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In this De-Natured segment of Nature League, Brit breaks down a recent scientific journal article about the increasing nocturnality of mammals around the globe.

Article citation:
Gaynor, K.M., Hojnowski, C.E., Carter, N.H., and Brashares, J.S.

The influence of human disturbance on wildlife nocturnality.

Science, 2018

Article link:

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On Nature League, we spend the third week of each month exploring a current trending article from the peer-reviewed literature.

Scientific information isn’t just for scientists- it’s for everyone! It just requires a bit of a break down. [CHEERY INTRO MUSIC].

For this month’s De-Natured segment, we’re going to look at an article released in June 2018 in the journal Science. In this month’s Lesson Plan, we talked about the different ways that organisms adapt to their environments. Most of the adaptations we discussed were physiological...but that’s just one type of adaptation.

Life on Earth can also adapt to changing environments by changing their actual behavior. In this paper entitled, “The influence of human disturbance on wildlife nocturnality”, the researchers investigated a very specific behavior: nocturnality. Nocturnality is the state of being nocturnal, meaning that an organism is active mostly during the night once the sun has set.

So why investigate what time of day an animal is active? In this particular study, the researchers wanted to know if animals are changing their activity levels throughout the day in response to a very specific factor: human disturbance. So here’s what’s already known.

The expansion of humans across the Earth has majorly impacted the rest of life on Earth. Things like habitat loss, fragmentation, and degradation have been documented as big factors in the current alarming trends of biodiversity extinction. However, animals don’t /quite/ have enough time to adapt to things like their homes being rapidly destroyed.

This means that a lot of the ways that animals adapt to humans being in their space is actually indirect. For example, several studies have demonstrated that animals will relocate, or move around to other areas when humans expand into their territories. This is called spatial avoidance, and we also see this sort of adaptation in a number of predator and prey relationships.

Basically, if you don’t want to be next to someone...ya move. But here’s the thing- the human population is getting massive, and we keep expanding to new areas. This means that it’s becoming harder and harder for species to avoid us, at least spatially.

So if certain species on Earth aren’t able to avoid humans in space, what about avoiding us in something like... time? This is something called “temporal partitioning”, and it’s when species are active during different times of the day in order to avoid things like competition or predation. In this brand new paper, scientists quantified how wildlife are spending their time in response to human disturbance.

To address this phenomena, the researchers conducted something called a meta-analysis. This is when a study studies a bunch of studies. Hence the whole “meta” thing.

The researchers when through the scientific literature and found studies that recorded the activities of mammals across gradients of human disturbance. They focused on 63 medium and large bodied mammals from across 6 continents. The goal was to measure the degree of nocturnality in response to human disturbance.

To do this, the researchers measured the effect size by calculating a risk ratio. The ratio compared the amount of nighttime activity of species in high human disturbance areas to nighttime activity of species in low human disturbance areas. A positive risk ratio meant a higher amount of nocturnal activity in response to humans, and a negative risk ratio meant reduced nocturnality.

So what did they find? Overall, the study found an increase in nocturnality by a factor of 1.36 in higher human disturbance areas or seasons when compared to lower human disturbance areas or seasons. But what’s that “factor of 1.36” mean in practice?

The authors give a great example of this increase in nocturnality in response to humans by re-framing this number as a percentage of time spent active during the day. Basically, if a species typically spends 50% of its active time during the day, and 50% during the night, an increase by a factor of 1.36 due to human disturbance means that the species now spends 68% of its active time during the night. Overall, there were some interesting nuances in the study design.

First off, there are many different things that fall under the category of “human disturbance”. To address these options, the authors analyzed a variety of human disturbances with varying degrees of harm to wildlife. These included things like lethal activity, agriculture, development, hiking, and vehicles.

One really interesting result of the study is that nocturnality significantly increased in response to all forms of human disturbance that the authors included. The authors also dug a bit deeper into the differences between the mammals they included in the study. They compared nocturnality in response to human disturbance across body sizes and trophic, or food web, levels.

In terms of body size, the researchers found that all body sizes responded strongly to human disturbance. However, the largest size class showed a slightly higher response. Similarly, all trophic levels showed responses to human disturbances.

Even apex carnivores displayed an increase in nocturnality, which is kind of amazing when you think about it. These are species like lions; you know, kings of their realms. Apex carnivores are used to being at the top of the food chain; and yet, this study found that even these creatures are changing their habits to avoid humans.

And this trend wasn’t just seen in one place or habitat type. The researchers found an increase in nocturnality in response to humans on all 6 continents they studied and across all types of habitats. So overall, this means that mammals around the world are preferring the night life.

However, this study was a meta-analysis, meaning that the authors analyzed many different studies, and some of those studies didn’t report an increase in nocturnality due to human impact. The authors explain that although they found this lack of increased nocturnal behavior in some studies, this /doesn’t/ mean that humans are not impacting the non-human animals around them. There are all sorts of factors that might limit a population from changing its daytime/nighttime activity routine, including differences in age, sex, personality, and food constraints.

But, overall the authors found significant increases in nocturnal activity among mammals closer to human disturbances. This led to the question of why. In the last piece of their article, the authors offer one big potential reason for the increase in nocturnality: fear of humans.

This article was published in the journal Science, which is highly prestigious. Not only that, but the results made some headlines in the news as well. Here are some reasons why I think this study is capturing both scientists and citizens.

For one, the topic is super timely. The extinction rate of species /and/ the expansion of the human population are both currently happening at rapid rates. This means that human and non-human species are all sort of trying to figure it out, but in super speed.

The human population relies upon other species on Earth, but there’s a constant conflict for coexistence. The results of this study are important because for they quantify for the first time time a true shift in behavior world wide. A smaller result mentioned in the study happens to be a big one in my book, and that’s what they found when comparing the effects of lethal vs non-lethal human disturbance.

The researchers actually found that non-lethal human disturbances, like hiking and walking, had just as much of an effect on increased nocturnality as lethal human disturbances, like hunting. This finding is fascinating, and certainly a big wake-up call for the ways in which we humans view our actions. Basically, we don’t have to be actively killing mammals for them to start preferring the night.

Our mere existence in their habitat is enough to potentially drive a fundamental change. As with any piece of new research, there are several areas of improvement that exist in this study for follow-up research. My first issue with the study is that is was based on mammals.

Not only that, but based on medium to large sized mammals. The problem here is that representation matters. Mammals make up a very small portion of life on Earth, and the largest group of mammals are the rodents, which are smaller.

Basically, this means the results aren’t completely applicable to the majority of life on Earth. Another spot of contention is that this study was a meta-analysis, meaning that there weren’t any new experiments conducted. This means we can only really discuss the correlations between nocturnality and human disturbance, and can’t talk about the potential for causation.

Improvements aside, the part of this paper I absolutely loved was the discussion of applications to ecology and conservation. The authors offer several points of interest:. First, if species are able to separate themselves from humans in time, it means that there might be fewer conflicts over space.

And if separation is occurring, that can lead to less instances of negative human-wildlife conflicts like attacks and transmission of disease. On the darker side of things, this shift to nocturnality could mean trouble for these species in terms of the way they adapt over time. It’s possible that the shift to nighttime activity could negatively impact crucial things like reproductive success and food web interactions.

After all, these species have been evolving traits over millennia that were selected for based on being diurnal, or active during the day. It’s hard to all of a sudden start selecting for traits that work in the night. Even though there’s a lot of doom and gloom surrounding the topic of humans disturbing wildlife, this study found remarkable levels of adaptations by mammal species all over the world to adjust to life in this human-dominated era.

Thanks for watching this episode of De-Natured here on Nature League. Nature League is a Complexly production: check out this episode from our sister channel SciShow if you’d like to learn more about wildlife and cities.