Lecture Six- Food webs and trophic cascades II

Question 1

What is an ecosystem?

Answer 1

An ecosystem consists of all the organisms living in a community, as well as the abiotic factors with which they interact.
Ecosystems range from a microcosm, such as an aquarium, to a large area such as a lake or forest.

Question 2

What is energy flow and energy cycling?

Answer 2

Regardless of an ecosystem’s size, its dynamics involve two main processes: energy flow and chemical cycling.
Energy flows through ecosystems while matter cycles within them.

Question 3

What is the second law of thermodynamics?

Answer 3

Second law of thermodynamics = every exchange
of energy increases the entropy of the universe.
In an ecosystem, energy conversions are not completely efficient and some energy is always lost as heat.
Entropy is the amount of disorder in a system.

Question 4

What is the law of conservation of mass?

Answer 4

The law of conservation of mass states that matter cannot be created or destroyed.
• Chemical elements are continually recycled within ecosystems.
• E.g. forest ecosystem, most nutrients enter as dust or solutes in rain, then carried away in water.
• Ecosystems are open systems, absorbing energy and mass and releasing heat and waste products.

Question 5

What is primary production?

Answer 5

1. Energy and other limiting factors control primary production in ecosystems.
   • Primary production is the amount of light energy converted to chemical energy by autotrophs during a given time period.
2. Ecosystem energy budgets.
   • Extent of photosynthetic production sets the spending limit for an ecosystem’s energy budget.

Question 6

What is the global energy budget?

Answer 6

• The amount of solar radiation reaching the Earth’s surface limits photosynthetic output of ecosystems.
• Only a small fraction of solar energy actually strikes photosynthetic organisms, and even less is of a usable wavelength.

Question 7

What is gross and primary production?

Answer 7

• Total primary production is known as the ecosystem’s gross primary production (GPP).
• Net primary production (NPP) is GPP minus energy used by primary producers for respiration.
• Only NPP is available to consumers.
• Ecosystems vary greatly in NPP and contribution to the total NPP on Earth.
• Standing crop is the total biomass of photosynthetic autotrophs at a given time.

Tropical rain forests, estuaries and coral reefs are among the most productive ecosystems per unit area.
Marine ecosystems are relatively unproductive per unit area but contribute much to global net primary production because of their volume.

Question 8

What is trophic efficiency?

Answer 8

Trophic efficiency is the percentage of production transferred from one trophic level to the next.
It usually ranges from 5% to 20%.
Trophic efficiency is multiplied over the length of a food chain.
Approximately 0.1% of chemical energy fixed by photosynthesis reaches a tertiary consumer.
A pyramid of net production represents the loss of energy with each transfer in a food chain.

Question 9

What are trophic pyramids?

Answer 9

• In a biomass pyramid, each tier represents the dry weight of all organisms in one trophic level.
• Most biomass pyramids show a sharp decrease at successively higher trophic levels.

Some aquatic ecosystems have upside down trophic pyramids, these are very rare.
Producers (phytoplankton) are consumed so quickly that they are outweighed by primary consumers.
Turnover time is a ratio of the standing crop biomass to production.