Ecosystems

Page
4

Slide 21

Primary

producers

100 J

1,000,000 J of sunlight

10 J

1,000 J

10,000 J

Primary

consumers

Secondary

consumers

Tertiary

consumers

Slide 22

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

Certain aquatic ecosystems have inverted biomass pyramids: 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.

Slide 23

Pyramids of biomass = standing crop:

(a) Most ecosystems (data from a Florida bog)

Primary producers (phytoplankton)

(b) Some aquatic ecosystems (data from the English Channel)

Trophic level

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

Trophic level

Primary consumers (zooplankton)

Dry mass

(g/m2)

Dry mass

(g/m2)

1.5

11

37

809

21

4

Slide 24

Dynamics of energy flow in ecosystems have important implications for the human population.

Eating meat is a relatively inefficient way of tapping photosynthetic production.

Worldwide agriculture could feed many more people if humans ate only plant material.

Most terrestrial ecosystems have large standing crops despite the large numbers of herbivores.

Slide 25

The green world hypothesis proposes several factors that keep herbivores in check:

Plant defenses

Limited availability of essential nutrients

Abiotic factors

Intraspecific competition

Interspecific interactions

Slide 26

Biological and geochemical processes cycle nutrients between organic and inorganic parts of an ecosystem

Life depends on recycling chemical elements.

Nutrient circuits in ecosystems involve biotic and abiotic components and are often called biogeochemical cycles.

Slide 27

Biogeochemical Cycles

Gaseous carbon, oxygen, sulfur, and nitrogen occur in the atmosphere and cycle globally.

Less mobile elements such as phosphorus, potassium, and calcium cycle on a more local level.

A model of nutrient cycling includes main reservoirs of elements and processes that transfer elements between reservoirs.