Ecosystem Energetics
• Limits on primary production
• Relationship between primary and secondary
productivity
• Trophic efficiency
• Nutrient Cycles
Energy flow in ecosystems
Ecosystem energetics - terminology
• Standing crop (=biomass) – amount of
accumulated organic matter found in an
area at a given time [g/m2]
• Productivity – rate at which organic matter
is created by photosynthesis [g/m2/yr]
• Primary productivity – autotrophs
• Secondary - heterotrophs
• Gross versus net primary productivity
Ecological Efficiency
• Ecological efficiency (food chain
efficiency) is the percentage of energy
transferred from one trophic level to the
next:
– range of 5% to 20% is typical, mean = 10%
– to understand this more fully, we must study
the utilization of energy within a trophic level
Intratrophic Energy Transfers
• Intratrophic transfers involve several
components:
– ingestion (energy content of food ingested)
– egestion (energy content of indigestible materials
regurgitated or defecated)
– assimilation (energy content of food digested and
absorbed)
– excretion (energy content of organic wastes)
– respiration (energy consumed for maintenance)
– production (residual energy content for growth and
reproduction)
Fundamental Energy
Relationships
• Components of an animal’s energy budget are
related by:
1) ingested - egested energy = assimilated energy
2) assimilated energy - respiration - excretion =
production
Assimilation Efficiency
• Assimilation efficiency =
assimilation/ingestion
• primarily a function of food quality:
– seeds: 80%
– young vegetation: 60-70%
– plant foods of grazers, browsers: 30-40%
– decaying wood: 15%
– animal foods: 60-90%
Net Production Efficiency
• Net production efficiency =
production/assimilation
• depends largely on metabolic activity:
– birds: <1%
– small mammals: <6%
– sedentary, cold-blooded animals: as much as 75%
Production Efficiency in Plants
• The concept of production efficiency is
somewhat different for plants because plants do
not digest and assimilate food:
– net production efficiency = net production/gross
production, varies between 30% and 85%
– rapidly growing plants in temperate zone have net
production efficiencies of 75-85%; their counterparts
in the tropics are 40-60% efficient
NPP =
Net Primary
Productivity
GPP Gross Primary
Productivity
R
Respiration
Secondary production
by trophic level n
Amt respired by
trophic level n
Amt assimilated (i.e.
absorbed into body)
by trophic level n
Amt ingested by
trophic level n
Amt produced by
trophic level n-1
Efficiency
of energy
transfer
Amt egested as
feces (waste) by
trophic level n
Assimilation
efficiency
70/200
Consumption
efficiency =
200/1000
Production
efficiency =
14/70
Detritus Food Chains
• Ecosystems support two parallel food chains:
– herbivore-based (relatively large animals feed on
leaves, fruits, seeds)
– detritus-based (microorganisms and small animals
consume dead remains of plants and indigestible
excreta of herbivores)
– herbivores consume:
• 1.5-2.5% of net primary production in temperate forests
• 12% in old-field habitats
• 60-99% in plankton communities
Exploitation Efficiency
• When production and consumption are not
balanced, energy may accumulate in the
ecosystem (as organic sediments).
• Exploitation efficiency = ingestion by one
trophic level/production of the trophic level
below it.
• To the extent that exploitation efficiency is
<100%, ecological efficiency = exploitation
efficiency x gross production efficiency.
Energy moves through
ecosystems at different rates.
• Other indices address how rapidly energy
cycles through an ecosystem:
– residence time measures the average time a
packet of energy resides in storage:
• residence time (yr) = energy stored in biomass/net
productivity
– biomass accumulation ratio is a similar index
based on biomass rather than energy:
• biomass accumulation ratio (yr) = biomass/rate of
biomass production
Biomass Accumulation Ratios
• Biomass accumulation ratios become
larger as amount of stored energy
increases:
– humid tropical forests have net production of
1.8 kg/m2/yr and biomass of 43 kg/m2,
yielding biomass accumulation ratio of 23yr
– ratios for forested terrestrial communities are
typically >20 yr
– ratios for planktonic aquatic ecosystems are
<20 days
Biomass Accumulation Ratios
Ecosystem Energetics
• Comparative studies of ecosystem
energetics now exist for various systems.
• Many systems are supported mainly by
autochthonous materials (produced
within system).
• Some ecosystems are subsidized by input
of allochthonous materials (produced
outside system).
Autochthonous versus
Allochthonous Production
• In streams assimilation of energy by
herbivores often exceeds net primary
production - difference represents energy
subsidy.
– autochthonous production dominates in
large rivers, lakes, marine ecosystems
– allochthonous production dominates in small
streams, springs, and caves (100%)
Ecosystem NPP
Energy allocation
Primary productivity limits secondary
productivity
Consumption
efficiency determines
pathways of energy
flow through
ecosystem
Note:
• Detrital food chain accounts for
most biomass produced in a
community
• Grazing plays greatest role in
phytoplankton-based food
chains
Energy loss between trophic levels
General Rules for
Energy Flow through Ecosystems
1) Assimilation efficiency increases at higher
trophic levels
2) Ecological efficiencies average about 10%
Thus, only about 1% of NPP ends up as
production in the third trophic level
Decomposition and Mineralization
• Most material is derived from plants
• Involves:
• Release of chemical energy
• Mineralization (= organic --> inorganic)
• Note immobilization = reverse of mineralization
• Net mineralization rate = mineralization immobilization
Terrestrial communities:
Nutrient sources
•
•
•
•
Weathering of rock (K, P, Ca and many others)
Fixation of CO2 (photosynthesis) and N2
Dryfall (particles in the atmosphere)
Wetfall (snow & rain); contains
– Oxides of S, N
– Aerosols
• particles high in Na, Mg, Cl, S
• produced by evaporation of droplets
– Dust particles from fires, volcanoes
• Ca, K, S
Terrestrial communities:
Nutrient losses
• Release to atmosphere
– CO2 from respiration
– Volatile hydrocarbons from leaves
– Aerosols
– NH3 (decomposition), N2 (denitrification)
• Loss in streamflow
– Dissolved nutrients
– Particles
Oceans
•
•
No outflow
Detritus sinks --> mineralization --> nutrients
end up:
1. Being carried back to surface in upwelling
currents, or
2. Trapped in sediment (e.g., phosphorus: 1% lost
to sediment with each cycling)
CARBON CYCLE
4 PROCESSES MOVE
CARBON THROUGH
ITS CYCLE:
CO2
1) Biological
2) Geochemical
3) Mixed biochemical
4) Human Activity
CO2
NITROGEN CYCLE
N2
in Atmosphere
Nitrogen-containing
nutrients include:
1) Ammonia (NH3)
2) Nitrate (NO3-)
3) Nitrite (NO2-)
4) ORGANISMS NEED
NITROGEN TO MAKE
AMINO ACIDS FOR
BUILDING
PROTEINS!!!
N03NH3
&
N02-
The nitrogen cycle
PHOSPHORUS CYCLE
PHOSPHORUS FORMS PART OF IMPORTANT LIFE-SUSTAINING
MOLECULES (ex. DNA & RNA)
The phosphorus cycle
We’re in the Driver’s Seat - Human Activities
Dominate Many Biogeochemical Cycles