Testing the parasite-mediated competition hypothesis between sympatric northern and southern flying squirrels (2022) O’Brien et al. 2022, International Journal for Parasitology: Parasites and Wildlife, https://doi.org/10.1016/j.ijppaw.2021.11.001
Image credit: Stephen Durrenberger, CC BY-NC-SA 2.0, Image Cropped
One consequence of climate change is that organisms move to new habitats, as they try and track suitable environmental conditions. This can result in closely related species coming into contact with one another, which in turns drives competition among these organisms. Competition between these organisms can manifest as either direct competition (where two organisms directly compete with one another for food or habitat), but it can also manifest as apparent competition.
Apparent competition happens when species A serves as a food source for predators or parasites, which increases the numbers of predators/parasites in the environment. This increase in predators or parasites then puts more pressure on species B. Apparent competition via parasitism was actually a major driver for the decline of red squirrels in the UK, as the introduced grey squirrel brought along squirrelpox virus that had severe effects on the red squirrels.
If one species is more tolerant to a parasite than another, this can result in competitive exclusion, where one species outcompetes the other species to such an extent that the outcompeted species goes locally extinct. This is particularly important when a climate-mediated range expansion brings two species together that share parasites. Today’s authors sought to quantify how infection by parasites affected a vulnerable population after a range expansion by a potential reservoir species.
Population size impacts host-pathogen coevolution (2021) Papkou et al. 2021, Proc B, https://doi.org/10.1098/rspb.2021.2269
Image credit: Kbradnam, CC BY-SA 2.5, via Wikimedia Commons
Host-pathogen interactions are maybe best characterized as a battle – a pathogen (a parasite that causes disease) doing what it can to maximize how much it can get from a given host organism, and a host doing what it can to defend itself from this endless attack. As a result, hosts and pathogens are locked in an endless evolutionary battle, whereby hosts evolve to better defend themselves and pathogens evolve to better attack the host. A key factor in this battle is population size, as this affects the evolutionary potential of a given population of organisms to respond to selection.
The larger a population of hosts, the more novel genetic variants there are, which are simply organisms with different genetic make-ups, which can be the result of mutations popping up or through combinations with other genetic variants within the population. The more variation there is, the more diverse the population is, and the more chance it has of carrying the genes that could help it respond to a new threat, like a pathogen.
This means that a larger host population is more likely to have a genetic variant that is able to defend itself from these pathogens. That variant will then be selected for and the host population will become more resistant to that pathogen over time. While a lot of theory has been dedicated to understanding these coevolutionary battles, actual experimental evidence is lacking. Today’s authors used a model system to conduct evolutionary experiments to test the effect of host population size on host-pathogen coevolution.
Arthropod predation of vertebrates structures trophic dynamics in island ecosystems (2021) Halpin et al., The American Naturalist, https://doi.org/10.1086/715702
Image credit: Bernard Dupont, CC BY-SA 2.0, via Wikimedia Commons
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.
Predicting how climate change threatens the prey base of Arctic marine predators, Florko et al., 2021 Ecology Letters. https://doi.org/10.1111/ele.13866
Image credit: Kingfisher, CC BY-SA 3.0
We are all (unfortunately) very familiar with the effects of climate change on arctic ecosystems. Horrifying images of polar bears on small blocks of ice and the shrinking polar ice caps are but two of the many results of a warming climate, yet a great deal of the work in the realm has focused on the the charismatic, apex species (like the aforementioned polar bear). These are obviously important things to consider, but it is also necessary to look into the effects of climate change on the lower positions within food webs, as any change to these organisms and processes are likely to cascade upwards to effect the upper trophic levels (like our friend the polar bear).
Hudson Bay in North America is one such area impacted by our warming climate. Due to the changes in temperatures, the energy flowing through ecosystems has shifted away from away from species living in the ice and on the bottom. As a result pelagic (free-swimming) species are favored over benthic species (those living on the bottom of the bay), which alters the rest of the food web itself. Specifically, the fish that feed on pelagic species are increasing, while those that feed on benthic species are decreasing. Today’s authors wanted to understand how these changes in fish numbers are will affect Arctic predators, namely the ringed seal (Pusa hispida).
The disruption of a keystone interaction erodes pollination and seed dispersal networks, Vitali et al., 2021 Ecology. https://doi.org/10.1002/ecy.3547
Image credit: Ennio Nasi, CC BY 4.0
Ecological communities are incredibly complex networks, made up of interactions between the species that reside in them. To properly understand how these interactions shape a community, researchers have to employ a variety of analytical methods and modelling approaches. This was something that I had to learn to appreciate in my work, because I always thought that studying ecology would involve a lot of time outdoors working with animals. While that does happen (and I spent months outside during my PhD), most of the ecological research I’m familiar with centers on math and statistics.
Using math and statistics to model ecological communities helps us to break down how various organisms are connected with one another. For example, keystone species are organisms that are connected to so many others within a given ecosystem such that any change to their populations will have consequences for the entire community. Understanding the processes that affect these keystone individuals (and all of the organisms linked to them) is vital to predicting how processes such as climate change and invasive species will affect natural communities in the future.
Today’s authors investigated how disruption of an important species interaction affected pollination and seed dispersal networks in Patagonia. A hummingbird species (Sephanoides sephaniodes) is the main pollinator for a mistletoe species (Tristerix corym-bosus), while the mistletoe provides the hummingbird with nectar in the winter. The colocolo opossum (Dromiciops gliroides) is a small marsupial that is vital for the mistletoe, as mistletoe seeds must pass through the opossum’s gut to trigger their germination. Additionally, the opossums defecate many seeds on branches in a “necklace” arrangement, which likely helps the mistletoe to parasitize their plant hosts. These three species are tightly connected to one another, and any reduction in abundance for one species may affect the other two, and even destroy the entire food web.
Biotic interactions are more often important at species’ warm versus cool range edges, Paquette & Hargreaves, 2021 Ecology. https://doi.org/10.1111/ele.13864
Image credit: Trey Ratcliff, CC BY-NC-SA 2.0
In nature, we usually refer to the given area in which a species is found as a species range. The size of these vary, even between species that are very similar in appearance. For example, many of the dragonflies and damselflies I worked with during my PhD research could be found all over the state of Arkansas, but others had more limited ranges, and could only be found in the more southern lakes that I visited. Often, species are limited to these areas because the environmental conditions, such as temperature, are favorable to them, and the change in those conditions beyond the boundaries of their range will lead to them suffering. Knowing which factors limit the range of a given species is important for management policies, as knowing the temperature limits can inform predictions about the effects of climate change, while knowledge of natural enemies (like predators) can help with the containment of invasive species.
Previous work on the constraints experienced by species at their range limits tend to focus on abiotic factors (temperature, precipitation, etc.), as these data are easily quantified and there are very extensive records available. However, biotic factors (interactions with predators/competitors, the availability of prey) can also limit the range of a species. Though biotic factors are important, they are more difficult to quantify than abiotic factors, and are often species-specific. That is, the effect of a competitor on limiting the range of one species won’t be the same on another species. Interestingly, biotic interactions may be more important in warmer range limits, while the abiotic may be more important in the cooler range limits. Today’s authors used data from a number of studies to test just that idea.
Image credit: Charles J. Sharp, CC BY-SA 4.0
When ecology fails: how reproductive interactions promote species coexistence (2021), Gómez-Llano et al., Trends in Ecology and Evolution. https://doi.org/10.1016/j.tree.2021.03.003
Scientific literature, like many different aspects of society and culture, goes through periods where a given subject/topic is more prominent in the public conscience than others. Lately, the question of coexistence has been at the forefront of the minds of many community ecologists. Coexistence is the state in which two or more species can each maintain a population in the same habitat as each other, provided that the environmental conditions and species interactions that they experience remain stable. Many studies of coexistence have investigated how differences among coexisting species allow them to maintain their coexistence, which makes sense, as it’s hard to coexist with another species if they require the exact same food or habitat as you do.
Yet there are a lot of examples of coexisting species that seem to be almost identical. Some researchers have suggested that these networks of similar species are unstable and should break down over time. But are these groups of species truly doomed? Or are there other processes maintaining this seemingly unlikely coexistence?
Today’s authors suggest that reproductive interactions among species are what may allow such similar species to continue coexisting. While much of the work in this area is theoretical rather than empirical (see Did You Know?), the authors reviewed what empirical evidence they could. Today’s paper is a review (a paper that summarizes lots of previously published papers with the goal of synthesizing knowledge), so I will briefly touch on the main points as put forward by the authors.
Image credit: Alex Proimos, CC BY-NC 2.0, Image Cropped
Experimental habitat fragmentation disrupts nematode infections in Australian skinks (2019), Resasco et al., Ecology. https://doi.org/10.1002/ecy.2547
Habitat destruction is an all-too-familiar side effect of human development and expansion. But another prevalent issue is habitat fragmentation, whereby habitat isn’t completely destroyed, but instead broken up into fragments and separated by developed areas. While some may think this is good, because there is still habitat available for wildlife to inhabit, the disconnected nature of what is left makes it very difficult for most wildlife to thrive, as they require much more connected landscapes.
Though fragmentation has been well studied in the past, less is known about how it affects parasites. Because they depend on other organisms for their own survival, parasites in particular are at risk of local or even extinction due to the cascading effects of species loss (i.e., coextinction, see Did You Know?). The complex nature of many parasite life cycles, in addition to a scarcity of experimental studies, makes it difficult to predict what effects that fragmentation will have on parasites. Today’s authors used a long-running, large-scale fragmentation experiment (The Wog Wog Habitat Fragmentation Experiment) to determine how fragmentation affects host-parasite interactions.
The indestructible insect: Velvet ants from across the United States avoid predation by representatives from all major tetrapod clades (2018), Gall et al., Ecology & Evolution. https://doi.org/10.1002/ece3.4123
Image credit: Adam Hasik, image cropped
Predation is a selective force that everyone is familiar with. One organism (the predator) kills and consumes another (the prey), and there is usually little nuance to the outcome of this interaction. The prey either escapes and survives, or it is killed and eaten. Due to this extreme pressure, prey organisms have evolved a remarkable array of defensive abilities and behaviors to attempt to reduce predation. Some colorful examples include the pufferfish and its ability to greatly increases its size, the octopus and its ink, or the hilarious (yet effective) behavior whereby the killdeer (a small bird here in North America) will make a lot of noise and fly a short distance before pretending its wing is broken in order to distract a predator from its offspring.
One animal that possesses a suite of such defensive abilities is the velvet ant (Dasymutilla spp.). Despite their name, velvet ants are a group of parasitoid wasps covered in a fine layer of setae (the velvet) where the females are wingless and look like ants. Because these females spend most of their time searching for ground-nesting insects to lay their eggs on/in and cannot fly, one might expect that these insects are particularly vulnerable to predators. But what’s really cool about these insects is just how many defenses that they have to ward off predators. First and foremost, they are brightly colored (just LOOK at that thing, nothing about that insect says “eat me”), which is usually enough of a warning in the natural world. Beyond their coloration, females also possess a venomous sting that is reputed to be one of the most painful stings in the world (see Did You Know?). I mean, that velvet ant in the featured image is colloquially known as the “cow killer” because of its painful sting. Velvet ants also possess a remarkably thick exoskeleton that is difficult to crush, and because it is rounded bites and stings tend to glance off of the abdomen. Today’s authors sought to understand just how effective all of these defenses were for reducing predation.
Host controls of within-host dynamics: insight from an invertebrate system (2021) Stewart Merrill et al., The American Naturalist. https://doi.org/10.1086/715355
Image Credit: Per Harald Olsen, NTNU, CC BY 2.0, Image Cropped
When it comes to understanding how parasites and pathogens spread, immune defenses may be an especially important factor. The immune system is the gatekeeper for parasites and pathogens (I’ll just use the term “pathogen” from here on out). Whether you are exposed to influenza, a parasitic worm, or a tick-borne bacterium, your immune response will determine the outcome of infection — either you will become infected (which benefits the pathogen’s reproduction) or you will not (which is a barrier to the pathogen’s reproduction). So now, picture a whole population of individuals. A room full of individuals with poor immune responses should result in more infections (and more transmission) than a room full of individuals with strong and robust immune defenses. By shaping the fate of pathogens, host immune defenses can shape transmission.