Camera traps reveal a hare-raising situation

Author Anthony Caravaggi describes new research that uses camera traps to explore the displacement of endemic Irish hares by introduced European hares.

Many ecologists consider the time spent collecting data in the field to be one of the most enjoyable aspects of their work, as indeed do I. Although manual wildlife surveys can be extremely useful, they are not without their problems. For example, they often require a substantial investment of time, the presence of the observer will almost certainly influence the subject, and, to quote Francis Bacon (albeit in a different context), “nature is often hidden”.

Camera traps provide a way around these issues as they require little time to install, have the potential to gather data that may not otherwise be easily collected, and they mitigate against the observer effect (though some human scent likely remains, and some models do make some noise or emit visible light).

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Ireland is home to both the Irish hare, an endemic subspecies of mountain hare which is of conservation concern, and the European hare, which was introduced for field sports. We know from previous studies that the European hare likely poses a threat to the Irish hare as the two species prefer similar habitats, there is a high degree of hybridisation, and the range of the European hare is expanding.

In our recent paper camera-trap-based Random Encounter Models (REM) were used to calculate population densities to investigate the potential replacement of the Irish hare by the European hare, with transect-based Distance sampling (a commonly-used method in hare surveys) for corroboration. This is the first time these models had been applied to hares.

Distance sampling was logistically straightforward: drive slowly along roads (we covered nearly 700km in all), stop when a hare is detected, and record the relevant data. In contrast, camera trap surveys required a little more consideration, not only because we needed the permission of a large number of landowners to work on their land, as well as contending with occasionally rambunctious livestock, but also, and most critically, to ensure that we met the assumptions of the models. Violating these assumptions would mean that our density estimates would be inaccurate.

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Fortunately, while we had to make some concessions due to the nature of the landscape, we were able to meet the assumptions and calculate reasonable density estimates. We also had to invent a method of extracting relevant data (the distance and angle to each animal) from camera trap captures. Previous REM studies captured still images and extracted data while on-site. We chose to capture video footage, making on-site extraction for each of our 20 cameras impractical. Happily, our rough-and-ready method of using a cane grid and computer-based image manipulation proved to be extremely effective.

Our results showed that the Irish hare has been displaced from much of the core invasive range, where the European hare now occurs in high densities. In this area, the native species now only occurs in the uplands where the habitat and food do not favour its non-native cousin. The species were found to co-occur elsewhere in the invasive European hare range.

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Camera trap photo of a hare

It seems likely, given the composition of the landscape, the example of the core range, and lessons from Scandinavia where the European hare has displaced the native mountain hare from much of southern Sweden, that the non-native species will continue to replace the native.

We concluded that the Random Encounter Model is certainly a useful conservation tool for monitoring native and non-native species, providing local-scale population density estimates which can be used to inform management and future monitoring efforts.

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