Ecosystem ecology
Energy Flows
open system
Nutrients cycle
closed system
earth
sun
Productivity and Energy Flow
Thermodynamics: energy cannot be created or destroyed, only transformed
earth
sun
>99% of incoming light is converted to heat
(driving global climate)
<1% is converted to
chemical energy by plants
1
Productivity is higher in the tropics where warm weather and
sufficient rainfall promote plant growth
What happens to that primary productivity?
5th trophic level
Food webs and trophic structure
in a prairie grassland
4th trophic level
(top carnivores)
3rd trophic level
(carnivores)
2nd trophic level
(herbivores)
1st trophic level (plants
producers)
decomposers
2
Diagrams for energy flow through ecosystems (steady state)
respiration
(heat loss) 12,000
6,500
350
10
30
38,000 Plants 8,000 Herbivores 400 Carnivore 20 Top carn.
400
1,500
40
6
<1%
1100
18
,0
00
10
decomposers
1,000
19,140
All energy must be accounted for either in biomass (amounts=kcal/m2; boxes)
or as fluxes transferred to other trophic levels (rates=kcal/m2/yr; arrows)
respiration
(heat loss) 12,000
6,500
350
10
30
38,000 Plants 8,000 Herbivores 400 Carnivore 20 Top carn.
400
1,500
40
6
<1%
1100
18
,0
00
10
decomposers
1,000
19,140
Biomass pyramids
20 top carnivores
40 carnivores
1500 herbivores
400 plants
3
Biomass pyramids - variable shapes
20 top carnivores
40 carnivores
1500 herbivores
400 plants
Energy pyramids (always a pyramid)
Decreasing energy passing to higher trophic levels because much is
lost to respiration at each transfer
Lots of plant production needed to support top carnivores
Limits communities to 4-5 trophic levels
Energy transfer between
trophic levels is not
100% efficient and
much energy is lost as
heat
Biomass pyramids (any shape)
20 top carnivores
40 carnivores
1500 herbivores
400 plants
4
Energy flow between trophic levels
Production
by
herbivores
Respired
Assimilated
Excreted (waste)
Ingested
Not caught (eaten)
Production
by plants
Energy flow between trophic levels : exploitation efficiency
% captured; 200/1000 kcal = 20% efficiency (prey defenses)
Production
by
herbivores
Respired
Assimilated
Excreted (waste)
Ingested
Not caught (eaten)
Production
by plants
}
Exploitation
efficiency
5
Energy flow between trophic levels : assimilation efficiency
% assimilated (digested); 70/200 kcal = 35% efficiency (not all of
the food is digestible, so some energy is lost as waste)
Production
by
herbivores
Respired
Assimilated
Excreted (waste)
Ingested
}
Assimilation
efficiency
Not caught (eaten)
Production
by plants
Energy flow between trophic levels : assimilation efficiency is
higher for animal food than plant food (indigestible cellulose and
lignin)
Carnivores have
assimilation
efficiencies of 90%
Herbivores have
assimilation
efficiencies of
20-60% (lower)
6
Energy flow between trophic levels : production efficiency
(growth and reproduction) / (energy assimilated); conversion of
assimilated food into new biomass; 14/70 kcal = 20%
Production
by
herbivores
Respired
Assimilated
Excreted (waste)
}
Production
efficiency
Ingested
Not caught (eaten)
Production
by plants
Energy flow between trophic levels : production efficiency depends
on metabolic rates of the consumer
Ectotherms (coldblooded) have
higher production
efficiencies of 1040%
Endotherms
(warm-blooded)
have lower
production
efficiencies of <5%
7
Energy flow between trophic levels : trophic efficiency
Overall efficiency of energy transfer between trophic levels (consumer
production)/(prey consumption) or the percent of energy in the prey which is
converted into consumer biomass; 14/1000 kcal = 1.4%
Production
by
herbivores
Respired
Most trophic
efficiencies
are 1-10%
Assimilated
Excreted (waste)
Ingested
Not caught (eaten)
Production
by plants
Population densities of organisms are related to their trophic level
Organisms at higher trophic levels need a larger energy base and
therefore are less abundant
Assuming a 1% trophic efficiency the following energy flows would be
needed to support one top carnivore
snake (top carnivore)
frog (carnivore)
insect herbivore
plants
1
100
10,000
8
Population densities of organisms are related to their trophic level
Herbivores are generally more abundant that carnivores of the same
body size
For a 65kg omnivore
human we predict almost
2 individuals per km2
Estimates for preagricultural humans are
1.5 individuals per km2
Actual human populations
are 44 individuals per km2,
which is only possible
because of unsustainable
agriculture and fishing
Humans are 0.5% of the animal biomass on earth and use 20% of the
global annual production for themselves (intermediate estimate)
(HANPP = human appropriated net primary productivity)
9
Exploitation efficiency (calories expended / calories harvested)
Energy use in modern agriculture
Hunting and
gathering: 1 cal in
metabolism
expended for 10
cals harvested
(=0.1)
Coastal fishing: 1 cal
in metabolism and
fuel expended for 1
cal harvested (=1;
break even)
1890
1940
1980
Exploitation efficiency (calories expended / calories harvested)
Energy use in modern agriculture
Agriculture today: 15
cals in metabolism
and fuel (to run
machinery, make
fertilizers, antibiotics
and pesticides) for 1
cal harvested (=15)
Modern agriculture
must be subsidized
by past primary
productivity (plants
that didn’t
decompose and
formed fossil fuels)
What can you do to
improve efficiency?
1890
1940
1980
10