Hourly movement decisions indicate how a large carnivore inhabits developed landscapes

1. What they did

As people build into wild places, some animals stay away while others move in. Black bears in North America are increasingly living among houses and roads, and conflicts with people are rising. In Connecticut, a recolonizing bear population includes some bears with houses scattered through their whole home range and others in almost entirely forested ranges. The authors asked how these bears manage to live in developed landscapes: do they get used to people (acclimation), or do they live there by carefully avoiding risky features (effective avoidance)?

They tested four ideas:

• Do bears see development as risky? If so, they should avoid houses, roads and highways, and females with cubs should avoid them more.
• Acclimation or effective avoidance? If bears acclimate, those living among more houses should select human features more strongly. If they avoid effectively, avoidance should hold steady or grow where there are more houses.
• Do bears change their behaviour with season and traffic? Bears might take more risks during hyperphagia, the late-summer period of intense feeding before hibernation.
• Are less wary bears in more conflicts with people?

State wildlife biologists fitted adult female black bears with Global Positioning System (GPS) collars that recorded a location every hour, and checked whether each female had cubs during winter den visits. The authors compared each real hourly step a bear took with many possible steps she could have taken from the same spot (a step-selection function). This showed which landscape features each bear was drawn to or avoided: housing density, distance to local roads, distance to highways and crossing a highway, alongside natural features. They also looked at how this changed with hourly traffic counts, with season, with whether the bear had cubs, and with how many houses were in her home range. Finally, they compared each bear’s choices with the number of times she was reported damaging property (such as garbage cans and birdfeeders) between 2012 and 2015.

2. What they found

Overall

• Bears varied a lot from one individual to another, and no single model fitted all of them best.
• Bears consistently avoided moving close to local roads, in every season.
• Housing density inside home ranges ranged from 0.51 to 214.43 houses per km², and did not differ between females with and without cubs.

Females with and without cubs

• Females with cubs were less variable in their choices than females without cubs.
• Females without cubs moved near highways and avoided residential areas during pre-denning. They also chose to cross highways in summer and hyperphagia, but avoided crossing during pre-denning. Females with cubs showed no clear preference either way.
• Females with cubs avoided local roads more strongly than females without cubs, mainly in summer and hyperphagia. In summer they also avoided highway crossings more strongly, and in pre-denning they avoided being near highways more strongly.
• Females with cubs avoided highways more as traffic increased, in all three seasons, while females without cubs generally did not change with traffic. They were also more sensitive to traffic in how they used residential areas.

Changes with season and traffic

• The best-supported models all included season, so bears changed how they responded to human features through the year, mostly with roads and highways.
• Bears avoided highway crossings more in pre-denning than in hyperphagia.
• Bears avoided local roads more when traffic was heavier, and this response to traffic grew stronger from summer to hyperphagia, for both local roads and highways.

More houses, more caution

• Bears whose home ranges held more houses selected residential areas more strongly. On average, bears with more than about 66 houses per km² in their home range moved towards residential areas, while bears in less developed ranges avoided them.
• At the same time, bears in more developed ranges avoided local roads more strongly (in summer and hyperphagia) and reacted more strongly to traffic on local roads.
• Responses to highways did not change with housing density.

Conflict

• All 35 collared females were reported in at least one conflict incident.
• Bears that chose to cross highways were involved in more conflicts. Bears that moved closer to local roads or into residential areas also had more conflicts, although the number of houses in their home range was a stronger predictor.

3. What they concluded

• Black bears appear to see development, and especially roads, highways and traffic, as risky. Yet they live in developed areas by avoiding those risks effectively and by changing their behaviour with season and traffic, rather than by simply getting used to people.
• Bears in the most built-up areas used residential areas more while avoiding roads more, which suggests they are skilled at exploiting human resources while dodging danger.
• Bears seemed to respond more to traffic and roads than to houses. The authors suggest noise, or the real risk of being killed on roads, may explain this.
• Because the population only recently recolonized the area, bear behaviour may not yet be settled. The wide differences between individuals give room for the population’s average behaviour to shift over time, for example if bolder bears do better.
• Some strategies may be costly: females with cubs that avoid roads could lose feeding opportunities during hyperphagia, and more highway crossings could raise the risk of death for breeding females.
• Getting used to people is an individual trait that may become more or less common in a population over time.

6. Other technologies, skills or innovations

• Lotek GPS 3300 collars taking hourly locations, with differential correction (GPS Pathfinder software).
• Step-selection functions with simulated steps drawn from each bear’s own movement patterns, fitted to each bear-year separately so individual differences could be measured.
• Hourly traffic counts from the state transport department, linked to each bear’s steps.
• Conflict reports linked to individually ear-tagged bears.

7. Implications for conservation (from the paper)

• Living near people does not mean a species or population has become used to people; tolerance of development varies between individuals and may change over time.
• Development that mixes housing with natural cover may help bears live close to people while still avoiding human features.
• Because bolder bears were involved in more conflicts, the authors suggest that preventing bold behaviour from spreading may help where bear populations have only recently become established. Their examples are community programs that reduce access to human food, and lethal management targeting bold individuals.
• Female survival and breeding success matter most for bear population growth, so seasonal trade-offs between feeding and avoiding roads may affect population viability.