A number of North American oaks are experiencing recruitment failure, with explanatory hypotheses including a range of consumer- or resource-based limitations. Using a factorial experiment with transplanted seedlings, we demonstrate how direct and indirect consumer effects prevent recruitment by Quercus garryana, a keystone savanna species of northwestern North America. Foremost, intense herbivory by mostly exotic small mammals severely damaged or killed 100% of unprotected seedlings during winter. Many damaged seedlings survived but were 64% smaller in size and produced 75% fewer leaves. Herbivory by deer had no detectable impact despite being long hypothesized as a major contributor to oak decline. Indirectly, herbivory altered the outcome of moisture limitation and competition. Without small mammals, summer drought and a dense exotic grass cover associated with fire suppression significantly reduced growth but caused little mortality. With small mammals, both significantly increased mortality of herbivore-damaged seedlings. Herbivore damage also increased the likelihood of severe insect attack, possibly due to reduced investment in foliar defense by recovering seedlings. These results show that no single factor necessarily prevents seedling establishment by oaks, but that the combination of herbivory, undisturbed exotic grass swards, and summer drought creates an almost insurmountable barrier for recruitment.

Summary1. Population decline is associated with increased vulnerability to extinction, but also with possible density-, frequency-or distance-related 'rarity advantages' that increase recruitment success as individuals become isolated from their congeners. Distinguishing between these alternatives (risk vs. recovery of rare populations via demographic processes) has become critical, given how anthropogenic disturbances are causing population declines globally. 2. Here, we demonstrate how distance-related rarity advantages are evident in spatially isolated recruits of a canopy-dominant but regionally rare species of oak that appears to be suffering recruitment collapse. As distance from parent trees increased, seedlings had significantly more leaves and experienced reduced insect browsing and intraspecific competition. Long-term field-based experimental treatments revealed these advantages to be associated with rapid rates of juvenile maturation and survival that are unobserved in natural settings. 3. The discrepancy between the experimental and natural settings was explained by trophic collapse and habitat loss -two changes ubiquitous to many terrestrial ecosystems -that combine to concentrate vertebrate herbivores in habitat remnants and cause 100% juvenile mortality via the browsing of taller juveniles. Exotic grass cover, long associated with oak recruitment failure, significantly suppressed seedling height and leaf production, but appeared to delay mortality by hiding shorter seedlings from vertebrate herbivores. 4. Synthesis. Our work demonstrates how rarity advantages have the potential to positively influence the population performance of a declining species, but are short-circuited by intense herbivory associated with human-based environmental change. Regionally, there appear to be few existing conditions on the contemporary landscape that favour juvenile survival, suggesting ongoing recruitment difficulties without intervention. Our work clarifies how extinction risk can in some cases be best defined by how anthropogenic disturbances affect, and are offset by, demographic-based persistence mechanisms, than simply by present-day abundance or distribution.

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