AP Environmental Science Study Guide Units 1-9

Ecosystem Structure and the BiosphereThe Earth's environment is categorized into abiotic and biotic components. Abiotic components refer to the nonliving elements of the planet, which include the atmosphere, the hydrosphere, and the lithosphere. Conversely, biotic components consist of living organisms such as animals, plants, fungi, protists, and bacteria, which collectively constitute the biosphere. Within these systems, organisms interact at various levels of biological organization. A population is defined as a group of organisms belonging to the same species residing in a specific area, while a community consists of populations of different species occupying the same geographic region. Within a community, every species occupies a specific habitat—the physical area or environment where it lives—and fills an ecological niche. The ecological niche represents the role and position of a species, encompassing how it utilizes biotic and abiotic resources, its habitat preferences, and its dietary habits.# Dynamics of Organismal InteractionsInteractions among organisms are fundamental to ecosystem function. Competition arises when two individuals or species vie for the same environmental resources. To coexist without conflict, species may engage in resource partitioning, where resources are shared or divided. Predation, the act of one species feeding on another, is a primary driver of population size changes; notable examples include the interactions between cats and mice, polar bears and seals, lions and wildebeests, and Great White Sharks and elephant seals. Furthermore, symbiotic relationships define close, prolonged associations between different species. In mutualism, both organisms benefit, such as pollinators getting nectar while plants achieve pollen transfer. Commensalism occurs when one organism benefits while the other remains unaffected, exemplified by barnacles growing on scallop shells. Parasitism involves one organism benefiting at the expense of another, such as mistletoe growing within the vascular system of a host tree, thereby harming the host.# Ecosystem Classification and Energy FlowEcosystems do not always have distinct boundaries; instead, biomes often blend into one another via ecotones, which are transitional areas where two biomes meet. These areas often exhibit edge effects, characterized by high species diversity and biological density. Within larger ecosystems, smaller regions with similar physical features are known as ecozones or ecoregions. Energy flow within these systems is studied through bioenergetics, noting that all energy on Earth originates from the Sun. Photosynthetic organisms utilize solar energy to convert carbon dioxide ($CO_2$) and water ($H_2O$) into carbohydrates, releasing oxygen ($O_2$) as a byproduct. These biological macromolecules store energy in chemical bonds. Cells then use respiration to transfer this energy into smaller high-energy molecules such as adenosine triphosphate ($ATP$), $GTP$, $NADH$, $NADPH$, and $FADH_2$, which power cellular processes and release $CO_2$ and $H_2O$ back into the environment. In plants, photosynthesis primarily occurs in the leaves, though it is also performed by algae, certain bacteria, and plant-like protists. The process involves light-dependent reactions, which convert sunlight and water into $O_2$, $ATP$, and $NADPH$, and light-independent reactions, which use that $ATP$ and $NADPH$ to build glucose ($C_6H_{12}O_6$) from $CO_2$. Cellular respiration, a series of redox reactions where glucose is oxidized to $CO_2$ to synthesize $ATP$, is a metabolic pathway used by all living organisms.# Trophic Levels and BiodiversityOrganisms are classified based on their nutritional strategies. Autotrophs, or primary producers, produce complex organic compounds from simple substances using light (photoautotrophs) or inorganic chemical reactions. Examples include plants and saprotrophs. Heterotrophs consume other organisms and include primary, secondary, and tertiary consumers, as well as decomposers and detritivores. Energy flow is mapped through food chains and food webs, which show the step-by-step or complex interactions among species, such as the chain from rice to humans to microorganisms. Energy pyramids illustrate the decreasing amount of energy available at successive trophic levels. Biodiversity, the result of evolution, refers to the variety of organisms in a habitat. Higher biodiversity translates to a larger gene pool and better chances of survival. Evolution occurs through natural selection, a theory popularized by Charles Darwin, where beneficial inherited characteristics are passed down through generations while unfavorable ones diminish. Genetic drift also influences population makeup through chance events. When species cannot adapt to environmental changes, they may face extinction. Biological extinction is the total elimination of a species (e.g., Dodo birds, Mammoths), while ecological extinction occurs when a population is too small to fulfill its ecological role (e.g., Wolves in Yellowstone before the $1990$s). Commercial or economic extinction happens when harvesting a species is no longer cost-effective, as seen with Groundfish in the Grand Banks.# Ecosystem Services and Natural ChangesEcosystems provide essential benefits known as ecosystem services. Provisioning services include physical items like food, water, and medicinal resources. Cultural services encompass non-material benefits like recreation, tourism, and spiritual experiences. Regulating services, which are often overlooked, include climate regulation, water purification, and pollination. Support services, such as nutrient recycling and soil formation, enable all other services to exist. Within these ecosystems, certain species play specialized roles. Keystone species, like fig trees in tropical forests or wolves in North America, maintain biotic balance and diversity. Indicator species, such as trout in freshwater, serve as standards to evaluate ecosystem health due to their sensitivity to change. Ecosystems undergo ecological succession; primary succession starts in lifeless areas like retreating glaciers, while secondary succession occurs in cleared areas with intact soil. Pioneer species with wide environmental tolerances appear first, eventually leading to a balanced climax community. However, habitat fragmentation can damage these balanced systems.# Population Ecology and Growth DynamicsPopulations are defined by their density (individuals per unit area) and dispersion patterns, which can be clumped (most common, e.g., fish), uniform (result of competition, e.g., trees), or random (uncommon). Population growth is influenced by biotic potential—growth under unlimited resources—and carrying capacity, which is the maximum population size supported by available resources. Growth models include the exponential J-curve and the logistic S-curve. The Rule of $70$ is used to estimate doubling time by dividing $70$ by the percentage growth rate (e.g., $70 / 25\% = 2.8$ years). Reproductive strategies vary between r-selected organisms, which reproduce early and provide little offspring care (e.g., bacteria), and K-selected organisms, which reproduce later and nurture few offspring (e.g., humans). Populations may experience boom-and-bust cycles or predator-prey cycles. Growth is limited by density-dependent factors (predation, disease, competition) and density-independent factors (storms, fires). Survivorship curves further categorize species: Type I has high survival until old age, Type II has a constant survival rate, and Type III has high early mortality.# Human Populations and Environmental ImpactThe human population is measured by birth and death rates per $1,000$ members. The growth rate is calculated using the formula $\frac{(\text{Birth rate} + \text{immigration}) - (\text{death rate} + \text{emigration})}{1000}$. The Total Fertility Rate ($TFR$) reflects the number of children a woman will bear, influenced by education, birth control, and cultural beliefs. Replacement birth rate, the number of children needed to replace parents, is approximately $2$ worldwide but up to $3.4$ in developing nations. The environmental impact of a population is modeled by the IPAT equation: $I = P \times A \times T$ (Impact = Population $\times$ Affluence $\times$ Technology). High population pressure can lead to overgrazing, desertification, and soil salinization. While the Green Revolution increased agricultural productivity through pesticides and fertilizers, it also led to environmental degradation. Water resources are also under threat; aquifers are being depleted by large-scale irrigation in countries like China and the U.S. Habitat destruction, often summarized by the acronym HIPPCO (Habitat destruction, Invasives, Population, Pollution, Climate change, Overexploitation), is the primary cause of species becoming endangered or extinct.# The Lithosphere and Geological EventsThe lithosphere consists of tectonic plates floating on the asthenosphere. Plate boundaries are categorized as convergent (pushing together), divergent (moving apart), and transform fault (sliding past). Subduction occurs when a dense plate sinks beneath a lighter one, often forming deep ocean trenches like the Cascade Range. Vibrations from plate movements cause earthquakes, measured by seismographs on the Richter scale, with the epicenter being the initial surface location. Volcanoes are classified as active (erupted within $10,000$ years), dormant, or extinct. They form at subduction zones (e.g., Ring of Fire), rift valleys (e.g., East African Rift), or hotspots (e.g., Hawaiian Islands). Volcano types include shield (broad, gentle slope, e.g., Kohala), composite (steep, e.g., Mount Fuji), cinder cones (small, symmetrical, e.g., Sunset Crater), and lava domes (e.g., Mount St. Helens).# Atmosphere, Hydrosphere, and Soil DynamicsThe atmosphere consists of several layers: the exosphere (thinnest), thermosphere (auroras), ionosphere (absorbs X-rays), mesosphere (meteors burn here), stratosphere (contains ozone layer), and troposphere (where weather occurs, containing $99\%$ of water vapor). Greenhouse gases in the troposphere include water vapor ($0-4\%$), $CO_2$ ($0.033\%$), and methane ($0.0002\%$). Climate refers to long-term patterns of temperature and precipitation, whereas weather is day-to-day. The Earth's tilt causes seasons by varying insolation. The hydrosphere covers $75\%$ of Earth, with freshwater bodies including deltas, estuaries, and wetlands. Water stress occurs when renewable supply is between $1,000$ and $2,000\,m^3$ per person, while water scarcity falls below $1,000\,m^3$. Soil, or the pedosphere, is a complex material made of $45\%$ rock, $25\%$ air, $25\%$ water, and $5\%$ organic matter, typically with a pH between $4$ and $8$. The rock cycle involves igneous (melted), sedimentary (compressed), and metamorphic (transformed by heat/pressure) rocks. Weathering can be physical, chemical, or biological. Agricultural soil conservation includes techniques like contour plowing, terracing, crop rotation, and the use of organic fertilizers.# Global Energy Resources and ConsumptionEnergy exists in various forms: potential, kinetic, radiant, thermal, chemical, electrical, and nuclear. The First Law of Thermodynamics states energy is conserved, while the Second Law states entropy increases, often through heat loss. Nonrenewable energy sources include fossil fuels—oil (hydrocarbons), coal (anthracite is the purest, lignite the least), and natural gas (methane)—as well as nuclear energy. Fossil fuels are extracted via primary, pressure, or heat extraction. Synfuels are non-petroleum synthetic fuels that offer a large supply but have low net energy yields. Nuclear energy utilizes fission (splitting atoms) in commercial plants, while fusion (combining atoms) is still in development. Renewable energy sources include hydroelectric, solar, biomass, wind, geothermal, and hydrogen fuel cells. Hydroelectric and solar are clean but have habitat and storage limitations, while biomass and wind are widely available but can be expensive or inconsistent.# Pollution and Environmental HealthPollution stems from various sources, primarily fossil fuel combustion and industrial processes. Anthropogenic water use is dominated by agriculture ($70\%$), followed by industry ($20\%$) and households ($10\%$). Wastewater increases biochemical oxygen demand ($BOD$), leading to dead zones or anoxic conditions. Eutrophication, often caused by fertilizer runoff, results in algal blooms that deplete oxygen. Pathogens in wastewater cause diseases like cholera and hepatitis, with fecal coliform bacteria serving as an indicator species. Treatment involves septic systems or centralized sewage plants. Heavy metals like lead, arsenic, and mercury are significant concerns, as is acid deposition ($SO_2$ and $NO_2$ converting to sulfuric and nitric acids). In the atmosphere, primary pollutants (e.g., $CO$, $SO_2$) can transform into secondary pollutants (e.g., $O_3$, sulfuric acid). Photochemical smog, prevalent in Los Angeles, forms from sunlight reacting with nitrogen oxides and VOCs. The Clean Air Act and EPA standards regulate these emissions using technologies like scrubbers and catalytic converters. Stratospheric ozone depletion is caused by CFCs, where one chlorine atom can destroy $100,000$ ozone molecules, allowing harmful UV-B radiation to reach Earth. Global warming is driven by rising Greenhouse Gas concentrations, with $CO_2$ reaching $400\,ppm$ in $2016$. Effects include melting ice sheets, rising sea levels, and shifting biotas, requiring technological, behavioral, and policy adaptations like carbon sequestration and international treaties.