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Understanding
Biodiversity

Explore the variety of life on Earth: from genes to ecosystems, and why biological diversity is essential for the survival of all living systems.

AP Biology

What Is Biodiversity?

Biodiversity is the variety and variability of life at every level of biological organization. It encompasses differences within species, between species, and across entire ecosystems. The term was coined in 1985 by Walter G. Rosen and popularized by E.O. Wilson.

Three Levels of Biodiversity

Genetic Diversity
Variation in alleles and genotypes within a population. Enables adaptation to changing environments.
Species Diversity
The number and relative abundance of species in a community. Measured by richness and evenness.
Ecosystem Diversity
The variety of ecosystems in a region: forests, wetlands, coral reefs, grasslands, and more.

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 landscapes, species diversity within communities and genetic variation within a species. At one diploid two-allele locus, AA, Aa and aa are three genotypes formed from two alleles.

Biodiversity depends on the scale

Biodiversity depends on the scale. Biodiversity includes ecosystem variety across landscapes, species diversity within communities and genetic variation within a species. At one diploid two-allele locus, AA, Aa and aa are three genotypes formed from two alleles.Biodiversity includes ecosystem variety across landscapes, species diversity within communities and genetic variation within a species. At one diploid two-allele locus, AA, Aa and aa are three genotypes formed from two alleles.

Why Biodiversity Matters

1
Ecosystem services: Pollination, water purification, carbon sequestration, and nutrient cycling all depend on diverse biological communities.
2
Resilience: Diverse ecosystems recover faster from disturbances like fires, floods, and disease outbreaks.
3
Medicine and agriculture: Over 50% of prescription drugs originate from natural compounds. Crop genetic diversity prevents famine.
4
Evolutionary potential: Greater genetic variation provides raw material for natural selection and long-term adaptation.

Explain how eDNA can complement direct observation.

A water sample can contain DNA from organisms that are difficult to observe. Transport, persistence and contamination mean detection does not establish live local abundance.

An illustrative stream-to-laboratory scene shows collection, filtration and assay equipment. Both the stream sample and a separate DNA-free field blank follow water, filter, extraction and assay. No result is shown.
A water sample can contain DNA from organisms that are difficult to observe. Transport, persistence and contamination mean detection does not establish live local abundance.

Measuring Species Diversity

Ecologists quantify species diversity using two components: species richness (the total number of species, \( S \)) and species evenness (how equally individuals are distributed among those species). A common index is the Shannon Diversity Index:

Richness vs. Evenness

Community A Sp1 Sp2 Sp3 Sp4 High Evenness \( H' = 1.38 \) Community B Sp1 Sp2 Sp3 Sp4 Low Evenness \( H' = 0.56 \) Same richness (\( S = 4 \)) Same richness (\( S = 4 \))

Richness vs. Evenness: Key Distinction

Species Richness Species Evenness
Definition Total number of different species Relative abundance of each species
What it measures How many species are present How equally individuals are distributed
Symbol \( S \) \( E = H' / \ln(S) \)
Example Forest with 50 tree species Each species has ~2% of individuals
Limitation Ignores relative abundance Ignores total species count

How to Calculate \( H' \)

1
Count individuals of each species in the sample. Record total \( N \) and count per species \( n_i \).
2
Calculate proportions: For each species, compute \( p_i = n_i / N \).
3
Compute each term: Multiply \( p_i \times \ln(p_i) \) for every species.
4
Sum and negate: \( H' = -\sum p_i \ln(p_i) \). The negative sign makes \( H' \) positive.
5
Interpret: Compare \( H' \) values between communities. Higher = more diverse.

Worked Example: Pond Community

A pond contains 3 species of fish. A sample of \( N = 100 \) individuals gives: Species A = 50, Species B = 30, Species C = 20.

1
Proportions: \( p_A = 0.50 \), \( p_B = 0.30 \), \( p_C = 0.20 \)
2
Each term: \( 0.50 \ln(0.50) = -0.347 \), \( 0.30 \ln(0.30) = -0.361 \), \( 0.20 \ln(0.20) = -0.322 \)
3
Sum: \( -0.347 + (-0.361) + (-0.322) = -1.030 \)
4
Negate: \( H' = 1.03 \)

Comparing Two Meadows

Meadow X: 5 species with counts 40, 30, 15, 10, 5 (\( N = 100 \)).
Meadow Y: 5 species with counts 20, 20, 20, 20, 20 (\( N = 100 \)).

Meadow X Meadow Y
Richness (\( S \)) 5 5
\( H' \) 1.40 1.61
Max \( H' = \ln(5) \) 1.61 1.61
Evenness (\( E \)) 0.87 1.00

Species-Area Relationship

Island biogeography predicts that larger areas support more species. The species-area curve follows a power law:

Area (A) Species (S) Small island Large island \( S = cA^z \)

Threats to Biodiversity

Biodiversity Loss Habitat Loss Invasive Species Overexploitation Pollution Climate Change

Habitat fragmentation, corridors and gene flow

Three qualitative forest landscapes show continuous woodland, fragmented woodland and a restored vegetated road crossing.Connected habitat can permit movement; gene flow also requires successful reproduction. Conservation plans balance habitat quality, core area, representation and shared disturbance exposure.

A corridor enables movement

A corridor enables movement. Connected habitat can permit movement; gene flow also requires successful reproduction. Conservation plans balance habitat quality, core area, representation and shared disturbance exposure.Connected habitat can permit movement; gene flow also requires successful reproduction. Conservation plans balance habitat quality, core area, representation and shared disturbance exposure.

Movement does not prove gene flow

Movement does not prove gene flow. Tracking movement tests connectivity. Parentage and reproduction measurements test gene flow, and repeated genetic sampling tests changes across generations.Tracking movement tests connectivity. Parentage and reproduction measurements test gene flow, and repeated genetic sampling tests changes across generations.

Habitat structure is species-specific

Habitat structure is species-specific. A corridor must provide the resources and habitat structure required by the focal species; movement alone does not establish suitable breeding habitat.A corridor must provide the resources and habitat structure required by the focal species; movement alone does not establish suitable breeding habitat.

Equal area does not mean equal core

Equal area does not mean equal core. For equal total area and a fixed edge depth, one large square retains 64% core habitat while four smaller squares retain 36%.For equal total area and a fixed edge depth, one large square retains 64% core habitat while four smaller squares retain 36%.

Reserve plans face more than edge effects

Reserve plans face more than edge effects. Reserve designs with equal total area can expose habitat to different shared disturbances; site arrangement is another consideration alongside core area.Reserve designs with equal total area can expose habitat to different shared disturbances; site arrangement is another consideration alongside core area.

Keystone Species & Trophic Cascades

A keystone species has a disproportionately large effect on its community relative to its abundance. Removing a keystone species triggers a trophic cascade: a chain of effects that ripples through the entire food web.

Top Predator (e.g., wolves) Herbivores (e.g., elk) Producers (e.g., willows, aspens) Remove wolves Elk increase Plants decline

Biodiversity Hotspots

A biodiversity hotspot is a region that meets two criteria: it contains at least 1,500 endemic vascular plant species, and it has lost at least 70% of its original habitat. Currently, 36 hotspots exist globally.

Tropical Andes
~30,000 plant species. Highest species richness of any hotspot on Earth.
Sundaland
Southeast Asian islands. Home to orangutans, tigers, and 15,000 plant species.
Madagascar
~90% of species found nowhere else. Over 12,000 endemic plant species.

Common Mistakes

✕ Confusing richness with diversity
A community can have high species richness but low diversity if one species dominates. Always consider evenness alongside richness.
✕ Using log base 10 instead of ln
The Shannon Index uses the natural logarithm (\( \ln \)). Using \( \log_{10} \) gives a different (smaller) value and makes comparisons invalid.
✕ Ignoring genetic diversity
Biodiversity is not just about counting species. A population with very low genetic diversity is vulnerable to extinction even if species count is high.

Biodiversity Summary

Concept Key Detail Why It Matters
Genetic Diversity Allele variation within populations Drives adaptation and evolution
Species Diversity Richness + Evenness (\( H' \)) Measures community health
Ecosystem Diversity Variety of habitat types Supports functional redundancy
Shannon Index \( H' = -\sum p_i \ln(p_i) \) Quantitative comparison tool
Species-Area \( S = cA^z \) Predicts island/fragment richness
Keystone Species Disproportionate ecological role Removal causes trophic cascades

Question 1 of 5

A forest community has 4 species with equal abundances of 25 individuals each (\( N = 100 \)). What is the Shannon Diversity Index \( H' \)?

Question 2 of 5

Which of the following best describes a keystone species?

Question 3 of 5

Two communities each have 10 species. Community P has one species comprising 91% of individuals. Community Q has all species at 10% each. Which statement is correct?

Question 4 of 5

A region qualifies as a biodiversity hotspot if it meets which two criteria?

Question 5 of 5

According to the species-area relationship \( S = cA^z \), what happens to species richness when habitat area is reduced by 90% (assuming \( z = 0.3 \))?