What is the difference between species richness and species evenness?

Species richness is the total number of different species in an area, while species evenness describes how close in numbers each species in an environment is. An ecosystem is most diverse when it has both high richness and high evenness.

How does habitat diversity influence species diversity?

Higher habitat diversity provides a greater variety of ecological niches and microhabitats. This allows more specialized species to coexist in the same general area, thereby increasing the overall species diversity of the ecosystem.

When should an ecosystem be considered 'resilient'?

An ecosystem is resilient when it can absorb disturbances—such as wildfires or disease outbreaks—and maintain its essential structure and functions. High biodiversity supports this by providing redundant species that can perform critical roles if others are lost.

What is a common error when evaluating the health of a recovered population after a bottleneck?

A common error is assuming that because the population size has returned to its original level, the population is healthy. In reality, the genetic diversity remains severely depleted, making the population vulnerable to future stressors.

Why is it a mistake to use only species richness as an indicator of biodiversity?

Richness ignores the relative abundance of species; a community dominated by a single species is less stable and diverse than one with balanced populations, even if they have the same number of species.

What error occurs when students confuse 'habitat loss' with 'bottleneck effect'?

While habitat loss often leads to a bottleneck, they are distinct: habitat loss is the physical reduction of living space, whereas a bottleneck is the specific genetic consequence of a drastic population reduction.

Define a population bottleneck.

A population bottleneck is a sharp reduction in the size of a population due to environmental events or human activities. This event limits the genetic variety available for future generations, regardless of subsequent population growth.

What is genetic diversity?

Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species. It serves as the raw material for adaptation, allowing species to survive changing biotic and abiotic factors.

Define species evenness.

Species evenness is a measure of the comparative abundance of all species in an ecosystem. High evenness indicates that no single species dominates the community, which generally leads to greater stability.

How does genetic diversity aid in surviving a new pathogen?

In a genetically diverse population, some individuals are likely to possess natural resistance to the pathogen due to unique gene combinations. These individuals survive and reproduce, ensuring the species does not go extinct.

Biodiversity in Ecosystems | College Board AP Environmental Science

Revision Notes

College Board

Environmental Science

Unit 2: The Living World: Biodiversity

Biodiversity in Ecosystems

Summary

Biodiversity is the measure of biological variation across multiple scales, from the genetic makeup of individuals to the variety of ecosystems in a biome. It serves as the primary indicator of ecosystem health and resilience, determining how well a biological community can withstand and recover from environmental stressors.

1. The Three Levels of Biodiversity

Genetic Diversity: This refers to the variety of genetic material within a single species or population. High genetic diversity allows a population to harbor a wide range of traits, increasing the likelihood that some individuals will possess the necessary characteristics to survive environmental changes.
Species Diversity: This level measures the number of different species in a community (species richness) and their relative abundance (species evenness). An ecosystem with high species diversity is typically more stable because multiple species may perform similar ecological roles.
Habitat Diversity: This is the range of different habitats, niches, and ecological processes within a particular area. Complex environments like coral reefs or tropical rainforests provide more 'ecological real estate,' supporting a wider array of specialized organisms.

A nested circle diagram showing the hierarchy of biodiversity from genetic diversity at the core, to species diversity, and finally habitat diversity at the largest scale.

2. Ecosystem Resilience and Stability

Resilience: This is the capacity of an ecosystem to maintain its core functions and structure after being disturbed by events like fires, floods, or human activity. High biodiversity acts as a 'biological insurance policy,' ensuring that if one species fails, others can fill its niche.
Ecological Tipping Points: These are thresholds beyond which an ecosystem undergoes a rapid and often irreversible shift in state. Diverse ecosystems are less likely to reach these points because they possess multiple feedback loops that stabilize the system.

3. Population Bottlenecks and Genetic Health

The Bottleneck Effect: A population bottleneck occurs when a significant event (natural disaster, overhunting, habitat destruction) drastically reduces the population size. Even if the population later recovers in numbers, the genetic diversity remains low because the 'survivor' gene pool is a limited subset of the original.
Consequences of Low Genetic Diversity: Small, genetically uniform populations are highly vulnerable to environmental stressors. They face a higher risk of inbreeding depression, where harmful recessive mutations become more common, leading to reduced fertility and higher mortality.

4. Quantifying Diversity: Richness vs. Evenness

5. Key Distinctions

6. Exam Strategy & Tips