Table of Contents
Energy and Matter in Ecosystems
Within the biosphere, every ecosystem is held together by two fundamental processes:
- A one-way flow of energy from the sun, through organisms, and out again as heat.
- A cyclic flow of matter (nutrients) between living organisms and their physical environment.
Understanding how these two flows differ is key to ecology: energy flows through ecosystems, nutrients cycle within them.
The Role of Energy in Ecosystems
Energy Sources
Most ecosystems rely on solar energy:
- Producers (autotrophs) such as green plants, algae, and many bacteria capture light energy and convert it into chemical energy in organic molecules.
- This stored chemical energy becomes available to other organisms when they eat or decompose these producers.
Some specialized ecosystems (e.g., deep-sea hydrothermal vents) rely on chemical energy instead:
- Certain bacteria and archaea use the energy released from chemical reactions with inorganic substances (e.g., hydrogen sulfide) to build organic molecules.
- These chemosynthetic organisms are still producers: they are the first link in the energy flow.
Energy Flow Is One-Way
Energy enters ecosystems as high-quality, concentrated energy (light or chemical energy) and leaves as low-quality heat:
- Input: Sunlight (or inorganic chemical energy).
- Transfer: Stepwise use and transfer of chemical energy as organisms feed on one another.
- Loss: At every step, some usable energy is transformed into heat and is no longer available to do biological work.
Because of this constant energy loss, ecosystems require continuous new energy inputs. Energy cannot be recycled in the way that atoms and molecules can.
Trophic Levels and Food Chains
Trophic Levels
A trophic level groups organisms by how they obtain energy:
- Primary producers
- Create organic matter from inorganic substances using light or chemical energy.
- Examples: plants, algae, cyanobacteria, chemosynthetic bacteria.
- Consumers (heterotrophs)
- Primary consumers: herbivores that eat producers (e.g., caterpillars, zooplankton, deer).
- Secondary consumers: carnivores that eat herbivores (e.g., frogs, small fish, spiders).
- Tertiary (and higher) consumers: carnivores that eat other carnivores (e.g., hawks, sharks).
- Detritivores and decomposers
- Detritivores (e.g., earthworms, woodlice) feed on dead organic matter (detritus).
- Decomposers (fungi, many bacteria) chemically break down organic remains and waste into inorganic nutrients.
These categories describe roles, not rigid groups. An animal may occupy different trophic levels depending on its diet.
Food Chains and Food Webs
- A food chain is a linear sequence showing who eats whom (e.g., grass → grasshopper → frog → snake → hawk).
- Real ecosystems consist of interconnected food webs, where most species feed on multiple kinds of organisms and are eaten by multiple predators.
Energy flow is easier to visualize in simple chains, but food webs more accurately show that energy branches and merges as it passes through an ecosystem.
Ecological Efficiency and Energy Pyramids
Inefficient Transfer of Energy
When energy moves from one trophic level to the next, only a fraction of the energy is passed on. The rest is lost as:
- Heat from metabolism (respiration).
- Undigested material in feces.
- Energy used for life processes (movement, growth, reproduction, maintenance).
The approximate rule:
- About 10% of the energy in one trophic level is passed to the next (this is a rough average, called the 10% rule).
- The ecological efficiency (also called trophic transfer efficiency) varies widely (often between 5–20%) depending on the organisms and environment.
Consequences: Energy Pyramids
Because of these losses, energy flow has a pyramidal shape:
- Pyramid of energy
- Always widest at the base (producers) and narrowest at the top (top predators).
- Shows the amount of energy entering each trophic level per unit area and time (e.g., kilojoules per m² per year).
- Never inverted: there is always less usable energy at higher trophic levels.
- Pyramid of biomass
- Shows the mass of living tissue at each trophic level at a given time.
- Often follows the same pattern as energy.
- Can be inverted in some aquatic systems (tiny, fast-growing producers like phytoplankton are consumed so rapidly that their standing biomass is low, even though they fix a lot of energy).
Implications for Food Chains
Because high trophic levels receive little energy:
- Energy limits the number of trophic levels; most ecosystems have 3–5 effective steps from producers to top predators.
- Top predators are rare and need large territories or large prey bases.
- Humans obtain more energy from ecosystems when eating at lower trophic levels (e.g., plant-based diets) than when eating high-level carnivores.
Primary Production and Productivity
Primary Production
Primary production is the formation of new organic matter by producers.
- Gross Primary Production (GPP): total amount of chemical energy fixed in organic compounds by producers per unit area and time.
- Net Primary Production (NPP): energy remaining in plant tissues after subtracting energy used for plant respiration.
Mathematically:
$$
\text{NPP} = \text{GPP} - R
$$
where $R$ is the energy used in respiration.
NPP represents the energy available to consumers and is often measured as biomass increase per unit area and time (e.g., g dry mass/m²/year).
Factors Affecting Productivity
NPP varies strongly between ecosystems. Major influences include:
- Light availability (especially in aquatic systems).
- Water availability (critical in terrestrial systems).
- Nutrient availability (e.g., nitrogen, phosphorus, iron).
- Temperature (affects enzyme activity and metabolism).
Some broad patterns:
- Tropical rainforests, estuaries, and coral reefs: very high productivity.
- Deserts, open ocean, and polar regions: low productivity.
- Agricultural fields: productivity heavily shaped by human inputs (fertilizers, irrigation) and harvest.
Nutrient Cycles: Matter Is Reused
In contrast to energy, matter (atoms and molecules) is not used up. Instead, nutrients constantly cycle between:
- Biotic components: organisms.
- Abiotic components: atmosphere, soil, sediments, rocks, and water bodies.
These cycles of essential elements (e.g., carbon, nitrogen, phosphorus, sulfur) are called biogeochemical cycles (“bio” = living, “geo” = earth, “chemical” = elements and compounds). Each major element has its own detailed cycle, but all share some general features.
General Features of Nutrient Cycles
- Reservoirs and Pools
- Reservoirs: Large, often long-term storage locations (e.g., atmosphere for nitrogen, oceans for carbon, rocks for phosphorus).
- Active pools: Smaller, more rapidly cycling parts (e.g., soil nutrients, biomass).
- Movement Between Pools
Nutrients move between pools through:
- Biological processes
- Uptake by producers.
- Consumption and digestion by consumers.
- Excretion and death.
- Decomposition by bacteria and fungi.
- Physical and chemical processes
- Dissolving in water, precipitation, sedimentation.
- Weathering of rocks.
- Volatilization and gas exchange with the atmosphere.
- Role of Decomposers
Decomposers are central to all nutrient cycles:
- They break down dead organisms and waste products.
- They release inorganic nutrients (e.g., nitrate, phosphate, sulfate) back into soil and water.
- Without decomposers, nutrients would quickly become locked in dead biomass and unavailable to producers.
- Closed Loops, Open System
- Within a given ecosystem, nutrient cycles form loops: producers → consumers → decomposers → inorganic nutrients → producers.
- But ecosystems are open: nutrients can enter or leave through wind, water flow, erosion, and biological movement (e.g., migrating animals).
Linking Energy Flow and Nutrient Cycles
Energy and nutrient flows are tightly connected but behave differently:
- Energy:
- Enters mainly as sunlight.
- Flows through trophic levels.
- Is degraded to heat and lost from the ecosystem.
- Requires constant new input.
- Nutrients:
- Enter mainly by geological and atmospheric processes (long timescales) or by import from other ecosystems.
- Cycle repeatedly between living organisms and the abiotic environment.
- Can be stored, mobilized, or lost from ecosystems.
Biological Control of Nutrient Cycles
Living organisms influence how fast and in what form nutrients cycle:
- Plants and algae:
- Capture nutrients from soil and water.
- Lock them into biomass.
- Animals:
- Redistribute nutrients through movement and feeding.
- Microorganisms:
- Transform chemical forms (e.g., nitrogen gas to ammonia, organic carbon to CO₂).
- Control the speed of decomposition and mineralization.
Changes in community composition (e.g., loss of decomposers, invasion of new plant species) can therefore reshape nutrient cycles and affect ecosystem productivity.
Human Impacts on Energy and Nutrient Dynamics
Human activities alter both energy use and nutrient cycles:
- Land-use change (deforestation, agriculture, urbanization):
- Changes the balance between GPP, NPP, and respiration.
- Alters the capacity of ecosystems to store carbon and other nutrients.
- Fertilization and pollution:
- Large inputs of nitrogen and phosphorus from fertilizers and wastewater can oversupply nutrients, leading to eutrophication of lakes and seas.
- This disrupts local nutrient cycles and can cause algal blooms and oxygen depletion.
- Combustion of fossil fuels:
- Transfers large amounts of stored carbon to the atmosphere.
- Speeds up carbon cycling on human timescales and affects climate, which in turn feeds back on productivity and other cycles.
These examples show that energy flow and nutrient cycles are not just abstract concepts but are directly affected by, and in turn influence, human societies.
Overview: Why Energy Flow and Nutrient Cycles Matter
To summarize the ecological significance:
- Energy flow:
- Determines how much life an ecosystem can support.
- Limits the number of trophic levels and the abundance of top predators.
- Depends on primary production and on the efficiency of transfer between trophic levels.
- Nutrient cycles:
- Ensure that essential elements are reused rather than exhausted.
- Depend on the interactions between organisms, atmosphere, water, and rocks.
- Strongly influence productivity and ecosystem stability.
Together, these two processes explain how the biosphere can sustain immense biological diversity on a finite planet with finite amounts of chemical elements but a continuous input of solar energy.