Biotic Interactions – Ecology Notes
Competition, Predation, Parasitism, Herbivory, Amensalism and Other Biotic Interactions
Organisms in an ecosystem do not live independently. Plants, animals, fungi, bacteria and other organisms constantly interact with one another. These interactions may help both organisms, benefit only one organism, harm one or both organisms, or have little visible effect on one of them. Such relationships are called biotic interactions. Understanding these interactions is important because they influence population size, community structure, species distribution, evolution and the stability of ecosystems.
📚 Index
Major Types of Biotic Interactions
1. Meaning of Biotic Interactions
A community is made up of many species living together in the same area. Every species needs resources such as food, water, space, nutrients and suitable environmental conditions. Because these resources are limited, organisms inevitably interact with each other.
A biotic interaction is an ecological relationship between two or more living organisms. The interaction can occur between members of the same species or between members of different species.
Important points
- Biotic interactions occur among living organisms.
- They may be intraspecific, occurring between members of the same species.
- They may be interspecific, occurring between different species.
- The interaction can be beneficial, harmful or neutral.
- Biotic interactions influence population growth and community structure.
- They can affect survival and reproductive success.
- Long-term interactions may contribute to natural selection and adaptation.
For example, two deer feeding on the same grass may compete with one another. A tiger hunting a deer is an example of predation. A tapeworm living inside a human intestine represents parasitism, while cattle feeding on grass represents herbivory.
2. Importance of Biotic Interactions
Biotic interactions are one of the major forces that determine which organisms survive in a particular habitat. They are also important in maintaining the organization of ecological communities.
- Population regulation: Predation, competition and parasitism can prevent populations from increasing indefinitely.
- Community structure: Interactions determine which species can coexist in a community.
- Food webs: Predator-prey and herbivore-plant interactions form important links in food webs.
- Evolution: Interactions create selection pressures that can lead to adaptations.
- Resource utilization: Competition encourages species to use resources in different ways.
- Species distribution: A species may be absent from an area because another species excludes it.
- Ecosystem stability: Complex interactions can contribute to stability and resilience.
- Co-evolution: Closely interacting species may influence each other's evolutionary changes.
3. Types of Biotic Interactions
The effect of an interaction on the two interacting species is commonly represented by the symbols +, − and 0.
| Interaction | Species A | Species B | Example |
|---|---|---|---|
| Mutualism | + | + | Mycorrhiza |
| Competition | − | − | Two species using the same limited food |
| Predation | + | − | Tiger and deer |
| Parasitism | + | − | Tapeworm and human |
| Herbivory | + | − | Caterpillar feeding on leaves |
| Amensalism | − | 0 | Chemical inhibition |
| Commensalism | + | 0 | Epiphyte using a tree for support |
4. Positive Interactions
In positive interactions, at least one organism benefits and no interacting organism is significantly harmed. Mutualism is represented by (+/+), while commensalism is represented by (+/0).
Mutualism (+/+)
- Both species receive a benefit.
- The association may be obligatory or facultative depending on the organisms.
- Examples include plant-pollinator relationships, mycorrhiza and legume-Rhizobium association.
- In mycorrhiza, the fungus helps the plant obtain minerals and water, while the plant provides carbohydrates to the fungus.
Commensalism (+/0)
- One species benefits while the other is not significantly affected.
- Epiphytes may grow on trees mainly for physical support and better access to light.
- The host tree may not receive a major benefit or harm in a typical commensal relationship.
5. Negative Interactions
Negative interactions are relationships in which at least one organism experiences a disadvantage. These interactions are particularly important in controlling population size and shaping community structure.
- Competition: − / −
- Predation: + / −
- Parasitism: + / −
- Herbivory: + / −
- Amensalism: − / 0
Negative interactions do not necessarily mean that an organism always dies. The effect may be reduced growth, reduced reproduction, loss of resources, physical injury or reduced survival.
6. Competition
Competition occurs when two or more organisms require the same resource and that resource is insufficient to satisfy all of them. The resource may be food, water, light, nutrients, territory, nesting sites or mates.
Basic characteristics
- Competition generally occurs because resources are limited.
- Both competing organisms may experience a reduction in fitness.
- Competition can occur within the same species or between different species.
- It can be direct or indirect.
- Competition is usually represented as −/−.
Intraspecific competition
This occurs between individuals of the same species. For example, two plants of the same species may compete for water, mineral nutrients and sunlight.
- It may become stronger as population density increases.
- It can regulate population size.
- It may lead to territorial behavior.
- It can influence mating success and reproductive output.
Interspecific competition
This occurs between individuals of different species that use a common limiting resource. For example, two bird species may feed on similar insects in the same habitat.
Resource competition
Competition may be reduced if organisms use resources differently. This idea is closely associated with resource partitioning. Different species may use different food types, feeding times or microhabitats, allowing them to coexist.
Competitive exclusion principle
The competitive exclusion principle states that two species with identical ecological requirements cannot coexist indefinitely under constant environmental conditions if they are competing for exactly the same limiting resource. One species may eventually exclude the other, or the species may evolve or shift their resource use.
Example
If two closely related bird species depend on exactly the same nesting sites and food resources, intense competition may occur. If one species is more efficient at using those resources, the other may decline or may shift to another resource.
7. Predation
Predation is an interaction in which one organism, called the predator, captures and consumes another organism, called the prey. The predator receives a benefit while the prey is harmed. Therefore, predation is represented as +/−.
Examples
- Tiger hunting deer.
- Spider catching insects.
- Frog eating insects.
- Ladybird beetles feeding on aphids.
- Fish consuming smaller aquatic organisms.
Role of predation in ecology
- Controls prey population size.
- Prevents a single prey species from becoming excessively abundant.
- Can influence community diversity.
- Creates selection pressure on prey.
- Creates selection pressure on predators to improve hunting efficiency.
- Forms an important connection in food chains and food webs.
Predator adaptations
- Sharp claws or teeth.
- Fast running or swimming.
- Camouflage.
- Excellent vision or hearing.
- Venom or toxins in some predators.
- Specialized feeding structures.
- Cooperative hunting in some species.
Prey defenses
Prey species have also evolved numerous adaptations to reduce the chance of being captured.
- Camouflage: Makes the organism difficult to detect.
- Warning coloration: Bright colors may indicate toxicity or unpalatability.
- Mimicry: A harmless species may resemble a harmful or protected species.
- Physical defenses: Spines, shells and hard coverings can discourage predators.
- Chemical defenses: Some plants and animals produce toxic or unpleasant compounds.
- Escape behavior: Running, hiding or forming groups can reduce predation risk.
Predator-prey relationship
Predator and prey populations often influence each other. When prey abundance increases, predators may obtain more food and their population can increase. Higher predator abundance can then increase prey mortality. As prey numbers decline, predator populations may subsequently decrease because food becomes less available. This creates a dynamic relationship rather than a fixed one.
8. Parasitism
Parasitism is an interaction in which one organism, the parasite, obtains food or other benefits from another organism, the host. The parasite benefits while the host experiences a negative effect.
Unlike a typical predator, a parasite generally does not kill its host immediately because the host is often required as a continuing source of nutrients or habitat.
Examples
- Tapeworm in humans.
- Plasmodium in humans.
- Lice living on mammals.
- Ticks feeding on blood.
- Cuscuta obtaining nutrients from host plants.
- Parasitic fungi infecting plants.
Ectoparasites
Ectoparasites live on the external surface of the host.
- Lice
- Ticks
- Fleas
- Some parasitic mites
Endoparasites
Endoparasites live inside the body of their host.
- Tapeworms
- Roundworms
- Plasmodium
- Several parasitic microorganisms
Parasite adaptations
- High reproductive capacity.
- Specialized attachment structures.
- Ability to avoid or suppress host defenses.
- Small body size in many parasites.
- Complex life cycles involving one or more hosts.
- Production of resistant stages in some parasites.
Host defenses
- Immune responses.
- Physical barriers such as skin.
- Chemical defenses.
- Behavioral removal of parasites.
- Protective secretions.
9. Herbivory
Herbivory is an interaction in which an animal feeds on plants, algae or other photosynthetic organisms. The herbivore obtains food, while the plant or algal tissue being consumed experiences damage.
Examples
- Cow feeding on grass.
- Goat feeding on leaves.
- Caterpillar feeding on plant leaves.
- Deer feeding on young shoots.
- Grasshopper feeding on crops.
Herbivory is commonly treated as a +/− interaction. It differs from typical predation because the consumer usually removes only part of the plant, and the plant may survive and continue growing.
Plant defenses against herbivores
1. Physical defenses
- Thorns.
- Spines.
- Trichomes or leaf hairs.
- Tough leaves.
- Thick bark.
2. Chemical defenses
- Alkaloids.
- Terpenoids.
- Phenolic compounds.
- Tannins.
- Latex.
- Other toxic or deterrent compounds.
3. Induced defenses
Some plants increase their defensive response after herbivore attack. Chemical signals can activate defense pathways in damaged and sometimes neighboring tissues.
Herbivore adaptations
- Specialized teeth or mouthparts.
- Digestive enzymes.
- Microbial communities in the digestive tract.
- Detoxification mechanisms.
- Ability to select less-defended plant tissues.
Herbivory is ecologically important because it transfers energy from plants to animals and can influence plant community composition. At moderate levels, herbivory may even stimulate compensatory growth in some plants. However, severe herbivory can reduce plant survival and reproductive success.
10. Amensalism
Amensalism is an interaction in which one species is negatively affected while the other species experiences no significant effect. It is represented by −/0.
A well-known mechanism associated with amensalism is the production of chemicals that inhibit another organism. This phenomenon is often discussed under the broader concept of chemical interference.
Examples
- Some microorganisms produce antibiotics that inhibit the growth of other microorganisms.
- Some plants release chemicals into the soil that reduce the growth of neighboring plants.
- Large organisms may accidentally damage smaller organisms without receiving a direct benefit from doing so.
Antibiosis
In antibiosis, one organism produces a chemical substance that inhibits or kills another organism. For example, some species of fungi and bacteria produce antimicrobial compounds.
Allelopathy
Allelopathy refers to chemical interactions between plants in which one plant releases biochemical substances that affect the germination, growth or development of another plant.
- Allelochemicals may be released through roots.
- They may enter the environment through decomposing plant material.
- They can affect seed germination.
- They may reduce root or shoot growth.
- They may alter nutrient or microbial interactions around the plant.
11. Comparison of Major Biotic Interactions
| Interaction | Effect | Basic Meaning | Example |
|---|---|---|---|
| Competition | − / − | Both organisms are negatively affected | Plants competing for light |
| Predation | + / − | Predator benefits and prey is harmed | Lion and deer |
| Parasitism | + / − | Parasite benefits and host is harmed | Tapeworm and human |
| Herbivory | + / − | Herbivore benefits from plant tissue | Caterpillar and leaf |
| Amensalism | − / 0 | One is harmed, other unaffected | Chemical inhibition |
| Mutualism | + / + | Both organisms benefit | Mycorrhiza |
| Commensalism | + / 0 | One benefits, other unaffected | Epiphyte on tree |
12. Ecological Importance of Biotic Interactions
1. Regulation of populations
Predators, parasites and competitors can prevent populations from growing beyond the available resources. In this way, biotic interactions contribute to density-dependent regulation.
2. Maintenance of biodiversity
Interactions among species can prevent one species from dominating all available resources. Resource partitioning and predation can sometimes allow several species to coexist in the same community.
3. Evolutionary change
Every interaction can create selection pressure. Predators select for prey defenses, while prey can select for better hunting abilities in predators. Similarly, parasites and hosts may continuously evolve new strategies against one another.
4. Food webs
Predation and herbivory connect trophic levels. Plants are eaten by herbivores, herbivores are consumed by predators, and energy moves through the food web.
5. Community organization
The presence or absence of important species can change the structure of an entire community. Predators may influence herbivore abundance, which can indirectly affect plant communities.
6. Co-evolution
When two species interact closely over many generations, evolutionary changes in one species may create selection pressure on the other. Predator-prey and host-parasite systems are common examples.
7. Ecosystem functioning
Biotic interactions affect decomposition, nutrient cycling, pollination, seed dispersal, primary production and energy flow. For example, plant-pollinator interactions are important for the reproduction of many flowering plants.
13. Important Exam Points
- Biotic interaction: Interaction between living organisms.
- Competition: −/−.
- Predation: +/−.
- Parasitism: +/−.
- Herbivory: +/−.
- Amensalism: −/0.
- Mutualism: +/+.
- Commensalism: +/0.
- Intraspecific competition: Competition within the same species.
- Interspecific competition: Competition between different species.
- Ectoparasite: Parasite living on the external surface of a host.
- Endoparasite: Parasite living inside the host.
- Allelopathy: Chemical interaction in which plants release compounds affecting other plants or organisms.
- Antibiosis: Chemical inhibition of one organism by another.
- Predator: Organism that captures and consumes another organism.
- Prey: Organism that is captured and consumed by a predator.
- Host: Organism that provides resources or habitat to a parasite.
- Herbivore: Animal that feeds on plants or other photosynthetic organisms.
Easy way to remember the signs
| Sign | Meaning |
|---|---|
| + | Organism receives benefit |
| − | Organism experiences harm or reduced fitness |
| 0 | No significant effect |
High-yield differences
- Competition vs predation: Competition generally harms both competitors, whereas predation benefits the predator and harms the prey.
- Predation vs parasitism: Predators usually consume prey directly and often kill it, whereas parasites generally exploit a host for an extended period.
- Parasitism vs herbivory: Parasitism involves exploitation of a host, while herbivory involves feeding on plant or algal material.
- Amensalism vs competition: Amensalism is −/0, while competition is −/−.
- Mutualism vs commensalism: Mutualism benefits both partners (+/+), whereas commensalism benefits one and has little effect on the other (+/0).
14. Practice MCQs
Test yourself before checking the answers.
- Mutualism
- Competition
- Predation
- Commensalism
Show Answer
Answer: B — Competition.
Both competing organisms generally experience a negative effect because they use a limited resource.
- +/+
- +/0
- +/−
- −/−
Show Answer
Answer: C — +/−.
The predator obtains food and benefits, whereas the prey is harmed.
- Flea
- Tick
- Tapeworm
- Butterfly
Show Answer
Answer: C — Tapeworm.
Tapeworms live inside the digestive tract of their host and therefore are endoparasites.
- Amensalism
- Mutualism
- Predation
- Parasitism
Show Answer
Answer: A — Amensalism.
In amensalism, one species is negatively affected while the other is considered unaffected.
- Herbivory
- Parasitism
- Amensalism
- Competition
Show Answer
Answer: A — Herbivory.
Herbivory involves animals feeding on plants or other photosynthetic organisms.
- Interspecific competition
- Intraspecific competition
- Predation
- Parasitism
Show Answer
Answer: B — Intraspecific competition.
The prefix “intra” means within, so intraspecific competition occurs among members of the same species.
- Allelopathy
- Predation
- Migration
- Mutualism
Show Answer
Answer: A — Allelopathy.
Allelopathy involves chemical substances released by plants that influence the germination, growth or development of other plants.
- Both species always benefit
- The parasite benefits and the host is harmed
- Both species are harmed equally
- Neither species is affected
Show Answer
Answer: B — The parasite benefits and the host is harmed.
The parasite obtains nutrients, shelter or another resource from the host.
- Competition — +/+
- Mutualism — −/−
- Predation — +/−
- Amensalism — +/+
Show Answer
Answer: C — Predation — +/−.
The predator benefits while the prey is negatively affected.
- Herbivores obtain energy by feeding on plant material
- Herbivory always kills the plant immediately
- Herbivory is a −/− interaction
- Herbivores are always parasites
Show Answer
Answer: A — Herbivores obtain energy by feeding on plant material.
Herbivory is generally considered a +/− interaction because the herbivore benefits while the consumed plant tissue is damaged.
⚡ One-Minute Revision
- Competition = −/−
- Predation = +/−
- Parasitism = +/−
- Herbivory = +/−
- Amensalism = −/0
- Mutualism = +/+
- Commensalism = +/0
- Intraspecific = same species
- Interspecific = different species
- Ectoparasite = outside host
- Endoparasite = inside host
- Allelopathy = chemical interaction involving plants
- Predator = consumer
- Prey = organism being consumed
- Host = organism exploited by parasite
Biotic Interactions – Ecology Study Notes
Useful for CSIR-NET Life Sciences, GATE Biotechnology, DBT-BET, university examinations and undergraduate/postgraduate ecology preparation.
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