BIOS 3010: Ecology
Lecture 12: Decomposition and Detritivory:
•  Lecture summary:
–  Decomposers &
detritivores:
•  Resources.
•  Characteristics.
•  Model of detritivory.
•  Size of detritivores.
•  Diversity &
abundance.
www.nfi.org.za/inverts/BIG12/dungbeetle.html
www.bbc.co.uk/nature/wildfacts/factfiles/498.shtml
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 1
2. Decomposers and detritivores:
–  Decomposers are saprobes like bacteria & fungi
that feed on dead or dying plant & animal tissue.
–  Detritivores feed on the same material once it has
been fragmented and processed to varying
extents by both decomposers and physical
events.
•  These interactions tend to be very general.
•  Taxonomic origin is usually unimportant.
•  Result in release of nutrients (Fig. 11.2).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 2
3. Resources of decomposers:
–  (1) Dead bodies of animals/carrion (Fig. 11.18)
•  Also manipulation of other interactions such as mites
reducing fly competitors of beetles (Figure 11.17).
–  (2) Feces & other excreted products (Fig. 11.15)
•  Australia was nearly covered with sheep/cow feces
because of a lack of dung beetles!
–  (3) Dead plant material
•  Trees, roots, stems, leaves as standing material or litter,
and ripe fruit separated from the parent (Fig. 11.11).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 3
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4. Resources of decomposers:
•  Resistance of resources to decomposition
increases in the order:
–  sugars < starch < hemicelluloses < pectins and
proteins < cellulose < lignins < suberins < cutins
•  As shown partially in Figure 11.2 for 2 different
ecosystems.
•  Cellulose is difficult to break down.
–  Cellulose catabolism (cellulolysis) requires cellulase enzymes
which most animals don t have.
–  Cellulases in 1 cockroach sp. & a few termite spp.
–  So complex mechanisms have evolved as in Figure 11.12
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 4
5. Differences from other consumers:
•  Decomposers/detritivores eat dead food.
•  Predators and herbivores also eat dead food
after they have caught and killed it.
•  Primary distinction is that:
–  Decomposers/detritivores do not affect the
rate at which their resources are produced
–  But of course, predators and herbivores do.
•  In addition, while mutualists may increase resource
availability, decomposers and detritivores do not
have an influence.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 5
6. A continuous model of detritivory:
•  If we represent:
–  Resource (R) renewal as F(R) or a function of the amount
of resource R
–  P as the number of predators, and
–  a as the efficiency with which individuals find and capture
their food resource
–  Then for exploiters, such as predators, herbivores and
parasites, the rate of resource renewal dR/dt is:
•  dR /dt = F(R) - aP
–  like the Lotka-Volterra predation model dN/dt = rN-aPN
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 6
2
7. A continuous model of detritivory:
–  For mutualists, where M is the number of
mutualists and δ is a measure of mutual benefit
dR/dt is:
dR/dt = F(R) + δM
–  For decomposers and detritivores that have no
influence on resource renewal, dR/dt is:
dR/dt = F(R)
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 7
8. Size classification and biomass
of detritivores:
•  Detritivores and microbivores:
–  Tiny detritivores that feed on bacteria and fungi
rather than larger particulate detritus
–  Their food is often alive!
–  Taxonomically diverse
–  Can be classified by size:
•  Microflora & fauna (<100µm)
•  Mesoflora & fauna (100µm-2mm)
•  Macroflora & fauna (2-20 mm)
(Fig. 11.3).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 8
9. Size classification and biomass
of detritivores
•  Micro-, meso-, and macro-detritivores vary
among biomes with latitude (Figure 11.4).
–  Most macrofauna in the tropics
–  Most microfauna in cold regions
–  Mesofauna dominant in temperate zones.
•  Darwin (1888) estimated that earthworms near
his house formed new soil layers at the rate of
18 cm/30 years and bring up 5.1 Kg soil/m2 each
year.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 9
3
10. Diversity & abundance of
detritivores:
•  Terrestrial:
–  In 1m2 of temperate woodland soil there could be:
•  10 million nematodes and protozoans
•  100,000 springtails (Collembola) and mites (Acari)
•  50,000 other invertebrates
–  In woodlands, microbial decomposition is highest
(Fig. 11.7)
•  Larger detritivores can enhance microbial respiration
and so function as a connected community (Fig. 11.8).
–  Perhaps allantoin excreted by woodlice at high density kills
microbes?
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 10
11. Diversity & abundance of
detritivores:
•  Aquatic:
–  Similarly in freshwater ecosystems, detritivores
are diverse and separate into different guilds
according to feeding methods as:
•  shredders , collector-gatherers , grazer-scrapers ,
and collector-filterers (Fig. 11.5).
–  Together this community breaks down detritus in
a stream as shown in Figure 11.6.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 11
Figure 11.2 (3rd ed.): Decomposition
of oak leaf litter.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 12
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Figure 11.18: Mouse burial by
Necrophorus beetles.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 13
Figure 11.17 (3rd ed.): Influence of mites on
Necrophorus beetle fitness at different soil depths
+ mites
- mites
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 14
BIOS 3010: Ecology
Lecture 12: slide 15
Figure 11.15:
African dung
beetles roll
balls of
mammalian
dung for
larval food
resources
Dr. S. Malcolm
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Figure 11.11: Decomposition of
detritus
❍
Dr. S. Malcolm
land, ● water
BIOS 3010: Ecology
Lecture 12: slide 16
Figure 11.12: Methods of detritivore
digestion of cellulose
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 17
BIOS 3010: Ecology
Lecture 12: slide 18
Figure 11.3:
Sizes of terrestrial
decomposers
Dr. S. Malcolm
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Figure 11.4: Latitudinal and biome
changes in decomposer communities
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 19
BIOS 3010: Ecology
Lecture 12: slide 20
Figure 11.7:
Significance of
microflora in forest
litter decomposition
Dr. S. Malcolm
Figure 11.8 (3rd ed.): Influence of woodlice
on microbial decomposition in microcosms
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 21
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Figure 11.5: Freshwater invertebrate
feeding guilds
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 22
Figure 11.6: Detrital energy flow
in a stream
(---- feces)
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 12: slide 23
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