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Transcript
Arrows show direction of energy flow from organism consumed to consumer.
Hypothesis:
more productivity à more trophic levels
(Bottom-Up Control)
But, why does biomass decrease at higher levels?
(E Trophic LevelN+1) / (E Trophic LevelN) = ~10%
1) Energy Sources: Energy in ecosystems originates
with primary production by autotrophs.
2) Energy Flow: Food webs and pyramids describe energy flow.
The influence of an organism on the movement of energy and
nutrients through an ecosystem is determined by the type
of food it consumes, and by what consumes it.
3) Global patterns in productivity indicate abiotic constraints.
4) Net primary productivity is also constrained by biotic factors:
top-down versus bottom-up control.
Terrestrial NPP is highest in the wet/warm tropics due to climate.
Marine NPP is highest along the coasts due to nutrient inputs
from the coasts and from upwelling ocean currents.
Net primary productivity is constrained by
both abiotic and biotic factors.
Nutrients:
Nutrient availability can affect local ecosystem productivity:
N, P, K, Ca, Fe, Mg
Species Composition:
Species vary in their capacity to respond to abiotic conditions
and resources (e.g., fertilizer).
1) Energy Sources: Energy in ecosystems originates
with primary production by autotrophs.
2) Energy Flow: Food webs and pyramids describe energy flow.
The influence of an organism on the movement of energy and
nutrients through an ecosystem is determined by the type
of food it consumes, and by what consumes it.
3) Global patterns in productivity indicate abiotic constraints.
4) Net primary productivity is also constrained by biotic factors:
top-down versus bottom-up control.
Top-Down Control and Trophic Cascades
HSS = Hairston, Smith, Slobodkin (1960)
Top-Down Control
Predators limit prey (herbivore) populations
“Green World” Hypothesis
Trophic cascades (top down control) have been described mostly
in (simple) aquatic (freshwater and marine) ecosystems.
Why?
Terrestrial: ~5-10% of NPP
consumed by herbivores;
most becomes detritus.
Aquatic: 35-40% NPP
consumed by herbivores;
much less becomes
detritus.
The amount of energy transferred from one trophic level to the next
depends on food quality, consumer abundance, and physiology.
Alternative hypotheses for lack of trophic cascades on land…
Terrestrial autotrophs have better defenses against herbivory such as
secondary compounds and structural defenses like spines.
(Alternative Hypothesis 1)
Terrestrial plants have nutrient-poor structural materials such as
stems and wood, which are typically absent in aquatic autotrophs.
(Alternative Hypothesis 2)
1) Sources: Nutrients enter ecosystems through the chemical
breakdown of minerals in rocks and through fixation of
gases in the atmosphere.
2) Transformations: Chemical and biological transformations in the
ecosystem alter the chemical form and supply of nutrients.
3) Controls: Transformation processes can be constrained by
climate and chemical composition.
All the organisms in a given area as well as the
physical environment in which they live;
an ecosystem can include one or more communities.
The path an element
takes as it moves from
abiotic pools through
producers and consumers
and back to abiotic pools.
All organisms share
similarities in their nutrient
requirements.
Variation reflects mode of
energy acquisition, mobility,
and thermal physiology.
Based on this table, why
do herbivores need to
consume more food than
carnivores?