Isotopic niche overlap between sympatric Australian snubfin and humpback dolphins

Abstract

Ecological niche theory predicts the coexistence of closely related species is promoted by resource partitioning in space and time. Australian snubfin ( Orcaella heinsohni ) and humpback ( Sousa sahulensis ) dolphins live in sympatry throughout most of their range in northern Australian waters. We compared stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) 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 δ 13 C and δ 15 N values and high (>50%) isotopic niche space overlap, suggesting that they feed at similar trophic levels, have substantial dietary overlap, and rely on similar basal food resources. Despite similarities, snubfin dolphins were more likely to have a larger δ 15 N value than humpback dolphins, indicating they may forage on a wider diversity of prey. Humpback dolphins were more likely to have a larger δ 13 C 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 the coexistence of Australian snubfin and humpback dolphins.

Trophic Niche Differentiation in Two Sympatric Nuthatch Species (Sitta yunnanensis and Sitta nagaensis)

“…The overlapping of food resources between S. yunnanensis and S. nagaensis is consistent with the adoption of strategies for food resources observed in other sympatric species. For example, a feather isotope analysis of three sympatric species of closely related seabirds revealed substantial overlap in food resources [21], and Australian snubfin and humpback dolphins that co-exist in the same domain have overlapping dietary resources but different dietary preferences [37].…”

Habitat partitioning, co-occurrence patterns, and mixed-species group formation in sympatric delphinids

“…This concurs with previous studies of niche partitioning between these species 44 , 46 . Elsewhere, it has been shown that, although habitat partitioning may occur, niche partitioning is mediated by differences in trophic niches 35 , 36 , 73 . Around the North West Cape, the trophic interactions of humpback and bottlenose dolphins are poorly understood, however, opportunistic observations of both species foraging suggest that the species may target different prey.…”

Too Close for Comfort? Isotopic Niche Segregation in New Zealand’s Odontocetes

“…As it is possible for different food resources to show similar isotopic composition [ 76 , 77 ], we cannot completely exclude the possibility of trophic segregation based on stable isotope data alone. Two coastal delphinid species in Queensland, Australia, the Australian humpback ( Sousa sahulensis ) and the snubfin dolphin ( Orcaella heinsohni ), also showed high isotopic niche overlap [ 78 ]. In this case, their co-existence was attributed to subtle differences in habitat use and prey selection [ 78 ].…”

“…Two coastal delphinid species in Queensland, Australia, the Australian humpback ( Sousa sahulensis ) and the snubfin dolphin ( Orcaella heinsohni ), also showed high isotopic niche overlap [ 78 ]. In this case, their co-existence was attributed to subtle differences in habitat use and prey selection [ 78 ]. However, those two species are similar in size and can exploit the same prey species.…”