LINKED PAPER
Consistent seasonal foraging niche segregation between critically endangered Whenua Hou Diving Petrels and abundant Common Diving Petrels. Tocker, G. E., Fischer, J. H., Bustamante, P., Guillou, G., Debski, I., Taylor, G. A., Ritchie, P. A., Wittmer, H. U. 2022. IBIS. DOI: 10.1111/ibi.13170. VIEW

When multiple species live in the same place, it is theorised that they reduce competition by partitioning resources in temporal, spatial, or tropic dimensions into separate ecological niches. Even ecologically similar species can coexist by breeding and foraging in different habitats and at different times. Ecological niches and interspecific interactions can be especially important to understand for threatened species, which are already under pressure from various threats. If threatened species also exhibit a small, inflexible niche they may be more vulnerable to competition and future environmental change (Barger & Kitaysky 2012).

In a 2022 study in Ibis, Grace Tocker and colleagues investigated the niche dynamics of the critically endangered Whenua Hou Diving Petrel (Pelecanoides whenuahouensis) and the abundant Common Diving Petrel (Pelecanoides urinatrix chathamensis) using stable isotope analyses.

Stable isotope analyses
The use of stable isotope analyses has helped to broaden our understanding of seabird foraging niches. Carbon (δ13C) isotope ratios reflect foraging locations, while nitrogen (δ15N) isotope ratios help us understand trophic positions, with these coming together to help us infer a species’ foraging niche. The different turnover rates of different tissues means that by sampling multiple tissue types (e.g. blood and feathers) we can determine temporal variations in the foraging niche (Bearhop et al. 2004).

The Whenua Hou Diving Petrel is critically endangered and restricted to Whenua Hou (Codfish Island), Aotearoa. During the breeding period, the species is at risk from terrestrial threats such as storms, storm surges eroding their breeding habitat, and competition for burrow sites with the non-threatened Common Diving Petrel (Fischer et al. 2017, 2021). However, any competition between the petrel species at sea and the vulnerability of the Whenua Hou Diving Petrel to environmental changes had not been assessed prior to this study. The researchers collected blood and feather samples from Whenua Hou Diving Petrels and Common Diving Petrels in 2017, 2018, and 2019, and used stable isotope analyses to quantify foraging niches, investigate intraspecific and interannual variation, infer interspecific competition, and investigate their vulnerability to environmental changes.

Figure 1. Isotopic (i.e. foraging) niches of Whenua Hou Diving Petrels (WHDP) and Common Diving Petrel (CDP) females and males during incubation periods (Oct–Dec, inferred from blood; a–c) and non-breeding periods (Feb–Aug, inferred from feathers; d–f) illustrated by single 95% standard ellipses (as an example for the iterative Bayesian Standard Ellipses used in analyses). Corrections were applied to account for different enrichment factors between blood and feather tissues (Cherel et al. 2014). Approximate locations of Fronts (dotted lines) based on Jaeger et al. (2010).

Foraging niche segregation
During the incubation period, the results showed that Whenua Hou Diving Petrels exhibited a larger foraging niche (i.e. were more generalist) than Common Diving Petrels. Although the two species foraged at similar average trophic positions, Whenua Hou Diving Petrels foraged further offshore and in oceanic subantarctic waters, while Common Diving Petrels foraged closer to shore and predominantly in subtropical, neritic waters. This suggests that there is clear niche segregation between the species when they are bound by central place foraging during the incubation period. It is unlikely that the subtle morphological differences between the species explains their foraging niche segregation, and it is suggested that high-resolution GPS tracking could help to provide further insights into the use of distinct foraging areas.

Figure 2. Density plots illustrating interspecific (a, b) and intraspecific (c–f) foraging niche overlap of Whenua Hou Diving Petrel (WHDP) and Common Diving Petrel (CDP) females and males during incubation (a, c, e) and non-breeding (b, d, f) periods, as estimated through pairwise overlap calculations of Bayesian 95% Standard Ellipses.

During non-breeding periods, both species dispersed into the Southern Ocean and their isotopic niches increased and overlapped, with Common Diving Petrels switching from an apparent specialist foraging strategy during breeding to a more generalist strategy. Results suggested that both species fed on a lower trophic level during the non-breeding period, which is expected as during breeding seabirds must balance the changing demands of self and offspring provisioning with the constraints imposed by central place foraging by feeding at higher trophic positions (Boersma et al. 2015, Booth et al. 2018).

Although a common source of intraspecific variation in foraging is sex, the results indicated that females and males of both species foraged at comparable tropic positions and in similar waters during the incubation period and non-breeding period. Patterns were also found to generally be consistent among the years of the study, however, Common Diving Petrel foraging niches contracted during El Niño conditions, suggesting sensitivity to environmental change.

Overall, the results suggest that the Whenua Hou Diving Petrel is a reasonably adaptable generalist feeder, with potentially limited sensitivity to competition and the impacts of environmental change at sea. The implications of these results are that Whenua Hou Diving Petrel conservation management interventions should focus on addressing terrestrial and near-shore threats, alongside monitoring for indirect, pelagic threats.

References

Barger, C.P. & Kitaysky, A.S. (2012). Isotopic segregation between sympatric seabird species increases with nutritional stress. Biology Letters 8: 442–445. VIEW

Bearhop, S., Adams, C.E., Waldron, S., Fuller, R.A. & MacLeod, H. (2004). Determining trophic niche width: a novel approach using stable isotope analysis. Journal of Animal Ecology 73: 1007–1012. VIEW

Boersma, P.D., Rebstock, G.A. & Garcia-Borboroglu, P.G. (2015). Marine protection is needed for Magellanic penguins in Argentina based on long-term data. Biological Conservation 182: 197–204. VIEW

Booth, J.M., Steinfurth, A., Fusi, M., Cuthbert, R.J. & McQuaid, C.D. (2018). Foraging plasticity of breeding northern rockhopper penguins, Eudyptes moseleyi, in response to changing energy requirements. Polar Biology 41: 1815–1826. VIEW

Cherel, Y., Jaquemet, S., Maglio, A. & Jaeger, A. (2014). Differences in δ13C and δ15N values between feathers and blood of seabird chicks: implications for non-invasive isotopic investigations. Marine Biology 161: 229–237. VIEW

Fischer, J.H., Debski, I., Taylor, G.T. & Wittmer, H.U. (2017). Assessing the suitability of non-invasive methods to monitor interspecific interactions and breeding biology of the south Georgian diving petrel (Pelecanoides georgicus). Notornis 64:13–20. VIEW

Fischer, J.H., Wittmer, H.U., Taylor, G.A., Debski, I. & Armstrong, D.P. (2021). Preparing for translocations of a critically endangered petrel through targeted monitoring of nest survival and breeding biology. Oryx 55: 564–572. VIEW

Jaeger, A., Lecomte, V.J., Weimerskirch, H., Richard, P. & Cherel, Y. (2010). Seabird satellite tracking validates the use of latitudinal isoscapes to depict predators foraging areas in the Southern Ocean. Rapid Communications in Mass Spectrometry 24: 3456–3460. VIEW

Image credits

Top right: Common Diving Petrel (Pelecanoides urinatrix) | JJ Harrison | CC BY-SA 3.0 Wikimedia Commons

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