In AP Environmental Science, species richness is the number of different species living in a particular area or ecosystem. It's one half of biodiversity (the other is species evenness) and higher richness generally means an ecosystem is more likely to recover from disruptions.
Species richness is simply a count: how many different species live in a given area? If a forest has 40 species of trees, birds, and insects, its species richness is 40. That's it. You're not counting individuals, just the number of distinct species.
Richness is one piece of biodiversity at the species level (EK ERT-2.A.1 lists genetic, species, and habitat diversity). The big idea in AP Enviro is that more species usually means a more resilient ecosystem. Per EK ERT-2.A.3, ecosystems with a larger number of species are more likely to recover from disruptions, because if one species gets wiped out, others can fill the gap. Think of it like a team with deep bench: lose a player and you're still functional.
Species richness lives in Unit 2: The Living World: Biodiversity, anchored to topic 2.1 (Introduction to Biodiversity) and topic 2.3 (Island Biogeography). It directly supports learning objective AP Enviro 2.1.A, which asks you to explain levels of biodiversity and why they matter to ecosystems. The payoff concept is EK ERT-2.A.3: high richness boosts an ecosystem's ability to bounce back from disturbances like oil spills, disease, or invasive species. This connects to the course's recurring theme that biodiversity equals stability, a claim you'll be asked to apply, not just recite.
Keep studying AP Environmental Science Unit 2
Species Evenness (Unit 2)
Richness counts how many species there are; evenness measures how balanced their populations are. A forest with 10 species split evenly is more diverse than one where a single species dominates and the other nine barely hang on. You need both to fully describe biodiversity.
Ecosystem Resilience (Unit 2)
This is the 'so what' behind richness. EK ERT-2.A.3 says more species means faster recovery from disruption. When a question shows you an ecosystem rebounding quickly after damage, high species richness is usually the reason.
Island Biogeography (Unit 2)
Islands typically have lower species richness than mainlands because fewer species can reach them. Topic 2.3 explains how island species often evolve into specialists, which makes their richness fragile when invasive generalists arrive and outcompete them (EK ERT-2.E.1).
Genetic Diversity (Unit 2)
Richness is diversity across species; genetic diversity is variation within a single species. Both buffer against environmental stress, and a population bottleneck can crash genetic diversity the same way habitat loss crashes species richness.
You'll see species richness in both multiple-choice and FRQs. MCQ stems often ask which scenario would decrease richness (like a coral reef die-off) or which sampling technique best measures it in a grassland. The classic application question gives you data showing that higher-richness areas recovered faster after an oil spill, and asks what that supports. The answer ties back to EK ERT-2.A.3 about resilience. On FRQs you may need to read a diagram of ecosystem zones, identify where richness is highest or lowest, and explain why using disturbance, dissolved oxygen, or habitat conditions. When asked to design a study, remember you're counting distinct species, so sampling methods like quadrats or transects that survey an area are appropriate.
Species richness is the number of different species; species evenness is how evenly distributed the individuals are among those species. An area with 5 species where one has 1,000 individuals and the rest have 5 each is rich but not even. Both together make up species diversity, so don't treat richness alone as the full picture of biodiversity.
Species richness is the number of different species in an area, not the number of individuals.
Higher species richness generally makes an ecosystem more likely to recover from disruptions (EK ERT-2.A.3).
Richness is only half of species diversity; species evenness, how balanced the populations are, is the other half.
Islands tend to have low richness, and that richness is fragile because specialist island species can be outcompeted by invasive generalists.
Habitat loss reduces richness by first eliminating specialist species, then generalists (EK ERT-2.A.4).
It's the number of different species present in a given area or ecosystem. If a wetland has 22 species of plants and animals, its species richness is 22, regardless of how many individuals of each there are.
No. Species richness is one component of biodiversity at the species level. Full biodiversity also includes species evenness, plus genetic and habitat diversity (EK ERT-2.A.1).
Richness counts how many species exist; evenness measures how balanced their population sizes are. Five species split evenly is more diverse overall than five species where one dominates and the rest are rare.
Because more species means more backup. EK ERT-2.A.3 states that ecosystems with more species are more likely to recover from disruptions, since if one species is lost, others can fill its role and keep ecosystem services running.
Use a sampling method that surveys an area, like quadrats or transects in a grassland, then count the number of distinct species found. The key is that you're tallying different species, not counting every individual organism.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.
Put the full course together before test day.