Ecology is all about understanding how the various parts of the natural world interact with one another. While we tend to think about things like predators, competitors, and parasites as separate entities that have their own effects, it is important to remember that these species interactions can interact with one another. Such interactions will have implications for the dynamics of natural populations.
Of interest is how predators and parasites interact with one another through their shared resources, prey/host species. Specifically, the Healthy Herds Hypothesis (HHH, see Did You Know?) predicts that predators reduce parasitism within the populations of their prey. While the HHH was based on a mathematical model, other theoretical models predict a range of effects, from predators decreasing parasitism to actually increasing parasitism. Because the empirical results from experimental studies show similar variation in their results, today’s authors wanted to determine if there is indeed a consistent, overall effect of predators on the parasitism of their prey.
Arthropod predation of vertebrates structures trophic dynamics in island ecosystems (2021) Halpin et al., The American Naturalist,https://doi.org/10.1086/715702
Predator-prey dynamics are (I think) the most well-known species interaction out there. Not everyone is a scientist, but almost everyone has seen an image of a cheetah running down a gazelle, a great white shark exploding out of the water as it hammers a seal from below, or wolves teaming up on a much larger herbivore.
These interactions are not only fascinating and captivating, they are also key to structuring communities. For example, the damselflies that I worked with during my PhD occur in two different kinds of lakes: fish lakes and dragonfly lakes. The type of predator alters the lake significantly: damselflies that live in fish lakes are adapted to “hide” from their fish predators by not moving. Not moving in a dragonfly lake means that a dragonfly will eat you.
Though these interactions have been (justifiably) studied to an extreme extent, there are still knowledge gaps out there. Of interest for today’s study is the effect of invertebrate predators on vertebrate prey. While these invertebrate predator/vertebrate prey interactions have been studied in marine and freshwater environments, little work has been conducted in terrestrial systems. This is especially hard to do with invertebrate predators of vertebrate prey, because such predators tend to be hard to find, nocturnal, and they also hunt in more “concealed” environments like leaf litter. To overcome these challenges, today’s authors utilized the Phillip Island centipede (Cormocephalus coynei, which is NOT the centipede featured in this post’s image) and stable isotope analyses (see Did You Know) to understand how invertebrate predators structure food web dynamics.
Do seasonal dietary shifts by Temminck’s pangolins compensate for winter resource scarcity in a semi-arid environment?(2022) Panaino et al., Journal of Arid Environments, https://doi.org/10.1016/j.jaridenv.2021.104676
The Crux
Climate change and habitat destruction are never positives for any species, but if that species is adaptable, they might be able to tolerate such hazards. Finding food in changing environments is one of the big challenges for a species, and there are a few things that species can do to adapt their diets when new conditions emerge. They can try and find more energy-rich food, or they could spend more time foraging to increase their energy intake (though this could also cost them more energy as well).
This is a particular problem for species which live in dry, arid environments, where food can already be scarce. In many environments vegetation and all the animal life that comes with it will often increase with temperature, but in an area that is already hot, at a certain point an increase in temperature will result in lots of vegetation dying off.
Today’s researchers selected a particularly charismatic species, the Temminck’s pangolin, and through some truly admirable fieldwork, monitored whether or not their foraging activities changed as the environment varied.
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.
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.
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 (Thamnopholisspp.) 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).
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.
The importance of functional responses among competing predators for avian nesting success (2019) Ellis et. al, Functional Ecology, https://doi.org/10.1111/1365-2435.13460
The Crux
In our ever-changing world, natural populations of different species are experiencing changes in both size and range. Part of the difficulty in predicting or responding to these changes is that ecological systems are made up of complex webs of species interactions, all of which have the potential to affect how populations respond to these changes. One of the most important interaction that occurs between species is predation.
Predators can affect the way prey species look, behave, and even where they live (see the Did You Know section). Different predator species can have varying effects on their prey, and as such it is important to consider these differences whenever wildlife managers make policy decisions on how to manage and control endangered populations. The authors of today’s paper were interested in uncovering how different predator species affected prey, using the snowy plover (Charadrius nivosus).
In an eat or be eaten world, the survival of the fittest can come down to who the most physically able is. Today’s paper investigated the athletic ability of sidewinder rattlesnakes relative to their kangaroo rat prey. (Image Credit: Tigerhawkvok, CC BY-SA 3.0, Image Cropped).
In nature, many animals are part of the predator-prey cycle. One animal is subject to being eaten by the other, and must escape in order to avoid this fate. Despite what you may have seen on a variety of amazing nature documentaries, most predator-prey interactions don’t involve some flashy takedown and subsequent meal for the predator. Predators usually fail far more often than they succeed, with one of the most successful animals on the planet (the killer whale) only succeeding HALF of the time.
These interactions between predators and their prey depend on two things: the predator’s physical attack ability/performance and the prey’s escape ability. Basically, who is more athletic? There are many different ways that predators try and take down their prey, but the authors of today’s paper wanted to know what the key aspects of the predator-prey interaction are, and which of them is most important for each participant. Read more
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 Clarke, CC 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
Ecology for the Masses 
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