2.1 - Introduction to Biodiversity

Learning Objectives

The Three Levels of Biodiversity

1. Ecosystem Diversity

• Definition: The number of different habitats available within a specific area.

• Importance: A variety of habitats, such as oceans, lakes, beaches, rivers, grasslands, forests, mountains, or deserts, lets the ecosystem support a wider diversity of plant and animal species, since each environment has unique conditions.

• Also called habitat diversity: The AP course calls this level habitat diversity. So the three levels of biodiversity you need to know are genetic diversity, species diversity, and habitat (ecosystem) diversity.

2. Species Diversity

• Species Richness (R): The total number of different species found in an ecosystem. Higher species richness generally indicates a healthier ecosystem, because supporting more species suggests quality resources like clean water or nutrient rich soil.

• Species Evenness: A measure of how well individuals are distributed among the different species present. Evenness tells you whether population sizes are balanced or whether one or two species dominate. An ecosystem with a balance across its species (high evenness) has higher overall species diversity than one dominated by a single species, even if their richness is the same.
• Example: Two forest plots each have 100 trees from the same 4 species. Plot A has 25 oak, 25 maple, 25 hickory, and 25 beech. Plot B has 85 oak and 5 each of maple, hickory, and beech. Both plots have a species richness of 4, but Plot A is much more even, so Plot A has higher species diversity.

3. Genetic Diversity

• Definition: How different the genomes (groups of genes) are among individuals within a given population. This leads to different traits within that population, like varying tail lengths or fur colors.

• Source of diversity: All populations naturally have some genetic diversity thanks to random mutations in DNA copying and recombination of chromosomes during sexual reproduction.

The Benefit of High Genetic Diversity

• Stressors include drought, famine, or disease like a blight.

• If diversity is high, there’s a better chance that at least some members of the population have traits that let them survive and thrive under new, harsh conditions.

• Example: If a farmer plants potatoes that are all genetic clones, a single disease (blight) will wipe out the entire population. If the potatoes have high genetic diversity, only a few individual plants may be wiped out and the majority will survive.

Threats to Genetic Diversity

1. Bottleneck Event

• Definition: An environmental disturbance, like a hurricane or fire, that kills a large number of a population’s members regardless of their genome. This is distinct from natural selection.

• Effect: The surviving population is much smaller and has a different genetic makeup, a different gene pool, than the original population. It no longer reflects the starting diversity.

• Vulnerability: Bottleneck events reduce genetic diversity and population size, which makes the remaining population more vulnerable to future environmental disturbances.

• Example (northern elephant seal): Bottlenecks can also be caused by humans. Hunters killed northern elephant seals for their oil until only a few dozen at most were left by the 1890s (some estimates say about 20). Once the seals were protected, the population grew to more than 200,000, but every seal alive today descends from those few survivors, so the species has very little genetic diversity. A 2024 genome study linked that loss of diversity to lower fitness in today's seals.

2. Inbreeding Depression

• Definition: This happens when a population is so small that organisms are forced to mate with close family members.

• Effect: The similarity in the parents’ genomes increases the chance that offspring receive two copies of a deleterious (harmful) recessive allele.

• Offspring often suffer decreased fitness, meaning they have a reduced likelihood of surviving and reproducing. Harmful mutations seen in inbred populations include heart defects, kinked tails, and decreased sperm counts.

• Example: The Florida panther population dropped to just 30 individuals because of hunting and habitat loss, which led to severe inbreeding depression. Scientists had to introduce pumas from Texas to increase the gene pool’s diversity, and the population grew.
• In 1995, eight female pumas from Texas were released in south Florida. Their genes reduced the inbreeding defects, and a 2017 state and federal estimate put the population at 120 to 230 adult and young adult panthers (not counting kittens). The Florida panther is still listed as endangered, so it has grown but hasn't fully recovered.

Ecosystem Resilience

• Just as high genetic diversity helps a population recover, high species diversity helps an entire ecosystem recover.

• If there’s a wide diversity of plant species, there’s a higher probability plants will quickly return after a disturbance.

• Real example (Cedar Creek, Minnesota): Ecologist David Tilman tracked more than 200 grassland plots that had different numbers of plant species. During a severe drought in 1987 and 1988, the plots with more species lost much less of their plant biomass than the plots with only a few species. Different species handle drought differently, so when some died back, others kept growing.

• The quick return of vegetation helps anchor the soil and prevent erosion, and it maintains the valuable decomposers and microbes needed to recycle nutrients. It also provides immediate food and shelter, which lets animal species return faster.

Habitat Loss: Which Species Disappear First

• Specialists go first: A specialist has a narrow niche, so it depends on one specific food or habitat. When that habitat shrinks, it can't switch to something else. Example: The northern spotted owl nests and hunts in old-growth forests in the Pacific Northwest. Logging of those forests helped push it onto the threatened species list in 1990.

• Generalists go next: A generalist can switch foods and live in many habitats, so at first it handles habitat loss well (raccoons and coyotes do fine in suburbs). But if habitat loss keeps going, eventually there isn't enough food or shelter left even for generalists, and their numbers drop too.

• Species with large territories decline: Animals that need a huge area to find enough food, like top predators, lose out because the remaining patches are too small to hold many territories. Example: A male Florida panther can roam a territory of up to about 200 square miles. Roads cutting through panther habitat have made collisions with vehicles a leading cause of panther deaths.