In shallow water ecosytems the abiotic environment is more important than prey abundance for foraging terns
Shallow freshwater aquatic ecosystems are discrete feeding patches for fish eating birds. A unique feature of these ecosystems is that their physical conditions can change dramatically in a short period of time, particularly temperature, turbidity, and dissolved oxygen. Based on previous research we predicted that increasing turbidity will reduce the availability of fish to birds due to reduced visibility, while increasing temperature and decreasing dissolved oxygen will increase their availability through increases in activity and movement towards the more oxygenated surface areas, respectively. We also predicted that overall abundance of fish should increase feeding activity by terns. We measured these environmental variables, bird activity, and fish abundance from May to August from 2006 to 2008 in a marsh in southern Manitoba, Canada. Our results showed that only variation in dissolved oxygen levels affected feeding activity by terns. Since there was no relationship between bird and fish abundance either within or among years, these results suggest that it is the availability of prey (i.e. the upward movement of fish into the water column) and not their abundance per se that influences the number of avian predators present and hence the risk of predation to fish. These data demonstrate how the physical environment of aquatic ecosystems can impact terrestrial avian predators, and the link that exists between the physical environment and predator-prey interactions.
KeywordsPredator-prey interactions Dissolved oxygen Avian predators Interannual variation
Funding for this research was provided by research grants to MA from NSERC and an internal grant from the University of Manitoba. MP was supported by an NSERG postgraduate scholarship. We are grateful to the assistance provided by the staff of the University of Manitoba’s Field Station at Delta Marsh.
Compliance with ethical standards
The research described in this manuscript was reviewed by the University of Manitoba’s Animal Care Committee and determined to conform to the ethical requirements of the Canadian Council on Animal Care (Protocol Number F03–041/1/2/3).
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