Maarg Manthan · Topic 3.6

Interaction Between Species

Environment & Ecology › Population Ecology and Species Interactions · Topic 3.6

Species in a community interact in several ways, which are classified by the effect on each partner as positive, negative or neutral. Mutualism is a positive-positive relationship in which both partners benefit, as in lichens, mycorrhiza and legume-Rhizobium. Commensalism benefits one and does not affect the other, as with an orchid growing on a mango tree. Predation benefits the predator and harms the prey. Parasitism benefits the parasite and harms the host. Competition harms both species, since they compete for the same limited resource. Amensalism harms one species and does not affect the other, as when Penicillium kills bacteria. In neutralism, neither is affected. Allelopathy is a chemical form of interaction in which one plant releases substances that hinder another, as Parthenium and black walnut do.

5 min read · UPSC Prelims · Environment & Ecology · Population Ecology and Species Interactions

At a Glance

Both gain Mutualism
One gains Commensalism
One gains, one loses Predation, parasitism
Both lose Competition
Chemical effect Allelopathy
Exam link UPSC Prelims, Environment & Ecology: Population Ecology and Species Interactions

Where Does It Fit?

This topic follows the account of homeostasis. It describes how species affect one another, the relationships that bind food webs and communities. Previous topic: Homeostasis. Next topic: Ecotone.

What Are the Types of Species Interactions?

Interaction	Effect and examples
Mutualism (+/+)	Both partners benefit. Examples: lichens (alga and fungus), mycorrhiza (fungus and plant roots), Rhizobium in legume root nodules, flowering plants and pollinators, fig and fig wasp, acacia and ants, coral polyps and zooxanthellae
Protocooperation (+/+)	Both benefit but can live without each other. Example: cattle egret and cattle, oxpecker and rhinoceros, sea anemone and hermit crab
Commensalism (+/0)	One benefits, the other is unaffected. Examples: orchid or fern on a tree, remora fish on sharks, barnacles on whales
Predation (+/-)	One kills and eats the other. Examples: tiger and deer, frog and insect, pitcher plant and insects
Parasitism (+/-)	Parasite lives on or in a host and harms it but usually does not kill quickly. Examples: tapeworm, lice, Cuscuta (dodder), mistletoe, Plasmodium
Competition (-/-)	Species use the same limited resource. Examples: Darwin’s finches, lions and hyenas, weeds and crops
Amensalism (-/0)	One is harmed and the other is not affected. Example: Penicillium and bacteria, large trees shading small herbs

What Are the Details of Parasitism, Predation, Competition and Allelopathy?

Ectoparasites: live on the outside, like lice and ticks
Endoparasites: live inside, like tapeworm and Plasmodium
Brood parasitism: the cuckoo lays eggs in crows’ nests
Hemiparasite: partly parasitic plant, like mistletoe, which makes some of its own food
Predator adaptations: speed, claws, sharp teeth and camouflage
Prey defences: camouflage, mimicry, warning colours, toxins, thorns and herding
Competition types: interference (direct fighting) and exploitative (using up resources); intraspecific (within species) and interspecific (between species)
Allelopathy: release of chemicals that prevent growth of other plants, as in Parthenium, black walnut and eucalyptus

What Else Should You Remember?

Mimicry: Batesian mimicry is a harmless species resembling a harmful one; Mullerian mimicry is among harmful species
Warning colouration: bright colours in poisonous frogs and monarch butterflies
Coevolution: reciprocal evolution of two interacting species, like flowers and pollinators
Keystone predator: controls competition among prey
Parasite effect: parasites can regulate host numbers
Plasmodium: parasite of malaria, with mosquito as vector
Cuscuta: a leafless climber that takes food from host plants
Rafflesia: giant parasitic flower
Epiphyte: commensal plant
Gause: competition experiments with Paramecium, as in Habitat and Ecological Niche
Lichens: useful as indicators of air quality
Mycorrhiza: increases phosphorus uptake by plants
Cleaner fish: clean parasites from larger fish
Honey guide birds: lead humans to bee nests
Clownfish and sea anemone: a mutualism or commensalism, depending on view
Seed dispersal: fruit eaters spread seeds
Predator-prey cycles: lynx and hare cycles
Biological control: uses predation and parasitism, as in Biological Control
Competition and invasive species: invasive species often outcompete natives
Symbiosis: a broader term for close long-term association, often meaning mutualism
Caution: textbooks differ on classifying cases like anemone-fish, so know the usual example
Pollination mutualism: about 75 percent of crop species benefit from animal pollination
Mycorrhiza share: over 80 percent of land plants form mycorrhizal links
Nitrogen fixation: Rhizobium fixes atmospheric nitrogen for legumes, which gives crop rotation value
Zooxanthellae: algae living in coral tissue give corals food and colour, and their loss causes bleaching
Predation and diversity: predators prevent one prey species from dominating, which maintains diversity
Host specificity: many parasites infect only one or a few host species
Parasites in humans: malaria, tapeworm and hookworm are parasitic diseases
Cooperation in animals: group hunting and cleaning behaviour are social interactions
Interaction strength: strong interactions have large effects on communities
Interactions change: relations can switch between positive and negative with conditions
Exam tip on signs: memorise the signs plus-plus, plus-zero, plus-minus, minus-minus and minus-zero with one example each
Insect-plant coevolution: plants make toxins and insects evolve ways to tolerate them

Why Does It Matter for the Exam?

Signs: +/+, +/0, +/-, -/-
Examples: lichen, mycorrhiza, Cuscuta
Allelopathy: Parthenium
Current affairs link: invasive plants
Common errors: calling commensalism mutualism or amensalism commensalism

As an analytical point, interactions regulate populations and shape communities. Many conservation tools, such as pollinator protection and biological control, are based on understanding these relationships.

Why Is This Topic Important?

Types Seven
Signs Effects
Examples Common

What Came Next?

The next topic is on the ecotone. Next topic: Ecotone.

Key People and Terms

Mutualism Both benefit
Commensalism One benefits, other unaffected
Amensalism One harmed, other unaffected
Parasitism Parasite harms host
Allelopathy Chemical inhibition
Mimicry Resemblance for protection

Exam Corner

Points to Remember

Lichen = mutualism.
Orchid on tree = commensalism.
Cuscuta = parasitism.
Penicillium and bacteria = amensalism.
Parthenium shows allelopathy.

Do Not Confuse With

Commensalism and mutualism One benefits versus both benefit
Predation and parasitism Kills quickly versus lives on host
Amensalism and competition One harmed versus both harmed

Memory Hook

Plus-plus mutual, plus-zero commensal, plus-minus eats or lives on, minus-minus competes.

Mains Angle

Questions ask for examples under each type. These points help.

Types Seven
Roles Population control
Use Biocontrol

A Question You May Face

An original practice question, not a past paper question.

“Explain the different types of interactions between species with suitable examples and discuss their role in ecosystems.”

How to Answer

List Types.
Examples Each type.
Effects Sign.
Role Regulation.

GS Relevance

Prelims: types of interaction with examples. GS Paper 3: ecology.

Frequently Asked Questions

What is mutualism?

Mutualism is an interaction between two species in which both partners benefit. Examples include lichens, which are an alga and a fungus living together, mycorrhizal fungi and plant roots, Rhizobium bacteria and legume plants, and flowering plants and their pollinators such as bees.

What is the difference between commensalism and parasitism?

In commensalism, one species benefits and the other is neither helped nor harmed, as with an orchid growing on a tree. In parasitism, one species, the parasite, benefits at the expense of the other, the host, which is harmed, as with a tapeworm in the gut or the climber Cuscuta on a plant.

What is amensalism?

Amensalism is an interaction in which one species is harmed and the other is neither harmed nor benefited. A common example is the mould Penicillium releasing penicillin that kills bacteria around it, or a large tree shading and stunting herbs growing beneath it.

What is allelopathy?

Allelopathy is the release by one plant of chemicals that inhibit the germination or growth of nearby plants of other species. Examples are black walnut, eucalyptus and Parthenium, which release substances into soil or air, and it helps these plants to dominate and reduces diversity.

What is competition between species?

Competition is an interaction in which two or more species use the same limited resource such as food, light or space, and each is harmed by the presence of the other. Interspecific competition occurs between species and intraspecific competition within a species, and the competitive exclusion principle says complete competitors cannot coexist.

PYQ Practice — Statement Analysis

1 Lichen is an example of mutualism.

True

Alga and fungus.

2 Orchid growing on a mango tree is parasitism.

False

It is commensalism.

3 Cuscuta is a parasitic plant.