Animals that are millions of years old being diurnal are moving into the night. Whether big or small, forest or savanna, predators or prey, species from all over the planet are transferring the bulk of their activity to the night time. A large study points to the expansive human presence as the cause of changes that can disrupt the dynamics of entire ecosystems.
The impact of humans on wildlife has many edges. The most obvious is the contraction of the space available to animals as the human race has been expanding throughout the planet. In addition, these natural spaces are increasingly reduced and quartered and their quality is reduced with each new infrastructure that surrounds them. One of the consequences of all this is that animals move less and less in areas with human presence and take refuge in increasingly smaller areas. But there is another way to hide from humans: leave when they go to bed.
A group of researchers from the United States has verified the global nature of this translation of animal life to the hours in which the great diurnal predator rests. Compiling the results of dozens of studies on the movements of some 60 species of mammals from five continents, scientists have found that, where there is a human disturbance, mammals are, on average, 1.36 times more nocturnal. This means that an animal that, without disturbances, equally distributes its activities between day and night, would increase its nocturnal activity up to 68%.
"There is evidence to suggest that animals everywhere are adjusting their daily activity patterns to avoid humans over time, as it is increasingly difficult to avoid them in space," says the researcher at the University of California at Berkeley. (USA) and principal author of the study, Kaitlyn Gaynor. "As people are more active by day, animals are moving into the night," he adds. This transfer occurs whether it is herbivores or large carnivores such as the tiger. The pattern is repeated both in smaller mammals such as the opossum, and in those weighing more than 3,500 kilograms, such as the African elephant.
The most striking aspect of this review, published in the journal Science, may be that animals are becoming more nocturnal regardless of the level of danger that humans pose. "We expected to find a tendency to increase nocturnality in the vicinity of humans, but We have been surprised by the consistency of the results. The animals respond to all types of human disturbance, regardless of whether it really poses a direct threat, "he adds.
Gaynor's work is based on dozens of studies that used various tracking techniques (beacons, collars with radio transmitters, GPS, phototraps or direct observation) of the movements of the animals before a range of human presences, from hikers to hunters, going through fields or roads. One of those studies tracked a species as opportunistic as the fox for lands of Castilla-La Mancha in a series of minor environments (Cabañeros national park) to greater human presence (around Ciudad Real).
"Although it is a twilight animal, the more human disturbance, the fox tended to reduce its diurnal activity," says the biologist at the University of Malaga and co-author of that study, Francisco Díaz. For the most nocturnal foxes, there was a temporary mismatch with their main prey, the rabbit, eminently diurnal. Fortunately for them, the foxes are among the most adaptive animals. "But there are other species with millions of years of adaptation to daytime behavior that are not so plastic," Díaz recalls.
The consequences of this transfer to the night of so many species are still uncertain. In principle, it would seem that the abandonment of the day in favor of humans would facilitate the coexistence between humans and animals. But such a widespread and rapid change of patterns of activity molded over millennia can alter an entire ecosystem. "In the case of predators not adapted to hunting at night, there could be an increase in the population of the ungulates that were their prey, which would affect the availability of vegetation cover, producing a cascading effect," says the Researcher at Radboud University, Niimega (The Netherlands), Ana Benítez.
For the Spanish ecologist, who has also investigated the different human impacts on animal life, the most relevant of this research is that it confirms a hypothesis raised in the 60s by the biologist Fritz R. Walther: "Animals respond equally to humans , they always see us as predators, "he says. This leads to the question whether the impact of a hunter can be the same as that of a nature-loving hiker. For Gaynor, his research "suggests that our mere presence is enough to interfere with natural behavior patterns".