Hedgehogs: recent evidence of their impacts on native fauna
It seems the more we look, the more we find when it comes to the impacts of invasive pests. European hedgehogs are no exception. Once thought to provide a service by preying on garden pests such as slugs and snails, hedgehogs are now known to also prey on a wide variety of native species, including invertebrates, lizards, and the eggs and chicks of a range of native birds. We have learnt this by sorting through the remains of prey in their droppings and stomachs.
For example, 21% of hedgehog guts (each reflecting a single night’s feeding) from Macraes Flat, north Otago, contained native skink remains; a single hedgehog dropping from near Alexandra contained 10 McCann’s skink feet; and two separate studies have shown that female hedgehogs are three times more likely than males to have eaten native lizards. Rare native invertebrates are also eaten widely, and a single hedgehog gut from the central South Island was found to contain 283 wētā legs!
Diet composition is one thing, but the real impacts on native species are often more difficult to measure. Research over the past 15 years has begun to clarify the picture.
Impacts on invertebrates and lizards
A field experiment undertaken by Chris Jones and colleagues in 2005/2006 in Otago enclosed various numbers of hedgehogs in six 0.5-hectare fenced areas containing naturally occurring populations of native ground wētā and McCann’s skinks. The team found increasing rates of decline of the populations of both wētā and juvenile McCann’s skinks as the density of hedgehogs rose from just under one per hectare to nine per hectare (see Figure).
Proportional changes in the number of ground wētā (left) and juvenile McCann’s skinks before and after exposure to a range of hedgehog densities.
Impacts on birds breeding in braided riverbeds
Video surveillance by Mark Sanders and Richard Maloney of the Department of Conservation (DOC) of 164 clutches of native shorebirds (mainly nationally vulnerable banded dotterel, but also nationally endangered black-fronted tern and nationally critical black stilt), located largely on one riverbed in the Mackenzie Basin, showed that 9% of all clutches were preyed on by hedgehogs. These losses were similar to those caused by more frequently considered predators (9% by ferrets, 15% by cats and 2% by stoats).
Research by Grant Norbury and colleagues found far greater rates of hedgehog predation of clutches in the upper and lower Tekapō riverbeds in the Mackenzie Basin. Video surveillance of 198 nests of mostly banded dotterels (but also of nationally vulnerable wrybills and declining South Island pied oystercatchers) across four riverbed sites revealed predation rates by hedgehogs as high as 51%, higher than those of all other vertebrate predators combined (see Table). These rates are a big blow to the recruitment of shorebird populations from this area.
|Percentage of nests preyed on by predators at each site|
|River||No. nests monitored||Hedgehogs||Ferrets||Cats||Stoats|
Trapping programmes by professional and community pest managers across New Zealand generally regard hedgehogs as annoying ‘by-catch’ that clog up traps set to catch other predator species. However, our increasing awareness of their impacts on a wide range of native wildlife means they are now targeted in many pest control programmes as one of the core suite of pests. Despite this, we still have no robust estimates of hedgehog densities in any New Zealand habitats. Trapping data suggest they are remarkably abundant. For example, of 3,636 introduced predators trapped by DOC at Macraes Flat over 3 years, an astounding 63% were hedgehogs. This pattern is repeated in trapping data from across the country.
Hedgehogs may not only be a significant threat to native wildlife; they may also play a part in transmitting Johne’s disease, an inflammatory gut infection in farmed stock. Research by Graham Nugent and colleagues detected the bacterium responsible for the disease in 36% of hedgehogs from three South Island farms. It was also found in their droppings, suggesting that hedgehogs may act as wildlife vectors of the disease.
When hedgehogs were introduced to New Zealand they encountered an environment with plentiful food, few competitors and still fewer predators. The only constraints on population growth were and remain climatic: cold winters drive them to hibernate, restricting the length of their breeding season and leading to losses of their young (who fail to make it through to the next spring). In the milder more benign environments of the country these constraints may not operate. It is also possible that they produce two litters each year in warmer northern areas , as they do in some parts of their native Europe.
Although more research is required, there is a risk that hedgehogs may be having a far greater impact on our native biodiversity than has been realised, given their likely abundance and their potential to eat large numbers of small native animals in a short time. What we do know is that managers need to re-think what they regard as by-catch, and target hedgehogs where they are suspected of preying on the eggs or nestlings of endangered shorebirds or on other native animals. Removing hedgehogs potentially has significant biodiversity benefits.
This work was funded by the Ministry of Business, Innovation and Employment.
For further information on hedgehog ecology and biodiversity impacts, contact Chris Jones: firstname.lastname@example.org 03 321 9869