Quantification of piscivorous predator presence in a beaver Castor fiber modified freshwater ecosystem

Abstract

1. By modifying the physical environment, ecosystem engineers influence a wide variety of ecological processes, affecting other species both directly (e.g., providing sites for reproduction, hibernation, and refuge) and indirectly (e.g., mediating availability of resources by altering food web dynamics). The beaver (Eurasian, Castor fiber, and North American, C. canadensis) provides a textbook example of an ecosystem engineer, restructuring the physical fluvial and riparian environment by constructing dams, bankside burrows, lodges, and canals.

2. The indirect secondary consequences of beaver activity for other species can be difficult to determine. Beaver ponds can benefit fish, such as through the provision of refuge from predators due to increased depth and instream woody structure. Conversely, the formation of deep pools may create habitat for large predatory fish and suitable foraging sites for piscivorous mammalian and avian predators. As a result, the relative costs and benefits for specific life-stages and species can be difficult to disentangle.

3. Focusing predominantly on Grey Heron (Ardea cinerea) and brown trout (Salmo trutta) as the predator–prey model, this study explored how predation pressure differed between four study reaches situated on two co-located streams connected to a common loch in Northern Scotland.

4. One of the two inflowing streams was (i) modified by Eurasian beaver activity. There were also three unmodified (control) reaches comprising: (ii) the second inflowing stream, (iii) a reach above the modified inflowing section, where the influence of dams was deemed to have ceased, and (iv) the stream flowing out of the loch. Trail cameras monitored how predator presence differed spatially (between the control and modified reaches) and temporally (between seasons). Heron abundance differed both spatially and temporally, being: (a) greater at the beaver modified than control sites; (b) positively correlated with 1+ age group trout (but not when all age groups were aggregated); (c) higher in deeper pool habitat; and (d) greater in the autumn and spring in the beaver modified and control sites, respectively.

5. Habitat modification by beaver can have complex secondary indirect consequences for predator–prey dynamics that should be accounted for in their conservation and management.

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Identifiers

ORCID for Robert J. Needham: orcid.org/0000-0003-4639-6031

ORCID for Paul S. Kemp: orcid.org/0000-0003-4470-0589

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