Introduction: The Interconnectedness of Life
Welcome to Lesson 16. Ecology is the scientific study of the interactions between organisms and their environment. In this lesson, we will explore the fundamental principles that govern ecosystems, from the flow of energy through food chains to the recycling of essential nutrients in biogeochemical cycles. We will also examine how populations of organisms change over time, a field known as population dynamics.
Part 1: Ecology and Energy Flow
Ecology studies life at various levels of organization, from individual organisms to the entire biosphere. An ecosystem consists of all the living organisms (biotic factors) in a community and the non-living (abiotic factors) with which they interact.
1.1 Food Chains, Food Webs, and Trophic Levels
The flow of energy in an ecosystem is described by food chains (simple, linear pathways) and more realistic food webs (complex, interconnected pathways). Each step is a trophic level.
- Producers (Autotrophs): Form the base of the food chain (e.g., plants, algae).
- Primary Consumers (Herbivores): Feed on producers (e.g., grasshoppers, deer).
- Secondary Consumers (Carnivores): Feed on primary consumers (e.g., frogs, foxes).
- Tertiary Consumers (Carnivores): Feed on secondary consumers (e.g., snakes, hawks).
- Decomposers (e.g., bacteria, fungi): Break down dead organic matter (detritus), returning nutrients to the ecosystem.
Diagram: A Simple Food Web
1.2 Ecological Pyramids and Biomagnification
Energy flows one way through an ecosystem, and its transfer is inefficient. The 10% rule states that only about 10% of the energy from one trophic level is incorporated into the next. This can be visualized with ecological pyramids.
Diagram: Ecological Pyramids (Energy, Biomass, Numbers)
An important consequence of this energy loss is biomagnification. Certain persistent toxins (like DDT or mercury) are not broken down and accumulate in organisms' tissues. As energy is transferred up the food chain, these toxins become more and more concentrated at higher trophic levels, reaching dangerous concentrations in top predators.
Part 2: Biogeochemical Cycles
Unlike energy, chemical elements are recycled within ecosystems. Biogeochemical cycles describe the pathways by which these essential elements move between biotic and abiotic components of the Earth.
2.1 The Carbon Cycle
Carbon is the backbone of organic molecules. The carbon cycle involves a reciprocal relationship between photosynthesis (removes CO₂) and cellular respiration (returns CO₂). Major reservoirs include the atmosphere, oceans, fossil fuels, and living organisms. Human activities, especially the burning of fossil fuels, are significantly increasing atmospheric CO₂.
2.2 The Nitrogen Cycle
Nitrogen is a key component of proteins and nucleic acids. The cycle depends heavily on bacteria to convert unusable atmospheric nitrogen (N₂) into forms that plants can assimilate.
- Nitrogen Fixation: N₂ → ammonia (NH₃) by nitrogen-fixing bacteria (e.g., *Rhizobium* in root nodules).
- Nitrification: Ammonia → nitrites (NO₂⁻) → nitrates (NO₃⁻) by nitrifying bacteria.
- Assimilation: Plants absorb nitrates and ammonia.
- Denitrification: Nitrates → N₂ by denitrifying bacteria, returning nitrogen to the atmosphere.
2.3 The Phosphorus Cycle
Phosphorus is essential for nucleic acids, ATP, and phospholipids. Unlike carbon and nitrogen, there is no major atmospheric component. The main reservoir is in rock and soil minerals. Weathering of rocks releases phosphate ($PO_4^{3-}$) into the soil and water. It is taken up by producers, transferred to consumers, and returned to the soil by decomposition. This cycle is very slow.
Diagram: The Phosphorus Cycle
Part 3: Population Dynamics
Population ecology studies how populations change over time. Growth is limited by various factors.
- Density-Dependent Factors: Effects intensify as population density increases (e.g., competition, disease, predation).
- Density-Independent Factors: Affect populations regardless of density (e.g., natural disasters, climate change).
3.1 Growth Models and Life History Strategies
Exponential growth (J-shaped curve) occurs under idealized, unlimited conditions. More realistically, populations exhibit logistic growth (S-shaped curve), where growth slows as it approaches the carrying capacity (K).
Organisms can be broadly categorized by their life history strategies:
- r-strategists: Produce many offspring with little parental care; thrive in unstable environments (e.g., bacteria, insects).
- K-strategists: Produce few offspring with significant parental investment; live near carrying capacity in stable environments (e.g., elephants, humans).
Diagram: Survivorship Curves
Interactive Practice Quiz
Test your understanding of ecology and ecosystems. Choose the best answer for each question (A-E) and then submit to see your results.