Ecosystem energetics
Outline:
• Limits on primary production
• Relationship between primary and secondary
productivity
• Trophic efficiency
Readings: Chapters 20
Laws of thermodynamics govern energy flow
Laws of thermodynamics govern energy flow
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
Estimating primary productivity in aquatic ecosystems
Factors limiting primary productivity in
terrestrial ecosystems
•
•
•
•
Temperature
Precipitation
Light
Nutrients
Controls on primary production in terrestrial
ecosystems
Controls on primary production in terrestrial
ecosystems
Controls on primary production in terrestrial
ecosystems
Controls on primary production in terrestrial
ecosystems
Controls on primary production in terrestrial
ecosystems
Primary production as a function of latitude
Despite much variation, there is a general trend of increasing net
primary productivity with decreasing latitude. a), Grassland and
tundra ecosystems. b) Cultivated crops. c) Lakes
Global map of primary productivity
Factors limiting primary productivity in
aquatic ecosystems
• Light
• Nutrients
Controls on primary production in aquatic
ecosystems
Controls on primary production in aquatic
ecosystems
Controls on primary production in aquatic
ecosystems
Global map of primary productivity
Energy allocation
Primary production varies with time
Primary production varies with time
Primary production varies with time
Primary productivity
limits secondary
productivity
Primary productivity limits secondary
productivity
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
Production
efficiency =
14/70
Amt egested as
feces (waste) by
trophic level n
Assimilation
efficiency
70/200
Consumption
efficiency =
200/1000
Efficiency of
production
I = ingested
A = assimilated through
gut wall
W = expelled as waste
product
Of A,
R = respired
P = production
Food chains
Consumption
efficiency determines
pathways of energy
flow through
ecosystem
FOREST
Note:
• Detrital food chain accounts for
most biomass produced in a
community
• LCS plays greatest role in
phytoplankton-based food chains
GRASSLAND
PLANKTON OCEAN
STREAM COMMUNITY
Energy loss between trophic levels
Secondary production
by trophic level n
Amt respired by
trophic level n
Amt assimilated (i.e.
absorbed into body)
by trophic level n
Amt egested as
feces (waste) by
trophic level n
Amt ingested by
trophic level n
Amt produced by
trophic level n-1
Efficiency of
energy transfer
Example: a
herbivore (level n)
feeding on a plant
(level n-1); values =
kilocalories.
Trophic Efficiency
= 0.2*0.35*0.2
= 14/1000
= 0.014
Decomposition and Nutrient cycling
Outline:
• Process of decomposition
– Types of decomposers
– Controls on decomposition
– Decomposition in lakes and
rivers
• Nutrient cycling: generalities
• Nutrient cycles
– Carbon
– Nitrogen
– Phosphorus
Readings: Chapters 21, 22
Decomposition
• Most material = plant
• Involves:
• Release of chemical energy
• Mineralization (= organic --> inorganic)
• Note immobilization = reverse of mineralization
• Net mineralization rate = mineralization immobilization
Decomposition involves a variety of
organisms
• Microfauna & microflora [<100 μm]–
bacteria and fungi; nematodes, protozoa
• Mesafauna [100 μm – 2mm] – mites,
potworms
• Macrofauna [2-20 mm] - millipedes
• Megafauna [> 20 mm]- earthworms, snails
Fungi: microfauna
Mites: mesofauna
Megafauna
Vertebrate scavengers
Consumers of animal carrion
Factors influencing decomposition rates
(highest lignin content)
(lowest lignin content)
Factors influencing decomposition rates
Decomposition of straw
Factors influencing decomposition rates
Factors influencing decomposition rates
Factors influencing decomposition rates
Immobilization vs. mineralization
Decomposition in aquatic environments
Rate of nutrient cycling
Rate of nutrient cycling
Zones of production and decomposition
Nutrient spiraling in rivers
Nutrient spiraling in rivers
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
The Carbon Cycle
Daily variation in CO2
Annual variation in CO2
The nitrogen cycle
The phosphorus cycle
Nitrogen saturation
Nitrogen saturation
For next lecture:
• Please read Chapter 6
81