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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
