The samango monkeys living near South Africa’s Lajuma Research Centre have learned that they can rely on humans as lookouts while they forage for food. The monkeys have learned that if humans are around, then they’re probably safe from leopards. When left alone, the monkeys spend less time foraging for food and more time scanning their surroundings to avoid becoming someone else’s dinner. The mountain lions of California have a different tale to tell.
For them, human presence isn’t helpful. Instead of focusing on eating and leaving the predator patrol to the humans, it’s the humans themselves that make mountain lions wary. The fear provoked by predators is known to affect their prey in ways that can affect entire food webs. What isn’t well known is how the fear provoked in predators by humans might likewise ripple through the food web.
Large carnivores, like mountain lions (which are also called pumas, cougars, or panthers, depending on where you are), are sensitive to human disturbances because they have slow life cycles, require large territories, and suffer persecution by humans for real, imagined, or anticipated losses of pets or livestock. As a result, developed landscapes tend not to serve as homes for these apex predators.
Their localized decline or extirpation commonly leads to two phenomena: mesopredator release, which is the proliferation of smaller carnivores, like coyotes or foxes, and the overpopulation of primary consumers, like deer. But some large carnivores have found a way to persist in human landscapes. Rather than running away, they modify their behavior to reduce run-ins with us. Take P22, the famous mountain lion photographed underneath the Hollywood sign. He’s lived in Los Angeles’s Griffith Park for several years, and if not for a series of camera traps, would generally remain undetected.
Even if they’re not driven away completely, the mountain lions’ modified behavior is still likely to alter their communities. That’s what University of California, Santa Cruz researchers Justine A. Smith, Yiwei Wang, and Christopher C. Wilmers wanted to find out.
Think of it as the tale of the city mouse and country mouse, but replace the rodents with very large, very hungry cats. Do mountain lions who spend their time in more developed areas have different foraging behavior than their more rural counterparts?
Wilmers’s group used data from thirty GPS-collared mountain lions (Puma concolor) who live along a gradient of urbanization in the Central Coast region of California, from mostly natural landscapes with no housing development to suburban regions with up to ten houses per hectare.
As typical solitary hunters, mountain lions expend quite a bit of energy bringing down large ungulates like deer, and gain a considerable amount of nutrition from the carcasses for several days. They’re physically constrained by the amount of food their guts can contain, which is why they keep returning to the same kill site to feast upon their leftovers.
In cities, carnivores like mountain lions can be displaced from their kill sites by human activity. That leaves the carcass open for scavenging by other predators or carrion eaters and, if left long enough, to decomposition. Taken together, the big cats might suffer from reduced caloric intake due to a perceived risk from human activities. Wilmers hypothesized that this would occur more often in more developed areas.
As a consequence of not being able to finish their meals, it turned out that city cats’ hunger simply drove them to take down more deer just to get the same amount of food. For the females, at least. Their overall time spent eating declined by 42% as housing density increased. Those in more urbanized areas killed 36% more deer than the females in the most natural habitats.
(The male lions killed the same number of deer per year regardless of their proximity to urban development. They may already spend less time at kills owing to their need to patrol their territories, which are three times larger than the females’.)
What that means is that the females are expending more energy to obtain an equivalent amount of nutrition, and more deer are dying as a result. And that could have consequences for their reproduction. “The tagged female living in the most developed habitat in our study area has lost at least three litters in the last 3 years, one of which was confirmed as abandonment,” write the researchers. Those living in more natural landscapes appear to have healthier litters. While there are many elements of city life that could result in this pattern, the increased energy expended while hunting is certainly a contributing factor.
The pattern is also likely to modify the entire wildlife community. More deer are dying to feed the mountain lions, and partially eaten carcasses provide a nutritional bounty for the ecosystem’s carrion eaters, scavengers, and smaller carnivores. Mesopredator release is usually associated with the extirpation of apex predators, but these findings suggest it could occur simply as the result of behavior changes among those apex predators.
Population declines and extirpations are both more intuitive and easier to explain to the public than the more subtle ways in which human behavior leads more resilient species to modify their behavior.
But if humans are to co-exist peacefully with wildlife, it is prudent to better understand how animals respond to our behavior. That requires a shift in thinking: humans are not superimposed onto otherwise natural ecosystems; we are full, active participants in a new sort of emulsified ecosystem, part wild and part domestic. – Jason G. Goldman | 30 January 2015
Source: Justine A. Smith, Yiwei Wang, & Christopher C. Wilmers. (2015). Top carnivores increase their kill rates on prey as a response to human-induced fear, Proceedings of the Royal Society B: Biological Sciences, 282 (1802) 20142711-20142711. DOI: http://dx.doi.org/10.1098/rspb.2014.2711
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