1Diversity and resilience

Resilience is an ecosystem's ability to withstand a disturbance (a drought, a disease, a fire, a new predator) and to return to something like its former state afterwards.

The CED's claim is that ecosystems with fewer component parts and little diversity among those parts are often less resilient. Three reasons:

1. Different tolerances. In a meadow of thirty plant species, some are deep-rooted and survive drought; some resist a particular fungus. In a field of one crop, every plant shares the same weakness.
2. Back-up roles. When several species do similar jobs (several pollinators, several grazers), the loss of one leaves others to keep the job going.
3. More links in the food web. A predator with five prey species can switch when one declines; a predator with one prey species starves with it.

Low diversity can mean few species, or one species with little genetic variety. A monoculture, a large area planted with one crop variety, often of near-identical genotype, is the extreme case. A single pathogen that can infect one plant can infect them all.

Worked example: reading resilience from data (invented). Grassland plots sown with different numbers of species were measured before and during a drought year.

The percent change is $\frac{\text{new} - \text{old}}{\text{old}} \times 100$; for the one-species plots, $\frac{160 - 420}{420} \times 100 = -62\%$. Claim: more diverse plots were more resilient to drought. Evidence: biomass fell by 62% with one species but only 18% with sixteen. Reasoning: in diverse plots some species tolerate drought and keep growing while others fail.

Fewer parts and less variety among them make an ecosystem fragile: one disturbance can hit every part at once.

2What keeps an ecosystem diverse

The CED names four kinds of factor that maintain diversity.

• Keystone species, described below.
• Producers. All the energy for every other species enters through them (8.2). Many kinds of producer support many kinds of consumer; lose producer diversity and consumer diversity follows.
• Essential abiotic factors: water, light, temperature, soil nutrients, pH. A stream kept cool and well oxygenated by shade supports insects and fish that a warm, still stream cannot.
• Essential biotic factors: pollinators, decomposers that recycle nutrients, nitrogen-fixing bacteria, the species that build habitat (corals, trees).

3Keystone species

A keystone species has an effect on its ecosystem that is far larger than its abundance would suggest. It is often a predator, but not always. The name comes from the central stone in an arch: a single stone, yet the arch falls without it.

How a keystone predator keeps diversity high: on some rocky shores, a predatory sea star eats mussels. Mussels are the strongest competitors for space on the rock. With the sea star present, it keeps mussel numbers down and many other species (barnacles, limpets, seaweeds) find room. When the sea star is removed, mussels spread over the rock, crowd the others out, and the number of species falls sharply.

Other keystone roles:

• A predator that controls a herbivore, which would otherwise strip the producers (sea otters, sea urchins and kelp: 8.5).
• A species that changes the habitat: beavers dam streams and create ponds and wetlands that many other species use.
• A mutualist on which many others depend: a fig tree that fruits all year and feeds many birds and mammals when little else is available.

Removal often collapses the ecosystem. Because a keystone species holds so many interactions in place, removing it can set off a chain of changes that ends with a very different, usually less diverse, ecosystem.

Abundance is not importance. A dominant species is the most abundant (the main grass of a prairie). A keystone species may be rare. In the exam, a species that is a small fraction of the biomass but whose removal changes everything is the keystone.

4Adding or removing a component: short term and long term

Skill 6.E asks for predictions. Separate the immediate effect from what follows.

Short-term effects are the direct, first-step changes: the species that ate the removed one lose food; the species it ate are released from predation; its competitors gain resources.

Long-term effects follow over many generations: populations settle at new sizes, the species composition changes, habitat structure changes, diversity rises or falls, and the ecosystem may reach a new, different balance or collapse.

Worked example (invented). Wolves are removed from a valley where they hunt deer.

• Short term: deer numbers rise because fewer are killed.
• Long term: deer graze young trees and shrubs along the river heavily; tree seedlings fail; riverbanks erode; songbirds and beavers that need the trees decline. Diversity falls.

The same thinking works in reverse for adding a component, such as returning a lost predator or introducing a new species (8.7).