From day into night
All around the world, humans are forcing other mammals to be more active at night, reports Amina Khan, of the Los Angeles Times.
THE growing impact of human activity and encroachment on wildlife domains are forcing an increasing number of mammals around the world to become more active at night, which could have some serious consequences for many species.
HUMAN encroachment is pushing wild mammals all over the globe to increasingly become creatures of the night, moving their daytime activities toward the darker hours, a new study finds.
This daytonight shift, described in the journal
Science, could have a host of implications for the health and survival of these species — and the structure of their ecosystems as a whole.
Roughly 75% of the world’s land surface has been impacted by humans, researchers say, and as animals have been trapped in these shrinking parcels of pristine land, they have had to adapt to living in the presence of cities or near human activity. Scientists have found some birds have had to change the frequency of their songs around loud urban environments; others have found that blackbirds become more sedentary.
But lead author Kaitlyn Gaynor, a wildlife ecologist and PhD candidate at the University of California, Berkeley, wondered if mammals were not just getting displaced in space, but also in time — if they were changing their routines to avoid humans, who primarily operate during the daylight.
That question had been hard to answer until recently, scientists said.
‘‘The effect of human disturbance on animal temporal activities has been difficult to assess, particularly in secretive wildlife,’’ Ana BenitezLopez, of Radboud University, who was not involved in the paper, wrote in a commentary.
‘‘In recent decades, the advent of technologies, such as satellite and GPS telemetry or camera traps, has made it possible to monitor wildlife activity more accurately.’’
Using these methods, researchers have now published studies documenting changes in wildlife activity regionally, but Gaynor wanted to find a global pattern. So she and her colleagues put together a metaanalysis of 76 papers on 62 different species studies spanning six continents.
They focused on mediumand largesized mammals, partly because these animals need a lot of space, have more potential to interact with humans, and are behaviourally very flexible. (Also, there was more data on their 24hour activity patterns.)
The researchers compared the ‘‘nocturnality’’ — that is, what share of an animal’s activity was conducted at night — of animals that lived in places with low and high human disturbance.
Gaynor found animals that lived in areas with high human activity were indeed shifting to more nocturnal activity by a factor of 1.36. (For example, this meant that an animal that used to spend 50% of its active time at night would see that share rise to 68%.)
‘‘We expected to find a trend towards increased wildlife nocturnality [across] species, but we were surprised by just how consistent the results were,’’ Gaynor said.
The trend held across continents, habitats, types of animals, and even types of human activity.
Whether that human activity was lethal (such as hunting) or largely harmless did not seem to matter, Gaynor said.
‘‘The response is of equal magnitude to activities that don’t actually pose a risk to animals, like hiking through the woods — activities that we think of leaving no trace.’’
The phenomenon was widespread — 83% of the 141 case studies they analysed saw an increase in nocturnality. Larger mammals appeared to shift more strongly, the scientists wrote, ‘‘either because they are more likely to be hunted or as a result of an increased chance of human encounter’’.
This shift could have a broad range of impacts that could ripple through an ecosystem, BenitezLopez said.
Among them: If apex predators cannot hunt as well at night as they can during the day, they may not effectively regulate the populations of prey species; a nighttime shift by one species could force them into competition with other animals who use the same resources but at different times; as some animals move into the nighttime, competitors might take over their daytime niches; animals that are sensitive to human presence might start to lose out to those that are not; seed sizes may have to evolve if the large mammals that usually disperse them during the daytime are no longer doing so; and for those animals that remain active mostly in the daytime, their stress levels might go up, which could have longterm physiological consequences that could affect their survival or reproduction rates but would be more difficult to observe.
‘‘Holistic approaches that take into account behavioural, physiological, population, and evolutionary responses to human disturbance across taxa are urgently needed to fully understand the consequences of human encroachment for the persistence of wildlife populations,’’ BenitezLopez said.
It is unclear whether the changes stop at the behavioural level, or whether having humans nearby is influencing deeper, more permanent changes.
‘‘That’s the next frontier in research,’’ Gaynor said.
‘‘We don’t really know whether these behavioural adaptations are accompanied by morphological or physiological adaptations in which animals are developing traits through natural selection that facilitate improved success at night.’’
Ultimately, the scientists said, the findings could be used to create protected times of day for wildlife, just as we already create protected spaces. In some ways, that was already done during certain times of the year for breeding seasons, the authors pointed out.
‘‘Approaches may include diurnal ‘temporal zoning’ . . . that would restrict certain human activities during times of the day when species of conservation concern are most active or when the likelihood of negative humanwildlife encounters is highest.’’