BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 1
BIOS 6150: Ecology Dr. Stephen Malcolm, Department of Biological Sciences • Week 9: Decomposers,
Detritivores & Mutualists. • Lecture summary:
• Decomposition & detritivory: • Examples & resources. • Comparisons. • Model of detritivory.
• Mutualism: • Non-symbiotic. • Symbiotic.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 2
2. Decomposers and detritivores:
• Decomposers are saprobes like bacteria and fungi that feed on dead or dying plant and animal tissues.
• Detritivores feed on the same material once it has been fragmented and processed to varying extents by both these decomposers and physical events.
• Interactions tend to be very general. • Taxonomic origin usually unimportant. • Result in release of nutrients (Fig. 11.2).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 3
3. Resources include:
• 1. Dead bodies of animals: • carrion (Fig. 11.18)
• 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 • Litter, and ripe fruit separated from the parent
• Fig. 11.11.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 4
4. Resource decomposition rates:
• Resistance of resources to decomposition increases in the order:
• sugars < starch < hemicelluloses < pectins and proteins < cellulose < lignins < suberins < cutins
• Shown partially in Fig.11.2 for 2 different ecosystems.
• Cellulose is difficult to break down: • Cellulose catabolism (cellulolysis) requires cellulase
enzymes which most animals don’t have: • 1 cockroach & a few termites.
• Complex mechanisms have evolved as in Fig. 11.12.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 5
5. Differences from other consumers:
• Although predators and herbivores also eat dead food after they have caught and killed it, the primary distinction between these consumers and decomposers/ detritivores is that the latter 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.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 6
6. A continuous model of detritivory:
• Represent resource (R) renewal as F(R). • P as the number of predators. • a as the efficiency with which individuals find and capture
their food resource. • For exploiters, such as predators, herbivores and
parasites, the rate of resource renewal dR/dt is: dR/dt = F(R) - aP
• for mutualists, where M is the number of mutualists and δ is a measure of mutual benefit dR/dt is:
dR/dt = F(R) + δ M • for decomposers and detritivores that have no influence
on resource renewal, dR/dt is: dR/dt = F(R)
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 7
7. Size classification and biomass of detritivores:
• Detritivores and microbivores (tiny detritivores that feed on bacteria and fungi rather than larger particulate detritus - but their food is often alive!).
• Taxonomically diverse and can be classified by size from:
• microfauna (<100µm) through, • mesofauna (100µm-2mm) to, • macrofauna (2-20 mm) (Fig. 11.3).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 8
8. Distributions of detritivore size classes:
• The relative distribution of micro- meso- and macro-detritivores among biomes related to temperature, rainfall and latitude is shown in Fig 11.4:
• Most macrofauna in 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 5.1Kg soil/m2.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 9
9. Diversity & abundance of detritivores:
• 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), but larger detritivores can enhance microbial respiration and so the different species function as a connected community (Fig. 11.8).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 10
10. Diversity & abundance of detritivores:
• In freshwater ecosystems, detritivores are also diverse.
• Different “guilds” according to feeding methods:
• “shredders”, “collecto-gatherers”, “grazer- scrapers”, and “collecto-filterers” (Fig. 11.5).
• Together this community breaks down detritus in a stream (Fig. 11.6).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 11
11. Mutualism:
• Mutualism is an interaction in which both partners benefit:
• “… individuals in a population of each mutualist species grow, survive and reproduce at higher rates when in presence of individuals of the other species”
• Note: it is not a “cosy” relationship - each species acts completely selfishly.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 12
12. Mutualism and symbiosis:
• Symbiosis just means "living together" in close association (excluding parasitism).
• Mutualism is a special kind of symbiosis, but mutualists don’t have to be symbionts to benefit each other.
• So mutualisms can be either symbiotic or non-symbiotic.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 13
13. Abundance of mutualists:
• Most of the world's biomass is made of mutualists:
• Most plants, coral reefs, pollinators. • Nonsymbiotic mutualisms are common:
• E.g. cleaner fish, ants tending aphids, or pollinator-flower interactions.
• Symbiotic mutualisms also common: • Include fungus-alga associations in lichens
(Fig. 13.21), fungus-plant associations in mycorrhizae, or animal-alga associations such as the flatworm Convoluta roscoffensis.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 14
14. Nonsymbiotic mutualisms:
• Bull's horn acacia and Pseudomyrmex ants: • Figs. 13.2 & 13.3 (4th)
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 15
15. Nonsymbiotic mutualisms:
• shrimps and gobiid fish • Fig. 13.3 (3rd)
• clown fish & anemones • cleaner fish & customers • honey guide and ratel
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 16
16. Nonsymbiotic mutualisms:
• Defense mutualisms: • E.g. Müllerian
mimicry in heliconiid butterflies:
• Eltringham (1916), from cover of Futuyma & Slatkin (1983) Coevolution.
• Group defense: • E.g. musk oxen or
sawflies.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 17
17. Agricultural/domestic mutualists:
• Are domestic crops and domestic animals examples of mutualisms with man?
• Is your dog or cat a mutualist? • If there are many more of a species than
there would have been without the association it must be a mutualism!
• “Farming” also occurs in termites and ants where they “tend” aphids and butterfly larvae and fungus gardens (Fig. 13.5).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 18
18. Fruit dispersal and pollination:
• Fruit dispersal is mostly a generalist phenomenon Fig. 13.7.
• Pollination: • Charles Darwin was fascinated by pollination and he
described the specialized floral structure of the Madagascar star orchid in 1859 with nectar tubes approx. 30 cm long.
• He suggested that a pollinator must exist with an appropriately long proboscis and 40 years later a hawkmoth with a 25cm proboscis was found: • see Fig. 8.5 Howe & Westley 1988 for floral diversity in
relation to pollinators & Fig. 7.7 for fig wasp mutualism.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 19
19. Symbiotic mutualisms:
• Degrees of symbiotic association. • Fig. 13.10.
• Such a range of association dependence implies that closer associations might benefit the interactants:
• Greater stability for the symbiont or the opportunity to control environmental conditions through the association.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 20
20. Gut mutualists:
• Gut inhabitants in plant-feeding vertebrates and invertebrates:
• Problem is coping with cellulose - a very hard to digest polysaccharide.
• Many animals have solved the problem by hosting bacterial microcosms in their guts.
• Ruminants (deer, cattle and antelope) have a 4-chambered stomach of which the rumen harbors a complete ecological community of protozoa and bacteria that compete, predate and cooperate through mutualisms, all driven by cellulose.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 21
21. Termite and aphid mutualists:
• Termites are “insect deer” with their own microflora that digest cellulose:
• Figs 13.11 & 13.14 of termite gut flora. • Aphids also have tightly associated
symbiotic mutualists: • Fig. 13.12.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 22
22. Mycorrhizae:
• Sheathing ectomycorrhizae and vesicular arbuscular (VA) endomycorrhizae:
• Found in majority of plant species (Figs 13.15 & 13.17). • Clearly benefit plants (Fig. 13.18) by providing P, N & Ca.
• Ectomycorrhizae: • Occur as a sheath most often on roots of trees. • Can be cultured in isolation from their hosts • Require soluble carbohydrates as carbon resource from
host (not cellulose like free-living fungi). • VA endomycorrhizae:
• Extremely widespread and penetrate host cells. • Makes them very difficult to culture.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 23
23. Nitrogen fixation:
• Bacteria associated with roots of some plants can fix atmospheric nitrogen and make it available to the plant:
• Fig. 13.19. • Benefits plant and influences outcome of
other ecological processes: • Fig. 13.21.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 24
24. Postscript on mutualisms:
• Ultimate symbiotic mutualism may be evolution of the eukaryotic cell.
• “tit-for-tat” • Axelrod & Hamilton demonstrated
theoretically the increased stability generated from cooperation.
• Sex may have evolved through parasitism that lead to cooperation and mutualism because of mutual benefit.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 25
Figure 11.2 (3rd). Release of resources during decomposition of oak leaves.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 26
Figure 11.18 (4th). Mouse burial by Necrophorus beetles.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 27
Figure 11.15 (4th). African dung beetle.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 28
Figure 11.11 (4th). Detritus decomposition rates.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 29
Figure 11.12 (4th). Mechanisms of cellulose digestion.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 30
Figure 11.3 (4th). Sizes of decomposers.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 31
Figure 11.4 (4th). Faunal variation of decomposers among biomes.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 32
Figure 11.5 (4th). Freshwater invertebrate feeding guilds.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 33
Figure 11.6 (4th). Energy flow in a stream.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 34
Figure 11.7 (4th). Forest litter decomposition.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 35
Figure 11.8 (3rd). Influence of isopods on microbial breakdown of leaf litter.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 36
Figure 13.21 (3rd). Lichen structure.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 37
Figure 13.5 (3rd). Feeding by Atta ants at their fungus garden.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 38
Figure 13.7 (3rd). • Diet
breadth of fruit feeders in Malaysia.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 39
Figure 8.5 (Howe & Westley, 1988). Floral diversity and pollinators.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 40
Figure 7.7 (Howe & Westley, 1988). Fig wasp mutualism.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 41
Figure 13.10 (3rd). • Degrees of
symbiotic association.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists
Figs 13.11 (4th) &13.14 (3rd). Symbiotic mutualists in termite intestines.
E – endospore-forming bacteria
S – spirochaetes Others are anaerobic,
flagellate protozoa
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 43
Figure 13.12 (4th). Congruent phylogenies of aphids and their bacterial endosymbionts.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 44
Figure 13.15 (4th). Sheathing ectomycorrhiza of pine roots.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 45
Figure 13.17 (3rd). Vesicular arbuscular mycorrhiza.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 46
Figure 13.18 (3rd). Effect of mycorrhizae on phosphate concentration in leek roots.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 47
Figure 13.19 (4th). Rhizobial bacteria in root nodules of legume roots.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 9: Decomposers, detritivores & mutualists Slide - 48
Figure 13.21 (4th). Influence of rhizobia on plant performance and competition.
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