What are Stressors?

On our planet, we live together with many types of organisms: animals, plants, and even very small organisms like fungi or bacteria. A group of organisms living in the same area forms what is called a community. A community interacts with its environment for food and shelter. Together, the community and its environment make up an ecosystem. A coral reef, a forest, and a river are all examples of ecosystems. Organisms within an ecosystem depend on each other and on their habitat. Let us look at the example of a river: in this freshwater ecosystem, shrimp provide food for fish. The same shrimp shred the leaves that fall into the river, converting them into smaller pieces and releasing nutrients that are needed for algae to grow. Algae provide food for river snails, which can also be eaten by fish. Bacteria and fungi act like a clean-up crew, taking care of all the leftovers. These organisms all play important roles in keeping the water clean and helping the river to stay healthy (Figure 1A). All parts of an ecosystem are connected and in a dynamic balance with each other, fitting together like pieces of a living puzzle.

Humans take part in river ecosystems too, but many human activities are harmful to rivers. Humans can destroy river habitats by building dams that block the flow of water, turning riverbeds into concrete channels, overfishing, or polluting the water with chemicals and trash. These disturbances happen along with natural events like storms, floods, and fires—events that natural communities are more adapted to. All these stressors make it hard for living organisms to survive and can damage their habitats, disrupting the balance that holds the ecosystem together and causing the ecosystem to become degraded (Figure 1B). Some stressors can be stronger than others and, in many instances, multiple natural and human-caused stressors occur at the same time [1].

Can Ecosystems Recover?

The good news is that ecosystems can recover once stressors are removed. Healthy ecosystems are important for humans as well as for many other species, and many people have been working to restore degraded ecosystems (Figure 1C). Some recover quickly without any human help once the stressors are gone. This is the response scientists most hope for. However, other ecosystems do not recover as well [2]. Some take so many years to recover that it is hard to tell if they are recovering at all. The asymmetric response concept (ARC) is a description of the possible trajectories (paths) ecosystems can take to recovery and the mechanisms explaining each type of path. “Asymmetric” in this name means that the rate at which ecosystems recover can differ, to the point that some never reach full recovery without help. The ARC helps us find out which ecosystems need our help to recover and how much effort is needed to restore them.

Ecosystem Trajectories

The ARC defines five ecosystem trajectories, which reflect the real-life results of restoration efforts (Figure 2) [3, 4]. The first two trajectories, called the rubber band model and the broken leg model, lead to complete recovery but differ in the amount of time it takes the ecosystem to get there. The others, the partial recovery, no recovery, and new state models, either recover partly or not at all, even after a long time.

For the rubber band and broken leg trajectories, communities recover completely but take different amounts of time to do so. If you pull on a rubber band, it immediately goes back to its original state after you release it. Similarly, in the rubber band trajectory, any species lost from the community can return shortly after the stressor is gone. On the other hand, if you break your leg, it takes much longer to heal. So, in the broken leg trajectory, species that are lost or reduced in number need more time to return to the community, compared to the time it took to lose them due to stress.

Sometimes full recovery is not possible, and communities can either be partially recovered, not recovered, or transformed into a new state. In the case of partial recovery, some of the lost species cannot return to the community after the stress is gone. So, in the final community, one or more species are missing compared to the original community. For example, if our river had two fish species, only one might come back. In some cases, recovery is not possible at all, and the community remains in a degraded state, in which none of the lost species can return. In this case, no fish return to our river. Both partial recovery and no recovery are caused by changes in the balance of how species fit together in the degraded community. Among the puzzle pieces of ecosystem function, these changes involve relationships between two or more species, known as biotic interactions. Examples are relationships between predators and their prey and competition between organisms. Another possibility is that, after stress release, the community can transform into a new state, in which new species take the place of those from the original community. For example, a new type of fish could take the place of the fish that were in our river before the stressor, and a salamander might take the place of a frog.

Ecological Mechanisms Shape Ecosystem Trajectories

As scientists, we want to find out which trajectory a degraded ecosystem will take once the stress disappears. To do so, we must understand what ecosystem responses occur during stress and how those responses change when the stress is gone. A community’s recovery depends on how many organisms survive the stressful period. A species’ ability to survive stress is known as its individual tolerance. For example, some frogs can freeze solid to survive cold winters, making them cold tolerant. Tolerance is most important during stress, and the individual tolerances of many species combined affect how the whole community changes under stress (Figure 3). A single species can also have different tolerances to different stresses—a very cold-tolerant frog can be very sensitive (intolerant) to chemicals in the water, for example. So, when there are multiple stressors at the same time, things can get complicated.

Even if organisms are not tolerant, they can still come back to the damaged ecosystem once the stress is gone. The ability to move, known scientifically as dispersal, can help organisms escape a stressor and come back when the stressor is released. For some species, coming back will be easy—for example, if they did not move very far away or if they are really good at flying long distances. For others, coming back to a recovering ecosystem is more difficult. Think about a tiny river snail trying to move upriver compared to a fish that can swim quickly, for example.

Even if an organism makes it back to the recovering ecosystem, the organism might not remain there because the ecosystem may have changed during the stress. If a fish that eats shrimp travels back to a recovering location where all shrimp have died, the fish will have nothing to eat and cannot survive there. This relationship between a predator and its prey is an example of a biotic interaction. These interactions are really important for ecosystems to function, but they can also prevent species from returning and prevent the ecosystem from recovering completely. Biotic interactions and dispersal are extremely important during recovery (Figure 3).

The combination of the tolerance of organisms, their dispersal, and the biotic interactions between them determines how many organisms die or leave during stress, and which ones can come back once the stress is gone. An ecosystem with many tolerant organisms and organisms that can disperse easily yet remain nearby (or can easily return) will be more likely to follow the rubber band trajectory of recovery. If many organisms are sensitive and all the food sources for important predators disappear, the ecosystem may not recover at all. If some species have high dispersal and others do not, partial recovery can occur, as only some animals will come back. A degraded community may also be invaded by new species that can deal better with the new balance of biotic interactions in the degraded ecosystem. Knowing which trajectory a stressed ecosystem will follow and which mechanisms lead to that trajectory helps scientists and ecosystem managers to create a plan of action to put the ecosystem on the path to recovery.

How Can the ARC Be Used?

With the asymmetric response concept, scientists can understand how the mechanisms of individual tolerance, dispersal, and biotic interactions work together to affect the trajectory of the ecosystem they are restoring. When they understand the trajectory an ecosystem will follow, they can give this information to ecosystem managers who take action to help return the ecosystem to a healthy state. Ecosystem managers include people working at water companies; natural parks; and city, state, and county governments, who make rules on how land and water are used. For ecosystems that follow the rubber band trajectory, nothing extra is needed. But for ecosystems that have no recovery, action is needed to avoid mistakes of the past. Are there not enough organisms living nearby? Managers can re-introduce them. Did the food source of a predator disappear? Managers can make sure the food source is back before re-introducing the predator. Once ecosystem managers know which conditions lead to no recovery, they can take steps to make sure those conditions do not happen again. Together, scientists and managers can use the ARC to restore more degraded ecosystems back to a healthy and happy state.

Glossary

Stressor: ↑ Any factor that causes organisms, communities, or ecosystems to become damaged, such as chemicals, high temperatures, pollution, or habitat destruction.

Degraded: ↑ An of unhealthy, bad condition following damage. In nature, a degraded ecosystem is no longer a nice place for many organisms to live.

Trajectory: ↑ The path an ecosystem takes, measured by how the community changes over time.

Restoration: ↑ The process of fixing something or making it new again. For ecosystems, this usually means cleaning up the habitat, removing stressors, and reintroducing species.

Biotic Interactions: ↑ A relationship between two or more species in a community, such as predator-prey relationships, or competition between species for a resource.

Tolerance: ↑ The ability to withstand environmental factors such as temperature or pollution. Sensitive species can only survive a small range of conditions, while tolerant species can survive in many different conditions.

Dispersal: ↑ The movement of species into or out of a habitat. Dispersal can be slow or fast.

Acknowledgments

We thank the entire Collaborative Research Center RESIST, which provided the basis for our research. All figures in this paper were created with BioRender.com. Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—SFB 1439/1 2021-426547801. The study was funded by CERN.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.