Biodiversity: Study Guide (Biology 8.6)

Why diversity matters

Topic 8.5 showed how to describe a community and measure its diversity. This topic asks what diversity does. The short answer: it helps an ecosystem survive change. Droughts, floods, heat waves, diseases and new species all disturb ecosystems. Some ecosystems barely change and bounce back quickly; others are badly damaged and stay damaged. The number of parts an ecosystem has, and how different those parts are, goes a long way to explaining the difference.

Biodiversity is the variety of life at three levels:

• genetic diversity: variation in alleles within a species (Unit 7 showed why this matters for evolution);
• species diversity: the number of species and how evenly they are represented;
• ecosystem diversity: the variety of habitats, communities and processes in an area.

Diversity and resilience

Two words describe how an ecosystem copes with a disturbance:

• Resistance: how little it changes when the disturbance hits.
• Resilience: how quickly and how completely it recovers afterwards.

Ecosystems with few component parts, and with little diversity among those parts, are often less resilient. This applies to natural ecosystems and to artificial ones, such as farms, plantations and fish ponds. Three reasons:

1. Back-up species (functional redundancy). In a diverse grassland, several plant species capture light and fix carbon. If drought kills a drought-sensitive species, a drought-tolerant one can keep production going. In a field of one crop, there is no back-up.
2. Alternative pathways in the food web. If a consumer has several food sources, the loss of one is survivable. In a simple food chain, losing one link breaks the chain.
3. Variety of responses. Different species, and different genotypes within a species, respond differently to heat, cold, disease or pests. The more variety, the more likely that something survives and can rebuild.

Low diversity in an artificial system. Picture a large field planted with a single crop variety. Every plant has almost the same genes, so every plant is vulnerable to the same pests and diseases. If one new disease arrives, nothing in the field resists it and the whole crop can fail. A field sown with several crops, or several varieties, is more likely to keep producing. Topic 8.7 returns to real cases of this kind.

What keeps an ecosystem diverse

Several kinds of component hold up the diversity of an ecosystem.

• Producers. Plants, algae and photosynthetic bacteria capture the energy that every other organism depends on. They also often provide the physical habitat: a kelp forest, a coral reef or a woodland canopy is built by its producers. Lose them and you lose both food and shelter.
• Essential abiotic factors. Water, light, temperature, nutrients, soil and seasonal events such as floods set which species can live there at all. Changing one can remove many species at once.
• Essential biotic factors. Pollinators, seed dispersers, decomposers and nitrogen-fixing bacteria do jobs that many other species rely on.
• Keystone species. These deserve a section of their own.

Keystone species

A keystone species has an effect on its ecosystem that is far larger than its abundance would suggest. It may be rare or make up a tiny share of the biomass, yet when it is removed, the ecosystem often collapses into a much simpler state. The name comes from the central stone at the top of an arch: small, but take it out and the arch falls.

Two well-studied examples:

• A sea star on rocky shores. The ecologist Robert Paine introduced the term in 1969. On the rocky coast of Washington State, he removed the predatory ochre sea star (Pisaster ochraceus) from areas of shore. Mussels, which the sea star had been eating, spread over the rock and crowded out other species; the number of species fell from 15 to 8. The sea star was keeping the strongest competitor in check, which left room for everything else.
• Sea otters in kelp forests. Sea otters eat sea urchins, and sea urchins graze kelp. Where fur hunting removed sea otters along parts of the North Pacific coast, urchins multiplied and stripped kelp forests, along with the fish and invertebrates that lived in them. Where otters returned, kelp recovered.

A keystone effect usually works indirectly. The keystone species controls one population (often a dominant competitor or a heavy grazer), and that control protects many others.

Adding or removing a component

Any component you add or remove changes the ecosystem’s structure. It helps to separate two time scales.

• Short-term effects fall on the species that interact directly with the changed component: its prey, predators, competitors or partners.
• Long-term effects spread through the whole community: changes in which species dominate, loss of habitat, loss of species that depended on the lost ones, and eventually a different community.

A good prediction follows the chain step by step: what does this component do → which population changes first → what depends on that population → what does the community look like after several years?

Worked example 1: which grassland is most resilient?

Question. Use the data in Figure 1. For each type of plot, calculate (a) the percentage fall in biomass during the drought and (b) the biomass in year 3 as a percentage of year 1. Then explain the pattern.

Plots	Year 1	Year 2 (drought)	Year 3
1 species	400	150	260
4 species	420	260	400
16 species	450	340	455

(a) Fall during the drought = (year 1 − year 2) ÷ year 1 × 100.

1. 1 species: (400 − 150) ÷ 400 × 100 = 250 ÷ 400 × 100 = 62.5%.
2. 4 species: (420 − 260) ÷ 420 × 100 = 160 ÷ 420 × 100 = 38.1%.
3. 16 species: (450 − 340) ÷ 450 × 100 = 110 ÷ 450 × 100 = 24.4%.

(b) Recovery = year 3 ÷ year 1 × 100.

1. 1 species: 260 ÷ 400 × 100 = 65.0%.
2. 4 species: 400 ÷ 420 × 100 = 95.2%.
3. 16 species: 455 ÷ 450 × 100 = 101% (fully recovered).

Interpretation. The more species, the smaller the fall (greater resistance) and the more complete the recovery (greater resilience). In the 16-species plots, drought-tolerant species probably kept growing while sensitive ones struggled, then the whole community regrew. The one-species plots had no back-up: when the single species was damaged, production fell sharply and recovery was slow.

Check. All falls are between 0% and 100%, and the order of the results (1 < 4 < 16) is the same for both measures, so the pattern is consistent. With only one drought and fictional means, the data support the idea; replicated plots and error bars would be needed to judge how reliable each difference is.

Worked example 2: spotting a keystone species

Question. In a fictional meadow, ecologists remove one species at a time from separate fenced plots. Before the experiment, every plot contains 20 plant species. They count the plant species again after 1 year and after 4 years.

Species removed	Share of total biomass	Plant species after 1 year	Plant species after 4 years
G, a tall grass	55%	21	23
H, a herb	12%	20	19
R, a small grazing mammal	1%	18	9
B, a leaf beetle	0.5%	20	20

(a) Calculate the percentage change in plant species after 4 years for each removal. (b) Identify the keystone species and justify your choice. (c) Explain the difference between the 1-year and 4-year results for R.

(a) Percentage change = (after − 20) ÷ 20 × 100.

1. G: (23 − 20) ÷ 20 × 100 = +15%.
2. H: (19 − 20) ÷ 20 × 100 = −5%.
3. R: (9 − 20) ÷ 20 × 100 = −55%.
4. B: (20 − 20) ÷ 20 × 100 = 0%.

(b) R is the keystone species. It makes up only 1% of the biomass, yet removing it more than halves the number of plant species. Grass G is far more abundant (55%), but removing it slightly increases richness. B is about as rare as R and has no effect, so rarity alone does not make a keystone species: the effect must be large relative to abundance.

(c) After 1 year, the loss is small (−10%): the direct, short-term effect is simply that the grass is no longer being grazed. Over 4 years, the ungrazed grass G grows taller and spreads, shading out small plants one after another. The long-term effect on the community is much larger than the short-term one. The result for G fits this: G is the dominant competitor, and R was keeping it in check, just as the sea star kept mussels in check.

Prediction. If R were reintroduced, grazing would reduce G, light would reach the ground again, and plant richness should rise slowly, as long as seeds of the lost species can still arrive from nearby.

Common misconceptions

• “A keystone species is the most common species.” It is the species whose effect is out of proportion to its abundance. The most common species is often a dominant competitor instead.
• “Removing any rare species makes an ecosystem collapse.” Many rare species have small effects (species B above). Only keystone species have a disproportionate effect.
• “More species always means total stability.” Diverse ecosystems are often more resilient, not immune. A large enough disturbance can damage any ecosystem.
• “Resilience means nothing changes.” That is resistance. Resilience is about recovery after change.
• “Only living things maintain diversity.” Essential abiotic factors such as water, light, temperature and nutrients set which species can live there at all.
• “Short-term and long-term effects are the same.” The first effects fall on direct partners; the community-wide effects often take years and can be much larger.
• “Artificial ecosystems are not affected by diversity.” A field of one crop variety is a low-diversity system, which is exactly why a single disease or pest can destroy it.

Where this leads

Topic 8.7 looks at disruptions in more detail: how variation in populations meets environmental change, invasive species, human activities, and geological and weather events. Continue with Disruptions in Ecosystems. For community structure and the diversity index, look back at Community Ecology. Test yourself with the practice questions, then use the revision notes and the checklist.