bios 6150: ecology - western michigan...

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.


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).


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.


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.


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.


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)


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).


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.


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).


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).


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.


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.


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.


14. Nonsymbiotic mutualisms:

•  Bull's horn acacia and Pseudomyrmex ants: •  Figs. 13.2 & 13.3 (4th)



•  shrimps and gobiid fish •  Fig. 13.3 (3rd)

•  clown fish & anemones •  cleaner fish & customers •  honey guide and ratel



•  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.


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).


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.


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.


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.


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.


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.


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.


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.


Figure 11.2 (3rd). Release of resources during decomposition of oak leaves.


Figure 11.18 (4th). Mouse burial by Necrophorus beetles.


Figure 11.15 (4th). African dung beetle.


Figure 11.11 (4th). Detritus decomposition rates.


Figure 11.12 (4th). Mechanisms of cellulose digestion.


Figure 11.3 (4th). Sizes of decomposers.


Figure 11.4 (4th). Faunal variation of decomposers among biomes.


Figure 11.5 (4th). Freshwater invertebrate feeding guilds.


Figure 11.6 (4th). Energy flow in a stream.


Figure 11.7 (4th). Forest litter decomposition.


Figure 11.8 (3rd). Influence of isopods on microbial breakdown of leaf litter.


Figure 13.21 (3rd). Lichen structure.


Figure 13.5 (3rd). Feeding by Atta ants at their fungus garden.


Figure 13.7 (3rd). •  Diet

breadth of fruit feeders in Malaysia.


Figure 8.5 (Howe & Westley, 1988). Floral diversity and pollinators.


Figure 7.7 (Howe & Westley, 1988). Fig wasp mutualism.


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


Figure 13.12 (4th). Congruent phylogenies of aphids and their bacterial endosymbionts.


Figure 13.15 (4th). Sheathing ectomycorrhiza of pine roots.


Figure 13.17 (3rd). Vesicular arbuscular mycorrhiza.


Figure 13.18 (3rd). Effect of mycorrhizae on phosphate concentration in leek roots.


Figure 13.19 (4th). Rhizobial bacteria in root nodules of legume roots.


Figure 13.21 (4th). Influence of rhizobia on plant performance and competition.

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