Action: Exclude or control ‘reservoir species’ to reduce parasite burdens
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- A literature review found no compelling evidence that culling mountain hares Lepus timidus (a carrier of the ticks that carry louping ill virus) increased red grouse Lagopus lagopus scoticus populations.
- A controlled before-and-after study from the UK did find that there was a significant increase in chick production on grouse moors with hare culling, compared to control sites but no change in population density. A comment on this paper argued that the controls used in it were not adequate.
If a bird population is threatened by a parasite or disease that can infect multiple species, it may not be enough to treat the population, because of the risk of constant re-infection from the other host, which can act as a ‘reservoir species’. Instead it may be necessary to treat the other hosts or to exclude them from particular habitats or control their numbers.
While we found studies describing the impact of controlling reservoir species through culling, we found no intervention-based studies describing the impact of excluding reservoir species from an area. However, a correlative study in Scotland in 2007 found that areas of forest that were fenced to exclude deer had fewer ticks (carriers of louping ill virus which attacks red grouse Lagopus lagopus scoticus) than unfenced areas, but that areas of adjacent moorland (where the grouse were actually found) were not affected (Ruiz-Fons & Gilbert, 2010).
Ruiz-Fons, F. & Gilbert, L. (2010) The role of deer as vehicles to move ticks, Ixodes ricinus, between contrasting habitats, International Journal for Parasitology, 40, 1013–1020.
Supporting evidence from individual studies
A controlled before-and-after study in the Scottish Highlands between 1993 and 2001 (Laurenson et al. 2003) found that there was no significant increase in the population density of red grouse Lagopus lagopus scoticus at a site with mountain hare Lepus timidus (a carrier of the ticks that carry louping ill virus) culling, compared to a control site without hare culling (approximately 25 grouse/km2 in 1993 and 100/km2 in 2001 at the experimental site vs. 140/km2 and 275/km2 at the control). However, there was a significant increase in the number of chicks produced/female at the treatment site, compared to the control (approximately 1.2 chicks/female in 1991 and 5 in 2001 at the experimental site vs. 3.5 and 3.0 at the control) and a significant reduction of louping ill virus at the treatment site, compared to a second control site. Hare densities were reduced from 8/km2 in 1993 to 0 in 1998. A comment on this paper in 2004 (Cope et al. 2004) argues that the control sites were not adequate, as they differed in either the pre-existing incidence of louping ill virus or in various environmental conditions.
Cope, D. R., Iason, G. R. & Gordon, I. J. (2004) Disease reservoirs in complex systems: a comment on recent work by Laurenson et al. Journal of Animal Ecology, 73, 807–810
A 2010 literature review (Harrison et al. 2010) found ‘no compelling evidence’ that culling mountain hares Lepus timidus (a carrier of the ticks that carry louping ill virus) increased red grouse Lagopus lagopus scoticus populations. The authors note that there is some evidence for an effect of culling on the prevalence of louping ill virus (e.g. in Laureson et al. 2003) but that evidence for population-level effects is uncertain, partly due to a lack of understanding of the population dynamics of both hares and grouse.
- Laurenson M.K., Norman R.A., Gilbert L., Reid H.W. & Hudson P.J. (2003) Identifying disease reservoirs in complex systems: mountain hares as reservoirs of ticks and louping‐ill virus, pathogens of red grouse. Journal of Animal Ecology, 72, 177-185
- Harrison A., Newey S., Gilbert L. & Thirgood S. (2010) Culling wildlife hosts to control disease: mountain hares, red grouse and louping ill virus. Journal of Animal Ecology, 47, 926-930