AP Environmental Science Unit 2 Vocabulary Review
Levels of Biodiversity and Population Resilience
Definition of Biodiversity:
Biodiversity represents the diversity of different life forms present within a specific ecosystem.
It encompasses three nested hierarchical levels: genetic diversity, species diversity, and ecosystem diversity.
Genetic Biodiversity:
Refers to the total variance of genes, alleles, or physical traits present among individual organisms within a population.
High genetic diversity increases population resilience against environmental changes or disturbances because it elevates the probability that certain individuals possess traits suitable for survival under altered conditions.
Example: In a sparrow population experiencing a drought, seeds become drier and harder to crack. Higher genetic diversity within the population increases the likelihood that certain individuals possess larger beak variations capable of cracking these hardened seeds, enabling population survival.
Species Biodiversity:
Refers to the variety and abundance of different species present across an ecosystem.
Measured using two distinct metrics:
Species Richness: The absolute count of different species found within a designated ecosystem. Richness alone does not fully describe species diversity.
Species Evenness: The relative distribution, balance, or proportion of population sizes among all present species.
Functional Impact of Evenness:
Ecosystems with high species evenness exhibit greater ecological stability.
In a comparison between two forests with identical species richness, Forest 1 (high evenness) features balanced proportions among tree species, while Forest 2 (low evenness) is heavily dominated ( dominance) by a single deciduous species.
If a host-specific disease strikes the dominant deciduous tree species, Forest 2 loses up to of its total canopy cover, causing severe ecosystem collapse.
Forest 1 retains sufficient proportions of alternative deciduous species and conifers to maintain canopy cover and repopulate the area.
Ecosystem Biodiversity:
Refers to the variety of distinct habitats, microhabitats, and ecological communities present across a broader geographic area or biome.
Biomes with high ecosystem diversity support significantly higher species richness.
Example: Tropical rainforest biomes exhibit exceptionally high ecosystem diversity, containing river systems, isolated ponds, varied tree canopy strata, and edge habitats where forest borders open land.
Example: Desert biomes feature low ecosystem diversity and consequently support fewer species and lower species richness.
Categories and Human Disruption of Ecosystem Services
Ecosystem Services Definition:
Direct or indirect financial, economic, and practical benefits that human societies derive from the natural functions and processes of healthy ecosystems.
Divided into four major functional categories:
Provisioning Services:
Physical goods and raw materials harvested directly from natural ecosystems for human use or economic profit.
Examples include timber extracted for housing and furniture manufacturing, food sources, medicinal plants, and clean freshwater.
Supporting Services:
Fundamental ecological processes performed by natural ecosystems that assist or enable valuable human activities, particularly agricultural production.
Examples include natural pollination of agricultural crops performed by insect vectors such as bees and moths. While crop production can occur without pollinators, crop yields and monetary profits are substantially enhanced by native pollinator activity.
Regulating Services:
Natural stabilization mechanisms where functional ecosystems control climate parameters, biochemical cycles, or physical environmental hazards, thereby reducing expensive economic cleanup or infrastructure damage.
Examples include forest carbon sequestration, where trees absorb atmospheric carbon dioxide (), dampening the monetary costs associated with severe global climate change impacts (e.g., agricultural crop damage from severe droughts or residential infrastructure destruction from violent storm events).
Cultural Services:
Non-material, recreational, educational, aesthetic, or intellectual economic benefits derived from interaction with natural environments.
Examples include eco-tourism revenue generated when visitors pay park entrance fees and utilize local lodging near natural landmarks like the Grand Canyon, as well as scientific field research that generates commercial products, publications, or technological innovations.
Human Disruption of Ecosystem Services:
Anthropogenic activities frequently impair or halt natural ecosystem functions, leading to economic losses across multiple service categories.
Case Example (Deepwater Horizon Oil Spill): The crude oil explosion and spill in the Gulf Coast of Mexico disrupted multiple ecosystem services simultaneously:
Provisioning disruption: Commercial fisheries suffered major financial losses due to toxic contamination and reduced fish catches.
Regulating disruption: Crude oil toxicity killed coastal saltgrasses and mangrove swamps, causing coastal soil erosion and leaving shorelines vulnerable to severe storm surge damage.
Cultural disruption: Oil deposits on public beaches reduced regional tourism revenue.
Theory of Island Biogeography and Specialist Species
Core Rules of Island Biogeography:
Distance Effect:
When comparing two islands of equal surface area, the island located closer to the continental mainland supports higher species richness.
Proximity increases the rate of successful migration by organisms flying, swimming, or rafting across ocean channels.
An inverse mathematical relationship exists between island distance from the mainland and overall species richness.
Area Effect:
When comparing two islands located at equal distances from the mainland, the larger island supports higher species richness.
Larger land area provides a wider range of physical landscapes (e.g., forests, grasslands, freshwater ponds), leading to greater habitat diversity.
A direct mathematical relationship exists between island surface area and the total number of supported species.
Specialist Species and Vulnerability on Islands:
Island ecosystems feature finite resources, offering a narrow selection of food sources and physical habitats.
Species colonizing islands undergo evolutionary specialization to adapt strictly to these restricted niche conditions.
Specialist characteristics on islands:
Evolutionary traits become highly tuned to specific local food sources and low-predation environments.
Island environments generally host a higher proportion of specialist species compared to mainland ecosystems.
Specialists lack competitive resilience when faced with non-native invasive species or rapid habitat alterations.
Case Study (The Dodo Bird - Raphus cucullatus):
Evolved on the island of Mauritius roughly years ago from an ancestral member of the pigeon family.
Due to an absence of mammalian carnivores and major herbivorous competitors, the species evolved increased body mass and lost flight capabilities, as energy expenditure for flight was no longer selection-favorable.
Upon the arrival of Dutch sailors and introduced invasive species, dodos exhibited no innate fear response toward predators.
Intense overhunting by humans at rates exceeding the species' reproductive potential caused rapid extinction.
Ecological Tolerance Ranges
Concept of Ecological Tolerance:
Refers to the spectrum of physical and chemical environmental conditions (e.g., temperature, salinity, pH, sunlight) that a species or individual organism can endure before experiencing severe physiological injury or mortality.
Zones of Ecological Tolerance:
Optimal Range:
The specific environmental range in which metabolic functions operate at peak efficiency.
Within this range, organisms experience maximum survival, growth, and reproductive output.
Example: The optimal core body temperature range for humans spans approximately to .
Zone of Physiological Stress:
Conditions drop below or rise above the optimal threshold.
Organisms experience reduced physical activity, impaired cognitive function, lower growth rates, and suppressed reproductive capabilities.
Prolonged exposure leads to permanent physical damage.
Example: Human body temperatures falling below or rising above induce severe physiological stress.
Zone of Intolerance:
Extreme condition deviations far outside the optimal baseline.
Organisms cannot survive in this zone; exposure results in rapid cellular collapse and death.
Example: Human body temperatures falling below or exceeding represent zones of intolerance.
Role of Genetic Diversity in Tolerance:
Genetic variation creates individual differences in tolerance thresholds within a single population.
Certain individuals possess wider optimal ranges or broader physiological tolerance bounds than the population average.
During sudden environmental shifts (e.g., thermal pollution or acidification), individuals possessing tolerant alleles survive, reproduce, and pass those traits to offspring, shifting the population's overall tolerance curve over generations.
Natural Environmental Disturbances and Earth's Climate Cycles
Classification of Natural Disturbances by Frequency:
Periodic Events:
Occur with predictable, recurring regular cycles over known time intervals.
Examples include seasonal rainy cycles, dry seasons, and ocean tides.
Episodic Events:
Occur with lower regularity, but display predictable occurrence when specific underlying environmental preconditions align.
Examples include forest fires during dry conditions and hurricanes during sea-surface warming trends.
Random Events:
Occur unpredictably with no discernable pattern, regular frequency, or seasonal trigger.
Examples include meteor or asteroid strikes and sudden volcanic eruptions.
Milankovitch Cycles and Natural Climate Change:
Earth's global climate naturally undergoes cyclic changes over timelines spanning tens of thousands of years due to minor periodic shifts in Earth's solar orbit.
Three orbital drivers (Milankovitch Cycles):
Eccentricity: Periodic alteration in the shape of Earth's orbital path around the Sun, shifting between more circular and more elliptical pathways.
Axial Precession: The continuous rotational wobble of Earth's axis of rotation.
Obliquity: Periodic changes in the angle of Earth's axial tilt relative to its orbital plane.
Mechanism and Impact:
These orbital variations alter the solar radiation reaching the Northern Hemisphere by changing Earth's distance from the Sun or adjusting axial tilt toward or away from solar rays.
Ice core temperature records spanning over confirm a direct correlation between Milankovitch cycles, global surface temperatures, and historical sea level fluctuations.
Sea Level and Coastal Ecosystem Alterations:
Global warm periods induce sea level rise via two distinct physical mechanisms:
Melting of continental glaciers and polar ice sheets, adding liquid water mass to oceans.
Thermal expansion of liquid water molecules as ocean temperatures rise.
Ecological impacts of sea level rise:
Inundation and complete submersion of low-lying coastal habitats such as estuaries.
Deepening of coastal marine shelf waters, preventing adequate sunlight penetration down to photosynthetic zooxanthellae within benthic coral reef ecosystems.
Evolutionary Adaptations in Response to Disturbances
Adaptation Mechanism:
An adaptation is a genetic mutation or inherited phenotypic variation that confers a survival or reproductive advantage on an organism under specific environmental conditions.
Environmental disturbances force biological populations to either adapt over generations through natural selection or face localized extinction.
Case Study (Hominid Evolutionary Adaptations):
Thumb Muscle Reorganization:
Approximately years ago, random genetic mutations in ancestral Homo habilis populations produced a unique thumb muscle arrangement.
This structural change enabled a precise, high-strength grip capable of crafting and manipulating fine stone tools.
Tools allowed Homo habilis to crack open animal skeletal remains to access energy-dense bone marrow.
The massive caloric intake from bone marrow provided an energetic advantage over competing primates, increasing individual survival and reproductive success.
Transition to Grasslands and Bipedalism:
As climate shifts transformed dense forest habitats into open tropical savanna grasslands, natural selection favored individuals capable of efficient ground movement.
In Homo erectus, bipedalism (standing and walking upright) allowed individuals to survey open grassland terrain for predators and prey while freeing upper limbs to carry tools and resources.
Primary and Secondary Ecological Succession
Primary Succession:
Occurs on newly exposed surfaces completely devoid of pre-existing soil, such as bare rock surfaces post-volcanic eruption or exposed bedrock following glacial retreat.
Step-by-Step Sequence:
Colonization by Pioneer Species: Spores or seeds of pioneer species like lichens and mosses are transported by wind currents onto bare rock.
Soil Formation via Chemical Weathering: Mosses and lichens secrete organic acids that chemically weather bare rock, extracting bound minerals and nutrients. The continuous cycle of pioneer growth, death, and organic breakdown builds a thin layer of soil.
Growth of Early Successional Plants: As soil accumulation deepens, wind-blown seeds of small grasses, weeds, and wildflowers germinate, expanding root structures and contributing organic matter.
Intermediate Successional Species: Deeper, nutrient-rich soil allows fast-growing, shade-intolerant shrubs and mid-successional tree species to establish.
Climax Community: Over long periods, slow-growing, shade-tolerant tree species (e.g., mature oaks and maples) overtop mid-successional trees. Climax communities exhibit high total biomass, complex food webs, and maximum species richness.
Secondary Succession:
Occurs in areas where an ecosystem disturbance destroys the standing vegetation canopy, but leaves the underlying soil layer intact.
Triggers: Forest fires, land clearing, agricultural abandonment, or severe storm blowdowns.
Process and Characteristics:
Bypasses the bare-rock chemical weathering phase required in primary succession.
Pioneer species consist of fast-growing, sun-tolerant grasses and wildflowers whose seeds are dispersed by wind currents or animal vectors into disturbed soil.
Reaches a climax community significantly faster than primary succession due to the initial presence of nutrient-rich soil.
Keystone Species and Ecosystem Engineers
Definition of Keystone Species:
A species whose impact on ecosystem structure, stability, and function is disproportionately large relative to its total population biomass or abundance.
The removal of a keystone species causes widespread structural breakdown and potential collapse of the surrounding food web.
Predator Keystone Case Study (Apex Wolves):
Removal of gray wolves from forest ecosystems leads to unchecked population growth of ungulate herbivores (e.g., deer and elk).
Overpopulated ungulates overgraze riparian vegetation and saplings.
Loss of vegetation destroys food sources and nesting habitats for birds and small mammals.
Stripping vegetation along river corridors destabilizes riverbanks, resulting in soil erosion and altered stream geometry.
Ecosystem Engineers:
Subcategory of keystone species that physically alter, construct, or maintain physical habitat structures, creating ecological niches that would otherwise not exist.
Example (Beavers): Beavers fell trees and build dams across stream channels, impounding flowing water to form extensive wetland ponds. These ponds create slow-water habitats necessary for numerous fish, amphibian, bird, and aquatic plant species.
Example (Mangrove Trees): Possess specialized stilt-like aerial root systems that trap ocean sediment, dissipate wave energy, and stabilize coastal shorelines, constructing complex mangrove swamp habitats that serve as nurseries for marine species.