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 anaerobesFermentation—sugars organic acids/alcohol

Decomposers

Microfauna/microflora – <1 m 100 m

Mesofauna – 100 m 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

3. Shredders attack CPOMFeed also on attached microbes

Becomes FPOM

4. Filterers / collectors gather FPOMFeed 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

Flowing water

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 displacementDownstream transport + nutrient cycling =

nutrient spiralingOne cycle = 1. Uptake of an atom from DOM2. Passage through food chain3. 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, forestedStrongly heterotrophicDominant 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