🎒 Base knowledge and understanding

✅ Tasks

🌿 Content

🔑 Key terms

Phase 1: Starter

One estate, two states

Here are two descriptions of the same 1,400 hectares of heavy clay in West Sussex, England.

The estate in 2000. A loss-making arable and dairy farm. The fields are ploughed, drained and fertilised every year. The soil is so heavy that one of the owners later described it as concrete in summer and porridge the rest of the year. After years of trying, the farm is deep in debt.

The same estate in 2025. The plough is gone. Free-roaming cattle, ponies, pigs and deer move through a mosaic of grassland, thorny scrub and open-grown trees. Nightingales sing where there were almost none, one of the largest colonies of a rare butterfly in Britain breeds on the willows, white storks nest on the oaks, and the estate runs a profitable wildlife tourism business.

https://www.youtube.com/watch?v=0JpilefdeBc

Knepp - Rewilding Pioneers

1. State two things that had to stop and one thing that had to start for the estate to change from the first state to the second.

2. Could the 2025 mosaic turn back into the 2000 farm on its own, with no human action? Explain your answer in one sentence.

3. Suggest one reason why some people might object to a productive farm being turned into a rewilding project.

Phase 2: How a system flips

Positive feedback and alternative stable states

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3.3.11 Positive feedback loops that enhance biodiversity and promote ecosystem equilibrium can be triggered by rewilding and habitat restoration efforts.

A positive feedback loop is a necessary condition for the emergence of alternative stable states at the community scale. Consider the role of enhanced growth and biomass, reproduction and survival of species on food web interactions.

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In Topic 1 you met two kinds of feedback. Negative feedback counteracts change and pushes a system back towards its original state, which is why ecosystems can be stable and resilient. Positive feedback does the opposite: it amplifies a change, driving the system further in the direction it has already started to move.

This distinction is the key to alternative stable states. A stable state is a configuration a system settles into and tends to return to after small disturbances. Many ecosystems have more than one possible stable state. A useful picture is a ball resting in a valley: a small push and negative feedback rolls it back, but a large enough push can send it over a ridge into a different valley, where a new set of self-reinforcing processes now holds it in place.

Getting from one valley to the other is the hard part. A gradual nudge is simply cancelled out by negative feedback. To cross the ridge, the system needs positive feedback to amplify the change past a threshold, or tipping point. This is why the syllabus says a positive feedback loop is a necessary condition for an alternative stable state to emerge: without amplification, the system just returns to where it started.

At the scale of a whole community, these loops run through food web interactions. When rewilding raises the growth and biomass of plants, or the reproduction and survival of animals, those changes feed back through who eats whom, who shelters whom and who fertilises what, in ways that can reinforce themselves.

1. Distinguish between negative feedback and positive feedback in terms of their effect on a change in a system.

2. Using the ball-and-valley image, explain why a small, gradual change rarely moves an ecosystem into an alternative stable state.

3. Explain what is meant by saying a positive feedback loop is a necessary condition for an alternative stable state.

Three loops at Knepp