Individual study: Effects of fire management practices on forest butterfly diversity, Ashland Watershed Reserve, Rogue River National Forest, Oregon, USA
Huntzinger M. (2003) Effects of fire management practices on butterfly diversity in the forested western United States. Biological Conservation, 113, 1-12
Forests of the western USA historically had frequent fires. Since the early 1900's, however, fire-suppression policies have caused changes in community compositions from fire-tolerant (e.g. ponderosa pine Pinus ponderosa, Douglas Fir Pseudotsuga menziesii, ceanothus Ceanothus spp., and manzanita Arctostaphylos spp.) to fire-intolerant (e.g. white fir Abies concolor) communities, and have increased tree densities and fuel loads. This has dramatically increased the chance of severe fire and, therefore, emergency programs of prescribed burning have been initiated. In this study, the effects of burning on butterfly communities in the Pacific northwest USA, are investigated.
Study site: Ashland Watershed Reserve in the eastern Siskiyou Mountains, Oregon (northwestern USA), is a late successional reserve dominated by a fir Pseudotsuga/hardwood plant community. It has a Mediterranean climate, with warm dry summers, cool moist winters, and annual precipitation of 51 cm. The study investigating the effects of burning on butterfly communities was conducted in the summer of 1998.
Prescribed burning: Transects were created on five prescribed burn sites and five unburnt control sites. One site was burned per year in 1991, 1993, 1995, 1996, and 1997, and the control sites had not been burned since at least 1978. Transects ranged in elevation from 900 to 1,450 m and were on slopes facing between east and south, for both the prescribed burn and control sites. All sites were dominated by Douglas fir Pseudotsuga menziesii, some also having hardwood/scrub components, and were near breaks in the forest (i.e. shaded roads, stand-replacing fire sites, fuel breaks), which are potential sources of butterflies.
From 28 June to 28 August 1998, transects were walked in regular rotation during conditions of peak butterfly activity (clear days between 10:00 and 15:00 hrs). Each transect consisted of eight connected 30 m sections that followed the contour of the mountainside. Butterflies with 10 m of either side of the transect were recorded, with each section walked for 5 mins. Each transect was sampled six times.
In absolute terms, burnt forest held more individuals and more butterfly species than unburnt forest controls. A total of 325 individuals were recorded in forest burn sites, compared to 56 in forest control sites. Furthermore, forest burn sites had a total of 24 species of butterfly, whereas control sites only had 13 species (see Table 1, attached, for breakdown to family level).
These results were mirrored at the site level. The average number of species per site (i.e. species richness) was significantly higher in forest burn sites (11.5 species per site) than unburnt controls (4.5 species per site) (numbers estimated from original figure). Furthermore, diversity measured by the Shannon index was also significantly higher in forest burn sites (1.4 per site) than unburnt controls (0.8 per site) (numbers estimated from original figure). Lastly, of the eleven species that occurred in both burnt and unburnt sites, the total abundance of these species was significantly higher in the burnt forest.
Conclusions: Fire management increases habitat heterogeneity. The reintroduction of fire management in forests greatly benefits butterfly diversity. (see also Cases 372, 373 and 374).
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