AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/8/8-6)
Unit 8 · Topic 8.6
8.6 Biodiversity
Ecosystems with more species and more variety are usually more resilient, meaning they handle and recover from disturbances better. Keystone species, producers and key abiotic factors hold ecosystem diversity together, and keystone species have effects far larger than their numbers would suggest.
Key terms
- biodiversity
- resilience
- keystone species
- producer
- abiotic factor
Biodiversity and resilience
Biodiversity is the variety of life at every level: genetic diversity within species, species diversity within communities, and ecosystem diversity across a region. Resilience is an ecosystem's ability to resist damage and recover after a disturbance such as drought, disease or fire.
Ecosystems with few parts, and little variety among those parts, are often less resilient. That applies to natural ecosystems and to artificial ones like farms. A field of one crop variety can be wiped out by one pest, while a mixed planting is more likely to include plants that resist it. In a diverse grassland, if drought kills some plant species, others that tolerate drought can keep producing food, so the whole system keeps running. Species that do similar jobs act as backups for each other.
What holds diversity together
Some parts of an ecosystem matter more than others:
- Producers capture the energy that every other organism depends on (8.2). Losing producers or their biomass shrinks every trophic level above them.
- Essential abiotic factors, such as water, sunlight, temperature, nutrients and soil, set what can live there at all.
- Essential biotic factors include pollinators, decomposers and nitrogen-fixing microbes, which keep energy and matter moving.
- Keystone species have an effect on the ecosystem that's disproportionately large relative to how many of them there are.
Keystone species
In Pacific kelp forests, sea otters eat sea urchins, and sea urchins graze on kelp. Where otters were hunted out, urchin numbers exploded and they ate the kelp down to bare rock, often called urchin barrens. The kelp forest, and the fish and invertebrates that lived in it, disappeared. Where otters returned, kelp forests recovered.
In a famous experiment on the rocky shore of Washington State, ecologist Robert Paine removed a predatory sea star (Pisaster) from test areas. Without it, mussels took over the rocks and crowded out other species, and the number of species in those areas dropped sharply. The sea star was keeping the dominant competitor in check.
The pattern: when a keystone species is removed, the ecosystem often collapses into a much less diverse state, even though the keystone species was never very abundant.
Short-term and long-term effects
Adding or removing any part of an ecosystem changes its structure. Short-term effects are usually direct: remove a predator and its prey increases within a season or two. Long-term effects come through chains of interactions: the prey overeat their food, plant cover drops, soil erodes, and species that depended on those plants decline. Adding a species, such as an invasive one (8.7), can have similar ripple effects.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Predicting a keystone removal
In a coastal ecosystem, sea otters eat sea urchins, and urchins eat kelp. Kelp forests shelter many fish species. Predict the short-term and long-term effects of removing sea otters, and justify your prediction.
Show the solutionHide the solution
- Step 1: Short-term (direct effect): with their main predator gone, sea urchin populations increase.
- Step 2: Next link: more urchins graze more kelp, so kelp biomass drops.
- Step 3: Long-term (indirect effects): as the kelp forest disappears, fish and invertebrates that depend on it for food and shelter decline, and overall species diversity falls. The area may become an urchin barren.
- Step 4: Justification: sea otters are a keystone species; their effect on the ecosystem (through urchins and kelp) is much larger than their numbers suggest.
Answer: Urchins increase in the short term, kelp declines, and over the long term kelp-dependent species decline and diversity drops, because otters are a keystone predator controlling urchins.
- Example 2
Using diversity to predict resilience
Two grassland plots are hit by the same severe drought. Plot 1 has 2 grass species. Plot 2 has 16 plant species, including several deep-rooted, drought-tolerant ones. Predict which plot's plant biomass will drop less and recover faster, and explain.
Show the solutionHide the solution
- Step 1: Claim: Plot 2 will lose less biomass and recover faster.
- Step 2: Evidence: Plot 2 has more species, including drought-tolerant ones; Plot 1 has only two.
- Step 3: Reasoning: in a diverse plot, it's more likely that some species can survive the drought and keep producing biomass. In Plot 1, if both grasses are drought-sensitive, nearly all production stops.
- Step 4: So Plot 2 is more resilient.
Answer: Plot 2 should be more resilient, because its greater diversity makes it likely that some species tolerate drought and keep the ecosystem productive.
Common mistakes
- Assuming the keystone species is the most abundant one. Its impact is large relative to its abundance, not because there are lots of them.
- Giving only the direct effect of removing a species. Strong answers trace the chain to the long-term effect on other species and diversity.
- Thinking only natural ecosystems are affected by low diversity. Farms and other artificial systems with little diversity are also less resilient.
On the exam
- Expect food-web scenarios where you predict what happens when one species is removed or added; describe each step in the chain.
- Link diversity to resilience explicitly: more diversity means more chance that some species withstand the disturbance.
Connected topics
Videos
Check yourself
5 questions on 8.6 Biodiversity. Pick an answer to see if you got it, and why.
Sea otters eat sea urchins, which eat kelp. When sea otters disappear from an area, urchin numbers rise sharply and the kelp forest is destroyed, along with many species that depend on it. Sea otters are best described as
| Number of plant species in plot | Mean decrease in plant biomass during drought (%) |
|---|---|
| 1 | 70 |
| 4 | 45 |
| 8 | 30 |
| 16 | 18 |
Experimental data: grassland plots were planted with different numbers of species. Plant biomass was measured before and during a severe drought. Each value is the mean of 10 plots.
Which of the following claims is best supported by the data?
Which of the following is the most likely explanation for the pattern?
| Plot | Predatory sea star | Mussel cover after 5 years (%) | Number of species after 5 years |
|---|---|---|---|
| Control | Left in place | 30 | 15 |
| Removal | Removed each month | 90 | 8 |
Experimental data: on a rocky shore, a predatory sea star that eats mussels was removed from one area and left in place in a similar control area. In the control plot, sea stars made up less than 1 percent of the animals present.
Which of the following best explains the drop in the number of species in the removal plot?
Why was it important to include the control plot, where the sea star was left in place?
0 of 5 answered