Chapter 7
Decomposers and
Decomposition
Decomposition?
Decomposition
Decomposition—breakdown of chemical bonds
formed during the construction of plant and
animal tissue.
Organisms that feed on dead organic matter or
detritus
Microbial decomposers—bacteria and fungi
Detritivores—animals that feed on dead material
Carbon
Carbon sequestration
• CO2 vs. organic matter
• Forests vs. barren land
• Atmosphere vs. biomass
Stages of decomposition
Leaching—loss of soluble
sugars/dissolved compounds
Fragmentation—reduction into smaller
particles
physical/chemical
fragmentation
Energy processing
Energy and nutrients from organic
compounds
oxidation of carbohydrates—respiration
Mineralization—organic  inorganic
Immobilization—inorganic  organic
Decomposers
Groups based on size
Microflora—most common decomposers
bacteria—animal material
fungi—plant material
Aerobic—respiration
Anaerobic—facultative/obligate anaerobes
Fermentation—sugars organic acids/alcohol
Decomposers
Microfauna/microflora –
<1 mm  100 mm
Mesofauna –
100 mm  2mm
Macrofauna –
2mm  20 mm
Megafauna –
20 mm  64 mm
Microbivores– feed on bacteria and fungi
Food Quality
Energy and nutrient source
Litter—dead plant material
Quality related to chemical bonds/structure
simple sugars vs. complex carbohydrates
Lignin—complex class of carbohydrates
–little net gain of energy for decomposers
Rate of decomposition
Inverse relationship between rate and
lignin content
Quality influences feeding of large
detritivores
Aquatic environments
Phytoplankton—low lignin content
Vascular plants—high lignin content
O2 dependent
Low O2 –
absence of fungi
Animal matter
Chemical breakdown easier than plants
Flesh consumed by scavengers
–70% decomposed by
bacteria and arthropods
(maggots)
–Temperature dependent
Fecal matter
Mostly decomposed
Herbivores—partially digested organic
matter
Specialized detririvores’ larvae incubate
and feed
Tumblebugs—incubate larvae
Physical influence
Temperature and moisture—
Influence rate of decomposition
Decomposition highest in warm/wet
climates
Temperature parallels
CO2 release
Nutrients
Nitrogen  nutrient value
Organisms require N for growth during
mineralization
Mineralization and immobilization taking
place simultaneously
Net mineralization rate
Stages of nutrient
concentration
• Water soluble compounds leached
– Dependent upon soil moisture
• N increases—immobilization from other
sources
• As C quality declines—
net release of N
• Dependent upon original
nutrient content
Aquatic decomposition
Similar to terrestrial ecosystems
Influenced by abundance of water
More stable environment favors
decomposition
More accessibility
to detritivores
Aquatic systems
Particulate organic matter (POM)
Coarse particulate organic matter (CPOM)
Fine particulate organic matter (FPOM)
Water depth determines organic makeup
Benthic organic matter –bacteria
Aerobic vs. anaerobic
Dissolved organic matter (DOM)
Aquatic sources
• DOM readily available
– Sources—algae, zooplankton
– Death of phyto/zooplankton
• Bacteria concentrate DOM
• Mineralization and immobilization of
nutrients
• Excretion of exudates and feces
Organic matter processing
1. Physical mechanism
•
•
Water soaked leaves sink
5 – 30% organic
matter leached
2. Biological mechanism
•
•
•
Covered with
bacteria & fungi
CPOM & FPOM
Degrade cellulose
and metabolize
lignin
Flowing water
3. Shredders attack CPOM
Feed also on attached microbes
Becomes FPOM
4. Filterers / collectors gather FPOM
Feed also on attached microbes
5. Grazers feed on algal coatings
“leftovers” enter stream as FPOM
6. Gougers feed on woody debris
7. Predators feed on all the above
Nutrient passes from water column
plants  consumer  another
consumer  poop = nutrient cycling
• Downstream flow = new dimension
• Physical retention
– Storage in wood detritus
– Leaf sediments
– Beds of macrophytes
• Biological retention
– Uptake and storage in plant/animal tissue
Recycling, retention &
downstream displacement
Downstream transport + nutrient cycling =
nutrient spiraling
One cycle =
1. Uptake of an atom from DOM
2. Passage through food chain
3. Return to water for reuse
• Spiraling = distance of
one cycle
–
–
shorter cycle = tighter spiral
longer cycle = more open spiral
River Continuum Concept
From headwaters to mouth 
continuum of changes in
conditions
1. Headwater streams
(1-3)
Swift, cold, forested
Strongly heterotrophic
Dominant organisms–
–
–
Shredders – CPOM
Collectors – FPOM
Midorder streams (4-6)
•
•
•
•
•
Riparian vegetation important
Canopy opens  primary production
Temperature increases / current slows
Primary production > community
respiration
Dominant organisms
– Collectors – FPOM
– Grazers – algae &
macrophytes
Higher order streams (6 – 10)
•
•
•
Channel wider & deeper
Volume of flow increases
Autotrophic production decreases
– Shift back to heterotrophy
•
Energy from FPOM
– Utilized by bottom dwellers
•
Phytoplankton &
zooplankton population
minimal