Testing the resource-matching hypothesis for a synanthropic species in protected areas

Background/Question/Methods

Mechanisms underlying the expansion and distribution of synanthropic species remain unclear. The consumer resource-matching rule suggests that individuals distribute themselves according to resource availability such that densities are relatively high in areas with food subsidies. However, because resource availability is density dependent, fitness should be similar between areas with and without food subsidies. Recent empirical work suggests that some synanthropic species may “over-match” in urban habitats, resulting in lower fitness despite greater resource availability. Nevertheless, the consumer resource-matching rule has not been tested for synanthropic species in protected areas, where they can threaten endangered species.

Populations of Steller’s jays (Cyanocitta stelleri) have increased in protected areas containing remnant populations of the threatened marbled murrelet (Brachyramphus marmoratus), presumably in response to human foods in recreational areas within these parks. To determine whether the distribution of Steller’s jays is consistent with the resource-matching hypothesis, we compared the population density, diet (based on stable isotopes), body condition, reproductive rates, and behavior of jays in campgrounds and interior forests in Big Basin Redwoods State Park, California from 2010 – 2013. Under the resource-matching hypothesis, we predicted jays in campgrounds and interior forests would be in similar body condition and have similar reproductive rates.  

Results/Conclusions

Jay densities were nearly nine times greater in campgrounds than interior forests. Higher jay densities in campground were likely due to greater use of human-derived food as δ¹³C was more enriched in campgrounds than forests. Body condition, as measured with total body lipids, was also significantly greater in jays in campground areas (n = 23) than interior forest jays (n = 22). Annual reproductive rates, based on the ratio of hatch-year to after-hatch-year jays during point counts, were also greater in campgrounds than interior forests. Thus, despite high jay densities in park campgrounds, preliminary results suggest that jays are “under-matching” to abundant food subsidies in these areas. Additional analyses of behavior observations indicate that under-matching occurs due to agonistic behaviors and territoriality that persists in campgrounds despite the prevalence of food resources. Thus, ongoing efforts to control Steller’s jay populations in marbled murrelet habitat by reducing access to food subsidies do not appear to be having their desired effect.