Stable isotopes reveal unexpected relationships between fire history and the diet of Spotted Owls

Abstract

Although the effects of shifting fire regimes on bird populations have been recognized as important to ecology and conservation, the consequences of fire for trophic interactions of avian species – and raptors in particular – remain relatively unknown. Here, we found that within national parks with long‐standing (40+ years) fire management programmes, California Spotted Owls Strix occidentalis occidentalis consumed predominantly Woodrats Neotoma spp. and Pocket Gophers Thomomys spp.; however, in contrast to our predictions, when their territories experienced more extensive and frequent fire, Spotted Owls consumed proportionally more Flying Squirrels Glaucomys oregonensis. We hypothesize this finding could have been driven by either changes to prey abundance following fires (e.g. increases in flying squirrels) or changes to prey availability (e.g. shifts in forest structure or flying squirrel spatial distribution that increased predation upon them by owls). Our work thus demonstrates that fire may have unexpected consequences for the trophic interactions of raptor species and provides valuable information for the conservation of Spotted Owls in fire‐prone forest landscapes.

Megafire causes persistent loss of an old‐forest species

“…In contrast, higher pyrodiversity has led to increased persistence at finer spatial scales but not at broader spatial scales. Consequently, pyrodiversity at finer scales may result in greater likelihood of nest stand structure preservation or lead to increased diversity of prey habitat near the nest stand (Hobart et al., 2021). The effect of pyrodiversity may diminish at broader scales because these larger scales are more associated with foraging habitat and foray behaviors (Blakey et al., 2019), the former of which is more flexible for spotted owls than is their choice of roosting and nesting habitats (Atuo et al., 2019; Call, Gutiérrez, & Verner, 1992; Williams et al., 2011).…”

“…We developed two predictions that would indicate support for this hypothesis. We predicted that (i) post‐fire persistence and colonization rates would decrease in areas with extensive severe fire via loss of larger trees that provide suitable nest sites (e.g., Jones et al., 2016) and (ii) post‐fire persistence and colonization would increase in areas with higher pyrodiversity, which may more closely reflect historical post‐fire conditions, as well as maintain nest structures and produce diverse prey habitat and hunting cover (e.g., Hobart et al., 2021). We also hypothesized that post‐fire owl dynamics were driven by the fire itself, rather than post‐fire management.…”

Pyrodiversity and biodiversity: A history, synthesis, and outlook

“…Woodrats tend to be associated with open, early‐seral forest, while flying squirrels tend to be associated with late‐seral, closed‐canopy forest (Roberts et al., 2015). Therefore, in fire‐suppressed forests, pyrodiverse areas characterized by smaller patches of severely burned forest may introduce forest conditions that either recruit woodrats or increase local densities of flying squirrels (Sollmann et al., 2016), and both outcomes could improve spotted owl hunting efficiency (Hobart et al., 2021). Pyrodiverse areas may also give owls access to “open” forest patches for hunting while maintaining concealment in surrounding green forest from predators such as great horned owls (Gutiérrez et al., 1995; Johnson, 1992).…”

Population decline in California spotted owls near their southern range boundary

“…With respect to reproduction, we hypothesize that adverse effects of low‐ to moderate‐severity fire would be mediated by changes in prey abundance or access. In the Sierra Nevada, for example, spotted owl diet has been strongly affected by fire history, in terms of extent and frequency (Hobart et al 2021). In addition, prey may show functional responses in our study area that are different from prey populations in spotted owl ranges where previous owl‐fire studies have occurred.…”