Improving stable isotope assessments of inter‐ and intra‐species variation in coral reef fish trophic strategies

Abstract

Fish have one of the highest occurrences of individual specialization in trophic strategies among Eukaryotes. Yet, few studies characterize this variation during trophic niche analysis, limiting our understanding of aquatic food web dynamics. Stable isotope analysis (SIA) with advanced Bayesian statistics is one way to incorporate this individual trophic variation when quantifying niche size. However, studies using SIA to investigate trophodynamics have mostly focused on species‐ or guild‐level (i.e., assumed similar trophic strategy) analyses in settings where source isotopes are well‐resolved. These parameters are uncommon in an ecological context. Here, we use Stable Isotope Bayesian Ellipses in R (SIBER) to investigate cross‐guild trophodynamics of 11 reef fish species within an oceanic atoll. We compared two‐ ( δ 15 N and δ 13 C) versus three‐dimensional ( δ 15 N, δ 13 C, and δ 34 S) reconstructions of isotopic niche space for interpreting guild‐, species‐, and individual‐level trophic strategies. Reef fish isotope compositions varied significantly among, but also within, guilds. Individuals of the same species did not cluster together based on their isotope values, suggesting within‐species specializations. Furthermore, while two‐dimensional isotopic niches helped differentiate reef fish resource use, niche overlap among species was exceptionally high. The addition of δ 34 S and the generation of three‐dimensional isotopic niches were needed to further characterize their isotopic niches and better evaluate potential trophic strategies. These data suggest that δ 34 S may reveal fluctuations in resource availability, which are not detectable using only δ 15 N and δ 13 C. We recommend that researchers include δ 34 S in future aquatic food web studies.

Carbon, nitrogen, and oxygen stable isotopes in modern tooth enamel: A case study from Gorongosa National Park, central Mozambique

“…Additional Bayesian analyses are becoming increasingly popular in ecological studies and could also be included in future multi-isotope tooth enamel studies if sample sizes are increased. For example, 3-dimensional isotopic analysis using SIBER has proven useful for determining isotopic niches of complex trophic systems such as coral reef atolls (Cybulski et al, 2022), and other analyses such as nicheROVER allow for incorporation of additional dimensions of isotopes or other continuous ecological indicators (Swanson et al, 2015). Even with uncertainties, isotopic investigations coupled with Bayesian statistics have significant implications for future (paleo)ecological reconstructions.…”

Dietary partitioning among three cryptobenthic reef fish mesopredators revealed by visual analysis, metabarcoding of gut content, and stable isotope analysis

“…There are several possible explanations for these patterns. The isotopic baselines can fluctuate across small spatial scales (Cybulski et al., 2022), which can be reflected in the isotopic values of the fish. While the two color morphs of P. flavivertex were collected in the same coastal habitat during the same sampling period, our sources revealed variation in the isotopic values within the same environment.…”

“…While the two color morphs of P. flavivertex were collected in the same coastal habitat during the same sampling period, our sources revealed variation in the isotopic values within the same environment. Pseudochromis flavivertex (M1 and M2) feed predominantly on cryptobenthic invertebrates such as cyclopoid copepods and mollusks, and larvae of coral reef fish, which may exhibit fluctuation in their own isotope values due to potential variation in the available nutrients (Cybulski et al., 2022). Fish with a more generalist diet, such as P. flavivertex (M1 and M2), present a more variable isotopic niche as they likely feed opportunistically on a higher variety of prey with more heterogeneous isotopic values, resulting in larger isotopic niches (Cybulski et al., 2022; Miller et al., 2019; Nagelkerken et al., 2009).…”

“…Pseudochromis flavivertex (M1 and M2) feed predominantly on cryptobenthic invertebrates such as cyclopoid copepods and mollusks, and larvae of coral reef fish, which may exhibit fluctuation in their own isotope values due to potential variation in the available nutrients (Cybulski et al., 2022). Fish with a more generalist diet, such as P. flavivertex (M1 and M2), present a more variable isotopic niche as they likely feed opportunistically on a higher variety of prey with more heterogeneous isotopic values, resulting in larger isotopic niches (Cybulski et al., 2022; Miller et al., 2019; Nagelkerken et al., 2009). On the contrary, small trophic niches are likely characteristic of species having either small foraging ranges or that consume prey with homogenous isotopic signatures and close to the source of the basal resource (e.g., zooplanktivorous) (Miller et al., 2019), as it was found for P. fridmani .…”

“…Isotopic niche overlap was calculated as the average percentage of isotopic space shared by species and morphs based on SEA C values and the area of ellipse overlap. Niche overlap was considered significant when the overlap in shared isotopic space between species was >60% (Cybulski et al., 2022; Matley et al., 2017). To statistically compare the area of the ellipse among species, Bayesian models were fitted using uninformative priors to estimate the posterior distributions of the ellipse areas (iterations: 20000; burn‐in: 1000, thin 10), and the mode of each distribution and the 95% credible intervals were calculated for each population.…”