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

What is an 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 to reproduce.

• Energy enters an ecosystem through the activity of primary producers (autotrophs), which convert energy from the environment into organic material, or biomass.

Primary Producers

• Autotrophs are self-feeding organisms with adaptations for converting energy sources (such as sunlight or inorganic chemicals) into organic molecules.

• In terrestrial environments, primary producers are mainly plants, algae, and photosynthetic bacteria.

• In aquatic environments, primary producers include phytoplankton and chemosynthetic bacteria that use chemical energy from inorganic compounds (e.g., methane, hydrogen sulfide).

Importance of Food: Energy and Elements

Why is Food Important?

• Energy: Food contains potential energy stored in molecular bonds of carbohydrates, fats, and proteins.

• Elements: Essential elements such as carbon, nitrogen, phosphorus, and calcium serve as building blocks for new cellular growth and function (e.g., amino acids, nucleic acids, polysaccharides).

Example: Glucose (C6H12O6) is a carbohydrate that provides energy and carbon skeletons for biosynthesis.

Flow of Energy in Ecosystems

Primary Productivity

Primary producers transform energy from sunlight or inorganic compounds into chemical energy stored in organic molecules. This process is measured as gross primary productivity (GPP), the total amount of chemical energy produced in a given area and time.

• Producers use GPP for:

  • Cellular respiration: Producing ATP for metabolic processes.

  • Growth and reproduction: The remaining energy after respiration is used for building new biomass, called net primary productivity (NPP).

Formula:

where is the energy used in respiration.

Energy Transfer Efficiency

• Energy transfer from sunlight to chemical energy by primary producers is very inefficient (~0.8%).

• Most energy is lost as heat or waste during metabolic processes, in accordance with the laws of thermodynamics.

Trophic Structure in Ecosystems

Trophic Levels and Food Chains

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

• Primary consumers (herbivores) eat primary producers.

• Secondary consumers (carnivores) eat primary consumers.

• Decomposers (detritivores) feed on remains or waste, recycling nutrients back to producers.

Organisms that obtain energy from the same type of source occupy the same trophic level.

Food Webs

• Most consumers feed at multiple trophic levels, forming complex food webs rather than simple food chains.

• Food webs provide a more complete representation of energy and nutrient flow in ecosystems.

Energy Transfer Between Trophic Levels

• Only about 10% of the energy from one trophic level is transferred to the next; the rest is lost as heat, waste, or used for metabolism.

• This inefficiency limits the number of trophic levels in an ecosystem and constrains population sizes at higher levels.

Example: A vegetarian diet (primary consumer level) can support more human biomass than a carnivorous diet (secondary/tertiary consumer level).

Biomagnification

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

• This process is called biomagnification and can have serious ecological and health impacts.

Nutrient Cycling in Ecosystems

Biogeochemical Cycles

Nutrients and elements (C, N, P, S, Ca) cycle between biotic (living) and abiotic (non-living) components of ecosystems. The path an element takes is called its biogeochemical cycle.

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

• Rates of decomposition and nutrient cycling vary with climate (temperature, moisture).

Sources and Losses of Nutrients

• Nutrients can be lost from ecosystems by leaching, erosion, or migration, and gained through weathering of rocks or atmospheric inputs.

• Human activities can accelerate nutrient loss or introduce excess nutrients, disrupting natural cycles.

Major Global Biogeochemical Cycles

Water Cycle

• Major reservoirs: Oceans, lakes, atmosphere, groundwater.

• Processes: Evaporation, precipitation, transpiration, percolation.

• Human impacts: Urbanization reduces groundwater recharge; deforestation increases runoff and nutrient loss.

Nitrogen and Phosphorus Cycles

• Nitrogen: Major reservoir is the atmosphere (N2 gas, ~78%). Enters ecosystems via nitrogen fixation by bacteria, lightning, and industrial processes.

• Phosphorus: Major reservoir is rocks and sediments. Enters ecosystems through weathering.

• Human impacts: Fertilizer use and fossil fuel burning cause nutrient imbalances, eutrophication, and dead zones in aquatic systems.

Carbon Cycle

• Major reservoirs: Oceans, atmosphere, fossil fuels, sedimentary rocks.

• Processes: Photosynthesis removes CO2 from the atmosphere; respiration and combustion return it.

• Human impacts: Deforestation and fossil fuel burning have rapidly increased atmospheric CO2, driving climate change.

Global Climate Change

Greenhouse Effect and Global Warming

• Greenhouse gases (CO2, CH4, N2O, H2O vapor) trap heat in the atmosphere, raising Earth's average temperature.

• Since the Industrial Revolution, atmospheric CO2 has risen from ~280 ppm to over 417 ppm, causing measurable global warming.

Climate vs. Weather

• Weather: Short-term, local atmospheric conditions.

• Climate: Long-term average patterns of temperature and precipitation.

Impacts of Climate Change

• Rising global temperatures, altered precipitation patterns, and increased frequency of extreme weather events.

• Effects on biodiversity include changes in development, physiology, and behavior of organisms; population growth or decline; altered species interactions; and shifts in community dynamics.

• Other impacts: range shifts, phenological changes, evolutionary adaptations, extinctions, ocean acidification, and changes in net primary productivity.

Table: Comparison of Major Biogeochemical Cycles

Additional info: The notes above expand on the provided slides by including definitions, examples, and a summary table for biogeochemical cycles, as well as clarifying the distinction between weather and climate and the mechanisms of biomagnification.