Impacts of acute and long-term vehicle exposure on physiology and reproductive success of the northern spotted owl

Abstract

Stress physiologists posit that multiple simultaneous demands faced by an organism may have non-additive effects on the magnitude of their response to disturbance. The environmental assessment literature emphasizes a similar phenomenon at the population level, arguing that populations can compensate for perturbations up to a threshold, beyond which disturbance impacts may be greatly magnified—and even cause system collapse. We integrated these two approaches to examine the roles of environment, life history stage, prior disturbance experience, and their interactions on vulnerability to disturbance in a free-living species. Specifically, we examined the effects of off-highway vehicle use on the federally threatened northern spotted owl (NSO), Strix occidentalis caurina, by measuring fecal glucocorticoid metabolites (fGCs), which reflect disturbance; fecal thyroid hormone metabolites (fT3), which reflect nutrition; and the number of offspring fledged within a season. We experimentally applied one hour of motorcycle exposure to NSOs during periods of incubation (May) and fledging (July), comparing fGC levels of treated NSO with those of non-exposed controls. Acute vehicle exposure generally increased fGCs in the short term. Males showed the highest glucocorticoid response to vehicle disturbance in May when they were typically solely responsible for feeding themselves, their mates and their nestlings. By contrast, response to motorcycle exposure among females depended on their level of fT3 and their number of young. Levels of fGCs were highest post treatment among females that lacked young and had high fT3 (good nutrition); fGC levels were lower in treated females compared to controls among females with two young and low fT3 (compromised nutrition), possibly reflecting allostatic overload. The correlational approach showed that NSO close to roads had higher levels of fT3, suggesting better nutrition. Surprisingly, fGC levels were unrelated to proximity of roads, irrespective of noise. Presumably, the tendency for traffic exposure to increase fGCs over the long-term was offset by nutritional gains (i.e., reduced fGCs and high fT3) associated with proximity to roads. Sound level meters enabled us to quantify road noise on a subset of NSO territories. NSO close to noisy roads fledged significantly fewer young than NSO near quiet roads, indicating that routine traffic exposure may decrease NSO reproductive success over time.