Lesson 04: Biodiversity & Ecosystem Services

Warm-Up

Imagine two farms. One grows a single variety of corn across a thousand acres. The other grows dozens of crop varieties mixed together. A new fungus arrives. On the first farm, if the fungus can kill that one variety, it kills everything. On the second, some varieties resist, and the farm survives. That's the whole argument for biodiversity in one image: variety is insurance. More kinds of genes, species, and habitats mean more ways for a system to absorb a shock and keep functioning. This lesson defines the three levels of biodiversity, shows why they make ecosystems resilient, and catalogs the free services nature provides that we'd otherwise have to pay for.

Core Concept

Three levels of biodiversity

Biodiversity is the variety of life, measured at three scales:

1. Genetic diversity — the variety of genes within a species. High genetic diversity lets a population adapt to change and resist disease. Low genetic diversity (e.g., cheetahs, or a monoculture crop) makes a population vulnerable to being wiped out by one disease or stressor.
2. Species diversity — the variety of species in an area. It has two components: - Species richness = the number of different species. - Species evenness = how equal the abundances are across species. A community with 10 species in roughly equal numbers is more diverse than one where a single species dominates and nine are rare.
3. Habitat (ecosystem) diversity — the variety of ecosystems in a region. More habitat types support more species and more services.

[DIAGRAM: Two communities, each with 4 species and 100 individuals. Community A: 25/25/25/25 (high evenness). Community B: 91/3/3/3 (low evenness). Same richness (4), but A has higher diversity. Label "richness = # species; evenness = balance of abundances."]

Diversity indices such as Simpson's and Shannon combine richness and evenness into one number — higher values mean more diversity. You won't compute Shannon by hand, but you should interpret that a higher index = greater diversity, and read comparative index values from a table.

Why biodiversity matters: resilience and stability

• Resilience is an ecosystem's ability to recover after a disturbance (fire, storm, disease).
• Resistance is its ability to withstand a disturbance without changing much.

Higher biodiversity generally increases both, because functional redundancy means if one species is lost, another can fill its role. Diverse ecosystems are more stable and productive over time. Species that are especially important include:

• Keystone species — have a disproportionately large effect relative to their abundance (sea otters controlling urchins; wolves in Yellowstone). Removing them collapses the community.
• Indicator species — sensitive organisms whose health signals ecosystem condition (amphibians, lichens, trout).

Ecosystem services

Ecosystem services are the benefits humans get from functioning ecosystems, often grouped in four categories:

Pollinators alone (bees, bats, birds) support a large share of global food crops. Wetlands filter water and buffer floods. Forests regulate climate and prevent erosion. Assigning economic value to these services (e.g., the cost to replace pollination or water filtration with technology) is a common way to justify conservation.

Threats to biodiversity

The major drivers, summarized by the acronym HIPPCO (developed further in Lesson 29): - Habitat destruction/fragmentation (the #1 cause) - Invasive species - Population growth (human) - Pollution - Climate change - Overexploitation (overharvesting/overhunting)

Why this matters

Biodiversity questions appear as data-table reading (compare richness/evenness or index values), as concept questions (keystone vs. indicator species), and as FRQ solution prompts (justify a conservation action by its ecosystem services). This unit is small in weight but high-yield because it's straightforward.

Worked Examples

Example 1 (easy): Richness vs. evenness

Plot A has 12 species with fairly equal abundances. Plot B has 12 species but one makes up 90% of individuals. Which has higher species diversity?

Solution: Same richness (12), but Plot A has higher evenness, so Plot A has higher species diversity.

Interpretation: Diversity isn't just how many species — it's how evenly individuals are spread among them.

Example 2 (medium): Keystone reasoning

Sea otters eat sea urchins, which graze kelp. Predict what happens to a kelp forest if otters are removed.

Solution: Without otters, urchin populations explode, urchins overgraze the kelp, the kelp forest collapses, and the many species depending on kelp habitat decline. The otter is a keystone species — its impact far exceeds its abundance.

Interpretation: Keystone = outsized role. Removing it triggers a trophic cascade.

Example 3 (AP-style): Genetic diversity and disease

Explain why a banana plantation of a single genetically identical variety is at high risk from a new fungal disease, and how genetic diversity would reduce that risk.

Solution: All plants share the same genes, so if the fungus can infect one, it can infect all — no individuals possess resistance genes. A genetically diverse crop would contain some individuals with resistance, so the disease could not wipe out the entire crop; survivors preserve the harvest and the population.

Interpretation: Monoculture = zero genetic diversity = maximum vulnerability. This links to Unit 5 agriculture.

Example 4 (AP-style): Valuing a service

A wetland upstream of a city naturally filters pollutants and controls floods. The city considers draining it for development. Give one economic argument for preserving it.

Solution: The wetland provides regulating services (water filtration and flood control) for free; replacing them would require building a water-treatment plant and flood-control infrastructure costing far more than the development gains. Preserving the wetland avoids those costs and preserves provisioning/cultural value.

Interpretation: Framing services as avoided costs is the standard APES economic-justification move.

Common Mistakes

• Equating diversity with richness only. Diversity = richness and evenness. A single dominant species lowers diversity even if many species are present.
• Confusing keystone and indicator species. Keystone = large ecological effect. Indicator = signals ecosystem health. A species can be both, but the definitions differ.
• Saying "biodiversity is good" without a mechanism. On an FRQ, connect it to resilience, disease resistance, or a specific ecosystem service.
• Forgetting habitat fragmentation. Habitat loss is the #1 threat; fragmentation (roads, farms cutting habitat into islands) reduces effective habitat even when total area seems preserved.
• Ignoring supporting services. Nutrient cycling and soil formation "support" the others — don't overlook them on a categorization question.

Practice Problems

11. (FRQ-style) Explain two distinct reasons (one ecological, one economic) that a government might protect a biodiverse rainforest rather than allow it to be logged.

12. (Data) A survey finds Forest X has a Shannon index of 3.1 and Forest Y has 1.4. (a) Which is more diverse? (b) Give one likely consequence for Forest Y's resilience to disease.

FRQ Practice — Analyze an Environmental Problem and Propose a Solution (10 pts)

A coastal region plans to convert 40% of its mangrove wetlands into shrimp farms and resorts.

(a) Identify two ecosystem services the mangroves currently provide, naming the service category for each. (2 pts) (b) Describe how converting the mangroves would affect local biodiversity, referencing at least one HIPPCO factor. (2 pts) (c) Explain how the loss of mangroves could increase the region's vulnerability to storms. (2 pts) (d) Propose two solutions that balance economic development with biodiversity protection, and justify each. (4 pts)