Action: Reduce adverse habitat alterations by excluding problematic terrestrial species
- Three studies from the USA and the UK found higher numbers of certain songbird species and a higher species richness in these groups when deer were excluded from forests. Intermediate canopy-nesting species in the USA and common nightingales Luscinia macrorhynchos in the UK were the species to benefit.
- A study from Hawaii found mixed effects of grazer exclusion, with some species showing population increases, some declines and other different long- and short-term trends.
One possible response to problematic, habitat-altering species is to create exclusion areas, allowing natural habitats to regenerate away from the problematic species.
Supporting evidence from individual studies
A replicated study in four hardwood forest sites in Pennsylvania, USA, between 1980 and 1991 (deCaslesta 1994) found higher species richness and abundances of intermediate canopy-nesting songbirds in plots with lower densities of white-tailed deer Odocoileus virginianus across sixteen experimental plots (deer density of 3.7 deer/km2: averages of 17.5 individuals and 11 intermediate canopy-nesting species in each plot; 7.9 deer/km2: 16 individuals and 11.5 intermediate canopy-nesting species; 14.9 deer/km2: averages of 13 individuals and seven ICN species; 24.9 deer/km2: averages of 10.5 individuals and 7.5 intermediate canopy-nesting species). Plots were 13 or 26 ha and contained between one and four deer. There were no changes in either species richness or abundance of ground-nesting or upper canopy-nesting species. Threshold densities for songbirds not found with high deer densities (eastern wood pewee Contopus virens, indigo bunting Passerina cyanea, least flycatcher Empidonax minimus, yellow-billed cuckoo Coccyzus americanus, cerulean warbler Dendroica cerulea, eastern phoebe Sayornis phoebe and American robin Turdus migratorius) appeared to be between 7.9 and 14.9 deer/km2.
A replicated, controlled study in northern Virginia, USA (McShea & Rappole 2000) found significantly higher numbers of 16 species of ground and intermediate canopy-dwelling songbirds in four 4 ha plots of deciduous forest with deer excluded between 1990 and 1998, compared to five control plots. However, there was no corresponding increase in bird diversity as species were gained and lost as understory vegetation developed. There was also no significant difference in the number of resident birds (eight songbird species and one woodpecker) caught.
A study in two koa Acacia koa forests in northern Hawaii, USA (Camp et al. 2010), found that all seven native birds recorded in an area of open forest from which feral grazers (cows and pigs) had been excluded showed long-term population stability or growth. However, all but two showed short-term declines. In a closed forest from which grazers were excluded, only two species showed an increase, with the rest either stable or declining. Birds were monitored between 1987 and 2007 in the open forest and 1999 and 2007 in the closed forest. This study is also discussed in ‘Habitat restoration and creation – Forest restoration’.
A replicated and controlled paired study in southeast England (Holt et al. 2010) found a significantly higher density of common nightingale Luscinia megarhynchos territories (monitored in 2000-8) in a coppiced woodland from which deer were excluded between 1999 and 2003, compared to control plots protected by an easily-breached brushwood fence (0.60 territories/ha in eight exclusion plots vs. 0.04 territories/ha in eight controls). The proportion of territories in exclusion plots also increased, from 0% in 2000 to 70-80% in 2005-7. A total of 48 territories were mapped. Native roe deer Capreolus capreolus and introduced fallow deer Dama dama and Revves’ muntjac Muntiacus reevesi were excluded by erecting 1.8 m steel fences. The authors argue that differences are due to the area of optimal-age coppice (3-8 year old) within plots.
- deCalesta D.S. (1994) Effect of white-tailed deer on songbirds within managed forests in Pennsylvania. The Journal of Wildlife Management, 58, 711-718
- McShea W.J. & Rappole J.H. (2000) Managing the abundance and diversity of breeding bird populations through manipulation of deer populations. Conservation Biology, 14, 1161-1170
- Camp R.J., Pratt T.K., Gorresen P.M., Jeffrey J.J. & Woodworth B.L. (2010) Population trends of forest birds at Hakalau Forest National Wildlife Refuge, Hawai'i. The Condor, 112, 196-212
- Holt C.A., Fuller R.J. & Dolman P.M. (2010) Experimental evidence that deer browsing reduces habitat suitability for breeding common nightingales Luscinia megarhynchos. Ibis, 152, 335-346