Ecosystem Productivity • Three hundred trout are needed to support one man for a year. • The trout, in turn, must consume 90,000 frogs, that must consume 27 million grasshoppers that live off of 1,000 tons of grass.
Ecosystem Productivity • The amount of energy available in an ecosystem determines how much life it can support
Ecosystem Productivity • Gross Primary Productivity (GPP) • Total amount of energy that plants capture and assimilate in a given period of time • Net Primary Productivity (NPP) • Plant growth per unit area per time • Represents the rate at which organic material is actually incorporated into the plant tissue for growth • NPP= GPP - respiration • Only NPP is available as food to organisms • Units: kg C/m2/day or g C/cm2/day
NPP Examples • Example: Two ecosystems Temperate forest: Primary producers = 1,500 g / m2 Desert: Primary producers = 100 g / m2 Food webs very complex, more tertiary consumers, some quaternary. Food webs very simple, very few tertiary consumers
Human Impact on NPP • Humans consume more of earth’s resources that any other animal • Humans represent 0.5% of land-based biomass • Humans use 32% of land-based NPP! • This may contribute to loss of species (extinction) • Humans’ high consumption represents a threat to planet’s ability to support both human and non-human inhabitants
SAMPLE NPP Calculation • • NPP = GPP -R • • %Efficiency = (output / input) x 100 • • % Efficiency of Photosynthesis = (NPP / insolation energy) x 100. • • 102 cm = 1m • In a rice paddy in Southeast Asia the insolation energy is 6.0 x 106 cal/m2/day. The gross productivity of the rice is 0.012 g / cm2 /day and the respiration loss is 20%. One gram of rice is equal to 1000 calories. • A) Calculate the net primary productivity of the rice. • B) Find the efficiency of photosynthesis by the rice.
Energy Transfer Efficiency and Trophic Pyramids • Biomass- The energy in an ecosystem is measured in terms of biomass. • Standing crop- The amount of biomass present in an ecosystem at a particular time. • Ecological efficiency- The proportion of consumed energy that can be passed from one trophic level to another. • Trophic pyramid- The representation of the distribution of biomass among trophic levels.
Ecological Pyramids • Graphically represent the relative energy value of each trophic level • Important feature is that large amount of energy are lost between trophic levels to heat • Three main types • Pyramid of numbers • Pyramid of biomass • Pyramid of energy
Pyramid of Numbers • Illustrates the number of organisms at each trophic level • Usually, organisms at the base of the pyramid are more numerous • Fewer organisms occupy each successive level-WHY?
Pyramid of Biomass • Biomass: measure of the total amount of living material • Biomass indicates the amount of fixed energy at a given time
Pyramid of Energy • Illustrates how much energy is present at each trophic level and how much is transferred to the next level • Most energy dissipates between trophic levels • Explains why there are so few trophic levels • Energy levels get too low to support life • 10% RULE
Why the World Should Be Vegetarians? That is 10 x more biomass available for (starving) humans
1 kg soybeans = 2.5 times calories 1 kg meat • 1 acre soybeans produces 25 times more calories when used for soy than beef production
13.5 kg coyote must range ~12 ha to subsist (30 acres). such as . . . Tertiary consumers must range over large areas to obtain enough energy to subsist. Desert Biomass Pyramid Tertiary consumers = 0.1 g / m2 Secondary consumers = 1.0 g / m2 Primary consumers = 10 g / m2 Primary producers = 100 g / m2
Temperate Forest Biomass Pyramid 13.5 kg coyote only needs ~1 ha to subsist (2.5 acres). Tertiary consumers = 1.5 g / m2 Secondary consumers = 15 g / m2 Primary consumers = 150 g / m2 Primary producers = 1,500 g / m2 Also, possibility of quaternary consumers, like bears.