual
of one species overlaps into the niche of another in
δ13C and δ15N space (Swansonet al ., 2015; Lysy et al ., 2021). We used a resampling
routine of 10,000 draws to randomly sample from each species pair,
maintained nicheROVER ’s default priors (flat
Normal-Independent-Inverse-Wishart distribution), and assessed model
convergence (all R-hat <1.01). From each species’ directional
niche overlap posterior probability distribution, we then calculated the
probability the difference between time periods was greater than zero.
To identify potential changes in the diversity of trophic resources used
by individual core species (i.e., collected in both time periods), we
employed the SIBER package which uses a Bayesian method to
calculate standard ellipses areas corrected for small sample sizes
(SEAB) from observed δ13C and
δ15N (Jackson et al ., 2011). We calculated
SEAB because it can be interpreted as a species’ core
isotopic niche breadth and because ellipses contain 40% of the sample,
the metric can be used when samples sizes differ between populations
(Jackson et al ., 2011; Layman et al ., 2012). Although
SEAB corrects for small samples, samples size
< 10 are prone to underestimates (Jackson et al .,
2011). Therefore, we included historical Colorado Pikeminnow (n = 6) in
our analysis with the understanding that the SEABestimates could be artificially low. We incorporated the same MCMC
algorithm parameters used to calculate fish community-wide trophic
structure metrics and assessed model convergence (all R-hat
<1.01). To assess whether niche dispersion differed for each
core species, we calculated the probability the difference between
historical and contemporary SEAB posterior probability
distributions were greater than zero using the same procedure described
above.