DOI: 10.5061/dryad.95x69p8n0
title: Isotopic niche overlap between sympatric Australian snubfin and humpback dolphins

description: Ecological niche theory predicts the coexistence of closely related
 species is promoted by resource partitioning and leads to the use of
 different ecological niches. Australian snubfin (Orcaella heinsohni) and
 humpback (Sousa sahulensis) dolphins live in sympatry throughout most of
 their range in northern Australia. We compared stable isotope ratios of
 carbon (δ13C) and nitrogen (δ15N) in their skin to investigate resource
 partitioning between these ecologically similar species. Skin samples were
 collected from live Australian snubfin (n = 31) and humpback dolphins (n =
 23) along the east coast of Queensland in 2014-2015. Both species had
 similar δ13C and δ15N values and high (&gt;50%) isotopic niche space
 overlap, suggesting that they feed at similar trophic levels, have
 substantial dietary overlap, and both rely on similar basal food
 resources. Despite similarities, snubfin dolphins were more likely to have
 a larger δ15N than humpback dolphins, indicating they may forage on a
 wider diversity of prey. Humpback dolphins were more likely to have a
 larger δ13C range suggesting they may forage on a wider range of habitats.
 Overall results suggest that subtle differences in habitat use, and prey
 selection are likely the principal resource partitioning mechanisms
 enabling co-existence of Australian snubfin and humpback dolphins.
description: Isotopic ratios were transformed into parts per thousand (‰)
 using delta notation (δ):   where δX
 is δ<sup>13</sup>C or δ<sup>15</sup>N, R sample is
 the ratio of stable isotopes in the sample and R standard is the ratio of
 stable isotopes in the standard reference materials (atmospheric nitrogen
 gas and carbon from Pee Dee Belemnite, a limestone from South
 Carolina). Statistical analysis We
 tested δ15N and δ13C data for each species for homogeneity of variance
 (non-parametric Levene’s test) and normality (Shapiro-Wilks Test). Tests
 revealed homogeneity of variance for species, but assumptions of normality
 were not met for δ<sup>13</sup>C for humpback dolphins (P =
 0.04). We, therefore, used a one-sided randomisation test with 10,000
 permutations at 0.05 significance level to investigate differences in
 isotopic values between species. This test compares the difference of the
 mean δ<sup>15</sup>N and δ<sup>13</sup>C per
 species with the difference obtained by randomly allocating the observed
 isotopic values among the two species (Manly 2018). We
 used six metrics proposed by Layman et al. (2007a) to compare the isotopic
 niches of Australian snubfin and humpback dolphins: 1.
 δ<sup>15</sup>N range, which is the difference between the
 highest and lowest δ15N values of each species. δ15N range provides
 information on the range of trophic levels at which each species has been
 feeding. 2. δ<sup>13</sup>C range is a
 measure of the difference between the highest and lowest δ13C values of
 each species. δ13C range provides an estimate of the variability of
 trophic sources of each species. 3. Total area (TA),
 which is a measure of the total amount of niche space occupied by a
 species in ‰2. TA was calculated from a convex hull drawn around the most
 extreme data points on an isotope
 δ<sup>13</sup>C–δ<sup>15</sup>N bi-plot. As TA is
 sensitive to differences in sample size because the area can only increase
 as new data points are added; we used the corrected version of the
 standard ellipse area (SEAC) as a measure of the mean core area (40%) of
 each species isotopic niche (A. L. Jackson, Inger, Parnell, &
 Bearhop, 2011). 4. Mean distance to centroid (CD) is
 the mean Euclidean distance of each individual of a population to the
 δ15N–δ13C centroid, where the centroid is the mean
 δ<sup>15</sup>N–δ<sup>13</sup>C value for all
 species in the food web. CD provides an estimate of overall dietary
 diversity. 5. Mean nearest neighbour distance (MNND) is
 the average nearest-neighbour euclidean distance between an isotopic
 coordinate relative to all other coordinates within a species. MNND
 provides an estimate of species packing and shows how similar or
 dissimilar the members of a population are to one another.
 6. Standard deviation of nearest neighbour distance (SDNND). A
 measure of the evenness of spatial density and packing of individuals. Low
 SDNND values indicate a more even distribution of trophic
 niches. We bootstrapped all Layman metrics with
 replacement (n = 10,000, indicated with a subscript
 ‘<sub>boot</sub>’) based on the smallest sample size in the
 data set (n = 23) to enable statistical comparison between dolphin species
 (M. C. Jackson et al., 2012; Manly, 2007). To further assess niche widths
 and isotopic niche overlap between species, we followed a Bayesian
 approach using multivariate ellipse-based metrics (A. L. Jackson et al.,
 2011). This method is particularly useful when comparing groups with small
 sample sizes, as it corrects for the influence of outliers. We calculated
 standard ellipse areas (SEAs), which are the bivariate equivalent to
 standard deviation in univariate analyses. We also calculated SEA
 corrected (SEA<sub>C</sub>) to minimize bias introduced by
 small sample sizes. In addition, we calculated SEAB (Bayesian SEA) using
 1000 posterior draws to statistically compare niche width between species.
 We used SEA<sub>B</sub> to calculate the niche overlap between
 Australian snubfin and humpback dolphins, calculated as the proportion of
 the total SEAB for each species respectively. We calculated all metrics
 using the R package SIBER (Stable Isotope Bayesian Ellipses in R (A. L.
 Jackson et al., 2011; R Core Team, 2021).