Energy and Ecosystem Ecology

Learning Objectives

• Compare and contrast the cycling and flow of nutrients and energy through biotic and abiotic reservoirs.

• Explain how resource acquisition of organisms is linked to their structure and function.

• Describe why energy transfer between trophic levels is inefficient and outline the constraints on trophic levels imposed by this inefficiency.

• Explain why decomposers are very important to ecosystem functioning.

• Describe how nutrients flow through the biotic and abiotic reservoirs of an ecosystem.

• Explain how humans have altered nutrient cycling, energy flow, and the water cycle, and have contributed to global warming and climate change.

Introduction to Ecosystem Ecology

Definition of Ecosystem

An ecosystem consists of all the organisms living in a community and the abiotic (non-living) components around them. A defining characteristic of all organisms is their need for energy to fulfill vital functions and reproduce.

• Primary producers are autotrophic (self-feeding) organisms that convert energy from abiotic sources (such as sunlight or inorganic chemicals) into organic material, or biomass.

Types of Primary Producers

• In terrestrial environments, primary producers include plants, algae, and bacteria that perform photosynthesis.

• In aquatic environments, some bacteria and archaea use chemical energy from inorganic compounds (e.g., methane , hydrogen sulfide ) to grow. This process is called chemosynthesis.

Importance of Food in Ecosystems

Energy and Nutrient Elements

Food provides both energy and essential elements required for cellular growth and maintenance.

• Energy: Stored as potential energy in molecular bonds of carbohydrates, fats, and proteins.

• Elements: Carbon, nitrogen, phosphorus, calcium, and others serve as building blocks for new cellular growth and biomolecules (e.g., amino acids, nucleic acids, polysaccharides).

• Example: Glucose () is a carbohydrate used for energy; amino acids are used to build proteins.

Flow of Energy in Ecosystems

Transformation of Energy by Primary Producers

Primary producers transform energy from sunlight or inorganic compounds into chemical energy stored in organic molecules.

• Gross Primary Productivity (GPP): The total amount of chemical energy produced by primary producers in a given area and time.

• Net Primary Productivity (NPP): The energy remaining after producers use some for cellular respiration. This energy is available for growth, reproduction, and consumption by other organisms.

Equation:

Where is the energy used in respiration.

• Energy transfer is not perfectly efficient; some energy is lost as heat and waste (Second Law of Thermodynamics).

Energy Budget and Efficiency

The total energy budget of an ecosystem depends on the efficiency of primary producers in converting abiotic energy sources into chemical energy.

• Photosynthetic efficiency is low (~0.8% of incoming solar energy is converted to chemical energy).

• Most energy is lost as heat or used in metabolic processes.

Trophic Structure in Ecosystems

Energy and Nutrient Flow Through Trophic Levels

Energy and nutrients move through ecosystems via trophic levels, which represent the feeding relationships among organisms.

• Primary producers (autotrophs) form the base of the food chain.

• Primary consumers (herbivores) eat primary producers.

• Secondary consumers (carnivores) eat primary consumers.

• Decomposers (detritivores) feed on dead organic matter and recycle nutrients back to producers.

Food Chains and Food Webs

A food chain summarizes the flow of energy and nutrients from one trophic level to the next. However, most consumers feed on multiple trophic levels, so food webs provide a more accurate representation of ecosystem interactions.

• Food webs include all organisms and their feeding relationships within an ecosystem.

• Energy transfer between trophic levels is inefficient; typically, only about 10% of energy is passed to the next level.

Energy Transfer Between Trophic Levels

Inefficiency of Energy Transfer

Most energy consumed at each trophic level is used for respiration and maintenance, with only a small fraction available for growth and transfer to the next level.

• On average, only about 10% of the energy from one trophic level is transferred to the next.

• This inefficiency limits the number of trophic levels and the biomass that can be supported at higher levels.

• Example: Human populations can be supported at higher densities on plant-based diets than on diets based on higher trophic levels (e.g., meat).

Biomagnification

While energy is lost at each trophic level, some harmful chemicals (e.g., heavy metals, pesticides) accumulate and magnify in concentration at higher trophic levels. This process is called biomagnification.

• Chemicals that do not break down easily or are not excreted accumulate in organisms and move up the food chain.

• Example: Mercury and DDT are known to biomagnify in aquatic food webs.

Nutrient Cycling in Ecosystems

Biogeochemical Cycles

Nutrients and chemical elements (e.g., carbon, nitrogen, phosphorus) cycle between biotic (living) and abiotic (non-living) reservoirs in ecosystems.

• Biogeochemical cycle: The path an element takes as it moves through producers, consumers, decomposers, and back to the environment.

• Decomposition of detritus (dead organic matter) is crucial for recycling nutrients.

Factors Affecting Decomposition

• High moisture and cool temperatures slow decomposition, leading to accumulation of organic matter in soil (e.g., temperate forests).

• High moisture and warm temperatures increase decomposition rates, resulting in little detritus accumulation (e.g., tropical forests).

Sources and Losses of Nutrients

• Nutrients can be lost from ecosystems via erosion, migration, and atmospheric release.

• New nutrients enter ecosystems through weathering of rocks, atmospheric fixation, and biological processes (e.g., photosynthesis, nitrogen fixation).

Global Biogeochemical Cycles

Major Reservoirs and Processes

At the global scale, nutrient cycles involve major reservoirs and processes that move elements between them.

• Water cycle: Major reservoir is the ocean; water moves via evaporation, precipitation, and percolation.

• Nitrogen cycle: Major reservoir is the atmosphere (as ); enters ecosystems via nitrogen fixation and precipitation.

• Phosphorus cycle: Major reservoir is rock formations; enters ecosystems via weathering.

• Carbon cycle: Major reservoirs include oceans, sediments, atmosphere, and fossil fuels; moves via photosynthesis and respiration.

Human Impacts on Cycles

• Alteration of surface permeability (e.g., urbanization) affects water cycle and groundwater recharge.

• Fossil fuel burning and deforestation increase atmospheric , contributing to global warming.

• Fertilizer use leads to nitrogen and phosphorus pollution, causing eutrophication and dead zones in aquatic systems.

Global Climate Change

Greenhouse Effect and Global Warming

Global warming refers to the increase in Earth's average temperature due to the accumulation of greenhouse gases (e.g., , , , water vapor) in the atmosphere.

• Greenhouse gases trap heat from solar radiation, leading to increased global temperatures.

• Human activities, especially fossil fuel burning and deforestation, have rapidly increased atmospheric levels.

• Recent records show 2024 as the hottest year on record, with ocean temperatures also at their highest.

Climate vs. Weather

• Weather: Short-term, variable atmospheric conditions at a specific place and time.

• Climate: Long-term average pattern of regional or global atmospheric conditions.

Effects on Biodiversity and Ecosystems

• Changes in temperature and precipitation patterns affect development, physiology, and behavior of organisms.

• Impacts include range shifts, changes in phenology (timing of biological events), evolutionary adaptations, extinctions, ocean acidification, and shifts in net primary productivity.

• Altered energy flow and nutrient cycling disrupt ecosystem dynamics.

Summary Table: Major Global Biogeochemical Cycles

Additional info: These notes expand on the original slides by providing definitions, examples, and a summary table for clarity and completeness.