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, https://doi.org/10.1098/rsbl.2022.0314
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).
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
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.
Nature is complicated and the environment is vast. How can we possibly learn all there is to know about our surroundings? Aspects of our natural world like life population dynamics and life histories influence the very survival of species, but understanding these requires data from long time periods. Luckily, technology and the remarkable commitment of some scientists have meant that we are making progress in data collection by establishing many long-term monitoring networks that collect a variety of information at many different locations. Weather sensors keep track of temperature, precipitation, and wind speed. Air and water-quality sensors keep track of what is in the air we breathe and the water we drink.
Turning an ecosystem that has been ruined by humans back into a thriving natural world is a long, difficult task, but it is possible. One method for making it easier is re-introducing species that we’ve wiped out. Often the reintroduction of the functions that these species perform helps restore many other species, and helps the ecosystem returns to a more ‘natural’ state.
But what happens when a really key species has gone extinct? One way of solving this conundrum is introducing a similar species that performs the same function. This sounds like a good workaround, but introducing a non-native species might have unexpected ecological repercussions.
This week’s researchers were based on Round Island, in Mauritius, where two species of giant tortoise (the saddle-backed and the domed Mauritius giant tortoise) had gone extinct. A third species, the Aldabra giant tortoise, was introduced in 2007. The main point of concern on the island is that the tortoise diet may overlap with that of a vulnerable species, the Telfair’s skink. This week’s team wanted to find out whether the tortoise was helping or hindering the island.
At this stage in the climate crisis, many of us are very aware that ecosystem destruction and biodiversity loss are huge problems, bringing about everything from rapidly expanding deserts to global pandemics. We are acutely conscious that something incredibly valuable is being destroyed, and we want to protect it.
However, there are also people who aren’t very aware of the scale of ecosystem destruction, and therefore don’t seem to be doing anything about it. Motivating these people to protect ecosystems – or at least stop destroying them – is a huge problem. A problem so big, some people have even tried to throw money at it.
When an animal is facing a lack of prey, or the weather is making it too difficult for them to keep on keeping on, they might choose to enter a state known as torpor. This occurs when the animal lowers its metabolic rate drastically, sometimes to less that 1% of its normal rate. It’s not a perfect solution though, as the costs of torpor include sleep deprivation and memory loss. Nevertheless, it’s a go-to for many small mammals, since they’re warmed up much more quickly than larger ones, and can snap out of torpor when they need to.
It might sound like this is cold-weather behaviour, but it can also occur in summer. Especially if you’re a nocturnal mammal living in part of the world where nights can be very short, or even non-existent, like Scandinavia. Long days means reduced hunting times, so using torpor might be necessary to get through summers as well as winters! This week’s researchers wanted to better understand how small bats survive in northern Norway by looking at how and when they awake from torpor.
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.
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.
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:
Both partners were receptive to mating and penis fencing was observed,
Both partners were receptive but no penis fencing was observed,
Only one partner was receptive and no penis fencing was observed however insemination was successful, and
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.
Blackadder: As cunning as a fox who’s just been appointed Professor of Cunning at Oxford University?
Baldrick: Yes, sir.
Even if you’re not familiar with the British classic Blackadder, if you’re an English speaker the expression “cunning as a fox” will need no explanation. Our fascination (or in some cases disregard for) the intelligence of animals, and our comparison of animals to our own levels of intelligence, have been a part of language for centuries.