The world is in the midst of a sixth mass extinction: current vertebrate loss is at a rate higher than any other extinction event in history. Even ecologically resilient species like the sloth bear aren’t safe from this human-caused crisis.
The sloth bear, Melursus ursinus, is a member of the Ursidae family and is instantly recognisable by the crescent marking that adorns its chest. Its home is the tropical dry and moist broadleaf forests of southern Asia, but human encroachment into this territory has sadly resulted in a projected 30-49% population size reduction in the next 30 years. This means the species is classified as “vulnerable” by the IUCN Red List – a list of flora and fauna species and their assessed endangerment levels.
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If the sloth bear’s downwards population trend isn’t reversed in the next few decades, it may soon join the 26,000 other species on the IUCN Red List that are currently threatened with extinction. Understanding the cause of this decline is key to developing conservation plans for both the sloth bear and threatened species facing similar issues.
At first glance, sloth bear ecology should give the species a low-likelihood of endangerment. Firstly, sloth bears have high dietary plasticity, meaning a flexible diet with many sources of nutrition like insects and fruit. This is beneficial because the sloth bear should be unaffected by the availability or seasonality of food sources. In fact, fruit makes up 75% of the sloth bear diet in the hot and dry season, decreasing to 27% in the wet season.
Low dietary plasticity is a common contributor to population decline in other at-risk species, almost as common as increased predation. However, predation is another threat that isn’t an issue for the sloth bear, whose only natural predator is the tiger. As tigers are themselves endangered, they form an infrequent threat. Furthermore, the sloth bear’s ability to occupy the same ecological niche in multiple habitats greatly increases its survival potential. This is because there are habitats, namely tall-grass savannahs and evergreen forests, available if their primary territory is partially degraded.
Additionally, environmental degradation results in habitat fragmentation, which usually causes genetic diversity loss. This is because the population sizes are smaller, meaning there are less mating options and more inbreeding. However, this has had no measurable impact on the sloth bear nor has it led to any genetic isolation. This is possibly due to the bear’s large home range, estimated to be an annual 129 square kilometres for an adult male.
So why is a species with such a high diet plasticity, few predators, and a large habitat range declining so quickly? The answer is anthropogenic forces. Human encroachment into the sloth bear habitat has resulted in severe habitat deterioration due to fire, overgrazing, and deforestation, which has in turn caused increased proximity to humans. This closeness has led to human-bear conflict which is considered the largest contributor to sloth bear mortality rates because the frequency of casualties has a significant positive correlation with the intensity of human use of local forests and daytime bear activity. For example, the highly degraded forests of Chhattisgarh, India are conflict hostpots with 137 human casualties from 1998 to 2000.
The single most significant action would be to enforce reforestation policies which would increase habitat range and reduce human-bear contact and conflict. Furthermore, by reducing human casualties by sloth bears, the stigma around conserving the species would diminish and increase conservation ethic among local communities.
With human-caused biodiversity decline accelerating on a scale never before seen in history, the plight of species like the sloth bear can help focus the world’s attention and provide insight into how human activity can be changed to preserve all threatened species worldwide.
Bethan HendersonBethan Henderson
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