Introduction to
Biodiversity

Explore the variety of life on Earth, from genetic variation within species to the richness of entire ecosystems, and discover why biodiversity matters for our planet.

AP Environmental Science

What is Biodiversity?

Biodiversity (biological diversity) is the variety and variability of life on Earth. It encompasses all living organisms: plants, animals, fungi, and microorganisms, as well as the genetic differences within them and the ecosystems they form.

The Three Levels of Biodiversity

Genetic Diversity

Variation in genes within a single species. Greater genetic diversity helps populations adapt to changing environments.

Species Diversity

The number and relative abundance of different species in a given area: both richness and evenness matter.

Ecosystem Diversity

The variety of ecosystems in a region: forests, wetlands, deserts, coral reefs, and the ecological processes within them.

Nested scales of biodiversity

A fox stands at a forest edge beside purple flowers, a wetland holds two ducks, and open grassland extends beyond the water toward distant wooded hills.

Biodiversity includes ecosystem variety across a landscape, species diversity within a community, and genetic variation within a species. At a two-allele locus, AA, Aa and aa are three genotypes formed from two alleles.

Three scales of biodiversity

Biodiversity includes ecosystem variety across a landscape, species diversity within a community, and genetic variation within a species. At a two-allele locus, AA, Aa and aa are three genotypes formed from two alleles.

Why Does Biodiversity Matter?

Species Richness vs. Evenness

Species richness is the total number of different species present. Species evenness describes how equally individuals are distributed among those species. True biodiversity depends on both.

Measuring Biodiversity: the Shannon Index

The Shannon Diversity Index (\( H \)) is one of the most widely used metrics to quantify species diversity. It accounts for both richness and evenness:

A higher \( H \) value indicates greater diversity. If all species are equally common, \( H \) reaches its maximum value of \( \ln(S) \).

Richness vs. Diversity Index

	Species Richness	Shannon Index \( H \)
What it measures	Count of species	Richness + evenness combined
Formula	\( S \) = number of species	\( H = -\sum p_i \ln(p_i) \)
Evenness sensitivity	None: ignores abundance	High: detects dominance
Use case	Quick field surveys	Detailed ecological analysis
Limitation	Incomplete picture	Requires abundance data

How to Calculate the Shannon Index

1

Count individuals per species. Record the number of individuals for each species in your sample.

2

Find the total \( N \). Sum all individuals across every species: \( N = \sum n_i \).

3

Calculate each proportion. For each species, compute \( p_i = n_i / N \).

4

Compute \( p_i \ln(p_i) \) for each species. Multiply each proportion by its natural log.

5

Sum and negate. \( H = -\sum p_i \ln(p_i) \). A higher \( H \) means more diversity.

Worked Example: Meadow Survey

A meadow contains 3 species of wildflower. A researcher counts: Species A = 40, Species B = 40, Species C = 20. Total \( N = 100 \).

Worked Example: Coral Reef vs. Tide Pool

Two marine habitats are sampled. Both contain 4 species, but their abundance distributions differ dramatically.

Species	Coral Reef	Tide Pool
A	25	85
B	25	5
C	25	5
D	25	5
Total	100	100

Biodiversity Across Biomes

Major Threats to Biodiversity

Habitat Loss

Deforestation, urbanization, and agriculture destroy or fragment the habitats species need to survive.

Invasive Species

Non-native organisms outcompete, prey on, or bring diseases to native species, disrupting ecosystems.

Pollution

Chemical runoff, plastics, and air pollutants degrade habitats and poison organisms across food webs.

Population Growth

Rising human demand for resources amplifies all other threats, increasing consumption and waste.

Climate Change

Shifting temperatures and weather patterns alter habitats faster than species can adapt or migrate.

Overexploitation

Overhunting, overfishing, and illegal wildlife trade push species toward extinction.

Biodiversity Hotspots

A biodiversity hotspot is a region that meets two criteria: it contains at least 1,500 endemic vascular plant species (species found nowhere else), and it has lost at least 70% of its original natural vegetation.

Ecosystem Services Powered by Biodiversity

Provisioning

Food, fresh water, timber, fiber, and medicines derived from biological resources.

Regulating

Climate regulation, flood control, water purification, pollination, and disease regulation.

Cultural

Recreation, aesthetic appreciation, spiritual enrichment, and educational opportunities.

Supporting

Nutrient cycling, soil formation, and primary production that underpin all other services.

Common Mistakes

✕ Richness = Biodiversity

Counting species alone ignores evenness. A forest dominated by one tree species has low functional biodiversity despite moderate richness.

✕ Forgetting the Negative Sign

The Shannon formula starts with a negative: \( H = -\sum p_i \ln(p_i) \). Omitting it gives a negative result, which is biologically meaningless.

✕ Using \(\log_{10}\) Instead of \(\ln\)

AP Environmental Science uses natural log (\(\ln\)) for the Shannon Index. Using \(\log_{10}\) changes the scale and gives incorrect values.

Biodiversity Summary

Concept	Key Idea	Example / Formula
Genetic Diversity	Gene variation within a species	Allele frequencies
Species Diversity	Richness + Evenness	\( H = -\sum p_i \ln(p_i) \)
Ecosystem Diversity	Variety of ecosystems	Forest, wetland, reef
Hotspot	>1,500 endemics + >70% loss	36 globally recognized
Threats (HIPPCO)	6 major drivers of loss	Habitat loss is #1
Ecosystem Services	Benefits from biodiversity	4 categories (MEA)

Question 1 of 5

Which level of biodiversity refers to the variety of ecosystems in a region, such as forests, wetlands, and coral reefs?

Ecosystem diversity describes the range of different habitats and ecological communities in an area. Genetic diversity is variation within species, and species diversity is about the number and abundance of species.

Question 2 of 5

A community has 4 species with equal abundance (\( p_i = 0.25 \) each). What is its Shannon Diversity Index \( H \)?

With 4 equally abundant species, each has a proportion \( p_i = 0.25 \). The formula is \( H = - \sum (p_i \ln p_i) \), which becomes \( H = -4 \times (0.25 \ln 0.25) \approx 1.386 \). This value is the maximum possible diversity for 4 species, equal to \( \ln(4) \).

Question 3 of 5

Which of the following is NOT one of the criteria for designating a biodiversity hotspot?

Biodiversity hotspots require high endemism (at least 1,500 endemic plant species) and significant habitat loss (at least 70%). There is no latitude requirement: hotspots exist from the tropics to temperate regions like the Mediterranean Basin.

Question 4 of 5

In the acronym HIPPCO, what does the first "P" stand for?

HIPPCO stands for Habitat loss, Invasive species, Pollution, Population growth, Climate change, and Overexploitation. The first P is Pollution.

Question 5 of 5

Two forests each have 10 tree species. Forest X has nearly equal numbers of each species, while Forest Y is 90% one species. Which statement is correct?

Species richness is identical (\( S = 10 \)), but Forest X has much higher evenness, giving it a greater Shannon Index \( H \). Biodiversity accounts for both richness and evenness, so Forest X is more biodiverse.