Fish may significantly affect habitat use by birds, either as their prey or as competitors. Fish communities are often distinctly size-structured, but the consequences for waterbird assemblages remain poorly understood. We examined the effects of size structure of common carp (Cyprinus carpio) cohorts together with other biotic and abiotic pond characteristics on the distribution of breeding waterbirds in a seminatural system of monocultured ponds, where three fish age classes were separately stocked. Fish age corresponded to a distinct fish size gradient. Fish age and total biomass, macroinvertebrate and amphibian abundance, and emergent vegetation best explained the differences in bird density between ponds. Abundance of animal prey other than fish (aquatic macroinvertebrates and larval amphibians) decreased with increasing carp age in the ponds. Densities of ducks and smaller grebes were strongly negatively associated with fish age/size gradient. The largest of the grebes, the piscivorous great crested grebe (Podiceps cristatus), was the only species that preferred ponds with medium-sized fish and was positively associated with total fish biomass. Habitat selection by bitterns and most rallids was instead strongly influenced by the relative amount of emergent vegetation cover in the ponds. Our results show that fish size structure may be an important cue for breeding habitat choice and a factor affording an opportunity for niche diversification in avian communities.
Aim: Theory predicts that the evolutionary adaptations of prey to reduce predator pressure often fail in confrontation with non-native predators; thus, their predation usually leads to sharp declines of prey populations. However, over time, prey can develop anti-predator adaptations, reduce predator impact and recover its population. We analyse the numerical response of multiple prey species to the impact of a non-native predator on a large spatiotemporal scale. Location: Poland.Methods: Long-term population dynamics of 13 waterbird species breeding in 123 sites were analysed with reference to the present and historical distribution of mink over 40 years. We rescaled the temporal waterbird dynamics to the time elapsed since the mink colonized each site. Results:The steepest waterbird declines were recorded up to 15 years after establishment of the mink population, and in that period, waterbird abundance averaged across the 13 studied species declined by 50% but certain species showed different patterns: the abundance of eight species declined linearly or nonlinearly, whereas the breeding populations of five other species did not change significantly. Over time, the abundance of some declining species increased slightly and tended to stabilize at a lower level. The temporal pattern of waterbird dynamics averaged across species correlated closely with temporal changes of mink density with a 4-year time-lag.A projection of the observed patterns shows that waterbird decline following the expansion of mink has the shape of a wave travelling from the north to the south of Poland. Main conclusions:We conclude that the impact of the invasive American mink should be considered an important factor affecting waterbird populations at a large geographical scale. The waterbird vulnerability to mink predation is species-specific, and over time the negative impact grows weak as mink population densities decline. K E Y W O R D SAmerican mink, biological invasion, Neovison vison, population dynamics, predation, spatiotemporal scale | 139 BRZEZIŃSKI Et al.
Common carp, a fish species blamed for profound adverse impacts on invaded freshwater ecosystems, can reach a large body size. We investigated the effects of carp size on water quality and on abundance of submerged macrophytes, zooplankton, and benthic macroinvertebrates in drainable ponds. Each pond was stocked with one of three age‐classes of carp (young‐of‐the‐year, 1‐year‐, and 2‐year‐old fish) corresponding to specific size ranges (classified as small, medium, and large carp). Water turbidity and chlorophyll a concentrations increased over time in the presence of all sizes of carp; ponds containing medium‐sized fish tended to be most turbid. Submerged macrophyte biomass was not significantly affected by fish size. Neither total zooplankton nor macrobenthos biomass differed between ponds stocked with different‐sized carp, but fish size was significant in determining the size composition of these communities. Copepods and small‐bodied cladocerans were least abundant in the presence of small carp, while large cladoceran grazers were most suppressed in ponds with medium‐sized fish. Among dipterans (primarily chironomids), which dominated the macrobenthos, densities of small larvae did not differ across fish sizes, while large larvae were more abundant in ponds stocked with small carp than in those with larger fish. Our results highlight the importance of fish body size in structuring aquatic communities, although the effects need not be linearly related to fish size. We caution against considering the effects of omnivorous fish on lower trophic levels to be unstructured and extrapolating the experimental results from one size range (developmental stage) to others.
Between-habitat distributions of prey shared by multiple predators depend on habitat use by the predators, whose own distributions may interact. We used a large-scale, whole-system natural experiment to examine distributions of anuran tadpoles and insect predators between pond microhabitats with contrasting complexity (open water vs emergent littoral vegetation) in drainable ponds that were either kept fishless or stocked with fish. Total relative densities of insect predators did not significantly differ with respect to the fish status of the ponds. Individual anuran taxa responded variously, but only fish-tolerant Bufo bufo densities were higher in the presence of fish. The densities of both insect predators and tadpoles showed positive interactions between fish presence and the use of complex littoral habitat. The habitat shift to littoral vegetation could be indirectly amplified by fish adverse impact on submerged macrophytes, the main structured microhabitat in open-water areas. Irrespective of mechanisms of fish effects (direct consumption, behavioral deterrence or alteration of habitat conditions), aggregation of both tadpoles and insect predators in littoral vegetation may put tadpoles at greater risk of predation by insects, a potentially important factor of amphibian mortality in waters containing fish.
Size relationships between fish and organisms from adjacent trophic levels are crucial for predicting the structure and dynamics of aquatic ecosystems. We compared macroinvertebrate communities along a fish-size gradient created by separate stocking of three age cohorts of common carp Cyprinus carpio in semi-natural ponds. The specific size range of fish (small, medium and large) corresponding to fish age in ponds was the factor most strongly associated with macroinvertebrate composition. The other significant habitat variables were dissolved oxygen concentration in the water and submerged vegetation abundance in the open-water zone. Among the most numerous taxa in the ponds, relative abundances of Hirudinea, Gastropoda, Odonata and Coleoptera were larger in the presence of smallsized than of larger-sized carp. However, fish size effect was not linear, in that macroinvertebrate assemblages were less similar between ponds containing medium-vs large-sized fish than between ponds with small-vs large-sized fish. The dissimilarity patterns were mainly determined by disparities in abundance of Corixidae, which unlike other taxa common in the ponds occurred in the greatest numbers in the presence of large-sized carp. Macroinvertebrate diversity was greatest in ponds with small-sized fish and was positively related to emergent macrophyte cover. Enhancement of emergent vegetation is recommended as the most effective management strategy to buffer adverse impacts of fish on macroinvertebrates. If fish are present in the system, assessment of the size structure of fish populations can be advantageous in unravelling the essential processes driving the variation in pond communities.
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