The ecology of human-carnivore coexistence

Abstract

With a shrinking supply of wilderness and growing recognition that top predators can have a profound influence on ecosystems, the persistence of large carnivores in human-dominated landscapes has emerged as one of the greatest conservation challenges of our time. Carnivores fascinate society, yet these animals pose threats to people living near them, resulting in high rates of carnivore death near human settlements. We used 41 y of demographic data for more than 2,500 brown bears-one of the world's most widely distributed and conflict-prone carnivores-to understand the behavioral and demographic mechanisms promoting carnivore coexistence in human-dominated landscapes. Bear mortality was high and unsustainable near people, but a human-induced shift to nocturnality facilitated lower risks of bear mortality and rates of conflict with people. Despite these behavioral shifts, projected population growth rates for bears in human-dominated areas revealed a source-sink dynamic. Despite some female bears successfully reproducing in the sink areas, bear persistence was reliant on a supply of immigrants from areas with minimal human influence (i.e., wilderness). Such mechanisms of coexistence reveal a striking paradox: Connectivity to wilderness areas supplies bears that likely will die from people, but these bears are essential to avert local extirpation. These insights suggest carnivores contribute to human-carnivore coexistence through behavioral and demographic mechanisms, and that connected wilderness is critical to sustain coexistence landscapes.

Keywords: coadaptation; demography; grizzly bear; source-sink; wilderness.

Fig. 1.

(A) Study extents (white polygons) for each of 12 telemetry and 29 genetic tagging studies on brown bear. Human Influence Index (HII) depicted with satellite images from across brown bear range on left. HII is a composite index derived by combining human population density, human land use and infrastructure (built-up areas, nighttime lights, land use/land cover), and human access (coastlines, roads, railroads, navigable rivers) (48). The index ranges from 0 (lowest human impact) to 64 (the most human-dominated category). For our purposes, we consider the range from 0 to 40 as brown bears generally don’t use—or survive in—habitats exceeding HII of 40 (SI Appendix, section 1.3). National borders in gray. Inset maps show the variation in human influence within and among studies. (B) Relationship between brown bear population density and HII within the study extents.

Fig. 2.

Per capita risk of annual mortality predicted from Cox proportional hazard model for subadult (3 to 6 y old) and adult (>6 y old) bears across HII gradients (A) and for HII and nocturnality (B). See SI Appendix, Section 2.2 for further details on hazard models. Uncertainty shown as SE. (C) Proportion of mortalities by cause by HII for animals > 2 y old; 76% of recorded mortalities were human-caused. Number of observed mortalities by cause is shown in brackets. Attract./conflict = mortality due to an attractant or conflict issue. Relationship between HII and (D) nocturnality (percent) between age classes, and (E) change in habitat use between age classes, indicating the degree to which animals changes their use of HII as they aged. For example, where HII = 20, this shows the change in habitat use as animals moved from subadults to adults when the area used by the subadult had an average HII area of 20, and (F) immigration required (percent of population) to sustain stable brown bear populations (population growth = 1).

Fig. 3.

(A) the relationship between probability of conflict and nocturnality across human influence gradients (SI Appendix, Analyses, Section 2.8). Uncertainty shown as SE. (B) annual number of dependent offspring observed (0 to 2 y old) per female (>5 y old), whether the female survived that year, annual measures of nocturnality, and use of human-dominated landscapes. We did not detect evidence for variation the number of offspring observed for females across the human influence gradient (effect of HII on offspring [n] = −0.018 [95% CI: 0.015 to −0.05], P = 0.29; but see SI Appendix, Analyses, Section 2.4 for more information on cub reproduction across HII gradients).

Fig. 4.

Spatial depiction of the landscape of coexistence and what sustains it across the southern range margin of brown bears. Black lines represent the current extent of the brown bear distribution and purple represents the contemporary recolonization frontier. (A) Percent immigration required to sustain the population, calculated by as the difference between a population that can sustain itself without immigration (in situ population growth = 1) and the observed in situ population growth rates for these areas (often <1). Tan lines represent a conservative extent of influence from localized sink areas (HII > 12) on the larger population (20-km buffer on sinks). (B) Estimated percent nocturnality displayed by adult bears (15 y old) across the landscape. Inset maps depict the coexistence landscape in a wilderness area, and in an area of high human influence and recolonization at the international brown bear range margin.

Fig. 5.

A basin of coexistence near Creston, British Columbia, Canada showing (A) human influence, towns, highways, and wilderness, demographic responses under two modeled scenarios of nocturnality (50% and 100%) for the following parameters: (B) projected population growth of brown bears, (C) number of immigrants (per 25 km² per decade) to support coexistence, and (D) survival to 10 y old (10yo).