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

Abstract

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.

The effect of industrial noise on owl occupancy in the boreal forest at multiple spatial scales

“…Species that do not show avoidance may still be negatively impacted from chronic noise, for example impacts on passerines include reduced pairing success (Habib et al 2007) and reduced fitness (Halfwerk et al 2011, Schroeder et al 2012. Spotted Owls nesting near noisy roads have been found to fledge fewer young than owls nesting near quiet roads (Hayward et al 2011). Future research should assess the reproductive success of owls living in noisy areas to determine if chronic industrial noise has other negative impacts.…”

“…A study on Spotted Owls (Strix occidentalis) found they will flush from nests more frequently with increased proximity to a noise source (Delaney et al 1999). Two other studies on physiological responses of Spotted Owls to anthropogenic noise found elevated fecal glucocorticoids (Hayward et al 2011), but no detectable increase in fecal corticosterone (Tempel et al 2003). These studies on Spotted Owls applied temporary noise stimuli, e.g., helicopters, chainsaws, or motorcycles, so it is not known how owls respond to a permanent noise source on the landscape.…”

Testing Cort-Fitness and Cort-Adaptation hypotheses in a habitat suitability gradient for roe deer

“…We assessed several non-invasive indicators of representative physiological functions related to condition (Hayward et al, 2011;Mormede et al, 2007). One of them was fecal cortisol metabolites (CORT) which were used to index stress levels.…”

“…Thus, the interpretation of GC levels might require complementary information on the state of reproduction and other relevant physiological functions such as immunity and nutritional conditions (Romero, 2004). In this sense, some authors have highlighted the importance of considering the trade-offs between stress response and fitness correlates of condition to accurately interpret GC or GCm results for conservation issues (Busch and Hayward, 2009;Hayward et al, 2011).…”

Effects of Exposure, Diet, and Thermoregulation on Fecal Glucocorticoid Measures in Wild Bears

“…Monitoring these impacts in nature can be difficult, however, because warmer ambient temperatures and environmental exposure may also hasten fGC degradation, and both temperature and exposure effects are likely to vary with time of year [15]. Nutritional status, which is likely to vary with time of year, can also be detected through fGC levels [2], [5], [10], [16], [17]. Yet, diet composition itself can impact steroid excretion rates [18]–[21].…”