Managing the Impact of Recovering Mobile Predators on Endangered Birds Through Prey Switching: A Field Experiment in a Habitat Restoration Landscape i

The restoration of ecosystems is being implemented in large areas in Scotland with the goal of reinstating lost biodiversity and ecosystem functions. The Cairngorms Connect partnership aims to restore natural habitats including Caledonian pine forests over nearly 600km2. Within this area, the density of over-abundant red deer is being reduced through culling, habitat is recovering, persecution of predators does not occur, and predator control is extremely limited. As a result, populations of hitherto persecuted predators are recovering.

While tree regeneration is spectacular in some glens, we lack any ability to predict how restoration will alter ecosystem processes. Specifically, there is much uncertainty about how iconic native species such as capercaillie and black grouse, which have been in decline and are the focus of significant conservation efforts, will fare in landscapes with a fuller complement of native predators. This is a conservation dilemma requiring evidence.

A corollary of the drive to reduce deer grazing pressure is that a substantial amount of deer grallochs (the viscera of culled deer) is discarded at the shooting locations and thus available as artificially introduced food to vertebrate predators, almost year round. We hypothesise this external input of resources allows predators to buffer variation in field vole density and maintain higher densities than in an unmanaged ecosystem.

The conservation dilemma is underpinned by the functional, aggregative and numerical responses of a guild of varyingly mobile, vertebrate generalist predators able to exploit different prey with varying degrees of profitability. Some of these prey experience multi-annual cycles (field voles and red grouse) and their distributions each follow distinct gradients from headwaters to valley floors.
In this project, we will evaluate a management intervention that involves

1. Removing grallochs to reduce predator density in key areas when resources dictate territory size.
2. Provisioning of carcasses and grallochs at regularly-spaced food dumps during the eight-week nesting and brood-rearing season to act as diversionary feeding for predators exploiting the area occupied by endangered ground-nesting birds;
The premise of this application is that the eggs and chicks of capercaillie and black grouse, by virtue of being ephemeral, are trophically trivial resources to predators, such that the frequency of spillover predation they experience reflects variation in abundance of other more important prey.

Our objective is to establish where in relation to the spatial distribution of prey species, and when in relation to the multi-cycles of abundance of other prey species, it is most efficient to manipulate predator food subsides. This requires empirical and modelling studies of the functional and numerical response of the predator community to landscape level changes in resource availability.
Capercaillie and/or black grouse population parameters, the main response variables, will be measured using various field surveys and genetic capture-recapture surveys, with DNA extracted from faeces and feather collected according to a structured sampling design, and analysed using spatial capture-recapture models. Predator and prey density will be estimated using standard field methods and trail cameras.

The research is important because it seeks to evaluate a management option that would make it possible to reconcile habitat restoration with priority species conservation and sheds light on how mobile predators exploit alternative prey with cyclical/transient dynamics in heterogeneous landscapes. It is impactful because it seeks to achieve this in a landscape under active restoration by a multi-institutional partnership in an iconic ecosystem of Scotland.