Question 1

What is a keystone species?

Accepted Answer

A species whose effect on its ecosystem is **disproportionately large relative to its abundance** — so removing it causes changes far beyond what its numbers would suggest.

**The origin.** The concept comes from Robert Paine's experimental work in the 1960s, removing a predatory starfish from rocky shore plots. The starfish was not abundant, but its removal allowed mussels to dominate and the number of species in the plots **collapsed** — from over a dozen to very few. The ecosystem restructured entirely because of one uncommon species.

The name comes from the wedge-shaped stone at the top of an arch: small, and the arch falls without it.

**The types:**

**Keystone predators**, which control a dominant competitor and thereby allow others to persist. The clearest and best-evidenced category.

**Ecosystem engineers**, which physically alter habitat. **Beavers** are the standard example — dams create wetlands supporting a completely different community, and their reintroduction has measurably altered hydrology, flood behaviour and biodiversity in several catchments.

**Mutualists**, such as key pollinators or seed dispersers on which many plants depend.

**Trophic cascades** are the mechanism by which the effect propagates. A change at one level cascades down: removing a predator increases herbivores, which reduces vegetation, which affects soil, water and everything depending on them.

**The wolves of Yellowstone** are the most famous example — and it is worth stating that the popular account is **substantially overstated**. Wolf reintroduction did produce ecological changes, but the widely circulated claim that wolves changed the course of rivers compresses a complex system into a story. Subsequent research has found the effects real but more modest and more contested than the narrative suggests, with other factors including bears, drought and elk management contributing.

**Why the concept is useful:** it means conservation effort is not simply proportional to abundance, and protecting or restoring one species can restore function across a system.

**Why it is criticised:** identifying keystone species reliably requires removal experiments that are rarely possible, the label is applied loosely, and ecosystems are networks in which many species matter.

**Umbrella** and **flagship** species are related but different ideas.

Question 2

What is a trophic cascade?

Accepted Answer

A chain of effects passing through a food web when something changes at one level — typically the addition or removal of a predator — producing changes several steps away that are not obviously

Question 3

What is a wildlife corridor, and do they work?

Accepted Answer

A strip of habitat connecting otherwise isolated areas, allowing animals to move between them — and the evidence that connectivity matters is strong, while the evidence about specific corridors is

Question 4

What is biodiversity net gain?

Accepted Answer

A requirement that development must leave biodiversity measurably better than before — in England, a legally mandated minimum uplift, calculated with a defined metric and secured for a long period.

Question 5

What are ecosystem services?

Accepted Answer

The benefits people obtain from ecosystems — a framework for making nature's contribution visible in decisions that would otherwise treat it as worth nothing. The four categories, from the Millennium

Question 6

Why are invasive species such a problem?

Accepted Answer

Because they arrive without the predators, parasites and competitors that constrained them where they evolved, and native species have no evolutionary history of dealing with them. What makes a

Question 7

What is rewilding and does it work?

Accepted Answer

Rewilding is an approach to conservation that aims to restore natural processes and allow ecosystems to organise themselves, rather than managing habitats toward a fixed target state. How it differs

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