Understanding Energy Flow and Trophic Levels
Solar energy enters ecosystems through producers (plants) via photosynthesis. Energy then moves through trophic levels: primary consumers (herbivores), secondary consumers (carnivores), and tertiary consumers (top predators).
The 10% Rule and Energy Loss
Only about 10% of energy transfers from one trophic level to the next. The rest is lost through cellular respiration and heat. This explains why ecosystems support far more plant biomass than herbivore biomass, and very few top predators.
Productivity Calculations
Gross primary productivity (GPP) is total solar energy converted to chemical energy through photosynthesis. Net primary productivity (NPP) is the energy remaining after plants use energy for their own respiration. These calculations appear frequently on exams.
Food chains rarely exceed four or five trophic levels because of energy loss. This also explains why eating lower on the food chain is more efficient for human populations. Master the math for calculating how much plant biomass supports a given animal population.
Biogeochemical Cycles and Nutrient Movement
Biogeochemical cycles show how essential elements move through living and non-living parts of ecosystems. You must master four major cycles.
The Carbon and Nitrogen Cycles
The carbon cycle involves CO2 being fixed by plants, consumed by animals, and released through respiration and decomposition. Fossil fuel combustion accelerates this cycle.
The nitrogen cycle depends on bacteria. Nitrogen-fixing bacteria convert atmospheric nitrogen into usable ammonia. Nitrifying bacteria then convert ammonia to nitrates. Unlike carbon, nitrogen lacks a significant accessible atmospheric reservoir.
The Phosphorus Cycle and Eutrophication
The phosphorus cycle has no atmospheric phase. Phosphorus moves through rock weathering, soil, organisms, and water. Excess phosphorus from agricultural runoff causes eutrophication, creating dead zones in water bodies.
Water Cycle Connections
The water cycle involves evaporation, condensation, precipitation, and transpiration. All nutrient cycles depend on water movement. Students often struggle remembering which organisms perform each transformation. Flashcards drill these process steps and build automaticity with complex sequences.
Population Dynamics and Limiting Factors
Population ecology examines how populations grow, stabilize, and decline. Carrying capacity (K) is the maximum population size an environment can sustain indefinitely with available resources.
Growth Patterns
Populations without resource limits follow exponential growth, creating a J-shaped curve. Real-world populations face limiting factors like food, space, predation, disease, and waste. These create logistic growth, producing an S-shaped curve approaching carrying capacity.
Density-Dependent vs. Density-Independent Factors
Density-dependent factors like disease and competition intensify as population density increases. Density-independent factors like hurricanes or freezes affect populations regardless of density.
Reproductive Strategies
R-selected species (rabbits, insects) produce many offspring with minimal parental investment. They reach high densities quickly. K-selected species (elephants, humans) produce fewer offspring with significant care. They approach carrying capacity slowly.
Age structure pyramids reveal whether populations are growing, stable, or declining. Understanding these dynamics helps with wildlife management, invasive species control, and population projections.
Community Ecology and Species Interactions
Communities consist of different species living together. The interactions between species shape ecosystem structure and function.
Five Major Species Interactions
You must know these five interactions:
- Competition: Two species use the same limited resource. This leads to competitive exclusion or resource partitioning.
- Predation: One organism kills another for food. Predator-prey relationships often show cyclical population patterns.
- Herbivory: Predation on plants. Plants evolve defenses like thorns, toxins, and tough leaves.
- Parasitism: One organism lives in or on another, benefiting the parasite while harming the host.
- Mutualism: Both species benefit, such as bees pollinating flowers or nitrogen-fixing bacteria in plant roots.
Ecological Succession
Primary succession begins on bare substrate like volcanic rock. Secondary succession occurs after disturbance in previously vegetated areas. Pioneer species like lichens colonize bare areas, gradually changing the environment. Later species colonize, eventually reaching a climax community.
Understanding disturbance and resilience explains why ecosystems vary and how they recover from damage.
Biomes and Terrestrial Ecosystems
Biomes are large-scale ecosystems with specific climate conditions, vegetation, and animal adaptations. Climate factors like temperature, precipitation, and seasonality determine biome distribution.
Major Terrestrial Biomes
- Tropical rainforests: High rainfall, high biodiversity, layered structure with canopy and understory.
- Temperate forests: Seasonal variation, deciduous trees drop leaves in winter, moderate biodiversity.
- Boreal forests (taiga): Cold long winters, dominated by conifers, limited biodiversity.
- Grasslands: Moderate rainfall insufficient for dense forests, maintained by grazing and fire.
- Deserts: Minimal precipitation, specialized plants with water-storage adaptations, nocturnal animals.
- Tundra: Polar regions with permafrost, limited growing seasons, low vegetation.
Aquatic Biomes
Freshwater ecosystems include rivers, lakes, and wetlands. Marine ecosystems include coral reefs, kelp forests, and open ocean. Each biome's characteristics determine which organisms thrive there.
Understanding biome characteristics, typical organisms, and human impacts is crucial for exams. Flashcards with biome images paired with characteristics, rainfall amounts, and species create strong visual memory links.