Do released pheasants affect invertebrates in woodland?

Summary

  • Pheasants are released into woodland pens across the UK lowlands. The potential impact of this on invertebrate populations in those woods is not fully understood.
  • Two recent studies have looked at ground invertebrates collected or sampled inside release pens throughout the summer, which together help answer some of these questions.
  • One of these studies also looked for a possible effect of releases on invertebrates outside but near to the release pen.
  • Lower numbers of certain invertebrate groups were found inside the release pens after birds were released in both papers. The second paper suggested an effect outside release pens two months after release as well. Whether this was due to impacts from gamebirds and their management, or from the sampling method used is not clear.
  • The relationship between pheasants and invertebrates is complex, and more studies are needed to understand whether the released gamebirds affect invertebrate populations near to but not inside release pens. There is no evidence that releases effect invertebrates in areas well away from release sites.

Background

Pheasant in woodland (www.davidmasonimages.com)Gamebird releasing is widespread across the UK’s lowlands, and both the presence of and management for these birds in the landscape affects our countryside in many ways. We know, for example, that pheasant releasing is one of the main reasons that landowners: 

  • retain, maintain and plant woodland
  • keep and plant hedges
  • plant game crops

These activities add much-needed diversity to the farmed environment, as well as providing seed and cover for birds and other wildlife as well as the gamebirds themselves. However, we also know, for example, that within pheasant release pens, the birds affect the profile of plants that grow there1,2.

There are important questions about other aspects of the effects of gamebird management, for example whether the release of high numbers of pheasants can help support high local predator populations. Overall, the evidence base suggests that when carried out according to best practice guidelines, gamebird releasing and management can be a positive force in the countryside, but there are still gaps in our knowledge and ongoing research helps us to explore these.

Pheasants can and do eat insects and other invertebrates such as spiders, although birds at the age at which they are released predominantly eat plant matter (chicks less than 7 weeks old are most likely to eat invertebrates3). Despite this, with high numbers in a small area, released pheasants could affect invertebrate populations either directly through eating them, or indirectly by affecting the local environment.

There have been two recent studies that have looked at this. Both looked at ground invertebrates (as opposed to flying insects or those which live in trees) inside release pens, while the more recent paper looked at the effect of pheasants on invertebrates just outside the release pens as well. This summary explores both of those papers, which help us understand more about the potential effect of pheasants on woodland invertebrates.

What they did

Pitfall TrapThe scientists used pitfall traps to sample invertebrates in both studies at a range of sites across southern England (see picture).

In the first study, published in 2015, pitfall traps were used to study ground invertebrate movement at pairs of release sites in game woodland inside and outside release pens, and in non-game woods. The scientists compared invertebrate samples from 10 pitfall traps at each of 37 sites in the central area of the release pen with woodland pitfall traps around 300m away. All sites were studied in the spring, in May/June of 2006 and 2008, and a subset of sites were revisited in October 2008 after pheasants were released4.

The second study, published in 20215, was carried out over two years, 2016 and 2017, and looked at 16 pairs of release pens each year. The pitfall traps were placed either inside the pen, or 25m away in the nearby woodland at three time points over the summer:

  • Just prior to release, before pheasants are present in the woodland
  • Four weeks after release
  • Nine weeks after release

Traps were left for approximately seven days to collect ground-moving invertebrates. Creatures that fall into the traps are preserved in a sample solution in the bottom for later identification and analysis.

The results were compared to understand whether there were differences between the invertebrates collected at different time points and at the different sites, which may have been due to the pheasants impacting woodland invertebrates.

What they found

The 2015 paper found that there was no difference in overall invertebrate abundance between the pens and woodland sites, but release pens had a different community of ground beetles with fewer large woodland carabid beetles and more common species that are characteristic of arable and grass fields. The number of different beetle species was not different, but the proportions were altered in release pens. There were also more detritivores such as snails in the release pens at sites that released more than 1,000 birds per ha4.

The clearest finding from the 2021 paper is that there were more invertebrates in the early summer before the pheasants were released at all sites, inside and outside the pens. This is an expected seasonal effect – at different times of year, populations fluctuate. It also found that four weeks after pheasant release, both the total number and total biomass of invertebrates caught was slightly lower inside the pens than outside5. This broadly agrees with Neumann’s results from the earlier study.

Then, nine weeks after pheasant release, their results show that the number of invertebrates caught in pitfall traps was lower again, more so outside the release pens than inside. When they looked in more detail at the different groups of invertebrates that were trapped – slugs, beetles, spiders or detritivores – no differences were found comparing inside to outside the release pens at any time throughout the year.

No differences were found when comparing areas with higher or lower densities of released pheasants, nor when looking at pheasants reared in an enhanced environment compared to the standard rearing system.

What does this mean?

Pheasant release penThe explanation offered for the main findings from both studies is that pheasants inside release pens reduce the numbers of some invertebrate groups after the pheasants are released, either directly by predation or indirectly by changing the habitat. The 2021 paper also suggests that as the pheasants gradually disperse from the pen to the surrounding area, they begin to affect insects outside the pen as well and numbers there are then lower than those inside the pen.

However, while the overall result that certain invertebrate groups within pens are supressed in the weeks after release seems clear, the second result outside the pens is less so. Even nine weeks after release, pheasant numbers within the pen will usually remain similar to or higher than 25m outside the pen, so the birds would likely still be affecting the pen itself more than the surrounding area at this time. Furthermore, some groups of insects are known to be more sensitive to the effect of pheasants that others, so it is sensible when studying these issues to look more closely at such groups, as they are likely to tell us more information about pheasant effects. When the scientists looked at these sensitive groups, the effect was not seen.

Pitfall trapping is a very useful method in some situations, but it is important to remember that it is a measure of insect activity or movement, rather than just the number of insects present. If insects travel around nearby, they may fall into the sample chamber, but if don’t, they won’t.

Because of this, pitfall trapping is susceptible to variations in the local environment, for example being heavily affected by the roughness of the ground surface, the vegetation profile around the trap and so on. The reason this is important is that we know that released pheasants can reduce ground vegetation and increase the amount of bare soil in the release pen compared to the areas outside the pen – therefore the presence of pheasants may affect the invertebrates themselves but will also affect the likelihood of them being caught, therefore impacting our ability to measure this effect.

Inside the pens in the most recent study, nine weeks after the release, this effect was likely to be at its peak, with ground insects moving around more and hence more likely to get caught in pitfall traps. This might have been the reason fewer insects were caught in pitfall traps 25m from the pen. It is important to bear this in mind when interpreting the results. In the only other study of this kind, a PhD study from 2009 looked at invertebrate populations away from release pens, at the wood-edge, and found no consistent differences in numbers between sites that did or did not have releasing, or when they compared before the release with later on in the year6.

In summary, we know from both of these papers that pheasants affect some invertebrate groups inside release pens, either directly or by changing the habitat. One paper also looked at effects close to but outside the release pen but the results that were found don’t fit with pheasant distribution at that time and may have been an effect of the methods used.

As concluded by the authors of both papers, these studies serve to emphasise once more that any effects pheasants have on invertebrate communities at release sites in woodlands are complex. More research, which should both be longer-term, focus on specific groups of insects, and use a different method of sampling, is needed to understand better what effects there may be that we are not yet seeing.

Read the original papers

Neumann, J.L., Holloway, G.J., Sage, R.B., & Hoodless, A.N. (2015). Releasing of pheasants for shooting in the UK alters woodland invertebrate communities. Biological Conservation, 191: 50-59.

Hall, A., Sage, R. A., & Madden, J. R. (2021). The effects of released pheasants on invertebrate populations in and around woodland release sites. Ecology and Evolution, 11, 13559-13569.

References

  1. Sage, R.B., Ludolf, C., and Robertson, P.A. 2005. The ground flora of ancient semi-natural woodlands in pheasant release pens in England. Biological Conservation, 122: 243–252.
  2. Capstick, L., Sage, R., and Hoodless, A. 2019. Ground flora recovery in disused pheasant pens is limited and affected by pheasant release density. Biological Conservation, 231: 181–188.
  3. Beer, J.V. 1988. Nutrient requirements of gamebirds. In: Recent Developments in Poultry Nutrition:  195–203.(eds. Cole, D.J.A. & Haresign, W.) Butterworths. London.
  4. Neumann, J.L., Holloway, G.J., Sage, R.B., et al. 2015. Releasing of pheasants for shooting in the UK alters woodland invertebrate communities. Biological Conservation, 191: 50–59.
  5. Hall, A., Sage, R.A., and Madden, J.R. 2021. The effects of released pheasants on invertebrate populations in and around woodland release sites. Ecology and Evolution, 11: 13559–13569.
  6. Pressland, C. 2009. The impact of releasing pheasants for shooting on invertebrates in British woodlands. PhD thesis, University of Bristol.

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