AP Environmental Science Unit 2 Review: Biodiversity

Biodiversity in an ecosystem includes genetic, species, and habitat diversity. Genetic diversity determines how well a population can respond to environmental stressors. A population bottleneck sharply reduces genetic diversity, leaving a population vulnerable. Species richness counts the number of different species in an area, and higher richness correlates with greater ecosystem resilience. Habitat loss first eliminates specialist species, then generalist species, and reduces populations of species with large territorial requirements.

• Genetic diversity: Variation in alleles within a population; higher diversity improves adaptive potential and stress response.
• Population bottleneck: A drastic reduction in population size that sharply lowers genetic diversity, increasing extinction risk.
• Species richness: The number of different species present in an ecosystem; a key measure of biodiversity.
• Specialist species: Species with narrow habitat or resource requirements; the first to disappear when habitat is lost or degraded.
• Ecosystem resilience: The capacity of an ecosystem to recover from disruption; generally increases with higher species diversity.

Ecosystem services are the benefits humans receive from functioning ecosystems, organized into four categories. Provisioning services supply tangible goods. Regulating services control environmental conditions. Cultural services provide non-material benefits. Supporting services underpin all other services. Anthropogenic activities such as deforestation, wetland drainage, and overfishing can degrade any category, producing cascading ecological and economic consequences.

• Provisioning services: Direct goods from ecosystems: food, freshwater, timber, and fisheries.
• Regulating services: Ecosystem processes that moderate conditions: carbon sequestration, flood control, water purification, and disease regulation.
• Cultural services: Non-material benefits including recreation, ecotourism, and aesthetic or spiritual value.
• Supporting services: Foundational processes like soil formation, photosynthesis, and nutrient cycling that enable all other services.
• Anthropogenic: Caused by human activity; anthropogenic disruptions to ecosystem services can trigger both ecological and economic harm.

Island biogeography examines how species colonize islands and how community structure develops over time. Species richness on an island reflects a balance between colonization rate and extinction rate. Larger islands support more species because they offer more resources and habitat. Islands closer to a mainland source receive colonists more frequently. Because island resources are limited, many island species evolve as specialists. When generalist invasive species arrive, they typically outcompete specialists, threatening their survival.

• Island biogeography: The study of ecological relationships, species distribution, and community structure on islands.
• Colonization rate: The rate at which new species arrive on an island; higher for islands closer to a mainland source.
• Extinction rate: The rate at which species disappear from an island; higher on smaller islands with fewer resources.
• Endemic species: Species found only in a specific geographic area, often islands; highly vulnerable to invasive species.
• Competitive exclusion: When a generalist invasive species outcompetes a specialist for the same resources, driving the specialist toward extinction.

Ecological tolerance is the range of environmental conditions, including temperature, salinity, flow rate, and sunlight, that an organism can endure before injury or death. Within that range, there is an optimum zone where the organism performs best, flanked by stress zones where performance declines. Tolerance applies to both individual organisms and entire species. Species with narrow tolerance ranges are more vulnerable to environmental change than those with broad ranges.

• Tolerance range: The full span of an environmental condition an organism can survive, from minimum to maximum.
• Optimum zone: The portion of the tolerance range where an organism functions most efficiently.
• Stress zone: Conditions near the edges of the tolerance range where the organism survives but at reduced performance.
• Salinity: Dissolved salt concentration in water; a key tolerance variable for aquatic and marine organisms.
• Ecological niche: The full set of conditions and resources an organism requires; tolerance ranges define the boundaries of the niche.

Natural disruptions include volcanic eruptions, wildfires, hurricanes, floods, droughts, and long-term climate shifts such as glacial cycles. These events can be periodic, episodic, or random, and they operate on timescales from hours to geological epochs. Their ecological consequences, including habitat loss, species displacement, and community restructuring, can equal or exceed those of human-caused disruptions. Sea level has varied significantly with glacial cycles, altering coastal habitats. Wildlife responds to disruptions through short- and long-term migration.

• Disturbance: A sudden, significant event such as a wildfire, flood, or volcanic eruption that disrupts ecosystem structure.
• Periodic disturbance: A disturbance that recurs on a regular cycle, such as seasonal flooding or predictable fire regimes.
• Episodic disturbance: A disturbance that occurs irregularly, such as a volcanic eruption or a major hurricane.
• Sea level rise: An increase in ocean surface height driven by glacial melt and thermal expansion; historically linked to glacial-interglacial cycles.
• Habitat loss: Destruction or degradation of living space for organisms; a primary consequence of both natural and anthropogenic disruptions.

Adaptations are inherited traits that improve an organism's survival and reproduction in its environment. They accumulate through small genetic changes over time via natural selection. When environmental conditions change suddenly or gradually, individual organisms may respond by altering behavior, shifting their geographic range, or, if neither is possible, declining toward extinction. Genetic variation within a population is the raw material for adaptation; populations with low genetic variation have fewer options when conditions change.

• Genetic variation: The diversity of alleles within a population; the foundation for natural selection and long-term adaptation.
• Natural selection: The process by which individuals with traits better suited to their environment survive and reproduce at higher rates.
• Behavioral adaptation: A change in an organism's actions in response to environmental pressure, such as shifting feeding times or migration routes.
• Gene flow: Movement of genetic material between populations through migration; reduced by habitat fragmentation, lowering adaptive potential.
• Inbreeding depression: Reduced fitness in offspring from closely related parents; a risk in small, isolated populations with low genetic diversity.

Ecological succession is the gradual, directional change in species composition of a community over time. Primary succession begins on bare substrate with no soil, where pioneer species such as lichens and mosses initiate soil formation. Secondary succession begins where soil remains after a disturbance like a wildfire or abandoned farmland. As succession proceeds, biomass, species richness, and net productivity generally increase. Keystone species disproportionately shape community structure at any successional stage. Indicator species signal the health or quality of the ecosystem by their presence, abundance, or scarcity.

• Primary succession: Succession beginning on bare rock or substrate with no existing soil; initiated by pioneer species like lichens.
• Secondary succession: Succession beginning in a disturbed area where soil already exists; generally faster than primary succession.
• Pioneer species: The first colonizers of a disturbed or bare area; they modify conditions to allow later species to establish.
• Keystone species: A species with a disproportionately large effect on community structure relative to its abundance.
• Indicator species: An organism whose presence, abundance, or scarcity signals a specific aspect of ecosystem condition or quality.