Ecosystem regeneration a challenge for effective restoration of natural resources

Regeneration of degraded ecosystems is the holy grail of modern ecology and conservation movements. Aware of the damage we have done to the environment, mainly in the last 100 years, we are taking steps to restore nature. Despite the efforts put into reducing pressure, the results are not spectacular, and sometimes there are none. Why is this happening? Because ecosystem regeneration is a complex, multifaceted process and is not at all necessarily the opposite of degradation. To explain this phenomenon, scientists have developed a theory called the Asymmetric Response Concept(ARC), which outlines various ecosystem restoration scenarios and their ecological consequences [1].

Why, despite efforts, does ecosystem regeneration often fail?

The degradation of the environment caused by human activity, particularly intense in the post-war period, has necessitated measures to reverse destructive trends. Remedial actions and mitigation measures implemented over the past 50 years have brought some improvement in water quality and biodiversity restoration, yet the results are still unsatisfactory. Further progress requires increased efforts. New and increasingly specialized strategies are emerging that aim to restore habitats and natural communities, reinitiate disturbed processes, and, above all, mitigate anthropogenic stressors. We hope that they will bring the expected results.

The problem begins when the corrective measures implemented through unavoidable sacrifices (something for something), usually at no small cost and effort, do not produce the expected results. The water quality is not improving, what was supposed to grow is not growing, the expected species are not coming back, the desired fish were not there as they are not there, and instead of a charming corner we have a mewling maelstrom. The rhetorical question remains whose mill this water is for. And who will have a case for not implementing corrective measures because they are expensive and ineffective.

And in fact, the failure is often due to insufficient understanding of the complex nature of ecological responses to degradation, as well as the restoration measures taken. Only an in-depth understanding of these processes can help predict ecosystem behavior and develop tailored countermeasures. Maybe then we can avoid environmental surprises?

What is degrading our environment? Disruptive factors, or stressors

In nature, everything affects everything, but some factors exhibit a disturbance effect, that is, an effect that causes environmental conditions, individuals, populations, communities or ecosystem functions to modify their range of variation relative to undisturbed reference conditions. Such factors are called stressors in the literature. It is rare that a single stressor affects the ecosystem. Most often, we are dealing with a whole range of factors that may act simultaneously or sequentially, and their effects reinforce or weaken each other at different times and to different degrees.

Very often in nature we encounter additive or synergistic interactions, when the effects of two stressors add up or amplify (producing an effect greater than their sum). Examples of such impacts in aquatic ecosystems abound, such as the amplification of the negative effects of eutrophication on biological communities as a result of increased water salinity or elevated temperatures. Similarly, the cumulative impact of multiple dams on a river can reduce its flow, alter temperature and oxygen levels, and affect biodiversity and ecosystem services. In lakes, the synergistic effects of climate change and nutrient pollution lead to excessive eutrophication, algal blooms, and the subsequent death of aquatic vegetation, oxygen depletion and fish die-off.

Antagonistic interactions occur when one factor favors a process (e.g., higher temperatures cause earlier and more luxuriant development of underwater macrophytes), but at the same time drives a factor that inhibits the process (an increase in the intensity of algal development due to higher temperatures causes shading of deeper water layers, limiting the development of underwater vegetation). Another example would be a decrease in pollutant concentrations due to an artificial increase in flow when a river channel is narrowed.

This seems beneficial to water quality, but the mechanical stress caused by the change in hydraulic force leads to a complete remodeling of biological assemblages through the elimination of organisms unsuited to high flow and the promotion of those with better adaptation. There are more such types of interactions, such as reversal, which occurs when the combined effect has an opposite effect to the effects of individual stressors. And we’re talking about a combination of just two stressors, while there are usually many more, and their interactions can be extremely complicated.

Interestingly, in different categories of waters, different types of interactions have a decisive impact. Studies of more than 170 examples of water degradation have shown that only 39 percent. cases it was a single stressor, in 28 percent. The combination of paired stressors produced an additive effect, and 33 percent an interactive effect (antagonistic, synergistic or opposite). For lakes, the overriding stressor appeared to be nutrient enrichment, the effects of which were generally superior to those of secondary stressors.

In the case of rivers, the effects of eutrophication varied in severity and depended on the specific combination of stressors and the biological variable analyzed [2]. These results confirm that for standing waters, restoration and management efforts should focus on reducing nutrient supply, while river management requires the development of case-specific solutions.

Tolerance, dispersion and biotic interactions – ecological mechanisms shaping trajectories of ecosystem regeneration

Recognizing the most likely trajectory of ecosystem recovery requires an understanding of the response of organisms to the stress factor and to its cessation. The recovery of biological assemblages, for example, depends on how many organisms survive an unfavorable period, and this in turn depends on the individual tolerance of the species to a given factor, such as temperature or pollution. Sensitive species (stenotopic) can survive only in a small range of conditions, while tolerant species (eurytopic) – in a much wider spectrum of them.

For example, some frogs have the ability to survive a state of complete freezing, enabling them to survive cold winters. The individual tolerances of many species together determine the sensitivity or resilience of the entire community to a particular stress factor. A given species may have different tolerances to different factors – for example, a very cold-tolerant frog may be very sensitive to certain pollutants in the water, such as salinity. So, in a situation where multiple stressors are present at the same time, things get complicated.

Even if organisms have a limited tolerance to an agent, they may flee to return to the ecosystem when the stress has passed. The ability to move is called dispersion or, more expertly, dispersion. Species have very different dispersal abilities, ranging from highly mobile, actively moving (mammals, fish, some aquatic insects) to completely sedentary (benthic organisms, including most plants, but also many macroinvertebrates), whose dispersal is basically reduced to passive transfer with the wind, water current or on mobile vectors. Thus, the least mobile species with the least tolerance to environmental conditions are potentially the most vulnerable to irreversible disappearance from a degenerating ecosystem and are least likely to return once the stress factor ceases.

In turn, even if an organism returns to a recovering ecosystem, it may not have a chance to repopulate it permanently if the change in conditions prevents it from fulfilling its vital needs. A mobile fish may return to a river after it has been decongested or water quality has improved, but it will not survive if it does not find a proper food base (e.g., poor recovery of the benthic macrofauna assemblage after the lethal effects of a certain ichthyotoxin) or the elements necessary to carry out its reproductive cycle (e.g., the lack of bivalves that are the key spawning organism of the rare and protected pinks in our country). Such relationships between species are examples of biotic interactions that ensure the proper functioning of ecosystems and whose disruption can effectively prevent their regeneration.

From rubber bands to new quality – different trajectories of ecosystem regeneration

The combination of organisms’ tolerance, dispersal ability and biotic interactions determines how many organisms can survive the effects of a stressor and how many will become extinct or leave the ecosystem, as well as how quickly their habitat can recover. And also what trajectory this regeneration will follow….

An ecosystem populated by a large number of tolerant organisms and those that can move easily and have the ability to return more quickly after the stressor has subsided will recover more quickly than those dominated by species with narrow ecological amplitudes and immobile species. Such a rapid return to the pre-degradation state has been called rubber band trajectory by the researchers. Recovery, but slower, occurring over a longer period of time, has been likened to the situation of a broken leg ( brocken leg trajectory) – it will knit together, but needs time to do so. Where there were many sensitive organisms and the food base of key predators was irreversibly lost, the ecosystem may not recover at all. This is a scenario referred to as the no recovery model.

In contrast, if some species show high dispersal and others do not, and only some of them return, there may be a partial recovery ( partial recovery model). A very interesting and often observed process is the emergence in the course of restoration of completely new species that are better able to cope with the balance of biotic interactions in a degraded ecosystem than the original ones. Such a phenomenon, known as the new state model, is often observed, for example, in re-watered peatlands, where tall grass species are encroaching instead of peat-forming mosses.

The concept of various recovery scenarios presented has been referred to by researchers as the Asymmetric Response Concept (ARC) [1] and is an extension of several previous ones developed in the first decade of the 21st century. [3-5]. Of the scenarios presented, only the rubber band model shows a virtually symmetrical response before and after release from a stressor or combination of stressors, both in terms of initial and final states after recovery and in terms of trajectory. The broken leg model is asymmetric in the sense that the trajectories are different, although the initial and final states are the same (hysteresis effect).

Models of partial recovery and no recovery are asymmetric in terms of both initial and final states, as well as trajectories before and after release from stressors. Similar asymmetries characterize the new state model, in which release from stressors causes a complete change in the original state.

What do we need concepts like ARC for?

According to ARC, the full restoration of community structure and ecosystem functions is one of several possible outcomes and is by no means a default expectation. When recovery from the release of a stressor fails, which is often the case, the question arises as to what obstacles impede this.

In view of the high uncertainty of success, it is not difficult to imagine the moderate enthusiasm of decision-makers to invest forces and resources in restoration activities, the results of which may be unsatisfactory and not as expected. But we have no other choice. The key to avoiding disappointment is to recognize the situation well and design measures accordingly. Knowing what trajectory the ecosystem will follow and what mechanisms lead to it can help scientists and wildlife managers develop a plan of action to put the ecosystem on the right path to recovery. The complexity of the determinants of ecosystem regeneration is immense, and perhaps recognizing all its aspects is beyond human capabilities. Maybe artificial intelligence algorithms will come to our rescue?

Photo. masthead: Hussain Niyaz/Unsplash

In the article, I used, among others. From the works:

[1] Vos M., Hering D., Gessner M.O. et al, 2023. The Asymmetric Response Concept explains ecological consequences of multiple stressor exposure and release. Science of The Total Environment, 872, 162196, https://doi.org/10.1016/j.scitotenv.2023.162196

[2] Birk S., Chapman D., Carvalho L. et al, 2020. Impacts of multiple stressors on freshwater biota across spatial scales and ecosystems. Nature Ecology & Evolution, Nature, 4, 1060-.1068. 10.1038/s41559-020-1216-4

[3] Sarr D.A., 2002. Riparian livestock exclosure research in the Western United States: a critique and some recommendations. Environ. Manag. 30, 516-526

[4] Duarte C.M., Conley D.J., Carstensen J., Sánchez-Camacho M., 2009. Return to Neverland: Shifting baselines affect eutrophication restoration targets. Estuaries and Coasts, 32, 29-36. 10.1007/s12237-008-9111-2

[5] Smith M.D., Knapp A.K., Collins S.L., 2009. A framework for assessing ecosystem dynamics in response to chronic resource alterations induced by global change. Ecology 90, 3279-3289