Energy and Ecosystem Ecology

Learning Objectives

This section introduces the foundational concepts of energy flow and nutrient cycling in ecosystems, emphasizing the roles of biotic and abiotic reservoirs, resource acquisition, trophic efficiency, decomposers, and human impacts on global cycles.

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

• Explain resource acquisition in relation to organism structure and function.

• Describe energy transfer inefficiency between trophic levels and its constraints.

• Discuss the importance of decomposers in ecosystem functioning.

• Trace nutrient flow through biotic and abiotic reservoirs.

• Analyze human impacts on nutrient cycling, energy flow, and climate change.

Introduction to Ecosystems

Definition and Components

An ecosystem consists of all organisms living in a community and the abiotic environment around them. All organisms require energy to perform vital functions and reproduce.

• Primary producers (autotrophs) convert energy from abiotic sources into organic material (biomass).

• Energy enters ecosystems primarily through photosynthesis or, in some aquatic environments, through chemosynthesis.

Types of Primary Producers

• Terrestrial environments: Plants, algae, and photosynthetic bacteria.

• Aquatic environments: Algae and bacteria using chemical energy from compounds like methane (CH4) or hydrogen sulfide (H2).

Importance of Food: Energy and Elements

Energy in Food

Food provides potential energy stored in molecular bonds of carbohydrates, fats, and proteins.

• Key elements: Carbon, nitrogen, phosphorus, calcium, etc., serve as building blocks for cellular growth and maintenance (e.g., amino acids, nucleic acids, polysaccharides).

• Example: Glucose (C6H12O6) is a primary energy source for cellular respiration.

Flow of Energy in Ecosystems

Primary Productivity

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

• Gross Primary Productivity (GPP): Total chemical energy produced in a given area and time.

• Net Primary Productivity (NPP): Energy remaining after producers use some for cellular respiration and metabolic processes.

Equation:

$NPP = GPP - R$

• R: Energy used in respiration.

• Only a fraction of incoming solar energy is converted to biomass (~0.8% efficiency).

Energy Budget and Efficiency

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

• Most energy is lost as heat or waste during metabolic processes.

• Only a small portion is available for higher trophic levels.

Trophic Structure in Ecosystems

Energy Transfer Across Trophic Levels

Primary producers incorporate energy and elements into their biomass, which is then transferred to other organisms through consumption.

• Consumers:

  • Primary consumers: Eat producers (herbivores).

  • Secondary consumers: Eat primary consumers (carnivores).

  • Decomposers: Feed on remains and waste, recycling nutrients.

• Food chains and webs: Summarize energy and nutrient flow, with most consumers feeding at multiple trophic levels.

Trophic Efficiency

• Only about 10% of energy from one trophic level is transferred to the next.

• Higher trophic levels support smaller populations due to energy loss.

• Example: Human populations can be sustained at higher numbers on plant-based diets than on diets of higher trophic level animals.

Biomagnification

While energy is lost at each trophic level, certain chemicals (e.g., heavy metals, pesticides) accumulate and magnify in concentration at higher trophic levels.

• Example: Mercury and DDT accumulate in top predators.

Nutrient Cycling in Ecosystems

Biogeochemical Cycles

Nutrients and elements (C, N, P, S, Ca) cycle between biotic and abiotic reservoirs, linking energy flow and ecosystem structure.

• Decomposition: Rate of detritus breakdown limits nutrient movement.

• Climate effects: Moisture and temperature influence decomposition rates and detritus accumulation.

Sources and Losses of Nutrients

• Losses: Respiration, migration, water and wind erosion, weathering of rocks.

• Additions: Photosynthetic fixation of carbon, nitrogen fixation by bacteria, weathering of rocks for phosphorus.

Global Biogeochemical Cycles

Major Cycles and Human Impacts

Global cycles of water, nitrogen, phosphorus, and carbon are crucial for ecosystem function and are affected by human activities.

• Water cycle: Driven by evaporation, precipitation, and percolation. Human impacts include altered surface permeability and deforestation.

• Nitrogen & phosphorus cycles: Nitrogen is abundant in the atmosphere but biologically inert; phosphorus is stored in rocks. Human impacts include pollution and eutrophication.

• Carbon cycle: Carbon is stored in oceans, sediments, and the atmosphere. Human activities (fossil fuel burning, deforestation) have rapidly increased atmospheric CO2 levels.

Global Climate Change

Drivers and Effects

Human activities have led to rapid environmental changes, including global warming and climate change, which affect physical, chemical, and biological processes.

• Global warming: Increase in average global temperature due to greenhouse gases (CO2, CH4, N2O).

• Climate change: Changes in temperature and precipitation patterns, storm frequency, and intensity.

• Weather vs. climate: Weather is short-term and variable; climate is long-term average patterns.

Greenhouse Effect

• Solar energy is trapped by atmospheric compounds, leading to increased global temperatures.

• CO2 levels have risen from 280 ppm to over 417 ppm in recent decades.

Recent Trends and Projections

• 2024 is the hottest year on record; ocean temperatures are at their highest.

• Human population growth and fossil fuel use are major contributors.

• IPCC projects continued temperature rise, with polar regions most affected.

• Changes in precipitation will impact water retention and productivity globally.

Effects on Biodiversity

Impacts of Climate Change

Even small increases in global temperature have significant effects on the biosphere.

• Altered development, physiology, and behavior of organisms.

• Population growth, decline, and evolution.

• Species interactions and community dynamics.

• Energy flow and nutrient cycling in ecosystems.

• Range shifts, phenological changes, evolutionary adaptations, extinctions, ocean acidification, and changes in net primary productivity.

Additional info: For further details, see Table 53.1 and e-text Chapter 53.3.