📚Study Guide: The Living World: Biodiversity
Unit 2: The Living World: Biodiversity
Biodiversity--the variety of life at genetic, species, and ecosystem levels--is the foundation of ecosystem resilience and the provision of ecosystem services. This unit explores the patterns and processes that create and maintain biodiversity, including evolution, natural selection, speciation, and island biogeography. Students must understand that biodiversity is not evenly distributed across the globe; it peaks near the equator (tropical regions) due to stable climates, high primary productivity, and complex habitat structures. The theory of island biogeography predicts that larger islands and those closer to mainland sources support more species due to lower extinction rates and higher immigration rates. The AP exam frequently asks students to interpret species-area curves and to explain why biodiversity hotspots--regions with high endemism and significant habitat loss--are conservation priorities. Genetic diversity within populations provides the raw material for adaptation, while species diversity (richness and evenness) determines ecosystem stability. Human activities, particularly habitat destruction, invasive species, overexploitation, pollution, and climate change, are driving the sixth mass extinction. Understanding these threats and the strategies to mitigate them--protected areas, captive breeding, and international agreements like CITES--is essential for the AP Environmental Science exam.
Key Concepts
- Levels of Biodiversity: Genetic diversity (variety of genes within a species), species diversity (number and abundance of species), and ecosystem diversity (variety of habitats and ecological processes).
- Evolution and Natural Selection: Genetic variation arises through mutation and recombination. Natural selection favors traits that increase survival and reproduction in a given environment. Adaptation leads to speciation when populations become reproductively isolated.
- Island Biogeography: Species richness on islands is determined by island size (larger = more species, lower extinction) and distance from mainland (closer = more immigration). The equilibrium model balances immigration and extinction rates.
- Species-Area Relationship: S = c A^z, where S = number of species, A = area, and c and z are constants. A tenfold increase in area typically doubles the number of species (z ~ 0.25-0.35).
- Biodiversity Hotspots: Regions containing at least 1,500 endemic plant species and having lost at least 70% of original habitat. They cover < 2.5% of Earth's land but support > 50% of plant species.
- Threats to Biodiversity: HIPPCO: Habitat destruction, Invasive species, Population growth (human), Pollution, Climate change, Overexploitation.
Vocabulary
- Endemic: A species that is native to and found only within a limited geographic area.
- Genetic Drift: Random changes in allele frequencies in a population, especially significant in small populations.
- Founder Effect: A type of genetic drift that occurs when a small number of individuals colonize a new area, resulting in reduced genetic variation.
- Ecosystem Services: Benefits that humans receive from ecosystems, including provisioning (food, water), regulating (climate, flood control), supporting (nutrient cycling, soil formation), and cultural (recreation, aesthetics).
- Indicator Species: A species whose presence, absence, or abundance reflects a specific environmental condition, such as amphibians indicating water quality.
- Keystone Species: A species that has a disproportionately large effect on its ecosystem relative to its biomass; removal causes cascading changes (e.g., sea otters, wolves).
Essential Formulas
- Species-Area: S = c A^z
- Shannon Diversity Index: H = -Sum(pi ln pi), where pi = proportion of individuals of species i
- Simpson's Diversity Index: D = 1 - Sum(pi^2)
Common Mistakes
- Confusing Species Richness and Evenness: Richness is the number of species. Evenness is how equally individuals are distributed among species. A community can have high richness but low evenness.
- Assuming All Islands Follow Island Biogeography Exactly: The theory is a model. Real islands are affected by human disturbance, historical factors, and habitat heterogeneity.
- Confusing Endemic and Native: Native species occur naturally in an area but may also live elsewhere. Endemic species are found ONLY in that area.
- Thinking Biodiversity Is Only About Species Count: Genetic and ecosystem diversity are equally important for resilience and function.
AP Exam Strategies
- Calculate Diversity Indices: Be prepared to compute Shannon or Simpson indices from a data table and interpret what the values mean (higher = more diverse).
- Apply Island Biogeography: When comparing two islands, explicitly reference size and distance from mainland to predict which will have more species.
- Explain HIPPCO Threats: For any biodiversity loss scenario, identify the specific HIPPCO factor(s) at work and explain the mechanism.
- Evaluate Conservation Strategies: Discuss trade-offs of in-situ (protected areas) vs. ex-situ (zoos, seed banks) conservation, and the importance of habitat corridors.
Real-World Applications
- Madagascar Biodiversity Hotspot: Over 90% of Madagascar's species are endemic, yet deforestation has destroyed > 90% of original forest, making conservation urgent.
- Wolf Reintroduction in Yellowstone: Restoring a keystone predator triggered a trophic cascade that improved riparian vegetation, beaver populations, and even river morphology.
- Coral Reef Conservation: Coral reefs support ~25% of marine species; bleaching from ocean warming threatens global marine biodiversity and fisheries.