1. Defining and Measuring Biodiversity★★☆☆☆⏱ 5 min
Biodiversity describes the variety of life across all levels of biological organization, from genetic variation within populations to ecosystem diversity across entire regions. For AP Biology exam questions, you will most often be asked to analyze species diversity within local ecological communities.
2. Biodiversity and Ecosystem Function★★★☆☆⏱ 6 min
Biodiversity is strongly linked to key ecosystem services and functions, including primary productivity, nutrient cycling, resistance to invasive species, and resilience after disturbance. Higher biodiversity allows more effective niche partitioning, increasing the overall efficiency of resource use by the entire community.
Keystone species have a disproportionately large impact on overall biodiversity relative to their population size or biomass. Removing a keystone species almost always leads to a dramatic drop in community-wide biodiversity and often total community collapse.
3. Disturbance and Biodiversity★★★★☆⏱ 5 min
Ecological disturbances are any events that remove organisms or alter resource availability in an ecosystem. Disturbances can be natural (wildfires, hurricanes, droughts) or human-caused (deforestation, pollution, urban development). The intermediate disturbance hypothesis describes how disturbance intensity and frequency impacts overall biodiversity.
At very high disturbance levels, only the fastest-growing, fastest-colonizing species can survive, so diversity is low. At very low disturbance levels, competitively dominant species exclude weaker competitors, so diversity is also low. Moderate disturbance removes enough dominant species to allow weaker competitors to persist, leading to maximum diversity.
Common Pitfalls
Why: Biodiversity depends on both richness and evenness. A very uneven community with high richness can have lower overall biodiversity than a less rich but more even community.
Why: The two terms have distinct meanings that are often swapped on multiple choice questions.
Why: Many students assume all human and natural disturbance is harmful, but moderate natural disturbance often increases biodiversity.
Why: While many keystone species are top predators, any organism can be keystone if it has a disproportionate impact on biodiversity.