Individual study: Effects of fire management practices on butterfly diversity in riparian forest, Yosemite Valley, Yosemite National Park, California, 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 riparian forest in the Pacific southwest USA, are investigated.
Study site: The study was undertaken around Yosemite Valley's 'northern rim' in Yosemite National Park, California. The area is dominated by mid- to late-seral stage firs and pines. It has a Mediterranean climate, with warm dry summers, cool moist winters, and annual precipitation of 190 cm (at 1,675 m). The study was conducted in the summer of 1999.
Transects: Transects were placed on each of four riparian burn sites (two treated in 1986, one in 1988, and one in 1998) and five unburnt riparian control sites. Each riparian site was on a perennial creek bordered by 1 – 3 m of riparian vegetation. Riparian burn sites ranged in elevation from 1,900 to 1,950 m and riparian unburnt sites ranged from 1,400 to 2,450 m. Most sites were near breaks in the forest (i.e. sunny or shaded roads, fire sites, meadows), which are potential sources of butterflies. Sites were dominated by Jeffrey pine Pinus jeffreyi or lodgepole pine P.contorta; one unburnt site was dominated by red fir Abies magnifica; and some sites also had significant white fir A.concolor or incense cedar Calocedrus decurrens components.
From 24 June to 25 August 1999, 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 within 10 m either side of the transect were recorded and each 30 m section was walked for 5 mins. Each transect was sampled five times.
In absolute terms, burnt riparian forest held more individuals and butterfly species than unburnt riparian forest controls. A total of 703 individuals were recorded in forest burn sites, compared to 189 in forest control sites. Furthermore, forest burn sites had a total of 38 species of butterfly, whereas control sites only had 26 species (see Table 1, attached, 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 (25 species per site) than unburnt controls (10 species per site) (numbers estimated from original figure). Furthermore, diversity measured by the Shannon index was also significantly higher in forest burn sites (2.5 per site) than unburnt controls (1.7 per site) (numbers estimated from original figure).
Conclusions: Fire management through creation of fire breaks increases habitat heterogeneity. The reintroduction of fire management in riparian forest greatly benefits butterfly diversity. (See also Cases 371, 372, and 373).
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