Effects of Woolsey Fire on Nesting Territories of Southern California Red-Tailed Hawks (Buteo

Effects of Woolsey Fire on Nesting Territories of Southern California Red-Tailed Hawks (Buteo jamaicensis) Jane Gao [email protected] 1 ABSTRACT: The relationship between fire and wildlife habitat is complex. Fires can create favorable conditions for some species and simultaneously extirpate entire populations of other species. Red-tailed hawks choose nesting habitats according to resources available. Natural disturbances such as wildfires change the availability of those resources. In late 2018, the Woolsey fire burned much of Ventura County, California, changing composition of the flora and fauna in areas affected by the fire. Nest site use for Red-tailed hawks (Buteo jamaicensis) has been tracked in the Santa Monica Mountains of Ventura county from the 1970s to 2019. The Woolsey fire area is surrounded by mountain ranges as well as suburban areas, providing hawks with a variety of nesting habitats consisting of both native and nonnative trees. The Woolsey fire affected these varied habitats differently. The area burned by the Woolsey fire had not been largely affected by fire since the 2003 Simi fire. The fact that this area has been free from damaging stochastic events for over a decade made it a prime area for studying the immediate effects of fire on raptor nesting. We investigated how fire affected nest habitability and whether several variables factored into nest persistence within the 2018 Woolsey fire area. Nest persistence in this context was defined by the presence of a nesting hawk. We hypothesized that the magnitude of the burn and the species of the tree in which the nest was built were correlated to nest persistence. In addition, we proposed that geographic position relative to the fire border, and not just local destruction of nests, was a key variable in nest persistence. While proximity to the edge of the fire’s border and magnitude of burn severity were found to be correlated with nest persistence, tree species was found to have no discernible correlation. 2 INTRODUCTION Wildfires have always been a part of Southern California’s ecosystems (Cheney et al. 1997). They engender a patchwork effect on local habitats. Some of the positive outcomes of wildfires include the release of nutrients back into the soil, clearing space for new seedlings, and removing nonnative species. Negative outcomes include the destruction of habitat, death of native species, and removal of erosion-preventing plants (Cheney et al. 1997). While the impact that wildfires have on California’s ecosystems has both positive and negative aspects, the overall increase in both number of fires and fire severity in recent decades has been the subject of major study. In more recent decades human suppression of fire increased, causing a buildup of vegetative fuel in fire-prone areas (Bowman 2010). This suppression was meant to limit the number of fires under the notion that fewer fires would create a safer environment for human settlements. Additionally, climatic controls have changed and altered size, severity, and fire frequency (Hurteau et al. 2014). Climate change created a longer fire season with more powerful winds and higher ambient temperatures (Westerling et al. 2008). Therefore, when fires do occur, they spread quickly and over a larger amount of terrain (Keeley et al. 2001, Syphard et al. 2007). The change in fire regime can have a profound change on the ecosystem, including vegetation shifts, invasive species, a decrease in biodiversity, and population shifts (Westerling et al. 2008, Hurteau et al. 2014, Williams et al. 2019). California’s growing population and increase in fire activity present a new stressor on Red-tailed hawk nesting habits. The goal of this study is to see how Red-tailed hawk nesting sites were affected by the 2018 Woolsey fire. Studying wildfire disturbance effects on apex predators can help future land management make informed management decisions. 3 In order to study how fires affect Red-tailed hawk nesting habits, it is prudent to understand California’s historical fire regime. The last few decades have seen a drastic rise in the number of acres burned in California wildfires (Figure 1). This increase in burned land area corresponds to a marked increase in California’s average temperature (Figure 2) and in California’s total number of fires per year (Figure 3). These longitudinal trends of increased intensity and frequency of California wildfires represent a significant threat to California’s wildlife. One group that can be especially affected by an increase in wildfire size and frequency is birds of prey. Fires can change how raptors use available habitat via direct and indirect cascading effects. While the fire itself may not kill the raptors themselves, increased wildfires may drive raptor prey to extinction or drive away animal species in lower trophic levels. These changes in the raptor’s food chain cascade up the trophic levels by simplifying the food web, thus limiting the raptor’s prey availability. Fire creates a mosaic of habitat patches with variable biotic and abiotic conditions. For example, fire disturbances can produce early seral plant communities that support different animal communities compared to pre-fire communities (Keane 2002). The new vegetative growth provides food and cover for early seral species that colonize the area (Keane 2002). In areas where fires are the most common disturbances, plants and animals have evolved various adaptations that allow them to survive or benefit from fire. Desert plants evolved pyriscence or flammable oils with fire-activated seeds (Brown 2000). Other flora such as oak trees and pine trees have adapted to fire with either more resistant bark or legacy sprouting (Barton 1999). One subgroup of fauna that is both affected by and even utilizes fire is raptors. Raptors take advantage of wildfires by searching for flushed out prey seeking to escape the flames (Sahores et al. 2004, Steenhof et al. 1999). After the wildfire is over, raptors scavenge burned areas for prey, taking advantage of the razed vegetation and consequent lack of cover 4 (Sahores et al. 2004, Steenhof et al. 1999). Raptors’ ability to capitalize on fire as a natural disturbance can maximize their productivity in an environment with variable resources. Figure 1. The number of acres burned per year in California since 1950 Source: Cal Fire. 2020 Number of acres burned per year (in millions) https://www.fire.ca.gov/stats-events/ 5 Figure 2. The average temperature in California shown from 1895 to 2018. Source: NOAA. 2019 Average temperature in California from January through October each year, 1895-2018. https://www.ncdc.noaa.gov/cag/ Figure 3. The number of fires reported per year in California starting from 1913 to 2013. Source: Cal Fire. 2020 Number of Fires Reported Per Year in California. https://www.fire.ca.gov/stats-events/ 6 Scientists in the Southern California National Park Service (NPS) and non-government organizations (NGOs) have been tracking the effects of urbanization on raptors in the Ventura County area since 1972. Red-tailed hawks (Buteo jamaicensis) were chosen for the study due to their position as apex avian predator, their wide trophic niche, and their large population size. These qualities made them good candidates for urban and wild comparative studies (Marti et al. 1993). The 2018 fire season was the deadliest and most destructive season on record in California (Cal Fire 2018, Figure 1, Figure 2, Figure 3). The Woolsey Fire was the largest fire in Southern California in 2018 and burned 96,949 acres of land, making the landscape more susceptible to invasive weeds and grasses (LA County 2018). The buildup of non-native vegetation before the fire increased the vegetative fuels which led to a more fire susceptible landscape, thus creating a feedback loop (NPS 2018). While the fire as a whole was severe, the severity varied greatly within Ventura County’s local microenvironments. Some habitats were completely razed to the ground, while others remained wholly unburned. This variation in severity within the study area created a mosaic of local environments. Overall, larger animals had more success in escaping the Woolsey fire, while smaller animals were more likely to succumb to the smoke and heat (NPS 2018). The Woolsey fire’s blaze occurred in November, before the Red-tailed hawk nesting season, which starts in early Spring (Cooper 2019). Red-tailed hawks would have been able to escape the fire easily, but many of their prey species populations declined. Perch sites necessary for Red-tailed hawks’ hunting also declined in burned areas. The circumstances of the Woolsey fire present a unique opportunity to study how the loss of perches, vegetation, and prey affect the nesting habits of the Red-tailed hawk species. 7 The study area that was affected by fire is undergoing early stages of post-fire succession that includes an influx of colonizing plants such as Lupines (Lupinus sp.) and Blue dicks (Dichelostemma capitatum). Early-seral plant species take advantage of the nutrient influx and open understory, which creates a more biodiverse landscape with pioneer plant species now able to grow (Arno 2000). These new plant species engender more biodiversity by replacing pre-fire habitats dominated by a few species with a variety of pioneering early seral species (Arno 2000). Plant biodiversity then invites R-select animal species (Arno 2000). Forest legacies such as Coast live oak and Sycamore have also started to sprout and will continue to grow and spread in the burned area, eventually outcompeting the pioneering species. The 2018 Woolsey Fire offers an opportunity to studyhow fire affects nest selection for Red- tailed hawks. We used previously published works (Lee 2004) of historical nest site locations, territory activity, nesting activity, and tree species in which the nest was located as a basis for our own data collection.

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