Geographic variation in the trophic ecology of an avian rocky shore predator, the African black oystercatcher, along the southern African coastline

Abstract

The reflection of baseline isotopic signals along marine food chains up to higher trophic levels has been widely used in the study of oceanic top predators but rarely for intertidal predators. We investigated variation in the δ 13 C and δ 15 N ratios of a sedentary, rocky shore predator, the African black oystercatcher Haematopus moquini, over ~2000 km of the southern African coastline, which is characterized by strong biogeographic patterns in primary productivity and intertidal communities. Blood and feathers from breeding adults and chicks and muscle tissues from primary prey items (mussels and limpets) were sampled between southern Namibia and the southeast coast of South Africa. 15N enrichment was observed between the southeast and west coasts in oystercatcher tissues and their prey, mirroring an isotope shift between the oligotrophic Agulhas Current on the east coast and the eutrophic Benguela upwelling system on the west coast. Oystercatcher blood showed δ 13 C values that varied between those of the carbon-depleted mussels and the carbon-enriched limpets along the coastline, which reflected changes in the proportion of grazers and filter feeders in the oystercatcher diet across the sampling range. The geographic shift in diet, dominated by mussels on the west coast and composed of mixed proportions of mussels and limpets on the southeast coasts, strongly reflected regionally high abundances of the invasive Mediterranean mussel Mytilus galloprovincialis. Finally, isotope signatures of blood and feathers displayed a strong correlation throughout the study area, indicating seasonal stability in environmental conditions and feeding habits of the adults. There were, however, local discrepancies on the south coast that indicated movement of adults occurred outside the breeding season possibly in response to a lower abundance of food in this region. Overall, the results indicate that the influence of regional oceanic conditions on the base of the food web can penetrate to the predator level, but that local effects can be incorporated within this pattern.

Comparison of ontogenetic trophic shift in two hake species, Merluccius capensis and Merluccius paradoxus, from the Northern Benguela Current ecosystem (Namibia) using stable isotope analysis

“…Sardine displayed the lowest mean δ 15 N values of all fishes (mean ± SD = 9.2‰ ± 0.7), comparable to zooplankton of copepods and euphausiids (mean δ 15 N = 9.6 ± 0.2‰, and 9.7 ± 1.2‰, respectively). When considering +3.4‰ per trophic level from the baseline of mytilid bivalves from Kohler et al. (2011, mean δ 15 N = 9.2‰), with the exception of Synagrops sp., all hake (both M. paradoxus and M. capensis ) >20 cm in total length had δ 15 N values above zooplankton and nekton prey, and generally had TLs of 3.0–3.8 (Table 1, Fig.…”

“…, 2008). Our use of bivalves from Kohler et al. (2011) as the δ 15 N base was based on its TL proximity to zooplankton (size 200–500 μm) in our study and that of Sholto‐Douglas et al.…”

Seasonal consistency and individual variation in foraging strategies differ among and within Pygoscelis penguin species in the Antarctic Peninsula region

“…Several studies have examined the isotopic values of feathers and blood synthesized under the same diet (Sanpera et al 2007;Bugoni et al 2008;Quillfeldt et al 2008;Kohler et al 2011;Cruz et al 2012;Cherel et al 2014). Based on a metaanalysis of these studies, we used the following regressions from Cherel et al (2014) to standardize values to directly compare between δ 15 N and δ 13 C values obtained from the feather and blood tissues of the same individual penguin:…”

The status of the Damara Tern in the Eastern Cape, South Africa

“…Unfortunately, no stable isotope data are available for these areas. Studies conducted from the primary producers up to top predators showed an increase in  15 N along the southern African coastline from the east coast up to Namibia reflecting the differences in oceanography and coastal hydrography (Hill et al 2006;Hill and McQuaid 2008;Kohler et al 2011). The oceanography and hydrography in the Gulf of Guinea and along the Angola coastline are complex (Rouault et al 2007;Rouault 2012), so one would expect variations in the  13 C and  15 N along the western African coast; variations that would be detected in the tern feathers.…”