Ecosystems

Chapter 52

Energy and chemicals flow through ecosystems

52.2 Energy Flow and
Ecosystem Energetics

Ecosystems receive a steady input of energy from an external source, which in virtually all cases is the sun

Photosynthesis converts less than 1% of the solar energy that arrives at Earth’s surface into chemical energy

Globally, primary producers create more than 150 billion metric tons of new biological material each year

Some of the solar energy that producers convert to chemical energy is transferred to consumers at higher trophic levels

Primary Productivity

The rate at which producers convert solar energy to chemical energy is an ecosystem’s gross primary productivity

After deducting the energy producers use for maintenance, the remaining chemical energy is the ecosystem’s net primary productivity

In most ecosystems, net primary productivity is 50% to 90% of gross primary productivity

Variation in Net Primary Productivity

Standing Crop Biomass
and Net Primary Productivity

Biomass and Net Primary Productivity

Energy Transfer

Net primary productivity ultimately supports all consumers in grazing and detrital food webs

Energy from primary producers that is stored in consumer biomass is secondary productivity

Energy is lost from the ecosystem every time it flows from one trophic level to another:

Some energy is used for maintenance or locomotion

No biochemical reaction is 100% efficient

Energy Flow is Inefficient 5-20%

Types of Pyramids

Pyramids of energy typically have wide bases and narrow tops – energy at each trophic level is reduced about 90%

Pyramids of biomass are proportional to the chemical energy temporarily stored there

In terrestrial ecosystems, the base is generally wide

Aquatic ecosystems sometimes exhibit inverted pyramids of biomass due to high turnover rates of phytoplankton

Pyramid of Energy: Silver Springs

Pyramids of Biomass

Trophic Cascades

• Primary productivity can be indirectly regulated by the consumers above it on the food chain through predator-prey effects called a trophic cascade

• Example: Cordgrass in a salt marsh

• Periwinkle snails preferentially eat cordgrass

• When predators (turtles and crabs) reduce snail populations, cordgrass grows abundantly

• When predators are removed, snail populations grow rapidly and virtually eliminate the cordgrass

Experimental Research:
Trophic Cascade in Salt Marshes

Experimental Research:
Trophic Cascade in Salt Marshes

Trophic Cascade

Consumers can influence primary productivity through food preferences (top-down regulation)

Producers can influence consumers by limiting available biomass (bottom-up regulation)

52.3 Nutrient Cycling in Ecosystems

Producers require carbon, hydrogen, oxygen, nitrogen, phosphorus, and other minerals

Earth is essentially a closed system with respect to matter – it already contains virtually all the nutrients that will ever be available for biological systems

Biogeochemical cycles constantly circulate nutrient ions or molecules between the abiotic environment and living organisms

Biogeochemical Cycling in Ecosystems

Generalized Compartment Model

Nutrients circulate in between living organisms and nonliving reservoirs

Nutrients accumulate in four compartments:

Available organic, available inorganic, unavailable organic, and unavailable inorganic

Nutrients move rapidly between available compartments, slowly in unavailable compartments

The Water Cycle

Volume of Water Reservoirs

The Carbon Cycle

Carbon in Major Reservoirs and Carbon Movement Between Reservoirs

The Nitrogen Cycle (Terrestrial)

Processes that Influence
Nitrogen Cycling

The Phosphorus Cycle