Blue Planet Prize Story : 2018 winner Prof. Brian Walker : 2. Resilience of ecosystems

2. Resilience of ecosystems

Encountering the concept of resilience

In 1974, which was before he left Rhodesia, he read a thesis that would change his life as a researcher. It was a thesis on ecological resilience written by a Canadian ecologist C.S. Holling.
Ecological theory at that time held that ecosystems change gradually over a wide range of variations. Even if an ecosystem changes into a different form, however, the theory held that it would return to its original state in a smooth and gradual manner.
However, Professor Walker often observed during the field work with students that ecosystems in savannas changed into different forms suddenly and did not return to their original state. The existing ecological theory could not explain this phenomenon.

Professor Holling's thesis made the following points:

Abrupt changes mean that the ecosystem is not functioning as it should. It is necessary for us to understand what is happening and why it is happening.
The key is to understand resilience, which is the ability of ecosystems.

Professor Walker was inspired by Professor Holling's work. In it he found answers to his questions.

Knowing limitations

What are the abrupt changes in ecosystems? Let's take a look at the desertification of pastures.
Imagine that you are raising cattle. If you increase the number of cows, the amount of grass eaten by cows will increase, which will change the state of the field. The grass will decrease and grow shorter.

Cows grazing in a field

If cows are taken from the field for a certain amount of time, the field will return to its original state. Grass absorbs nutrition from the roots; therefore, it can grow back even if everything above the ground is eaten. As explained above, resilience is the ability to flexibly respond to change. Resilience is maintained as long as the grass can grow back, and the field can return to its original state.
However, if grazing depletes the grass beyond a certain point, the field will not return to its original state even if the cows are removed. Eventually, the field of grass will turn into a desert where no grass can grow. This has happened in many regions of Africa south of sub-Sahara Africa.

Why did this happen? This has to do with the relationship between the amount of grass covering the ground and the amount of rain water that permeates the soil. If the grass covering the ground is depleted past a certain limit, a limit known as the threshold, the ground becomes unable to absorb and store rain water. Without water in the ground, grass cannot grow. As a result, the land becomes desert.

Desertification process

The amount of water stored in the ground changes depending on the amount of grass; and the amount of grass is determined by the amount of livestock. It is humans, however, that determine the amount of livestock. For sustainable grazing, it is necessary for us to understand how much grass must be maintained to ensure regeneration.

Desertification process

Studies on resilience spreading throughout the world

Professor Walker wrote to Professor Holling in the hope of learning more about the theory. In response, Professor Holling invited him to conduct joint research. This was the beginning of their collaboration.
Many others specializing in a wide range of research on lakes, forests, and insects joined the research on resilience, which created an expanding global network.

Researchers studying lakes observed the process of clear water becoming murky to determine the threshold of nutrients (the cause of algae) a lake can accommodate before the amount of algae prevents the water from returning to its original state.
At first, the lake ecosystem could maintain resilience, which prevented the propagation of algae and sustained the original ecosystem even with nutrients flowing into it. After the amount of nutrients exceeded the threshold, it rapidly changed and could not return to its original state. Their observations found that the changes prevented nature from recovering.

Clear lake (Top) and Murky lake caused by algae blooms (Bottom)

We tend to think that change in nature is gradual, but this is not true. Abrupt change can and does happen in nature around us and around the world.

3. Resilience of social-ecological systems (SESs)