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Supplementary Material
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Supplement 1. Evaluation of pooling species into ‘trout’ and ‘galaxiid’ groups
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We tested whether or not pooling species into broader taxonomic groups (i.e., trout and galaxiid) was
statistically justified using data from the subset of study streams containing both species of galaxiid
(Galaxias paucispondylus and G. vulgaris, n = 6 streams) and/or trout (Salmo trutta and
Oncorhynchus mykiss, n = 1 stream) together. For the two galaxiid species, we first tested differences
in the individual stable isotope metrics (i.e., trophic position and 13C) using mixed-effects
ANCOVA, with length as the covariate and stream as the random effect. We conducted a similar
analysis for S. trutta and O. mykiss, but given that these species co-occurred at one site only (Slip
Spring) used ANCOVA without a random site effect. Secondarily, to determine whether niche width
estimates varied by species identity or in the presence of more than one species, we compared this
metric between sites containing G. vulgaris as the only galaxiid, G. paucispondylus as the only
galaxiid, and both species together using ANOVA; we similarly compared trout niche width estimates
between sites comprised of O. mykiss alone and S. trutta.
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All statistical tests indicated that pooling Galaxias vulgaris and G. paucispondylus into the coarser
‘galaxiid’ category was reasonable for broader analysis purposes. In particular, there was no evidence
of a trophic position (difference < 0.1; 2 = 1.7, df = 1, P = 0.189) or 13C difference (difference =
0.3‰; 2 = 0.1, df = 1, P = 0.724) between species; further, galaxiid niche width calculations were
unaffected by the identity or number of Galaxias species involved in their computation (F2,15 = 0.7, P
= 0.491). For trout, there was no evidence of species differences for 13C (difference = 0.1‰; F1,7 =
0.1, P = 0.819) or an effect of trout species identity on niche width computations (F1,8 = 0.1, P =
0.814). However, there was a significant difference in the trophic position for the two trout species
(F1,7 = 10.8, P = 0.013), with brown trout averaging slightly higher for this metric than O. mykiss.
Though statistically significant, the level of difference (i.e., 3.5 vs. 3.4) was negligible in biological
terms. Based on these findings, all analyses described in the main body of the manuscript were
conducted using pooled galaxiid and trout ‘species’ categories.
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Supplement 2. Sample code for computing the Layman et al. (2007) niche width metrics in R. Note
the ‘splancs’ package must be downloaded and installed in your R library for the code to work.
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# Load necessary packages (be sure it is installed in your library first)
library(splancs)
# your data go here (can be added from a data frame that you've read in)
d13C<- c(-32.30,-32.54,-33.11,-32.90,-32.34,-32.19,-32.22,-32.38,-32.36,32.04,-31.41,-31.17)
d15N<-c(6.48,7.22,7.03,7.60,7.82,7.57,6.88,7.42,7.70,7.50,7.52,8.26)
data1<-matrix(cbind(d13C,d15N),ncol=2) #create a n*2 13C & 15N matrix
hpts1<-chull(data1) # derives points bounding the convex hull polygon
hpts1<-c(hpts1,hpts1[1]) # duplicate first point to close polygon
Area<-areapl(data1[hpts1,])
CR<-max(d13C)-min(d13C)
NR<-max(d15N)-min(d15N)
# calculates the area of the polygon
# print values to screen
print(paste("Area is", Area))
print(paste("C Range is",CR))
print(paste("N Range is",NR))
################################################################
# code for making a d13C-d15N bi-plot of a single site/species combination
# with the convex hull polygon overlaid
par(mar=c(5, 4.65, 4, 1.65))
yax=expression(paste(delta^{15},"N (\211)"))
xax=expression(paste(delta^{13},"C (\211)"))
plot(d13C,d15N,pch="",xlab=xax,ylab=yax,cex.lab=1.5,cex.axis=1.3,cex=2)
points(d13C,d15N,pch=24,bg="black",cex=2)
lines(data1[hpts1,],lwd=1.5)
################################################################
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Supplement 3. Model selection results for mixed-effects model analysis of trophic position and
standardized 13C (i.e., 13Cs = 13Cfish-13Cbase).
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Table 1. Model selection results for candidate trophic position (TP) models. Models highlighted in
bold-faced font were taken as the top-model in the candidate set. Site was included as a random effect
in all models. group = species (trout, galaxiid); L = individual length; csa = cross-sectional area; rdi =
river disturbance index.
Model
structure
Model
K
n
AICc
AICc
Deviance
Individual level
TP=intercept only (NULL)
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177
-135.8
35.8
-142.0
TP=group
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-132.1
39.5
-142.5
TP=L
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-150.4
21.1
-158.7
TP=group+L
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-154.7
16.9
-167.2
TP=group+L+group*L
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-171.6
0.0
-188.4
TP=group+L+group*L (NULL)
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-171.6
18.5
-188.4
TP=group+L+group*L+csa
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-171.9
18.2
-191.0
TP=group+L+group*L+rdi
9
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-173.0
17.1
-192.1
TP=group+L+group*L+csa+group*csa
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-174.0
16.1
-197.6
TP=group+L+group*L+rdi+group*rdi
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-190.1
0.0
-213.7
Site level
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Table 2. Model selection results for candidate standardized 13C (13Cs = 13Cfish-13Cbase).models.
Models highlighted in bold-faced font were taken as the top-model in the candidate set. Site was
included as a random effect in all models.
Model structure
Model
K
n
AICc
AICc
Deviance
Individual level
13C=intercept only (NULL)
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599.0
17.2
592.8
13C=group
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586.5
4.7
576.2
13C=L
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600.8
19.0
592.6
13C=group+L
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177
581.8
0.0
569.3
13C=group+L+group*L
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582.2
0.4
565.3a
13C=group+L+group*L (NULL)
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582.2
12.3
565.3
13C=group+L+group*L+csa
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584.3
14.4
565.3
13C=group+L+group*L+rdi
13C=group+L+group*L+csa+group*csa
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11
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177
569.9
585.3
0.0
15.4
550.8
561.7
Site level
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13C=group+L+group*L+rdi+group*rdi
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0.9
547.2
Given that it was similarly supported as the additive model, we used this model structure for site-level modelstructure determination and final inference in order to maintain consistency with TP analyses.
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Supplement 4. Summary of feeding links detected for galaxiids and benthic prey detected in our
study streams.
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Table 1. Occurrence (‘X’ = present in diet) of prey taxa in diets across study streams.
Site
Acheron
Bradley Mid
Bradley Upper
Broken
Cass
Coach Lower
Coach Upp
Cora Lynn
Dry
Flock Main
Flock Trib
Ghost
Lower Farm
Porter River
Roadmarker
Slip
Thomas
Waimakariri Spring
n
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0
1
6
1
X
X
2
X
X
X
X
X
3
4
X
X
X
X
X
X
X
X
X
--
5
X
X
X
-X
X
--
--
--
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X
X
X
X
X
X
X
X
X
X
X
X
X
X
-X
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Prey number
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13
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
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16
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
-X
--
--
X
--
X
X
--
--
--
X
--
X
--
--
X
X
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Table 2. List for prey taxa corresponding to numbers in the Table 1 heading. Note that prey item 17,
miscellaneous unidentified taxa, comprises an unknown number of taxa but is effectively treated as
one for analysis.
Prey
number
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Order
Diptera
Ephemeroptera
Coleoptera
Trichoptera (free)
Trichoptera (cased)
NA
Taxon
Austrosimulum
Chironomidae
Muscidae
Austroclima
Coloburiscus
Deleatidium
Nesameletus
Elmidae
Hydrobiosidae
Other non-cased
Orthopsyche
Olinga
Oxyethria
Plecoptera
Pycnocentria
Other cased
Misc. unid'd taxa
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17
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
-X
X