AP Environmental Science: Ecosystems, Biomes, Aquatic Systems, and Biogeochemical Cycles

Ecological Fundamentals and Levels of Organization

Individual: A single, distinct organism.
Population: Organisms of the exact same species living within a particular area at the same time.
Community: Populations of all different species living and interacting with one another in a designated area.
Ecosystem: The biological community of living organisms (biotic factors) interacting with their non-living physical environment (abiotic factors).
Habitat: The physical home or specific environment inhabited by an organism.
Niche: The functional role, position, and interactions of an organism within its ecosystem.
Biotic vs. Abiotic Components:
- Biotic Factors: All living or once-living elements in an ecosystem.
- Abiotic Factors: All non-living chemical and physical factors in an ecosystem.

Energy Flow and Trophic Dynamics

Major Trophic Levels:
- Producers (Autotrophs): Primary producers that convert energy sources into organic matter.
- Primary Consumers (Herbivores): Organisms that feed directly on producers.
- Secondary Consumers (Carnivores/Omnivores): Organisms that feed on primary consumers.
- Tertiary Consumers: Apex predators that feed on secondary consumers.
Energy Flow in Food Webs:
- Solar energy is absorbed and converted into chemical energy by producers via photosynthesis, and eventually returned to the environment as heat.
- Only approximately $10\%$ of usable energy is transferred from one trophic level to the next higher level; the remaining $90\%$ is lost as heat through metabolic processes.
Thermodynamic Laws Governing Energy Flow:
- First Law of Thermodynamics: The Law of Conservation of Energy states that energy cannot be created or destroyed, only transformed from one form to another.
- Second Law of Thermodynamics: Energy conversions are inefficient; whenever energy is transformed, the entropy (disorder) of the system increases over time, degrading high-quality energy into low-quality heat.
Ecological Efficiency:
- The percentage of usable energy transferred as biomass from one trophic level to the next.
- Because of cumulative energy loss at each level, it is substantially more energy-efficient for organisms to consume lower on the food pyramid (eating producers rather than high-level consumers).
Modes of Primary Production:
- Producer / Autotroph: An organism capable of synthesizing its own food molecules from inorganic compounds.
- Photosynthesis: The process by which plants, algae, and cyanobacteria convert carbon dioxide ( $CO_2$ ) and water ( $H_2O$ ) into energy-rich carbohydrates using solar light energy, effectively removing carbon from the atmosphere.
  - Chemical Equation: $6CO_2 + 6H_2O + \text{Light Energy} \rightarrow C_6H_{12}O_6 + 6O_2$
- Chemosynthesis: The biological conversion of one or more carbon molecules into organic matter using the chemical oxidation of inorganic compounds (such as hydrogen sulfide) or methane as an energy source rather than sunlight.
  - Chemical Equation: $6H_2S + 6H_2O + 6CO_2 + 6O_2 \rightarrow C_6H_{12}O_6 + 6O_2$
Ecosystem Productivity:
- Measured in units of energy production per unit area per unit time (e.g., $kcal\,m^{-2}\,yr^{-1}$ ).
- Net Primary Productivity (NPP): The rate of energy storage by photosynthesizers in a given area, after subtracting the energy lost to respiration.
Consumer Dynamics and Respiration:
- Consumer / Heterotroph: An organism incapable of synthesizing its own food, requiring ingestion of other organisms.
- Aerobic Respiration: The metabolic process used by producers, consumers, and decomposers to break down complex organic compounds using oxygen ( $O_2$ ) to release stored energy.
  - Chemical Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$
- Anaerobic Respiration (Fermentation): The metabolic breakdown of carbohydrates without oxygen, yielding end products such as methane ( $CH_4$ ), alcohol, and organic acids.
- Decomposer: Organisms (e.g., bacteria, fungi) that excrete digestive chemicals directly onto organic matter and absorb nutrients from the decomposed material.
- Detritivore: Organisms that physically ingest or feed on dead plant and/or animal matter (detritus).
  - Scavenger: Detritivores that specifically ingest or feed on dead animal carcasses.

Species Interactions and Ecological Niche

Symbiotic Relationships:
- Mutualism: An interaction between two species in which both species benefit.
- Parasitism: A relationship in which one partner (the parasite) obtains nutrients at the physical expense of the host.
- Commensalism: An interaction where one partner benefits while the other is neither harmed nor benefited.
Competition:
- Occurs when organisms compete for limited environmental resources.
- Intraspecific Competition: Competition occurring among individuals of the same species.
- Interspecific Competition: Competition occurring between individuals of different species.
Competitive Exclusion Principle:
- States that the inevitable elimination of one of two different species will occur if both attempt to occupy the exact same niche within a habitat.
Resource Partitioning:
- When competing species divide a niche, spatial region, or temporal period to avoid direct competition.
Edge Effect:
- An ecological concept describing the greater diversity and density of life present in the region where the edges of two adjacent ecosystems overlap (e.g., land/water interfaces or forest/grassland boundaries).

Terrestrial Biomes

Definition:
- Large geographical regions having a similar climate and plant life, determined primarily by temperature patterns and annual precipitation.
Deserts:
- Characterized by extremely low precipitation.
Grasslands:
- Characterized by distinct wet and dry seasons.
- Temperate Shrubland (Chaparral): Arid region with dense, scrubby vegetation adapted to periodic fires.
- Polar Grassland (Tundra): Treeless, cold climate featuring permafrost and low-growing vegetation.
Forests:
- Characterized by high rainfall capable of supporting tree growth.
- Tropical Rainforest: Features nutrient-poor soil due to heavy leaching, but maintains extremely high species diversity.
- Temperate Deciduous Forest: Features nutrient-rich soil driven by seasonal leaf drop and decomposition.
- Polar Forest (Taiga / Boreal / Evergreen Coniferous Forest): Cold forest dominated by cone-bearing evergreen trees.

Freshwater Ecosystems

Thermal Stratification in Lakes:
- In temperate regions, lakes become stratified into distinct temperature layers during summer.
- Lake Overturn: Occurs in the fall and spring when water temperatures equalize, causing vertical mixing that exchanges dissolved oxygen ( $DO$ ) and nutrients between surface and deep layers.
Lentic (Standing) Freshwater Systems:
- Euphotic (Sunlit) Zones:
  - Littoral Zone: Shallow water near the shore where light reaches the bottom; rooted plants grow here. Inhabited by green frogs, painted turtles, pond snails, diving beetles, yellow perch, muskrats, and blue-winged teals.
  - Limnetic Zone: Open, sunlit, offshore surface area. Inhabited by plankton and free-swimming fish like northern pike.
- Aphotic (Dark) Zones:
  - Profundal Zone: Deep, open water that is too dark for photosynthesis to occur.
  - Benthic Zone: The bottom floor of the water system; nourished by falling dead organic matter (detritus). Inhabited by organisms like bloodworms.

Diagram of Lake Zones and Associated Wildlife

Lotic (Flowing) Freshwater Systems:
- River System Zones:
  - Source Zone: Headwater area with cold water and high concentrations of dissolved oxygen ( $DO$ ); features rain, snow, glaciers, lakes, rapids, and waterfalls.
  - Transition Zone: Mid-elevation streams where tributaries join; warmer water and gentler slopes.
  - Floodplain Zone: Meandering rivers in flat valleys; warmer water with lower concentrations of dissolved oxygen ( $DO$ ) due to slower-moving water and increased decomposition. Features oxbow lakes, salt marshes, deltas, and deposited sediment flowing into the ocean.
- Ecological Services of Lotic Systems:
  - Delivers nutrients to the ocean to help sustain coastal fisheries.
  - Deposits silt that maintains deltas.
  - Helps purify water naturally.
  - Renews and re-nourishes wetlands.
  - Provides vital habitat for wildlife.

Diagram of Lotic System Zones from Source to Ocean

Freshwater Wetlands:
- Act as nature's sponges by absorbing excess water and filtering pollutants (e.g., the Florida Everglades system, which is actively undergoing restoration).
- Marsh: Grassland ecosystem that is temporarily flooded with water.
- Swamp: Ecosystem permanently covered with water, dominated by woody plants and trees.
- Hardwood Bottomland Forest: Lowland forest area that is temporarily flooded, where hardwood trees grow.
- Prairie Pothole: Shallow wetland depression formed by glaciers; provides essential duck habitat.
- Peat Moss Bog: Accumulates peat; collects salt which increases acidity levels and prevents microbial decomposition.

Marine Ecosystems and Coastal Wetlands

Marine Aquatic Zones:
- Coastal Zone:
  - Estuary: A partially enclosed coastal area where freshwater from rivers mixes with seawater to create brackish water; acts as a breeding ground for many marine organisms and purifies water.
  - Intertidal Zone: Coastal area between high and low tide; characterized by harsh conditions due to shifting water levels on rocky and sandy shores.
- Pelagic Zone (Open Water):
  - Euphotic Zone: Top, sunlit ocean layer where photosynthesis occurs.
  - Bathyal Zone: Dimly lit middle ocean layer.
  - Abyssal Zone: Deepest ocean layer; cold, dark, under immense pressure, and containing little dissolved oxygen ( $DO$ ).
- Benthic Zone: The ocean floor; organisms here feed on organic waste drifting down from upper layers, known as marine snow.
Key Marine Habitats:
- Kelp Forests: Composed of large brown algae growing in cooler waters along rocky coasts; provides vital habitat and food for numerous species and marine invertebrates.
- Coral Reefs: Built in warm seawater from layers of calcium carbonate ( $CaCO_3$ ); maintains a mutualistic relationship with photosynthesizing algae called zooxanthellae.
  - Ecological & Economic Importance: Provides coastline protection against storms, creates habitats, serves as genetic reserves, and provides major economic benefits from tourism.
  - Risks & Threats: Threatened by overfishing, tourism, and shifts in water quality parameters (such as elevated temperature, altered salinity, and declining pH).
Saltwater Wetlands:
- Salt Marshes: Coastal wetlands with low levels of dissolved oxygen ( $DO$ ), causing organic decay to happen slowly.
- Mangrove Forests: Coastal ecosystems dominated by salt-tolerant mangrove trees that protect coastlines from erosion.

Biogeochemical Cycles

Hydrologic (Water) Cycle:
- Key Processes: Evaporation (including plant transpiration), condensation into atmospheric clouds, and precipitation.
- Fate of Precipitation: Moves across land as surface runoff, or enters the soil via infiltration and percolation.
- Watershed (Drainage Basin): The area of land from which all water drains into a common body of surface water.
Nitrogen Cycle:
- Biological Importance: Essential structural component of proteins, amino acids, and nucleic acids.
- Main Reservoir: Atmospheric nitrogen gas ( $78\%$ $N_2$ ); primary producers cannot directly absorb or use nitrogen gas.
- Steps of the Nitrogen Cycle:
  - Nitrogen Fixation: Conversions of atmospheric $N_2$ gas into ammonia ( $NH_3$ ). Carried out by specialized bacteria (e.g., Rhizobium in legume root nodules) and lightning strikes.
  - Nitrification: Soil bacteria convert ammonia ( $NH_3$ ) into nitrite ions ( $NO_2^-$ ) and subsequently into nitrate ions ( $NO_3^-$ ); nitrous oxide ( $N_2O$ ) can also form.
  - Assimilation: Plants absorb ammonia and nitrate ions through roots to synthesize biological molecules like amino acids and nucleic acids.
  - Ammonification: Decomposer bacteria break down organic nitrogenous compounds from dead organisms and waste into ammonia ( $NH_3$ ).
  - Denitrification: Specialized denitrifying bacteria convert soil nitrogen compounds back into gaseous nitrogen ( $N_2$ ) and nitrous oxide ( $N_2O$ ), releasing them back into the atmosphere.
Sulfur Cycle:
- Natural Sources: Discharged naturally into the atmosphere through volcanic eruptions.
- Human Impacts: Discharged as sulfur dioxide ( $SO_2$ ) when humans:
  - Refine sulfur-containing petroleum products.
  - Burn coal and oil for electricity and industrial power.
  - Convert sulfur-containing metallic ores into free metals (such as copper, lead, and zinc).
- Environmental Impact: Directly linked to atmospheric acid rain formation.
Phosphorus Cycle:
- Biological Importance: Essential component of nucleic acids, cell membrane phospholipids, and cellular energy units (ATP).
- Cycling Rate: Cycles very slowly; acts as the primary limiting factor for plant growth.
- Unique Feature: Has no gaseous atmospheric phase; primarily bound in sedimentary rocks as phosphate ( $(PO_4)^{3-}$ ) and released through rock weathering.
- Environmental Impact: Excessive phosphorus inputs into aquatic environments (from animal waste, synthetic agricultural fertilizers, or untreated sewage) trigger excessive algal blooms and cultural eutrophication.
Carbon Cycle:
- Biological Importance: Fundamental structural component of all organic molecules.
- Reservoirs:
  - Largest Earth Reservoir: Sedimentary rocks containing carbonate compounds ( $(CO_3)^{2-}$ ).
  - Largest Active Reservoir: The ocean; carbon dioxide ( $CO_2$ ) is in constant physical exchange between ocean surface waters and the atmosphere via diffusion.
- Ocean Acidification: Increased absorption of dissolved $CO_2$ into ocean water lowers ocean pH through carbonic acid formation:
  - Chemical Equation: $CO_2 + H_2O \rightarrow H_2CO_3$
- Cellular Respiration: Organisms break down carbohydrates to release stored metabolic energy, returning carbon to the atmosphere:
  - Chemical Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$
- Carbon Releasing Processes: Cellular respiration, decomposition of organic matter, forest fires, combustion of fossil fuels, and volcanic activity.
- Human Impacts & Fossil Fuel Combustion: Atmospheric carbon dioxide concentrations have elevated significantly since the Industrial Revolution, primarily driven by human activities like fossil fuel burning:
  - Coal Combustion: $C(s) + O_2 \rightarrow CO_2$
  - Natural Gas Combustion: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$
  - Gasoline Combustion: $2C_8H_{18} + 25O_2 \rightarrow 16CO_2 + 18H_2O$