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Individual study: Effects of fire management practices on forest butterfly diversity in Yosemite Valley, Yosemite National Park, California, USA

Published source details

Huntzinger M. (2003) Effects of fire management practices on butterfly diversity in the forested western United States. Biological Conservation, 113, 1-12

Summary

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 southwest USA, are investigated.

Study site: The study was undertaken in Yosemite Valley's 'northern rim' in Yosemite National Park in the western Sierra Nevada, California. It 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: One transect was placed on each of five prescribed burn sites, one site burned per year in 1980, 1986, 1988, 1989, and 1998, and each of seven unburnt control sites. Burnt sites ranged in elevation from 1,850 to 2,150 m and unburnt sites ranged from 1750 to 2450 m. All sites were on slopes facing between south-east and south, or were flat, and all 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 red fir, 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). 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 forest held more individuals and butterfly species than unburnt forest controls. A total of 300 individuals were recorded in forest burn sites, compared to 190 in forest control sites. Furthermore, forest burn sites had a total of 34 species of butterflies, whereas control sites only had 20 species (see Table 1 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 (14 species per site) than unburnt controls (7 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 (1.3 per site) (numbers estimated from original figure). Lastly, of the fifteen 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 371, 373 and 374).


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