Individual study: Effects of woodland fuel breaks on butterfly diversity in the 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 woodland fuel breaks on butterfly communities in the Pacific northwest of USA, are investigated.
Study site: Ashland Watershed Reserve in Oregan, is a late successional reserve dominated by a fir Pseudotsuga/hardwood tree community. It has a Mediterranean climate, with warm dry summers, cool moist winters, and annual precipitation of 51 cm. The study was conducted in the summer of 1998.
Transects: Transects were placed on each of five ridge-top fuel breaks and four untreated wooded ridge-tops as controls. Fuel break sites (areas with combustable material removed) ranged in elevation from 1,050 to 1,450 m and control sites ranged from 1,050 to 1,350 m. Sites were dominated by hardwoods, especially Pacific madrone Arbutus menziesii, or Douglas fir, and some also had significant pine Pinus spp. components. Fuel break understories were dominated by grasses, forbs, or woody shrubs. All sites were near breaks in the forest (i.e., sunny or shaded roads, or fuel breaks), which are potential sources of butterflies. In general, fuel break controls were more densely forested with less understory cover than controls.
From 28 June to 28 August 1998, transects were walked in a 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, ridge-top fuel breaks held more individuals and butterfly species than wooded ridge-top controls. A total of 635 individuals were recorded in fuel breaks, compared to just 20 in control sites. Furthermore, fuel breaks had a total of 34 species of butterfly, whereas control sites only had four species (see Table 1 for breakdown to family level).
These results were mirrored at the site level. The mean number of species per site (i.e. species richness) was significantly higher in forest burn sites (16.5 species per site) than unburnt controls (2.0 species per site) (numbers estimated from original figure). Furthermore, diversity measured by the Shannon index was also significantly higher in forest burn sites (2.1 per site) than unburnt controls (0.3 per site) (numbers estimated from original figure).
Conclusions: Fire management through creation of fire breaks increases habitat heterogeneity. The managed fuel breaks reveal a dramatically higher butterfly diversity compared to untouched ridge-top controls. (see also Cases 371, 372 and 374).
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