Being a prey can sometimes be advantageous

Breaking new ground 27. apr 2023 3 min PhD Fellow Anastasios Marantos Written by Kristian Sjøgren

Simulation provides surprising insight into the relationship between prey and predators. Conventional wisdom sees no advantage in being hunted by predators when the prey is already under pressure in its biological niche, but research shows cases in which being hunted is advantageous.

Evolution has proceeded for billions of years, developing countless species, some of which live on grass and leaves and others that eat the weakest animals.

Prey and predators have a deadly relationship throughout nature. This applies to lions and gazelles on the savannah, killer whales and seals in the oceans and even among bacteria and viruses, with bacterial viruses called bacteriophages hunting bacteria and killing them so that the bacteriophages can multiply.

Conventional wisdom sees minimising predators as being advantageous for prey, but reality begs to differ.

This is highlighted by a new study in which researchers examined the balance between prey and predators over time, when resources are scarce, and the prey not only struggle to avoid being eaten by predators but also fight with other prey for limited food.

“The fundamental concepts of ecology are similar, whether it is a study on viruses and bacteria or on prey and predators on the savannah. We want to understand these dynamics better for several reasons but from a broader ecological viewpoint because they affect how we approach promoting biodiversity, which is currently under pressure. In this context, understanding the interactions between species is important for determining how to optimally ensure the conservation of biodiversity in the future, which is essentially liked with humanity’s future as well,” explains a researcher behind the study, Anastasios Marantos, PhD Fellow, Niles Bohr Institute, University of Copenhagen.

The research has been published in PLoS Computational Biology.

Understanding ecosystem balance is key

Anastasios Marantos says that the study aimed to understand balanced diversity in an ecosystem when species compete for the same scarce resources while being hunted by a predator.

For example, in the Arctic regions, two species could compete for the few resources available, and one species could reproduce more rapidly and thereby increase in number more rapidly than the other. This species can thus outcompete the other species, which will disappear over time.

Adding a predator with a taste for the slowly reproducing species will only accelerate the rate at which this species becomes extinct.

For the predator, however, developing a taste for the prey that reproduce rapidly may have many advantages because there are potentially more of them.

“We study ecosystems in which the kill-the-winner dynamics is at play, according to which a predator predominantly attacks the otherwise winning species in a competition for scarce resources. This can help maintain high biodiversity by creating a balance between the prey animals. In our study, we also show that when a predator attacks both the winner and the loser in the battle for the scarce resources, this can actually be advantageous for the loser, which can even end up being the winner, eliminating the otherwise winner with the help of the common predator,” says Anastasios Marantos.

Model elucidates biology’s engine room

The researchers created a model that simulates the development of a microbial community with bacteria and bacteriophages in an environment with limited resources.

The researchers fed the model various input data on the growth rates of both bacteriophages and bacteria, how much they consume, how quickly they die and other data.

The model was an open model in which the magnitude of the various input parameters could vary when new bacteria or viruses invaded the system.

“This means that the various digital species have some characteristics that we do not predetermine but are biologically plausible. We then ran the models and saw how the relationship between the bacteria and bacteriophages developed over time,” explains Anastasios Marantos.

Predators make being the weaker party advantageous

The model shows how biological systems can develop and go through different periods of highly varying diversity. Sometimes a system is in temporary balance with many species coexisting, and at other times there are only a few species.

In their model, the researchers also found that being a rapidly growing bacterium can be advantageous, but this bacterium can still be outcompeted by slowly growing bacteria when specific conditions are present.

In the model, the bacteriophages could create periods when being a slowly growing bacterium was advantageous, when the bacteriophages attacked both the slowly and rapidly growing species. This applied, for example, when slowly growing species were less exposed to bacteriophage attack than a rapidly growing species. This only required that the bacteriophages attacked both species and that resources were limited.

Wolves maintained the balance in Yellowstone

Many may find the connection between computer-simulated bacteria and the real-world battles between gazelles and lions somewhat tenuous, but the same mechanisms are present throughout nature.

Anastasios Marantos says that the predator’s role in an ecosystem is often to create balance in the struggle between species competing for the same resources. If the predator is not there, the balance shifts and one type of prey may be able to outcompete another.

“One can say that the weaker prey should be grateful that is being hunted if it shares a common predator with another prey species that would drive it to extinction in the absence of this predator,” adds Anastasios Marantos.

In the real world, conservationists have already recognised that the dynamics of biology are difficult to understand and to manipulate.

Anastasios Marantos reports that many wolves were shot in Yellowstone National Park at one point because the wolves hunted and ate many endangered prey that conservationists wanted to protect.

The problem was that when the park rangers removed many of this leading predator in Yellowstone, this adversely affected the intended biodiversity precisely because of the mechanisms that Anastasios Marantos and his colleagues identified in their study.

With the predators gone, the balance between the prey animals disappeared, and those that were already losing the struggle for resources thereby became even more marginalised and outcompeted.

“Many of the endangered species simply disappeared once the wolves that normally hunted them were eliminated, and then Yellowstone had to reintroduce the wolves,” concludes Anastasios Marantos.

From kill the winner to eliminate the winner in open phage–bacteria systems” has been published in PLoS Computational Biology. The European Research Council supported the project. In 2021, the Novo Nordisk Foundation awarded a grant to co-author Namiko Mitarai for the project Universal Growth Laws of Viruses of Bacteria.

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