Fish stocking creates an ecological trap for an avian predator via effects on prey availability

Abstract

In anthropogenic landscapes animals may be lured into low‐quality habitats where they survive or reproduce poorly (‘ecological traps’). I investigated breeding habitat selection in relation to intra‐seasonal changes in food availability and reproductive output in red‐necked grebes Podiceps grisegena, a size‐limited predator, of common carp Cyprinus carpio ponds. Carp farms constitute highly heterogeneous habitat mosaics due to separate stocking of different age/size fish. Pond features significant for grebe settling decisions, i.e. hydroperiod and emergent vegetation cover, had no obvious effects on prey abundance for chicks and on fledging success. Breeding grebes avoided ponds containing fish too large for them to ingest but exhibited little preference between ponds with medium‐sized one‐year‐old carp that could be exploited by pre‐laying birds, and ponds designated for young‐of‐the‐year carp, where only invertebrates and amphibians were available as prey in early spring. Red‐necked grebes settling on ponds with medium‐sized fish failed to predict future shifts in interactions with carp stocks; carp exceeded the prey‐size threshold of chicks and adversely affected their non‐fish prey levels. The resulting food shortage led to severe egg‐to‐fledging mortality rates compared to fishless ponds or those containing young‐of‐the‐year fish. This study shows that waterbirds vulnerable to competition from fish can risk maladaptive habitat selection due to unrecognised spatial and temporal variation in food resources caused by fish stocking practices. Ecological traps created by perturbations to trophic interactions may be common but difficult to detect because altered dynamics of trophic resources can affect wildlife indirectly. As with other types of ecological traps, manipulation of habitat features identified as attractive cues for settling animals, but not related to critical food resources, may help to reduce perceptual pitfalls. For example, wetland management to mitigate trap effects driven by commercially stocked fish should preserve abundant emergent vegetation in habitats with weak fish impact and extend their hydroperiod.

Habitat selection and reproductive success of coot Fulica atra on ponds under different fish size and density conditions

“…ponds and suffered heavy food-dependent mortality of chicks on the densely stocked 1? ponds, where fish proved unavailable to young birds due to size constraints and suppressed the non-fish prey of grebes (Kloskowski, 2012). In the present study, 0?…”

“…Staicer et al, 1994;Paracuellos, 2006;Broyer & Calenge, 2010). However, in human-managed aquatic systems, bird habitat preferences may not match the actual habitat suitability due to unpredictable habitat dynamics, such as seasonal changes in food supplies or water-level fluctuations (Anteau et al, 2012;Kloskowski, 2012). Here we used separate stocking of ponds with carp age cohorts as the context for a natural experiment to investigate coot habitat selection and breeding success in relation to the age structure of fish.…”

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

“…Holboelli, however, even on lakes with fishes, has been reported to rely on a mixed diet of macroinvertebrates and fish (Paszkowski et al 2004; see also Newbrey et al 2012). Similarly, in our study area, grisegena did not avoid ponds dominated by fish (contra Wagner and Hansson 1998) when the stocked carp was available as prey (Kloskowski 2012); although grebes were capable of preying on relatively large-bodied carp, on the ponds with plentiful fish insects formed a significant proportion of the diet (see also Onno 1960;Fjeldså 1982). Addition of fish, a large-sized and energetically highly profitable prey, to the ponds might help prelaying grebes to maximise their net energy gain (McParland and Paszkowski 2006).…”

“…within biomass density ranges cited for carp in natural systems (Crivelli 1983). Carp at the lower end of the size range stocked could be preyed on by adult grebes (Kloskowski 2012). …”

“…Data for conventional dietary analyses were collected from a larger area of ponds where grebe breeding performance was monitored (see Kloskowski 2012). Owing to expected inter-site differences in baseline d 13 C and d 15 N values, we checked for isotopic variation at the base of the food web using snails Lymnea stagnatilis, long-living primary consumers effective at representing isotopic signatures of organisms occupying low positions in littoral food webs (Post et al 2000;Quevedo et al 2009).…”