Mutualism, Commensalism, Neutralism, Amensalism and Allelopathy – Interdependence Among Organisms in an Ecosystem (Senior 3 Biology) | Ganzaa

Interdependence Among Organisms in an Ecosystem

Lessons

Senior 3 · Biology

Interdependence Among Organisms in an Ecosystem

5 lessons

Lesson 5 of 5

Mutualism, Commensalism, Neutralism, Amensalism and Allelopathy

25 min

Lumps on a bean root

Pull up a healthy bean plant carefully and wash the soil from its roots. You will find small, round lumps, pinkish inside when cut open. They are not a disease. Each lump is a home for millions of bacteria, and both the bean and the bacteria gain from living together. Other pairs of species live side by side with very different results: one gains while the other is unaffected, or one is harmed by a chemical the other releases. This lesson completes the map of relationships you began in Lesson 3.

Definition

Symbiosis: a close, long-lasting relationship between individuals of two different species that live together. Mutualism, commensalism and parasitism are the three kinds of symbiosis.

Mutualism (+ +): both species benefit. Example: a bean plant and the Rhizobium bacteria in its root nodules.

Commensalism (+ 0): one species benefits and the other is neither helped nor harmed. Example: an orchid growing on the branch of a forest tree.

Neutralism (0 0): two species live in the same place and neither affects the other.

Amensalism (0 −): one species is harmed and the other is not affected. Example: small plants crushed under the feet of an elephant.

Allelopathy: the release by a plant of chemicals that reduce the germination or growth of other plants nearby. It is a form of amensalism.

Mutualism: both partners gain

Figure 1: Mutualism in a bean root nodule. The plant supplies sugars; the bacteria fix nitrogen gas from the soil air and supply nitrogen compounds.

Legumes and Rhizobium. The bacteria turn nitrogen gas, which plants cannot use, into ammonium compounds that the plant uses to make proteins. The plant gives the bacteria sugars and a protected home. This is why beans, peas and groundnuts grow well in poor soil and leave the soil richer in nitrogen for the next crop. You will meet it again in the nitrogen cycle (Unit 3).
Lichens. A lichen, the crusty grey, green or orange patch on tree bark and rocks, is a fungus and a green alga (or cyanobacterium) living as one. The alga makes food by photosynthesis; the fungus holds water and mineral salts and shelters the alga.
Cattle and rumen microorganisms. Cattle, goats and sheep cannot make enzymes to digest cellulose. Bacteria and protists in the rumen (part of the stomach) digest it for them, and get a warm, food-filled home.
Termites and gut protists. In the same way, protists in a termite's gut digest the wood the termite eats.

Commensalism, neutralism and amensalism

Commensalism. Cattle egrets follow grazing cattle, buffaloes and hippos. As the big animal walks and bites the grass it disturbs grasshoppers and other insects, which the egrets catch. The egret gains a meal; the cow is not affected. Orchids, ferns and mosses growing on the branches of forest trees (epiphytes) gain a place in the light without taking anything from the tree.

Neutralism. A dung beetle and a butterfly in the same Akagera grassland probably have no effect on each other. True neutralism is hard to prove, because two species in one ecosystem are nearly always linked somehow, for example through a shared predator.

Amensalism. Elephants walking through the savanna crush small plants and insects; the elephant is not affected. Tall trees shade the ground so much that some small plants cannot grow beneath them. The mould Penicillium releases penicillin, which kills bacteria around it: this chemical form of amensalism is called antibiosis, and it gave us the first antibiotic.

The relationships compared

Relationship	Effects	Symbiosis?	Example
Mutualism	+ +	yes	bean and Rhizobium; fungus and alga in a lichen
Commensalism	+ 0	yes	orchid on a tree branch; cattle egret and cow
Parasitism	+ −	yes	tapeworm and human; tick and cow
Neutralism	0 0	no	dung beetle and butterfly
Amensalism (including allelopathy and antibiosis)	0 −	no	elephant and trampled plants; Penicillium and bacteria

Allelopathy: chemical effects of eucalyptus

Eucalyptus trees grow fast and are planted all over Rwanda for firewood, charcoal, poles and timber. According to the 2015 National Forest Inventory, most of Rwanda's plantation forest (256,065 hectares) is eucalyptus. Crops planted close to eucalyptus often grow poorly. One reason is competition for water and light, but eucalyptus leaves also release chemicals (phenolic compounds and oils) that reduce the germination and growth of other plants. A class can test this.

Aim: to find out whether eucalyptus leaf extract affects the germination of bean seeds.

Soak 100 g of fresh eucalyptus leaves, cut small, in 1 litre of water for 24 hours and filter it: this is the 100% extract. Dilute it with water to make 50% and 25% extracts.
Line four Petri dishes with filter paper and put 20 bean seeds in each.
Wet the paper with 10 cm³ of distilled water (the control, 0%) or of each extract, and keep the dishes together in the dark at room temperature.
After five days, count the seeds whose root has emerged.

Figure 2: The germination investigation after five days (illustrative results).

Warning

Safety. Eucalyptus oil irritates the eyes and should not be swallowed: wash your hands after handling the leaves and extract. Do not eat seeds used in experiments. Wipe up spills at once.

Worked example 1: analysing the germination results

Calculate the percentage germination for each dish, plot a graph, and state a conclusion.

Formula: $\text{germination} = \frac{\text{seeds germinated}}{\text{seeds sown}} \times 100\%$
0%: $\frac{19}{20} \times 100\% = 95\%$ . 25%: $\frac{15}{20} \times 100\% = 75\%$ . 50%: $\frac{9}{20} \times 100\% = 45\%$ . 100%: $\frac{3}{20} \times 100\% = 15\%$ .
Graph: concentration (the variable you changed) on the x-axis, germination on the y-axis (Figure 3).
Conclusion: the higher the concentration of eucalyptus extract, the lower the germination of bean seeds. The control shows that almost all the seeds were alive, so the drop is caused by the extract.

Figure 3: Germination falls as the concentration of eucalyptus extract rises (illustrative results).

Evaluate: the result agrees with field studies, which found that eucalyptus leaf extracts reduce the growth and yield of crops such as wheat. But a lab dish is not a field: rain washes the chemicals away and soil microbes break them down, so the effect in a real farm may be smaller.

Worked example 2: naming the relationship

Name the relationship in each case and give the +, − or 0 effects. (a) A fungus grows around the roots of a pine tree: it passes mineral salts to the tree and receives sugars. (b) Barnacles attach to the skin of a whale, which is unaffected, and are carried to new feeding areas. (c) A sunflower's roots release a chemical that stops weed seeds germinating nearby, and the sunflower is unaffected.

(a) Both gain: mutualism (+ +). This kind of root fungus is called a mycorrhiza.
(b) Barnacle gains, whale unaffected: commensalism (+ 0).
(c) Weeds harmed by a chemical, sunflower unaffected: allelopathy, a form of amensalism (0 −).

Method: decide the effect on each partner separately, then match the pair of signs to the table.

Ecological Relationships (Amoeba Sisters). Watch the section on symbiotic relationships, and pause to classify each example as mutualism, commensalism or parasitism before the answer is given.

Tip

Memory hook: "mutual friends both gain". Mutualism + +; commensalism + 0 (the "commensal" eats at the same table, com-mensa, without being a nuisance); amensalism 0 − ("a-" = the one who gains nothing). In exams, write the two signs beside the name.

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Practice: symbiosis, amensalism and allelopathy

Define symbiosis and name its three kinds.
Explain how both partners benefit in a bean root nodule.
Give the + / − / 0 effects for commensalism and for amensalism.
Classify: (a) an orchid on a tree branch; (b) Penicillium killing bacteria; (c) a fungus and an alga forming a lichen.
In a repeat of the germination investigation, 12 of 20 seeds germinated in the 25% extract. Calculate the percentage germination.
Why was a dish watered with distilled water included in the investigation?
Suggest two reasons why crops planted right next to a eucalyptus woodlot grow poorly.
A student says the cattle egret and the cow are an example of mutualism. Evaluate this claim.

Show answersHide answers

A close, long-lasting relationship between individuals of two different species living together: mutualism, commensalism and parasitism.
The bacteria fix nitrogen gas into nitrogen compounds that the plant uses to make proteins; the plant gives the bacteria sugars and a protected home in the nodule.
Commensalism: + 0. Amensalism: 0 −.
(a) commensalism; (b) amensalism (antibiosis); (c) mutualism.
$\frac{12}{20} \times 100\% = 60\%$ .
It is the control: it shows how many seeds germinate without extract, so any drop in the other dishes can be blamed on the extract.
Competition for water, light and mineral salts; allelopathic chemicals from the eucalyptus leaves and litter.
Mostly wrong. The egret gains food; the cow gains nothing that has been shown, so the usual classification is commensalism. It would be mutualism only if the egret also helped the cow, for example by removing its parasites, which cattle egrets do not usually do.

Key points

Symbiosis: mutualism (+ +), commensalism (+ 0) and parasitism (+ −).
Neutralism (0 0) and amensalism (0 −) are not symbiosis; allelopathy and antibiosis are chemical amensalism.
Legume–Rhizobium mutualism enriches soil with nitrogen; eucalyptus allelopathy can reduce crop germination.

1Lumps on a bean root

2Mutualism: both partners gain

3Commensalism, neutralism and amensalism

4The relationships compared

5Allelopathy: chemical effects of eucalyptus

Lumps on a bean root

Mutualism: both partners gain

Commensalism, neutralism and amensalism

The relationships compared

Allelopathy: chemical effects of eucalyptus