About the authors: Dr. Breeanne Jackson is the Director of the Yosemite and Sequoia Field Stations at University of California Merced, and served previously as a Wildlife Biologist for the National Park Service at Yosemite National Park. Her research focuses on wildfire and riparian ecology. Dr. Sean Schoville is an Associate Professor of Entomology and Director for the Center of Ecology and the Environment at the University of Wisconsin-Madison. His research focuses on insect population genetics and conservation.
Wildfire plays an important role in determining species abundance, community composition and structure, and ecosystem function of forests in the Western United States. Many of these forested ecosystems are adapted to disturbance from wildfire. However, there are important knowledge gaps with respect to how fire impacts wildlife species, and in particular invertebrate species, in these forests.
Yosemite is a world-renowned destination for nature lovers, and as part of the United States National Park System, park managers have the long-term goal of preserving natural resources within the park. Humans have been using fire to promote growth of plants used for food, medicine, and materials since their arrival to Yosemite 4,000 years ago. However, the arrival of Euro-Americans in the 1850s led to fire suppression and an end to traditional burning practices that lasted until the 1970s. Yosemite National Park managers were some of the first to embrace reintroduction of fire to the landscape, and the current Fire Management Plan signed in 2017 allowed for the use of wildland fire suppression, wildland fire used to benefit natural and cultural resources, prescribed fire, fire prevention, fuel reduction through mechanical methods, and fire ecology research to reduce fuels, protect life and property, and protect and restore historic ecosystem processes. This management strategy led to a mosaic of fire effects on the landscape that supports a diversity of plant and animal species. Furthermore, this mosaic provides ecosystem-level resistance to large severe fires and may have been partly responsible for slowing the spread and attenuating the severity of the Rim Fire as it entered Yosemite National Park in 2013. The Fire Management Plan is a living document that will continue to be revised. Currently, invertebrates are not considered in the plan except for the Valley Elderberry Longhorn Beetle (Desmocerus californicus dimorphus) whose range is limited to the El Portal Administrative area. Furthermore, the plan fails to integrate how management techniques impact the ‘hidden’ biodiversity represented by species genetic diversity.
Genetic diversity provides the raw materials for adaptation to novel environmental challenges, and often becomes a limiting factor if inbreeding erodes variation and allows deleterious mutations to increase in frequency. By necessity, management practices often embrace the philosophy that “if you build it they will come”, and focus on how landscape composition and vegetation structure influence wildlife diversity and ecosystem function. In the western United States, prescribed fire often increases habitat diversity and recruits species that favor open spaces and abundant understory plants. However, it is not clear whether fire management has positive or negative effects on the genetic diversity and genetic connectivity of wildlife species that respond to landscape change following fire.
Our study (Gates et al. 2021) represents the first attempt to relate fire management to genetic diversity of an invertebrate. The limited research on wildlife genetic responses to fire has revealed mixed effects of frequent fires – in some cases increasing or decreasing genetic diversity (Banks, et al., 2017; Ragsdale, et al., 2016; Schrey, et al., 2016; Steinitz, et al., 2012), leaving considerable uncertainty about the persistent effects of fire management on wildlife populations. Our research attempted to address this gap by assessing how a gradient of fire history and severity in Yosemite impacts genetic diversity and connectivity of two butterfly species.
Butterflies are among the organisms that become more abundant following wildfires in western U.S. forests (Huntzinger, 2003; Pavlik, et al., 2017). Thus, an expectation might be that fire enhances genetic diversity as population size is increased in burned habitats, and this may be further reinforced through increased population connectivity in post-fire landscapes. Alternatively, fire could lead to negative impacts on butterfly genetic diversity through local population extinction.
We examined genomic diversity for two species of small butterflies from mid-elevation habitats in Yosemite, the Propertius Duskywing Erynnis propertius (Figure 1) and Boisduval’s Blue Plebejus icarioides (Figure 2). While we found that fire is beneficial for maintaining genetic diversity in both species, different aspects of fire history impacted genetic variation. Fire appeared to counteract genetic drift and the loss of variation in the Propertius Duskywing, as genetic variation was reduced in sites with fire suppression. In contrast, fire appeared to directly enhance genetic diversity in Boisduval’s Blue, as multi-locus heterozygosity was increased by the size and severity of fires. The different responses may reflect hostplant use: Boisduval’s Blue utilizes perennial lupines (Lupinus spp.), which grow abundantly in post-fire landscapes, whereas the Propertius Duskywing utilizes oak trees (Quercus spp.) that are slow-growing and respond over a longer timescale to fire.
The outcome of this research should allow fire managers to predict that fire reintroduction will be more beneficial to butterfly genetic diversity than fire suppression. But it also suggests that differences in life history traits might impact wildlife responses, making decisions about habitat management practices more complex.
References