Author Archives: Adam Hasik

Charting the Spread of Disease Ecology

Image Credit: Davian Ho, Maya Peters Kostman, and Philippa Steinberg for the Innovative Genomics Institute, CC BY-NC-SA 4.0, Image Cropped

There’s a certain poetry to the popularity of disease ecology. Once a quirky biological sub-field, the study of diseases in an ecological context had spread steadily in popularity over the last two decades. Then COVID hit, and much like the disease itself, disease ecology rocketed into the forefront of natural sciences.

This wasn’t just contained to university and hospital corridors. Before COVID, how often did you hear words like “transmission”, “virulence” and “pathogen”? While disease ecology is the crux of my professional life now, there’s little chance I would have been able to make a career of it twenty years ago.

To get some perspective, I decided to talk to people who have been there for the surge in relevance disease ecology has experienced in that time. I was recently in Kruger National Park, South Africa for the 4th International Congress on Parasites of Wildlife, and had the pleasure of sitting down with two prominent disease ecologists, Dr. Sandra Telfer and Dr. Vanessa Ezenwa, in separate meetings to talk about how the field has changed over the course of their careers.

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The Water of Life

Image credit: Muséum de Toulouse, CC BY-SA 4.0, via Wikimedia Commons

Top-down response to spatial variation in productivity and bottom-up response to temporal variation in productivity in a long-term study of desert ants (2022) Gibb et al., Biology Letters,

The Crux

Ecosystem productivity can tell us a lot about how an ecosystem functions. The more productive an ecosystem is, the more life it can support. But productivity doesn’t just affect the diversity or number of species within an ecosystem, it affects how those species interact, from the large carnivores you find at the upper levels, to the plants and bacteria down the ‘bottom’.

Within ecosystems, the strength of a top-down process (something influencing those upper levels) vs. a bottom-up process (something influencing the lower levels) depends on how much primary productivity there is. Primary production occurs when a species makes its own energy instead of eating something else, and when there is a lot of it going around, it often allows the carnivores at the upper trophic levels to suppress the population numbers of herbivores. That means that while a bottom-up process may end up affecting the herbivores, a top-down process (like the hunting of carnivores) might impact the entire ecosystem.

On the other side of the spectrum, when there is little primary productivity, there aren’t usually as many carnivores suppressing the herbivore populations. A bottom-up process will increase herbivore numbers, making these bottom-up processes more important in these low-productivity systems. This is known as the Exploitation Ecosystem Hypothesis (EEH).

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Invasive Snails and Their Hippo Victims

Image credit: Muhammad Mahdi Karim, CC BY-SA 4.0, Image Cropped

Invasive snails, parasite spillback, and potential parasite spillover drive parasitic diseases of Hippopotamus amphibius in artificial lakes of Zimbabwe (2021) Schols et al., BMC Biology,

The Crux

Artificial lakes can be a huge plus for the regions where they are constructed. People come to hang out at them, they can serve as habitat for local or migrating species, and they can also improve water accessibility. In fact, the majority of the research that I did for my PhD took place in artificial, human-made lakes (see here and here). Yet, these artificial lakes can also wreak havoc by destroying local ecosystems and introducing invasive species. Furthermore, because humans build communities around these lakes there is a risk of increased transmission of parasites to livestock and humans alike.

One group of common invasive species in these artificial lakes are snails, which serve as intermediate hosts for many parasites (see Did You Know?). Introduced water plants (like hyacinth) often harbor invasive species like the snails, and dams built to make artificial lakes often block snail predators from accessing the lakes, which means that the snails increase in number due to the release from predation pressure. Today’s authors wanted to understand how invasive snails modified parasite transmission within an artificial lake.

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Bad Neighbors

Hidden effects of habitat restoration on the persistence of pollination networks (2022) Gaiarsa & Bascompte, Ecology Letters,

Image credit: dronepicr, CC BY 2.0, via Wikimedia Commons

The Crux

It’s no secret that the world is undergoing a biodiversity crisis. This comes not only from climate change and human land use, but also invasive species – non-native species that cause harm to native ecosystems. Specifically, there are seven times more invasive species now than there were 75 years ago. Because of how many there are, and just how fragile ecosystems have become, it’s important to know what effects that invasive species have.

Ecological restoration (see Did You Know?) is one effective solution that can be used to mitigate the biodiversity crisis. Reestablishing native species can often help with this restoration, as does removing invasive species, but it usually requires human intervention. By removing these invasive species, the idea is that the native species will be released from competition and benefit from better access to necessary resources.

Yet to monitor invasive species removal, you need long-term data on population persistence, which is very difficult (logistically and financially) to collect. Understanding how the removal of invasive species benefits restoration requires not only measuring how such removal benefits ecosystem function, but also how it can benefit population persistence in the long term. Today’s authors wanted to understand how the removal of an invasive species benefited local community resilience.

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Move Over Wolves, it’s Time for Cougars and Donkeys

A novel trophic cascade between cougars and feral donkeys shapes desert wetlands (2022) Lundgren et al., Journal of Animal Ecology,

Image credit: CHUCAO, CC BY-SA 3.0, via Wikimedia Commons

The Crux

Trophic cascades (see Did You Know?) are an important part of many ecological systems. However, most of the world’s large predator species were lost around 10,000 years ago (potentially due to human impacts), thus limiting the role that predators could play in driving trophic cascades. Though large predators were lost, many large herbivores are still around, which means it is difficult for a smaller predator to take down/consume these herbivores, much less have an effect large enough to drive a trophic cascade.

In the United States, large felines such as cougars (Puma concolor) are known to predate large equid species (such as feral horses or donkeys), but much of the ecological literature assumes/claims that cougars do no exert a strong enough pressure to consider them “significant” predators of these equid species. Specifically, some reports state that these species don’t have any natural predators, and other reports echo the claim. Today’s authors report on a novel trophic cascade between the cougar, feral donkeys (Equus africanus asinus), and wetland vegetation.

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A role for biotic interactions in limiting species’ range limits

Biotic interactions are more often important at species’ warm versus cool range edges (2021) Paquette & Hargreaves, Ecology Letters,

Image credit: Malonecr7, CC BY-SA 3.0, via Wikimedia Commons, image cropped

The Crux

In the natural world, most organisms are limited by the environment as to where they can live. While this can be as drastic as a whale being limited to the ocean and humans being limited to the land, there are also more subtle limitations. That is, black and grizzly bears live in temperate environments, but polar bears are inhabit the arctic where it is MUCH colder. Due to the limitations imposed by the environment, black and grizzly bears cannot live further north.

Historically, most studies have focused on abiotic variables (i.e., non-living), like temperature and precipitation, as there is a clear role for the climate in determining where and when a species can live. However, biotic variables (i.e., living) like predation or competition can also play a role in defining the limits of a species range, though this has proven more difficult to test than abiotic factors, as many tests of biotic variables produce species-specific results. Charles Darwin proposed a framework in 1859 that the importance of biotic interactions would vary predictably with latitude and elevation. That is, at cooler, high-altitude locations abiotic interactions would be more important, while biotic interactions would be more important at warmer, low-altitude locations. Although a number of studies have attempted to test the three predictions (see Did You Know? ) derived from this framework, the results are contradictory and come from data testing different predictions using different data. Today’s authors sought to test all three predictions at once in order to resolve these contradictory results.

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It’s Who and Where You Are

A role for the local environment in driving species-specific parasitism in a multi-host parasite system (2022) Hasik & Siepielski, Freshwater Biology,

Image credit: Adam Hasik, image cropped

The Crux

Parasites are an ever-present part of every ecological community on Earth, yet there are some species that harbor more parasites than others. In systems where parasitism is density-dependent, meaning parasitism increases with host density, the most common/numerous species will harbor the greatest amount of parasites. Yet there are also cases of species-specificity, whereby parasites specifically target a single host species. In other host-parasite systems, local-adaption plays a role in parasitism dynamics, whereby parasites are better at attacking their local hosts than they are attacking foreign hosts and/or hosts are better at defending themselves from local parasites than foreign parasites.

With all of these different factors affecting how host-parasite systems operate, it is important to identify when and if each one is operating within specific ecological communities. This is especially necessary when ecological communities are comprised of multiple host species and multiple parasite species, all of which can/do interact with one another.

To investigate the above factors, we first conducted a survey of parasitism in damselflies (Enallagma spp.) and their water mite parasites (Arrenurus spp.). From there, we then carried out to field experiments to understand why parasitism operates the way it does within this system.

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My Enemy is Not the Enemy of My Other Enemy

Do predators keep prey healthy or make them sicker? A meta- analysis (2022) Richards et al., Ecology Letters,

Image credit: Angah hfz, CC BY-SA 4.0, via Wikimedia Commons

The Crux

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.

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Hunting and Evolution

Hunting alters viral transmission and evolution in a large carnivore (2022) Fountain-Jones et al., Nature Ecology & Evolution,

Image credit: Joachim S. Müller, CC BY-NC-SA 2.0

The Crux

It’s no secret than humans have had an enormous impact on the native wildlife of our planet, and we have looked into many of these complicated relationships and effects before on Ecology for the Masses. One common interaction is that of hunting, whereby humans hunt and kill an animal for recreation and/or food. Regardless of your feelings on hunting, such removal of animals can be an issue in systems where there is density-dependent transmission, meaning the more animals there are, the more likely there is to be parasite transmission within the populations of these animals. Reducing animal populations via hunting can either decrease, have no effect on, or even increase density-dependent transmission.

These changes in transmission dynamics (and subsequent changes in infection patterns) will have effects on the evolution of the parasites infecting these animals, making it easier for researchers to detect if (and how much) transmission is occuring. To investigate these patterns, today’s authors studied data on feline immunodeficiency virus (FIV) and its puma (Puma concolor) hosts. FIV is mostly benign and infects its hosts for life, though puma hosts can become infected with different strains of FIV. The goal of today’s study was to understand how hunting affects transmission dynamics of FIV within populations of puma that are hunted.

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Your Immune Defenses Are What You Eat

Condition‐dependent immune function in a freshwater snail revealed by stable isotopes (2022) Seppälä et al., Freshwater Biology, link to article

Image credit: Bj.schoenmakers, CC0, via Wikimedia Commons

The Crux

There are myriad factors at play when it comes to parasitic infections, but the primary physiological barrier for the parasite is the immune function of host organisms. Despite its importance and usefulness, the immune function is costly to maintain. Building and effectively using immune defenses relies on the host being able to secure enough food to properly fuel its defenses. As a result, individuals in poor condition are more susceptible to parasites. Building off of that, if the conditions in a given area are poor/worsening, then an entire population of organisms may be vulnerable to disease outbreak.

Many studies have investigated the dependence of immune function, including one of my own, but many of those studies take place in lab settings where the food given to a host is carefully controlled. While there are obvious benefits to controlling experimental conditions, it can be hard to generalize the findings of a lab study to the natural settings that organisms actually live in. Today’s authors utilized an observational study of a freshwater snail (Lymnaea stagnalis) to better relate host condition in nature to immune function.

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