Unit 2: Ecological Concepts and Energy Flow

I. Orientation: Ecosystems as Open, Interconnected Systems

Ecology examines relationships among organisms and between organisms and their physical surroundings. An ecosystem is governed by the interaction of biotic communities with abiotic conditions, while energy moves mainly in one direction and nutrients are repeatedly recycled through biological, geological, and chemical processes.

• Interdependence: Organisms depend on one another for food, pollination, shelter, decomposition, and regulation of populations.
• Energy principle: Solar radiation is captured by producers, transferred through feeding, and ultimately dissipated as heat; energy is not recycled.
• Nutrient principle: Elements such as carbon, nitrogen, phosphorus, and water circulate between organisms and the atmosphere, soil, rocks, and water.
• Systems boundary: An ecosystem may be as small as a rotting log or as large as a rainforest, but it always includes interacting living and non-living components.
• Dynamic equilibrium: Ecosystems change through succession, disturbance, seasonal variation, and feedback while often maintaining functional stability.

II. Ecosystem and Its Components — Structure and Function

A. Ecosystem and its components

An ecosystem is a functional unit in which a community of organisms interacts with its abiotic environment through energy flow and material cycling.

• Biotic components: These are living organisms classified by their ecological roles:
  • Producers: Green plants, algae, and some bacteria synthesize organic matter, usually through photosynthesis.
  • Consumers: Herbivores, carnivores, omnivores, and parasites obtain energy by feeding on organisms or organic matter.
  • Decomposers: Fungi and bacteria break down dead material and release mineral nutrients.
• Abiotic components: Temperature, light, water, air, soil, pH, salinity, minerals, and nutrients influence survival and productivity.
• Functional processes: Photosynthesis stores energy in glucose, respiration releases usable energy, and decomposition returns nutrients to soil or water.
• Example: In a pond, algae are producers, zooplankton are primary consumers, fish may be secondary consumers, and bacteria decompose dead organisms.
• Productivity: Gross primary productivity is total photosynthetic production; net primary productivity is the energy remaining after producer respiration.

III. Trophic Level of Organisms — Feeding Position

A. Trophic level of organisms

A trophic level is the feeding position occupied by an organism in a food chain or food web.

• First trophic level: Producers, such as grasses and phytoplankton, convert external energy into chemical energy.
• Second trophic level: Primary consumers, such as rabbits and zooplankton, feed directly on producers.
• Third trophic level: Secondary consumers, such as frogs eating insects, feed on primary consumers.
• Higher levels: Tertiary consumers and apex predators occupy progressively higher positions, although many ecosystems do not contain all possible levels.
• Omnivory: Humans, bears, and crows may occupy different trophic levels depending on the food being eaten.
• Detrital pathway: Detritivores and decomposers obtain energy from dead leaves, carcasses, and waste rather than from living prey.
• Energy limitation: Less energy is generally available at higher trophic levels because much energy is used in metabolism and released as heat.

IV. Ecological Pyramids — Quantifying Ecosystem Structure

A. Ecological pyramids

Ecological pyramids represent the quantitative relationship among trophic levels using numbers, biomass, or energy.

• Pyramid of numbers: It shows the number of organisms at each level. One tree may support thousands of herbivorous insects, so the pyramid can be inverted.
• Pyramid of biomass: It measures the dry mass of living material, commonly in grams per square metre. Aquatic systems may show inversion because phytoplankton reproduce rapidly despite having low standing biomass.
• Pyramid of energy: It shows energy available at each trophic level, commonly in kilojoules per square metre per year. It is always upright because energy is lost during transfer.
• Ten-percent approximation: Often only about 10% of energy at one trophic level becomes biomass at the next; this is a broad ecological approximation, not a fixed law.
• Example: If producers contain 10,000 kJ m⁻² yr⁻¹, primary consumers might receive about 1,000 kJ and secondary consumers about 100 kJ.
• Limitation: Pyramids simplify complex food webs and may not represent organisms that feed across multiple trophic levels.

V. Ecological Niche — The Functional Role of a Species

A. Ecological niche

An ecological niche is the total range of environmental conditions, resources, and interactions that define how a species lives and functions in an ecosystem.

• Habitat versus niche: Habitat is the physical place where an organism lives; niche describes its role, resource use, tolerance limits, and interactions.
• Fundamental niche: The potential range of conditions a species could occupy without competition, predation, or other biological restrictions.
• Realized niche: The actual range occupied after competition, predation, disease, and resource limitation are considered.
• Resource partitioning: Similar species reduce competition by using different foods, spaces, or times; warblers may feed in different parts of the same tree.
• Competitive exclusion: Two species with identical niches cannot indefinitely coexist in a stable environment if they depend on the same limiting resource.
• Niche breadth: A generalist, such as a rat, uses many resources; a specialist, such as a koala dependent largely on eucalyptus leaves, has a narrower niche.

VI. Energy and Nutrients Flow in the Ecosystem — Transfer and Recycling

A. Energy and nutrients flow in the ecosystem

Energy and nutrients move through ecosystems differently: energy flows one way from an external source, whereas nutrients circulate between living and non-living reservoirs.

• Energy capture: Producers absorb solar radiation; photosynthesis can be represented as:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Symbols: CO₂ is carbon dioxide, H₂O is water, C₆H₁₂O₆ is glucose, and O₂ is oxygen.

• Energy transfer: Feeding transfers chemical energy, but respiration releases much of it as heat:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP + heat

ATP is adenosine triphosphate, the immediate energy-transfer molecule in cells.

• Nutrient movement: Carbon, nitrogen, phosphorus, and water move through producers, consumers, decomposers, soil, air, and water.
• Open-system character: Ecosystems receive energy, usually sunlight, and lose heat; nutrients can enter through rainfall or weathering and leave through erosion or export.
• Limiting nutrients: Growth may be restricted by the scarcest required nutrient, such as nitrogen in many terrestrial ecosystems or phosphorus in freshwater systems.

VII. Food Chains — Linear Feeding Pathways

A. Food chains

A food chain is a simplified linear sequence showing the transfer of food, energy, and matter from one organism to another.

• Grazing food chain: It begins with a living producer, such as grass → grasshopper → frog → snake → hawk.
• Detritus food chain: It begins with dead organic matter, such as leaf litter → earthworm → bird → hawk.
• Arrows: An arrow points from the food to the feeder, indicating the direction of energy transfer; grass → rabbit means the rabbit obtains energy from grass.
• Trophic representation: The grass in the example occupies level 1, the grasshopper level 2, the frog level 3, and the snake level 4.
• Chain length: Chains are usually short because usable energy declines at every transfer.
• Limitation: A food chain cannot fully show organisms with several food sources or predators; that complexity requires a food web.

VIII. Food Webs — Interconnected Feeding Relationships

A. Food webs

A food web is a network of overlapping food chains that represents the multiple feeding relationships within a community.

• Interconnection: A single producer may be eaten by several herbivores, while one predator may consume several prey species.
• Stability: Greater diversity of feeding links can provide alternative pathways when one population declines, although complexity does not guarantee stability.
• Top-down effects: A predator can reduce herbivore abundance, allowing producer biomass to increase; removal of wolves in some systems illustrates a possible trophic cascade.
• Bottom-up effects: Changes in nutrients or producer productivity can affect consumers at every higher level.
• Keystone species: A species with an unusually large effect relative to its abundance can reshape a food web; sea otters can influence kelp forests by controlling sea urchins.
• Human impacts: Overfishing, habitat loss, invasive species, and pesticide use can remove links and cause indirect effects across the web.

IX. Biogeochemical Cycles — Circulation of Essential Elements

A. Biogeochemical cycles

Biogeochemical cycles describe the movement of chemical elements between the biosphere, atmosphere, hydrosphere, and geosphere.

• Water cycle: Evaporation and transpiration add water vapour; condensation forms clouds; precipitation returns water, while infiltration, runoff, and groundwater storage redistribute it.
• Carbon cycle: Plants remove atmospheric CO₂ through photosynthesis; respiration, decomposition, combustion, and ocean exchange return carbon. Fossil-fuel burning transfers geological carbon rapidly to the atmosphere.
• Nitrogen cycle: Atmospheric N₂ is converted by nitrogen fixation into usable compounds; nitrification produces nitrates, plants assimilate nitrogen, decomposers ammonify organic matter, and denitrification returns N₂ to the atmosphere.
• Phosphorus cycle: Weathering releases phosphate from rocks; plants absorb it, animals obtain it through food, and decomposition returns it to soil. Unlike nitrogen, phosphorus has no major gaseous atmospheric phase.
• Sulfur cycle: Weathering, volcanic activity, decomposition, and combustion move sulfur; excessive sulfur dioxide emissions can contribute to acid deposition.
• Human alteration: Fertilizer runoff can cause eutrophication, while deforestation and fossil-fuel combustion disrupt carbon and nutrient balances.
• Ecosystem significance: Cycling maintains the supply of elements needed for proteins, nucleic acids, membranes, chlorophyll, and other biological structures.