Tag Archives: ecology

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, https://doi.org/10.1186/s12915-021-01093-2

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, https://doi.org/10.1111/ele.14081

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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To Stab, Or Not To Be Stabbed: The Sex Lives of Flatworms

Image Credit: Hectonichus, CC BY-SA 3.0, Image Cropped

Would you rather stab, or be on the receiving end of a stab? This may seem like a confronting question, but it’s the dilemma many flatworms face when a mating opportunity arises. 

Option 1: You are a flatworm and have just been stabbed by a stubby penis. You now have puncture wounds that must heal, after which you must carry fertilized eggs which you need to lay and protect upwards of 24 hours. Oh, the energy demands! 

Option 2: Flatworm victory! You have successfully stabbed your opponent with your stubby penis before they could stab you. Your sperm has now fertilized their eggs. With this win, you move on with life and wait for your next mating “opponent”.

Which option do you choose? If you still can’t choose, it’s a good thing you aren’t a simultaneous-hermaphroditic flatworm. These flatworms have both fully functional male and female reproductive capabilities that can be used interchangeably, unlike other hermaphroditic species who switch back and forth during different phases of life. One might say these individuals have the capability to “choose” what role they want to play, male or female. Although, those forced into the role of reproductive female may disagree…

It is believed that individuals fight to “remain male” (i.e., not be fertilized) because sperm is biologically cheaper to produce than eggs, and males can produce more offspring than females over a lifetime. This type of fight has been thought to be “pure evolutionary selfishness”.

It was only discovered recently, after Dr. Leslie Newman and Dr. Nicholas Michiels spent 20 hours continuously watching pairs of captured flatworms. They observed that when an individual encounters another, both assume a fighting stance, curling their bodies back to display their penises. Next, they began to fight, each attempting to stab the other, which could last from 20 to 60 minutes.

Two Pseudobiceros bedfordi  facing off (Image Credit: Whitfield et al., 2004, Everything You Always Wanted to Know about Sexes, CC BY 2.5)

Different species fight with different strategies. For example, racing-line flatworms (Pseudocerotidae bifurcus) use their penis to repeatedly strike at one another until one succeeds, injecting sperm under the skin of the other. Once the sperm is injected, it moves through the body to find and fertilize the eggs. Persian carpet flatworms (P. bedfordi, pictured above) instead use their penis like a water gun, ejaculating anywhere on their opponent’s body. With a sperm cocktail that dissolves flesh, it burns its way through various tissues until it reaches and fertilizes the eggs. 

Penis fencing is the term scientists use to describe this behavior to “remain male. This mating behavior isn’t seen amongst all flatworm species, only certain species within the family Pseudocerotidae. In the 1990’s there were only two species of flatworm known for this behavior, however as of 2020, the number has grown to 16. 

Evolution of Penis Fencing

Species of flatworms can use sexual reproduction (need both gametes; sperm and egg), asexual reproduction (does not require both gametes, obtain all DNA from parent), or both. Those that use both, do so depending on which strategy is favoured by the environmental conditions. For example, sexual reproduction is favored under harsher, more unpredictable conditions, since genetically variable offspring are often better able to adapt and survive these conditions. Asexual reproduction may be favored when individuals are scarce, however it tends to be avoided as there is on average a 50% loss of genetic diversity per generation, subsequently increasing the probability of inbreeding in future generations. If asexual reproduction does occur, it can occur through budding or transverse fission. Budding occurs when ‘buds’ (i.e., outgrowth) grow out of the flatworm’s body until they are large enough to break off as new individuals. Fission, on the other hand, involves an individual being cut in half, with each half becoming a new individual. 

For more wholesome Ecology comics, check out more of Tanya Strydom’s work on this site!

A species may employ different hermaphroditic strategies of cross-fertilization depending on their ecological niche. These include delivery of sperm to a sperm-receiving organ of the mating partner, or hypodermic insemination of sperm into the cellular tissue by a modified penis that enables individuals to pierce the body wall of their partner. It is believed that the willingness to invest as little resources as possible into their offspring is very strong in hermaphroditic species, leading to these extreme mating behaviors such as penis fencing.  

Yet penis fencing does not always occur when individuals meet. Four possible scenarios have been observed when individuals encountered one another: 

  1. Both partners were receptive to mating and penis fencing was observed, 
  2. Both partners were receptive but no penis fencing was observed, 
  3. Only one partner was receptive and no penis fencing was observed however insemination was successful, and 
  4. Neither were receptive to mating. 

If penis fencing occurs, it typically leads to successful sperm insemination for one or both individuals. Number 3 may be the result of other mating behaviors. For example, mating Starry flatworms (P. stellae) will curl around each other, swimming in circular motions in attempts to inseminate each other. 

Outcomes of Penis Fencing

A more recent study in 2020 found that penis fencing results in three outcomes; 1) both individuals were inseminated, 2) one individual was inseminated, or 3) neither were inseminated. These researchers found penis fencing to be more of a duel or contest mating ritual, rather than an aggressive, violent behavior as was originally thought. This is because they found different scenarios where penis fencing occurred that resulted in neither individual being inseminated, or where no penis fencing occurred resulting in at least one individual being inseminated. Although we may think of penis fencing a little differently now, one thing that will forever remain constant are the words of David Attenborough, “its only solace is knowing it’s young will carry the genes of a master swordsman”.  


Jennifer Merems is a writer and researcher focusing on behavioral and nutritional ecology. She is currently a PhD candidate in the Department of Forest and Wildlife Ecology with the University of Wisconsin-Madison. You can learn more about Jennifer by following her on Twitter at @atyourcervid.

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, https://doi.org/10.1111/1365-2656.13766

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, https://doi.org/10.1111/ele.13864

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, https://doi.org/10.1111/fwb.13961

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, https://doi.org/10.1111/ele.13919

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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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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Getting Hot Hot Hot

How melanism affects the sensitivity of lizards to climate change (2022) Mader et al. , Functional Ecology, https://doi.org/10.1111/1365-2435.13993

Image credit: Tony Rebelo, CC BY-SA 4.0, via Wikimedia Commons

The Crux

Climate change is a fact of life. Every day we uncover more of the negative effects it will have on the various animals, plants, and fungi in the natural world. Species range contractions are one such effect, and they occur when the area that a given species normally occupies shrinks. They are directly linked to a species’ risk of extinction, with this risk growing as a species inability to adapt to new environments grows. Though the theory sounds logical, many of the exact mechanisms behind range contractions are still unknown.

Ectotherms are organisms that depend on the surrounding environment to regulate their own body temperature, making them particularly vulnerable to climate change. Many different biological mechanisms are involved in regulating temperature, but the ability to reflect solar radiation is a key player. Indeed, the ability of organisms to reflect solar radiation (aka energy from sunlight) is part of the thermal melanism hypothesis (see Did You Know?). Melanistic (darker) organisms may be favored under climate change, due to the protection against UV radiation provided by melanin. However, melanistic individuals are more prone to increased heating, which can be bad. Today’s authors sought to understand how climate change would affect melanistic organisms.

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Why Are There So Many Species?

The causes and ecological context of rapid morphological evolution in birds (2022) Crouch & Tobias, Ecology Letters, https://doi.org/10.1111/ele.13962

Image credit: Andrej Chudý , CC BY-NC-SA 2.0

The Crux

One of the biggest questions facing evolutionary ecologists is why some groups of organisms contain SO MANY species, while others are relatively sparse in comparison. We’ve discussed adaptive radiations on Ecology for the Masses before, which is when a burst of speciation occurs within a group, with new species adapting to fill new ecological niches. It could be that the reason for such uneven groups is that some clades, or related groups of organisms, are more prone to such adaptive radiations than others. If this is true, it would mean that such clades experience not only an increase in the number of lineages (species) that they contain, but also the number of traits they exhibit.

Increases in the speciation rate and trait evolution are the hallmarks of adaptive radiations, but they may not occur at the same time, which can lead to some different outcomes. Clades may diversify rapidly, without really evolving new traits, and this is known as a “non-adaptive radiation“. In contrast, a lineage may quickly evolve new traits without speciating, which is known as an “adaptive non-radiation“. To understand the causes and context of such evolutionary scenarios, today’s authors studied the history of bird evolution.

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