More wildlife turn nocturnal
For their first 100 million years on planet Earth, our mammal ancestors relied on the cover of darkness to escape their dinosaur predators and competitors. Only after the meteor-induced mass extinction of dinosaurs 66 million years ago could these nocturnal mammals explore the many wondrous opportunities available in the light of day. Fast forward to the present, and the honeymoon in the sun may be over for mammals. They are increasingly returning to the protection of night to avoid the Earth's current terrifying super-predator: Homo sapiens. A new study in the journal Science has documented a powerful and widespread process by which mammals alter their behaviour alongside people. Human disturbance is creating a more nocturnal natural world. Many catastrophic effects of humans on wildlife communities have been well-documented. Man is responsible for habitat destruction and overexploitation that have imperilled animal populations around the world. Many animals fear humans. Animals often go out of their way to avoid encountering man. But it's becoming more and more challenging for wildlife to seek out human-free spaces as the human population grows and its footprint expands across the planet. Might animals all over the world be adjusting their daily activity patterns to avoid humans? To explore this question, a meta-analysis, a study of studies was conducted. The focus was on mammals because their need for plenty of space often brings them into contact with humans. Data for areas or seasons of low human disturbance, that is, more natural conditions, and high human disturbance was sought. For example, studies compared deer activity in and out of the hunting season, grizzly bear activity in areas with and without hiking and elephant activity inside protected areas and outside among rural settlement. Based on reported data from remote camera traps, radio collars or observations, each species' nocturnality was determined, which was defined as the percentage of the animal's total activity that occurred between sunset and sunrise. The difference in nocturnality between low and high disturbance was quantified to understand how animals changed their activity patterns in response to people. Overall, for the 62 species in the study, mammals were 1.36 times as nocturnal in response to human disturbance. An animal that naturally split its activity evenly between the day and night, for example, would increase its nighttime activity to 68 per cent around people.
83 per cent of the case studies examined showed some increase in nocturnal activity in response to disturbance. The finding was consistent across species, continents and habitat types. Antelope on the savanna of Zimbabwe, tapir in the Ecuadorian rainforests, bobcats in the American southwest deserts, all seemed to be doing what they could to shift their activity to the cover of darkness. Perhaps most surprisingly, the pattern also held across different types of human disturbance including activities such as hunting, hiking, mountain biking and infrastructures such as roads, residential settlement and even agriculture. Animals responded strongly to all activities, regardless of whether people actually posed a direct threat. It seems that human presence alone is enough to disrupt their natural patterns of behaviour. Over millions of years, many of the animals included in the study have evolved to adapt to life in the daylight. Sun bears, for example, are typically diurnal and sun-loving creatures; in undisturbed areas, less than 20 per cent of their activity occurred at night. But they increased their nocturnality to 90 per cent in areas of the Sumatran forest where intensive forest research activity created a disturbance. Such diurnally adapted animals may not be as successful at finding food, avoiding predators or communicating in the darkness, which could even reduce their survival or reproduction.
As long as animals are able to meet their needs during the night, they may actually thrive in human-dominated landscapes by avoiding daytime direct encounters with people that could potentially be dangerous for both parties. Dividing up the day, through what researchers call temporal partitioning, may be a mechanism by which people and wildlife can coexist on an ever more crowded planet. An increase in nocturnality among certain species may also have far-reaching consequences for ecosystems, reshaping species interactions and cascading through food webs. Even if so many large mammals cannot be seen during the day, they may still be living alongside man. In areas where threatened species live, managers may consider restricting human activity to certain times of the day, leaving some daylight just for wildlife. And there is need to preserve wilderness areas entirely free of human disturbance to conserve the most vulnerable and sensitive mammal species. Those that try to avoid human disturbance entirely may be most vulnerable to the consequences of the expanding human footprint.