What is the interaction between two species called?

NCERT Exemplar · Q12

Q. What is the interaction between two species called?

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Concept understanding — Ecological Interactions

Ecological Interactions: A First Look

Imagine a city. People live in apartments, work in offices, buy food from shops, and throw away garbage that someone else collects. Some people compete for the same job. Others help each other—a landlord provides space, a tenant pays rent. Now shrink that city to a forest, a pond, or even your backyard garden. Replace people with plants, animals, fungi, and bacteria. What you get is a web of relationships between living organisms—that is what ecologists call ecological interactions.

The Core Idea

Every living thing on Earth is connected to others. No organism exists in isolation. A tiger needs deer to eat; the deer needs grass; the grass needs sunlight and soil nutrients made available by bacteria and fungi. These connections are not random—they follow patterns. Some interactions help both partners, some help one at the expense of the other, and some harm both.

The NCERT textbook defines ecological interactions as the relationships between different species in a community. These interactions shape who lives where, how many of them exist, and how energy flows through an ecosystem.

Why Should a Commerce/Humanities Student Care?

You might think this is only for biology students. But consider this: every business operates within an ecosystem of suppliers, competitors, customers, and regulators. A startup that helps its suppliers grow (mutualism) survives longer than one that squeezes them dry (parasitism). A company that competes aggressively with rivals (competition) may drive prices down for consumers. The same logic that governs a forest governs a market—because both are systems of interacting agents.

Note

Ecological interactions are not just "nature facts." They are a lens to understand any system where different entities depend on, compete with, or exploit each other—including economies, societies, and organizations.

The Main Types of Interactions

Ecologists classify interactions based on who benefits (+) and who is harmed (−). A zero (0) means no effect.

1. Mutualism (+ / +)

Both species benefit. Think of bees and flowers: bees get nectar (food), flowers get pollinated (reproduction). In human terms, this is a win-win partnership—like a coffee shop and a bookstore sharing the same space to attract more customers.

2. Competition (− / −)

Both species are harmed because they fight for the same limited resource—food, water, space, light. When two businesses open identical stores on the same street, both may earn less profit. In nature, two species of birds eating the same insect will both struggle if insects become scarce.

3. Predation (+ / −)

One species (predator) kills and eats the other (prey). The predator benefits; the prey is harmed. This is the tiger-deer example. In business, think of a large corporation acquiring a smaller competitor—the acquirer grows, the acquired loses independence.

4. Parasitism (+ / −)

One species (parasite) lives on or inside another (host) and feeds on it, usually without killing it immediately. The parasite benefits; the host is harmed. A tick on a dog, or a virus in a human. In economics, this resembles a monopolist that extracts value from suppliers without providing fair returns.

5. Commensalism (+ / 0)

One species benefits; the other is neither helped nor harmed. Barnacles attaching to a whale get a free ride to food-rich waters; the whale is unaffected. In a city, a street vendor setting up near a popular metro station benefits from foot traffic without affecting the station.

6. Amensalism (− / 0)

One species is harmed; the other is unaffected. A large tree casts shade that kills smaller plants beneath it—the tree does not care. In business, a giant retailer opening next to a small shop may drive the shop out of business without intending to.

Important

|-------------|---------------------|---------------------|---------------------|

| Mutualism | + | + | Joint venture |

| Competition | − | − | Price war |

| Predation | + | − | Acquisition |

| Parasitism | + | − | Exploitative contract |

| Commensalism | + | 0 | Free rider |

| Amensalism | − | 0 | Collateral damage |

The Bigger Picture

No interaction exists in isolation. A single species may be involved in multiple relationships simultaneously. A tree competes with other trees for sunlight, provides shelter for birds (commensalism), hosts fungi that help it absorb nutrients (mutualism), and is eaten by insects (predation). This complexity is what makes ecosystems resilient—or fragile. …

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The interaction between two different species is known as an interspecific interaction, which can be beneficial, harmful, or neutral for each species involved.

Life on Earth is not a solitary affair. Every organism exists within a complex web of relationships, not only with its physical environment but, crucially, with other living beings. These relationships are fundamental to ecology, shaping the structure and function of ecosystems. When we specifically consider how two different species influence each other, we are talking about interspecific interactions.

These interspecific interactions are diverse and can have a wide range of outcomes for the species involved. Ecologists often classify these interactions based on the effect they have on each species, using symbols: a '+' sign for benefit, a '-' sign for detriment or harm, and a '0' for a neutral effect (neither benefit nor harm). Understanding these interactions is key to comprehending population dynamics, community structure, and the very process of evolution.

Here are the primary types of interspecific interactions:

Mutualism (+/+): In this interaction, both interacting species benefit from the association. The relationship is often obligate, meaning one or both species cannot survive without the other.
- For example, lichens are a classic case of mutualism, representing an association between a fungus and photosynthetic algae or cyanobacteria. The fungus provides shelter and absorbs nutrients, while the algae/cyanobacteria perform photosynthesis to produce food.
- Another example is the relationship between mycorrhizae (fungi) and the roots of higher plants. The fungi help the plant absorb essential nutrients like phosphorus, while the plant provides carbohydrates to the fungi.
- Pollination is also a mutualistic interaction where animals (like insects, birds, or bats) get nectar or pollen as food, and in return, they help the plant in transferring pollen for reproduction.
Competition (-/-): This interaction occurs when two species compete for the same limited resources, such as food, space, light, or water. In competition, both species are negatively affected, as the presence of one reduces the resources available to the other, leading to reduced growth, survival, or reproduction for both.
- A common example is when two different species of birds compete for the same type of insect in a particular area. Both species experience a reduction in their food intake due to the presence of the other.
- Important
  
  Competition is often considered the most potent force in evolution, as it drives species to adapt to minimize overlap in resource use, leading to resource partitioning or competitive exclusion.
Predation (+/-): Predation is an interaction where one species, the predator, kills and consumes another species, the prey. The predator benefits, while the prey is harmed (and often killed).
- Examples include a tiger hunting a deer, a frog catching an insect, or a hawk preying on a mouse.
- Predation is crucial for transferring energy across trophic levels in an ecosystem and plays a vital role in controlling prey populations, preventing them from overpopulating and destabilizing the ecosystem.
Parasitism (+/-): In parasitism, one species, the parasite, lives on or in another species, the host, deriving nourishment from it. The parasite benefits, while the host is harmed, though typically not immediately killed, as the parasite depends on the host for survival.
- Examples include human liver fluke, malarial parasites in humans, ticks on dogs, or parasitic plants like Cuscuta (dodder) growing on other plants. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 56 on this concept.

CBSE 2026Set 57/2/11 markMCQ
$Q. What could be the reason of extinction of Abingdon Tortoise from Galapagos Islands ? (A) Intraspecific Competition (B) Predation (C) Parasitism (D) Interspecific Competition$
›Reveal solutionSolution

The Abingdon tortoise was driven to extinction primarily by interspecific competition — specifically, competition from introduced feral goats that destroyed its food supply.

The story of the Abingdon tortoise is one of the most poignant examples of how a single introduced species can unravel an entire ecosystem. To understand what happened, we need to look at the Galapagos Islands not as a pristine paradise, but as a fragile web of life that evolved in isolation for millions of years. The Abingdon tortoise, a subspecies of the giant Galapagos tortoise, lived only on Pinta Island (also called Abingdon Island). Its extinction was not caused by a dramatic volcanic eruption or a sudden disease. It was caused by something far quieter, and far more human-driven.

The key event was the arrival of feral goats. Sailors and whalers, over centuries, deliberately left goats on islands as a future source of fresh meat. On Pinta, these goats found a paradise with no natural predators. They multiplied explosively. And here is where the ecological interaction becomes critical: goats and tortoises both eat vegetation — grasses, shrubs, and low-lying plants. But goats are far more efficient and destructive. They can climb, they can strip bark, they can eat plants down to the root. The tortoise, slow and with a limited diet, simply could not compete.

This is a textbook case of interspecific competition — competition between individuals of different species for the same limited resource. The goats did not attack the tortoises directly. They did not parasitise them. They simply outcompeted them for food. As the goat population boomed, the island’s vegetation was decimated. The tortoises, which had evolved with no such competitor, starved. Their population collapsed. By the time scientists realised what was happening, only a single individual — famously named Lonesome George — remained. Despite decades of effort, he never reproduced, and with his death in 2012, the subspecies was gone. …
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CBSE 2026Set V11 markMCQ
$Q. Herbivores in a broad ecological context not very different from$
(a) Commensals
(b) Predators
(c) Parasites
(d) Producers
›Reveal solutionSolution

In a broad ecological sense a herbivore that feeds on a plant is, functionally, a predator on that plant.

Predation is any interaction in which one organism (the predator) consumes another living organism (the prey) for its nutrition. When this idea is applied broadly, a herbivore eating a plant is essentially a "predator" of that plant, because it is a consumer that lives on another living organism and transfers energy from the producer level to higher trophic levels. This is why the NCERT text states that, in a broad ecological context, herbivores are not very different from p …
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CBSE 2026Set A1 markMCQ
$Q. Which of the following was used to control the cactus that had been introduced in Australia?$
(a) Moth
(b) Calotropis
(c) Phytophagous insects
(d) Virus
›Reveal solutionSolution

The cactus moth Cactoblastis controlled the introduced Opuntia cactus in Australia — a classic biological-control example.

When the prickly-pear cactus (Opuntia) was introduced into Australia in the early 1900s, it spread uncontrollably because it had no natural enemies there. It was brought under control by introducing a natural predator (a herbivore) — the cactus-feeding moth Cactoblastis cactorum. Thi …

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CBSE 2026Set A1 markMCQ
$Q. What is lichen an example of?$
(a) Mutualism
(b) Competition
(c) Parasitism
(d) Commensalism
›Reveal solutionSolution

Lichens are a mutualism between a fungus and an alga, where both partners benefit.

A lichen is an intimate, mutually beneficial partnership between a fungus and a photosynthetic partner (an alga or a cyanobacterium). The alga carries out photosynthesis and supplies food, while the fungus provides shelter, anchorage, water and min …

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CBSE 2026Set BOTANY1 markMCQ
$Q. In terms of population interactions, mycorrhiza is an example of _____.$
(a) competition
(b) predation
(c) parasitism
(d) mutualism
›Reveal solutionSolution

Mycorrhiza is a mutualistic association between a fungus and the roots of a higher plant, in which both partners benefit.

Population interactions between two species can be classified by their net effect (+, -, 0) on each partner: mutualism (+ +), competition (- -), predation/parasitism (+ -), commensalism (+ 0), and amensalism (- 0). Mycorrhiza is the intimate association of a fungus with the root system of a vascular plant. The fungal hyphae greatly increase the absorptive surface area of the root system, so the fungus helps the plant take up phosphorus and other minerals (and sometimes water) from the soil more efficiently. In return, the plant supplies the fungus with photosynthetically fixed carbohydrates (sugars) that the non-photosynthetic fungus cannot make for itself. Because BOTH pa …

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CBSE 2026Set ANNUAL1 markMCQ
$Q. If species 'A' + (Positive) and species 'B' - (Negative) in population, the name of interaction will be$
(a) Mutualism
(b) Competition
(c) Predation
(d) Parasitism
›Reveal solutionSolution

A +/- interaction, where one species benefits and the other is harmed, describes parasitism; of the given options 'Parasitism' is the standard population interaction named for this sign pattern.

Ecological population interactions are often summarised by the effect on each species: Mutualism is (+,+) - both benefit; Competition is (-,-) - both are harmed; Commensalism is (+,0) - one benefits, the other unaffected. When one species benefits (+) at the cost of the other, which is harmed (-), the interaction is parasitism - the parasite (species A) derives nutrition from the h …

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CBSE 2026Set ANNUAL1 mark
$Q. What is brood parasitism? Give one example.$
›Reveal solutionSolution

Brood parasitism is a special case of exploitative interaction where a parasitic bird lays eggs in a host bird's nest, and the unsuspecting host raises the parasite's chick as its own.

Brood parasitism is an interesting interspecific interaction (studied alongside predation, parasitism, and other population interactions) in which a bird species lays its eggs in the nest of another species, so the host bird ends up incubating and rearing the parasite's young, often at the expense of its own offspring.

…

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CBSE 2026Set ANNUAL1 markMCQ
$Q. This type of interaction confers benefits on both interacting species are called:$
(a) Commensalism
(b) Mutualism
(c) Parasitism
(d) Competition
›Reveal solutionSolution

An interaction that benefits both species is mutualism.

Interspecific interactions are classified by their effect on the two species. Mutualism is the interaction in which both species are benefited (a +/+ relationship), for example the pollination of flowers by bees, or lichens (fungus and alga). In commensalism one benefits and the other is unaffected (+/0), in parasitism one benefits …

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CBSE 2026Set ANNUAL1 mark
$Q. How does an endoparasite differ from an ectoparasite?$
›Reveal solutionSolution
Endoparasites live inside the host body; ectoparasites live on the host's external surface.

Parasites are classified by their site of attack on the host:

Endoparasite — lives inside the host's body: in the alimentary canal, blood, liver, tissues or even inside cells. Examples: liver fluke, tapeworm, Plasmodium. They often have simplified body organisation but complex life cycles. …
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CBSE 2025Set A1 mark
$Q. Write True / False: Lichen represent an intimate parasitic relationship between a fungus and a cyanobacteria.$
›Reveal solutionSolution

The statement is False: the fungus-cyanobacterium association in a lichen is mutualism, where both partners benefit.

A lichen is a classic example of mutualism, an interspecific interaction in which both participating species benefit. In a lichen, the fungal partner provides a protective structure, absorbs water and minerals, and shelters the photosynthetic partner (a cyanobacterium or green alga); in return, the photosynthetic partner supplies the fungus with organic nutrients (food) made via photosynthesis. Because …

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CBSE 2025Set ANNUAL1 mark
$Q. Fill in the blank: Parasites that feed on the external surface of the host organisms are called ___.$
›Reveal solutionSolution

Parasites living and feeding on the outer body surface of a host are called ectoparasites.

Ectoparasites attach to and feed on the external body surface of their host, drawing blood or tissue fluids — common examples include lice on humans, and ticks and mites on other animals. This is …

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CBSE 2025Set ANNUAL1 mark
$Q. Observe the first pair and fill in the blanks. Population interaction in which one species benefits and other is neither harmed nor benefited : Commensalism. Population interaction where one species is harmed and other is unaffected : _____.$
›Reveal solutionSolution
In commensalism one species benefits while the other is neither harmed nor benefited; in amensalism, by contrast, one species is harmed while the other remains unaffected.

Population interactions between two species can be classified by their effect (+ benefit, − harm, 0 no effect) on each species:

Commensalism (0, +): one species benefits, the other is unaffected — e.g. an orchid growing as an epiphyte on a mango branch, or barnacles growing on the back of a whale. …
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Q.What is the interaction between two species called?

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