Topic 8.6 Notes – Biodiversity

1. What Biodiversity Is

Biodiversity = the variety of life in an ecosystem.
It exists at more than one level, and each level affects how stable the system is.

Genetic Diversity

This is variation in alleles within a population.

• Individuals in a population are not genetically identical.
• Different alleles → different traits (disease resistance, drought tolerance, etc.).
• Natural selection acts on this variation.

Why it matters:

• If a disease hits, some individuals may survive because they carry resistant alleles.
• If conditions change, some genotypes may be better suited.
• Higher genetic diversity reduces inbreeding depression (fewer harmful recessive traits expressed).

Think of it this way: more genetic variation means a higher chance that some individuals survive a disturbance → the population continues.

On FRQs, if you’re asked why a population survived a change, connect it directly to existing genetic variation that allowed natural selection to act.

Species Diversity

This is the number and distribution of species in an ecosystem.

It includes:

• Species richness = how many species are present
• Relative abundance = how evenly individuals are distributed among species

An ecosystem with 20 species where one dominates 95% of individuals has lower effective diversity than one where individuals are more evenly distributed.

Why it matters:

• Different species fill different ecological roles.
• Energy flows through multiple pathways.
• Nutrients are recycled by many types of organisms.

More species = more interacting parts = more stability.

2. Key Components That Maintain Ecosystem Diversity

Biodiversity doesn’t just “happen.” Certain components help maintain it.

Producers

Producers (autotrophs) form the base of all food webs.

• Plants, algae, some bacteria
• Convert solar or chemical energy into organic molecules

Energy enters ecosystems through producers and then moves up through consumers and decomposers.

Energy flow through trophic levels

Without producers:

• No energy enters the ecosystem
• Consumers cannot survive
• The entire system collapses

Any time you analyze ecosystem stability, always identify the producers first. They support everything else.

Keystone Species

A keystone species has a disproportionately large effect relative to its abundance.

Even if there aren’t many of them, removing them causes major change.

Common types:

• Predators
  • Control prey populations
  • Prevent overgrazing
  • Their removal can cause a trophic cascade
• Ecosystem engineers
  • Physically modify the environment
  • Create habitats used by other species
• Mutualists
  • Critical interactions like pollination
  • Support reproduction of many species

A classic trophic cascade involves a top predator affecting herbivores, which then affects plant communities and other organisms.

Example of a trophic cascade

When keystone species are removed:

• Effects ripple through multiple trophic levels
• Biodiversity often decreases
• Ecosystem structure may permanently shift

The AP loves asking you to predict what happens when a top predator disappears. Always think several steps down the food web.

Essential Abiotic and Biotic Factors

Abiotic (nonliving) factors:

• Temperature
• Water availability
• Soil composition
• Nutrient levels
• pH and oxygen

Biotic (living) factors:

• Competition
• Predation
• Disease
• Symbiosis

These determine which species can survive. Change one factor, and community composition shifts.

3. How Biodiversity Increases Resistance and Resilience

Two key terms:

• Resistance = ability to withstand disturbance without major change
• Resilience = ability to recover after disturbance

They are related but not identical.

Why More Diversity = More Stability

1. Functional redundancy
   • Multiple species perform similar roles.
   • If one declines, others can compensate.
2. Different stress responses
   • Not all species respond the same way to drought, disease, or temperature shifts.
3. Multiple energy pathways
   • More complex food webs buffer disruptions.

Artificial systems like monoculture farms have:

• Low genetic diversity
• Low species diversity
• High vulnerability

One pathogen can wipe out the entire crop. That’s low resilience.

On data-based questions, if you see a graph showing faster recovery in a diverse system, connect it to redundancy and variation in response.

4. Effects of Adding or Removing Ecosystem Components

Ecosystems are networks. Changing one part affects others.

Short-Term Effects

• Rapid population increases or decreases
• Resource availability shifts
• Altered competition or predation
• Immediate food web disruption

Example chain:
Remove predator → prey increases → vegetation declines.

Long-Term Effects

• Trophic cascades
• Species migration or extinction
• Altered nutrient cycling
• Formation of a new stable state

Keystone species removal is especially dramatic. Because their effect is disproportionate, their loss can restructure the entire ecosystem.

When answering, think in layers:

1. Immediate population change
2. Effects on other species
3. Long-term structural consequences

That full chain earns points.