Evaluating population impacts of predation by owls on storm petrels in relation to proposed island mouse eradication

Abstract

We quantify the expected demographic benefit to a seabird of conservation concern, the ashy storm‐petrel Oceanodroma homochroa, from the proposed eradication of introduced house mice Mus musculus on the South Farallon Islands, California. A key objective of the eradication is to reduce storm petrel predation by burrowing owls Athene cunicularia, which stopover on the island during their fall migration. Mouse trapping and field surveys of both owls and depredated storm petrel carcasses conducted during 2000–2012 reveal a strongly seasonal, inter‐related pattern among the three species: When owls arrive during the fall, mice are super‐abundant and the overwhelming choice of prey for those owls that remain. In the winter, the mouse population crashes just as storm petrels begin to arrive in large numbers; owls that remain on the island switch to preying upon storm petrels until May, when they depart to breed. Capture–recapture analyses of storm petrels showed (1) annual adult survival was inversely related to owl abundance, especially during January–April, and (2) storm petrels demonstrated a declining trend in abundance 2006–2012. The latter was associated with low rates of adult survival, high abundance of overwintering burrowing owls, and high incidence of depredated storm petrels. To evaluate projected impacts to storm petrels of a change in owl predation, we developed a Leslie matrix model, incorporating environmental stochasticity. We modeled future storm petrel population trajectories, allowing for different levels of owl‐mediated predation. Our results suggest that a reduction in owl abundance, a projected consequence of the elimination of mice, has the potential to substantially reduce overall storm petrel mortality, thereby reducing storm petrel declines and increasing the likelihood of stable trends in the future. While long‐term benefits to storm petrels of mouse eradication are apparent, the risk of increased predation due to prey‐switching by owls also needs to be addressed. This study highlights uncertainty of outcomes, which must be considered in evaluating management impacts. This study demonstrates the value of concurrent, continuous, long‐term datasets in providing a quantitative basis for management to aid the conservation of species of concern.

Projecting long‐term impacts of a mortality event on vertebrates: incorporating stochasticity in population assessment

“…For projecting into the future, we used parameter estimates from the period 1999 to 2011, the most recent period for which we had estimates for all our demographic parameters, as our focal period. We note that there was a substantial, well‐documented change in the oceanographic conditions in 1998/1999 (Peterson and Schwing 2003, Bestelmeyer et al 2011) which likely had had an important influence on Western gull demography, as it did on other seabirds of the South Farallon Islands (Nur et al 2019). Population trends differed in the two time periods, with a relatively stable trend in the more recent period (1999–2011) compared with greater fluctuation in earlier period (up to 1999).…”

Population Dynamics and Resource Availability Drive Seasonal Shifts in the Consumptive and Competitive Impacts of Introduced House Mice (Mus musculus) on an Island Ecosystem

“…Specifically, during the summer snipe feed significantly lower in the food chain due to competition with mice for invertebrates, which may prevent snipe reaching adequate breeding condition (Russell et al 2020). While mice have been shown to have an indirect impact on adult ashy storm petrel populations via hyper-predation by owls on SEFI (Nur et al 2019), no studies to date have explored the potential for resource competition among mice and other native fauna on SEFI.…”

“…We created an index of mice abundance based on monthly trapping success on 4 transect lines spread across available habitats at SEFI (Irwin 2004; Nur et al 2019). Trapping was conducted for each of 3 nights per month between March 2001 and March 2004, and again from December 2010 to March 2012, and finally September 2016 through January 2018.…”

“…While predation on eggs by mice can be difficult to detect in a crevice-nesting species, these observations suggest the frequency of direct seabird egg or chick predation in this population may be low (Ainley & Boekelheide 1990). Even so, there is a growing body of evidence that house mice facilitate migratory burrowing owls ( Athene cunicularia ) to overwinter on SEFI (Chandler et al 2016; Mills 2016) which can lead to significant hyper-predation by owls on adult ashy storm petrel ( Hydrobates homochroa ) when the mouse population crashes (Nur et al 2019).…”

Population dynamics and resource availability drive seasonal shifts in the consumptive and competitive impacts of introduced house mice (Mus musculus) on an island ecosystem

“…Moreover, the presence of house mice facilitate migratory burrowing owls ( Athene cunicularia ) to overwinter on SEFI ( Chandler et al, 2016 ; Mills, 2016 ). When mouse populations seasonally crash, burrowing owls switch from feeding primarily on mice to adult ashy storm-petrels ( Hydrobates homochroa ) which results in significant predation on this species of conservation concern ( Nur et al, 2019 ).…”