Tag Archives: predator

Predators Under Nightlights

This is a guest post by Dr. Mark Ditmer.

Streetlights like these have meant that for some animals, hiding in the dark is now increasingly difficult (Image Credit: ME Stoner, CC BY 2.0, Image Cropped)

Artificial nightlight alters the predator–prey dynamics of an apex carnivore (2020) Ditmer et al., Ecography, https://doi.org/10.1111/ecog.05251

The Crux

The earth is no longer dark at night – artificial lighting has degraded the dark nighttime conditions that many species have evolved with throughout their evolutionary history. This change is only accelerating, with human expansion and intensity of radiance continuing to increase annually. We already know that elevated light levels can disrupt ecological processes like pollination or migration, as well as have a litany of negative effects on individual species, from physiological stress to predation risk. But it’s hard to get an idea of how the increase in ‘light pollution’ affects free-roaming wildlife, especially large mammals, and especially at scales relevant for making conservation policy.

In areas like the American west, the rapid growth of urban areas and the accompanying spread of light pollution create a rapidly changing ecosystem, one that sees many conflicts between humans and wildlife. One particularly species of particular interest is the mule deer (Odocoileus hemionus), which seeks out sources of forage on the edges of and within towns and cities (e.g. parks, farms), especially in arid regions. The primary predator of mule deer – the cougar (Puma concolor) – also navigates and hunts near human development where their prey congregate, but tend to avoid human presence more so than deer.

Today’s authors wanted to assess how artificial lighting, both where it occurs and its intensity, can shape the behaviors and predator-prey interactions of these species across the American West ranging from the edges of bright urban regions, such as Salt Lake City (Utah) and Reno (Nevada), to areas receiving minimal light pollution like Grand Canyon National Park.

What They Did

The authors used a massive dataset that included GPS-locations from 263 mule deer, 56 cougars, and 1,562 locations where cougars successfully killed mule deer. The resulting location data were combined with estimates of anthropogenic light pollution (more on this in Did You Know?).

Several different analyses were performed on the combined light and GPS-location data, along with other variables representing environmental (e.g., snow cover, land cover, terrain) and human factors (e.g., distance to roads, housing density). The aim was to figure out whether A) light has any influence on the behavior of each species, B) cougars avoid areas with high light pollution, allowing deer to forage freely wherever and whenever they want (the ‘predator shield hypothesis’), or C) cougars exploit the higher densities of deer seeking forage around areas with elevated light pollution (e.g., parks, golf courses, agriculture; the ‘ecological trap hypothesis’).

Did You Know: A Space Agency’s Ecological Impact

In this study we used remote sensing data to determine the amount of light pollution in a given environment. Yet the sensors only pick up the total amount of light, and can’t tell us what is a product of our activity and what is a natural source of light. To separate the two, we used light data which was recently developed by the U.S. National Aeronautics and Space Administration (NASA). This dataset removes the contributions of natural sources of light (e.g., moonlight, fire, atmospheric spray) from our data and results in values of just the human-created nighttime light emissions.

What They Found

The behaviors of both species changed greatly with levels of light pollution, as did the predation risk for deer. The behaviour changed across different scales as well. Cougars killed deer in study sites with the high amounts of light pollution, but within those sites (e.g., edge of Salt Lake City, Utah) cougars selected to hunt and kill in the relatively darkest locations. In contrast, in the darker study areas, cougars killed deer in areas with the relatively more light pollution than the surrounding area. However, even though cougars killed deer in the darkest spots within the bright urban interface, those locations generally had much higher levels of light pollution than the brightest kill sites in the low light pollution study areas.

The study found that cougars made kills in bright regions, but generally only within the darker parts of those regions (Image Credit: ME Stoner, CC BY 2.0)

Deer living in brighter urban areas tended to forage at night, potentially to avoid direct human interactions. This shift might have benefited deer by avoiding humans, but as they sought out more natural and dark locations in these areas, cougars would wait in ambush.

In the end, the authors concluded that their findings fell in a gray zone between the predator shield and ecological trap hypotheses dependent on scale. Areas with high levels of light and subsequent human activities provide excellent foraging opportunities for ungulates (as this study measured as well), but adaptable predators can follow and take advantage – at least in environments that they feel are safe enough.

Problems

This is an observational study, so it’s hard to fully tease apart what effects are driven by light and what are driven by other human factors. We did our best to account for the other more traditional sources of the human footprint, reporting effect sizes for each, but there’s always a chance we’re attributing some effects to light pollution that could be caused by some other aspect of our presence.

So What?

Work like this shines a light on (pun intended) how different species will respond to the ongoing urbanization trends humans are driving in much of the planet.

Although many wildlife ecology studies consider various human alterations to habitats and the consequent changes in animal behavior, most studies fail to consider the sensory environment and the pollutants (e.g., noise, light) that can impact wildlife populations in their analyses. How wildlife use an ecosystem can impact everything from human-wildlife interactions to pulses of nutrients to the soil based on shifting areas of kill sites/carcasses.

Dr. Mark Ditmer is a post-doctoral researcher at the Centre for Human-Carnivore Co-Existence and Wittemyer Lab at Colorado State University. You can read more about him at his profile here or follow him on Twitter @MDitmer.

Predator Poop Propagating Plant Persistence

Image Credit: Rene Rauschenberger, Pixabay licence, Image Cropped

An omnivorous mesopredator modifies predation of omnivore-dispersed seeds (2021) Bartel & Orrock, Ecosphere, https://doi.org/10.1002/ecs2.3369.

The Crux

The evolution of different methods of seed dispersal has played a huge role in shaping plant diversity and distribution. Earlier plants could only use the water or wind to disperse their offspring, but eventually plants evolved the ability to harness the movement of animals, letting their seeds disperse often further and more efficiently than before.

Seeds are also a vital form of food for many species, including small rodents and insects. Larger animals too, including wild boars, bears, and coyotes who will get stuck into berries when there’s plenty around. This leads to them leaving berry seeds mixed in with their faeces. We might be deterred by the idea of picking dinner out of another animals poop, but many of those rodents and insects don’t mind.

But what about when those faeces are from one of your predators? Do you still want that seed, or should you get the hell out of an area clearly inhabited by a threat to your livelihood? The answers to these questions can determine which seeds get left where, which in turn can determine where plants end up taking root and spreading to. That’s the focus of today’s study.

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Risk it for the Biscuit

Environmental controls on African herbivore responses to landscapes of fear (2021) Davies et al., Oikos. https://doi: 10.1111/oik.07559

Image Credit: Olga Ernst, CC BY-SA 4.0, Image Cropped

The Crux

Despite the incredible variation seen in nature when it comes to flora and fauna, it always seems like the two types that most people know are predators and prey. Prey animals being those that eat plants (or other animals), and the predators being those that eat those prey animals. Because prey animals must not only eat food, but try to avoid becoming food for something else, they must always be on the lookout. This watchfulness and awareness is what creates a “landscape of fear” (See Did You Know?), but variation is inherent to the natural world, and there are likely many things that prey animals consider when they pick where they decide to forage. Today’s authors wanted to investigate what factors influence the prey animals choice of foraging areas, and if that selection varies with the environment during the dry season when there isn’t much food available.

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Form Versus Function

Image Credit: Graham Wise, CC BY 2.0, Image Cropped

Sexual differences in weaponry and defensive behavior in a neotropical harvestman (2018) Segovia et al., Current Zoology, https://doi.org/10.1093/cz/zoy073

QUICK NOTE: Harvestmen (aka Daddy Long Legs in North America) are NOT spiders! Despite the false myth that they can’t bite you due to short fangs, harvestmen aren’t even venomous. They can’t hurt you! There, now that I got that off my chest…

The Crux

Sexual dimorphism is a common phenomenon in nature whereby male and female members of a given species differ from one another physically. Think of the large bull moose or elk with its antlers, peacocks and their colorful tails, or the larger horns of male stag beetles. Because of these differences, natural selection is able to act on both their behavioral and functional differences. That is to say, differences in performance and morphology mean that males and females of the same species may experience differential selection pressures. As a result, males and females could be expected to react differently to the same challenge, such as a predator.

Harvestmen (known in North America as Daddy-Long-Legs) are a group of arachnids that, although bearing a resemblance to and being commonly mistaken for spiders, are not actually spiders. They belong to a group called Opiliones. Some males of this group have thicker legs with pronounced spines, used in male-to-male competition and anti-predator defenses. In addition to using these spines against predators, these arachnids also engage in thanatosis (“playing dead”, see Did You Know?) and use chemical defenses. Due to these morphological differences, the authors hypothesized that males and females would differ in their response to predators.

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Don’t Compete If You Don’t Want to Get Eat(en)

Image Credit: Judy Gallagher, CC BY 2.0, Image Cropped

Predators weaken prey intraspecific competition through phenotypic selection (2020) Siepielski, Hasik et al., Ecology Letters, https://doi.org/10.1111/ele.13491

The Crux

We are all familiar with predator-prey relationships in nature, those in which one organism (a predator) kills and consumes another (the prey). Besides these direct effects on prey via consumption, predators can also impose indirect effects on their prey. An indirect effect is one in which the predator changes some aspect of the prey, such as their behavior or the way that they look, but these changes are brought about just by the predator being around. These predator-mediated effects are known to affect the relationships between prey organisms themselves, such as how prey organisms compete with one another, whether its for food, mates, or other resources.

Predators are known to affect how active their prey are, and this selection on activity results in a trade-off between how much prey can grow and their risk of predation. Being more active can allow you to find and eat more food, but that also means that a potential predator is more likely to see you. Today’s paper used larval damselflies and their fish predators to study how selection of fish on their damselfly prey based on the damselfly activity rates affected competition between the damselflies.

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Protection from Two Enemies with One Defense

Image Credit: Connor Long, CC BY-NC-SA 3.0, Image Cropped

Of poisons and parasites—the defensive role of tetrodotoxin against infections in newts (2018) Johnson et al., Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.12816

The Crux

Many organisms in nature produce powerful (and sometimes deadly) toxic substances, often taken as evidence that prey evolved chemical defenses against predators. Interestingly, these chemical defenses are deadly not only to predators, but also to parasites. This complementary defense, in addition to the ubiquity of parasites themselves, indicate that parasites may have had a hand in the evolution of host toxicity.

One particularly potent toxin found in the animal kingdom is tetrodotoxin (TTX). It can cause paralysis, difficulty with breathing, and even death in some cases. Newts in the genus Taricha are notorious for having high concentrations of TTX in their skin and eggs, and this has long been thought to have evolved as a defense against predators. In particular, Taricha newts and garter snakes (Thamnopholis spp.) are a classic example of arms-race dynamics (see Did You Know). Despite this relationship, newt toxicity and snake resistance to the toxin don’t always match up perfectly in nature, suggesting that other factors may influence newt toxicitiy. The goal of today’s study was to study parasitic infection and compare it to variation in toxicity among two newt species, the rough-skinned newt (T. granulosa) and the California newt (T. torosa).

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Brains Over Brawn

Image Credit: Kevin Pluck, CC BY 2.0, Image Cropped

Brain expansion in early hominins predicts carnivore extinctions in East Africa (2020) Faurby et al, Ecology Letters, https://doi.org/10.1111/ele.13451

The Crux

We’ve covered humans and their harmful effects many times here on Ecology for the Masses (see my recent breakdown from last month). Despite all of the colorful examples of our current effects on the wildlife of our planet, a significant amount of research has implicated Homo sapiens as the driver of the extinction of some of the megafauna of the prehistoric world, events that happens millions of years ago. Another possibility is that we as organisms (hominins, not Homo sapiens specifically) have been impacting other species for a very, very long time.

Today, East Africa is home to the most diverse group of large carnivores on the planet (though it is still less diverse than what was once seen in North America and Eurasia). Millions of years ago East Africa had an even more diverse assemblage of large carnivores, including bears, dogs, giant otters, and saber-toothed cats. The change in climate since that time may have caused the decline in large carnivore diversity, but another explanation is the rise of early hominins (our ancestors). Using fossil data, the authors of today’s paper wanted to figure out if it was indeed early hominins that drove many large carnivores extinct.

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The Early Mouse Gets the Cheese

For small animals like the mouse, predators are a constant concern (Image Credit: Jess, CC BY-NC 2.0)

Maximising survival by shifting the daily timing of activity (2019) van der Vinne et al., Ecology Letters, https://doi.org/10.1111/ele.13404

The Crux

All animals need to eat food to survive and maintain their energy balance, but unlike us they can’t just order a pizza and have the food brought to them. They must always forage for food themselves, and every time that they do they expose themselves to predators. Small mammals like mice balance this trade-off by foraging for food at night, when their risk of predation is lowest.

One interesting strategy that mice can employ is to switch their foraging from the nighttime to the day, if they cannot get enough resources during the night or if their nighttime predation risk increases. The authors of today’s paper wanted to develop a model to predict under what conditions these temporal switches would occur, a model which they then tested with mice in the field.

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Outdoor Cats are a Problem

Outdoor cats are a contentious issue for cat-owners, cat-lovers, and those that are concerned about the environment. Like it or not, Fluffy is doing a LOT of damage (Image credit: Cat Outside in Sweden-148884.jpg by Jonatan Svensson Glad, CC BY-SA 4.0, Image Cropped).

I hate to be the bearer of bad news, but domestic cats are bad for the environment. Sure, we as a species have adopted and incorporated them into our society (I live with two, myself), but that doesn’t mean we aren’t responsible for them and their actions.

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Do As I Do

Predators like these Great tits (Parus major) eat a wide variety of insects, but some of those insects are so unpleasant to eat that birds tend to avoid them. How does this trait evolve in prey animals when its maintenance and origin depend on the predators learning by eating them? (Image Credit: Shirley ClarkeCC BY-SA 3.0).

Social information use about novel aposematic prey is not influenced by a predator’s previous experience with toxins (2019) Hämäläinen et al., Functional Ecology, https://dx.doi.org/10.1111/1365-2435.13395

The Crux

Many animals in nature have evolved a defense strategy known as aposematism, meaning that they display warning colors or patterns that tells predators that they are not worth eating due to their toxicity. Predators can learn to avoid aposematic prey by either sampling different prey animals and learning for themselves, or they can watch other predators eat different prey species and, depending on the reaction of that predator, learn what may or may not be good to eat.

The paradox of the evolution of this aposematic trait is that toxic prey species are not only highly visible and easily noticed by predators, but they must be attacked in order for predators to learn that they shouldn’t eat them, meaning that these prey species may not even survive long enough for them to enjoy the benefits of predator avoidance. The question then becomes are aposematic prey able to persist in nature because predator learn to avoid them? The authors of today’s paper wanted to investigate how predators that have learned to avoid toxic prey will watch and learn from other predators eating new, possibly toxic prey.  Read more

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