Large carnivores, animals who require meat to survive like the gray wolf or the mountain lion, are some of the most vulnerable animals to human activity. They often have low reproductive rates and require a significant amount of land to search for prey, which can be affected by roads and housing development. Large carnivores also face threats from hunters and farmers who see them as a threat to their farmed animals. Due to the important role they play in environmental balance, scientists are interested in understanding the causes of large carnivore mortality.

Two common theories used to predict large carnivore mortality are the compensatory mortality theory and the additive mortality theory. The compensatory mortality theory suggests that human-caused mortality reduces natural causes of death for large carnivores, as there will be greater resources like food available for the surviving animals. The additive mortality theory suggests that human-caused mortality, like hunting and traffic accidents,  increases the overall mortality of large carnivores. This theory claims that human-caused mortality disrupts actions key to survival, like travel and mating.

Previous research has found some amount of evidence for each. However, past studies tend to focus on small areas and struggled to identify causes of death. This study tracked 590 mountain lions across California from 1974 to 2020, monitoring their movement and survival using radio trackers attached to collars. California is unique because it is the only western state with a permanent ban on hunting mountain lions, which passed in 1990. As a result, mountain lions have access to large amounts of land, despite still facing significant risk.

In the study, the researchers also tracked the level of human presence near the mountain lions, including roads and buildings, as well as animals farmed for food. Finally, researchers measured attitudes towards the environment in areas around the mountain lion territory through local ballot votes for environmental causes.

Researchers recorded 263 deaths, with 76% of those recorded after 2000. They were able to determine the cause of death for 199 of those, which were divided into human-caused mortality (135) and natural mortality (64), leaving 64 unknown. The leading causes of mortality were: killing to protect farmed animals (61), vehicle-caused deaths (39), natural deaths (33), deaths caused by other mountain lions (31), and hunting (26).

Researchers found that overall mountain lion survival declined when human-caused mortality increased, which seems to confirm the additive mortality theory. In other words, human-caused mortality disrupts mountain lion survival more than previously assumed. As more mountain lions are killed through hunting or vehicle collisions, their population-level survival decreased.

When mountain lion mortality was compared to human presence (amount of housing, roads, etc.), researchers found that mortality was highest near low and medium levels of human presence. Surprisingly, the amount of goat and sheep farms did not affect mountain lion mortality. Researchers suggest that it is human attitudes, not the level of human presence or the need to protect farmed animals, that affect mountain lion survival. In areas where more voters supported environmental causes, mountain lion mortality from intentional killing was reduced, although vehicle-caused deaths were unaffected.

Interestingly, researchers found that in areas where more voters supported environmental causes, mountain lions were less likely to be killed by other mountain lions. Although it’s uncertain why this is the case, they claim that hunting can increase aggression in disputes over new territory between young male mountain lions, which may result in death.

Although this study offers some interesting results, its findings are specific to a population of mountain lions in California from 1974 to 2020, and can’t be applied to large carnivores broadly. However, this study tracked large carnivores over a greater area and time than other studies, providing new understanding of large carnivore behavior.

For advocates, this study serves as a reminder that a hunting ban alone may not protect mountain lions from vehicles or hunters. It also highlights the effects of human activity on mountain lions and offers a strong response to those who argue that human activity doesn’t significantly affect large carnivore populations. The study also reveals the importance that changing human attitudes can have. Areas with more positive attitudes toward environmental causes had greater mountain lion survival, even in areas with farmed animals. This suggests that spreading education and awareness of the threats large carnivores face is an essential part of protecting these vulnerable species.