Energy and Nutrient Flow in Ecosystems (Chapter 42 Study Notes)

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Energy and Nutrient Flow in an Ecosystem

Introduction

Understanding how energy and nutrients move through ecosystems is fundamental to ecology. This topic covers the roles of autotrophs and heterotrophs, the flow of energy, and the cycling of nutrients, with a focus on terrestrial ecosystems.

Methods of Acquiring Energy and Nutrients

Autotrophs

Autotrophs are organisms capable of synthesizing complex organic molecules from CO2 and other simple inorganic molecules using energy from a non-biological source.

Primary autotrophs: Plants (on land) and algae (in water).
Photosynthesis: Most autotrophs use photosynthesis to transform light energy into chemical energy (glucose).
Important nutrients for autotrophs:
- Carbon (C): Acquired as carbon dioxide (CO2) from the air; forms the backbone of all organic molecules.
- Oxygen (O): Obtained from water and air; used in structural and metabolic processes.
- Hydrogen (H), Nitrogen (N), Phosphorus (P): Obtained from water, soil, and air; essential for macromolecules.
There are trade-offs between obtaining energy, nutrients, and water in plants.

Heterotrophs

Heterotrophs are organisms that must obtain nourishment (complex organic molecules containing energy and nutrients) from other organisms.

Types of Heterotrophs:
- Predators: Attack and consume another organism (prey) at the time of the initial attack.
- Parasites: Live on or in another organism and obtain nutrients from it.
- Scavengers: Consume dead organisms or organic matter.
Saprotrophs: Consume another organism (or a portion of another organism) that is dead at the time of "attack." Includes decomposers such as bacteria and fungi.

Flow of Energy Through an Ecosystem

Energy Input and Transfer

Energy enters ecosystems primarily through sunlight, which is captured by autotrophs via photosynthesis.

Less than 1% of the sun's energy that reaches Earth's surface is used for photosynthesis; the rest is lost as heat or reflected.
Energy flows through ecosystems in a one-way stream, from primary producers to various consumers.

Energy Transfer Efficiency

Only about 10% of energy captured by autotrophs is transferred to the next trophic level (primary consumers).
The rest is lost due to:
- Use of energy in biomass to fuel metabolism (work or heat).
- Death without being consumed by a predator (energy supports decomposers).
- Loss as waste or excreted substances.
Significant energy is lost at each trophic level, limiting the number of higher-level consumers.

Example: In a food chain, energy captured by plants is passed to herbivores, then to carnivores, with only a fraction transferred at each step.

Cycling of Nutrients Through Ecosystems

Overview of Nutrient Cycles

Unlike energy, nutrients can cycle through ecosystems multiple times. The biogeochemical cycle describes the movement of nutrients through both living (biotic) and non-living (abiotic) components.

Geochemical cycle: Movement of nutrients in their inorganic form via the atmosphere, soil, and water.
Biochemical cycle: Movement of nutrients in their organic form through living organisms.
Movement from inorganic to organic and back again.

Major Nutrient Cycles

Nutrient	Major Inputs	Major Outputs
Carbon	Atmospheric CO2 via photosynthesis	Respiration, decomposition
Nitrogen	Atmospheric N2 via nitrogen fixation	Denitrification, leaching
Phosphorus	Weathering of rocks	Runoff, sedimentation
Water	Precipitation, absorption	Evaporation, transpiration

Nutrient Budgets and Human Influence

Ecologists quantify nutrient gain and loss in ecosystems by creating nutrient budgets. Human activities, such as agriculture and pollution, can significantly alter nutrient cycling.

Atmospheric inputs: Precipitation, fixation, and deposition.
Atmospheric outputs: Release of gases during decomposition and combustion.
Hydrospheric inputs: Nutrients deposited in floodplains and backwaters.
Hydrospheric outputs: Nutrients dissolved in water and lost via runoff.
Lithospheric inputs: Weathering of rocks releases nutrients.
Lithospheric outputs: Nutrients lost to the hydrosphere in solution.
Human influence: Fertilizer use, fossil fuel burning, and land use changes can increase nutrient inputs and outputs, sometimes leading to environmental problems such as eutrophication.

Key Terms and Concepts

Autotroph: An organism that produces its own food from inorganic substances.
Heterotroph: An organism that obtains food by consuming other organisms.
Photosynthesis: The process by which plants and some other organisms use sunlight to synthesize foods from CO2 and water.
- Equation:
Biogeochemical cycle: The flow of chemical elements and compounds between living organisms and the physical environment.
Trophic level: The position an organism occupies in a food chain.
Energy pyramid: A graphical representation of energy flow in an ecosystem.

Summary Table: Energy vs. Nutrient Flow

Feature	Energy Flow	Nutrient Cycling
Direction	One-way (lost as heat)	Cyclic (reused)
Efficiency	~10% transferred per trophic level	Can be recycled multiple times
Main Source	Sunlight	Atmosphere, lithosphere, hydrosphere

Conclusion

Energy and nutrient flow are central to ecosystem function. While energy moves in a one-way stream and is lost as heat, nutrients cycle repeatedly through biotic and abiotic components. Human activities can disrupt these cycles, impacting ecosystem health and productivity.