Fish reduce habitat coupling by a waterbird: evidence from combined stable isotope and conventional dietary approaches

Abstract

Aquatic consumers can function as habitat couplers by using allochthonous subsidies of prey that migrate across ecosystem boundaries. We examined the relative use of allochthonous (invertebratesterrestrial or living on littoral vegetation; immigrating amphibians) versus autochthonous (aquatic invertebrates, fish) resources by the red-necked grebe Podiceps grisegena, a generalist predator, on fishless ponds versus ponds stocked with common carp Cyprinus carpio. We combined conventional methods of diet estimation with stable carbon and nitrogen analyses of egg yolks and putative prey of grebes. Prelaying grebes were observed to take mainly adult amphibians on fishless ponds and fish on stocked ponds. Alimentary tract analyses gave more weight to invertebrate prey, especially leaf beetles Donaciinae, apparently picked off water or emergent plants. Bayesian isotopic mixing models did not reveal predominance of a single food source but indicated that in the presence of fish grebes received relatively less energy for egg formation from amphibians and leaf beetles. Overall, our results show that grebes relied more on allochthonous resources (range of means 50-97 % of the biomass contribution estimated by different assessment methods) in the absence than in the presence of fish (8-23 %). We suggest that habitat coupling by waterbirds may be controlled by fish, which can suppress external prey subsidies, apart from being an attractive food for piscivorous birds.

Stable isotope tracing of links between marine wintering and freshwater breeding habitats of Red-necked Grebes

“…Earlier research on the trophic ecology of Red-necked Grebes (the North American subspecies Podiceps grisegena holboellii) on boreal lakes in Canada suggested that females incorporated locally derived nutrients into eggs (Paszkowski et al 2004). Our previous study on exploitation of local resources by grebes, using yolks from eggs collected mainly in mid-season, led to basically the same conclusion (Kloskowski and Trembaczowski 2015). Here we used eggs laid early in the season.…”

“…Interannual differences in the location of wintering grounds and diet composition may have affected the results (cf. Hobson et al 2011), but on the breeding grounds food sources were determined by pond culture regime and did not vary isotopically between years (Kloskowski and Trembaczowski 2015). Sample sizes also differed between analyses, mainly because of the varying amount of material for isotopic assays.…”

“…Live birds were captured for sampling on the same or closely adjacent ponds. We identified the major dietary items of grebes during the early breeding season by examination of alimentary tracts, behavioural observations, and isotopic mixing models (Kloskowski and Trembaczowski 2015), and assigned these prey taxa to four categories: adult beetles (Coleoptera; individuals > 1 cm body length), aquatic leaf beetles (Donaciinae), Common Carp, and adult anurans breeding in the ponds (Table 1). Whole body homogenates of prey invertebrates were used for analyses.…”

“…Pond food webs become more complex with the progress of the season, which may result in lengthened food chains and thus increased δ 15 N levels in tissues of top predators (McParland et al 2010). The isotopic patterns reflecting nutrient acquisition by grebes might be additionally complicated because a varying proportion of their prey originated from terrestrial food webs, potentially differing in δ 13 C and δ 34 S signals from freshwater biomes (Kloskowski and Trembaczowski 2015; see also Croisetière et al 2009). We observed particularly large variation in amphibian δ 34 S signatures, presumably because most of the sampled species are terrestrial and enter small waterbodies only for reproduction, while others (water frogs Pelophylax sp.)…”

Higher reproductive performance of a piscivorous avian predator feeding on lower trophic-level diets on ponds with shorter food chains

“…Data were combined across years due to low sample sizes within each year. We assumed that given pond management procedures, including regular pond drainage and refilling, isotopic signatures of primary producers that supported the pond food webs did not change significantly over the study period (Kloskowski and Trembaczowski 2015).…”