Table of Contents
Types of Interactions Between Organisms
Within the broader idea that “life means living together,” organisms interact in several recurring ways. Four especially important types are competition, symbiosis, commensalism, and antibiosis. They differ in how they affect the partners’ survival and reproduction (their “fitness”).
A simple overview:
- Competition: both partners are harmed in terms of fitness ($-$ / $-$)
- Symbiosis (mutualism): both partners benefit ($+$ / $+$)
- Commensalism: one benefits, the other is (approximately) unaffected ($+$ / $0$)
- Antibiosis (amensalism): one is harmed, the other is (approximately) unaffected ($-$ / $0$)
These interaction types form a continuum; real examples often lie between categories or can shift from one type to another depending on conditions.
Competition
What competition is
Competition occurs when two or more individuals or species use the same limited resource—such as food, space, light, water, or mates—so that access to that resource for one reduces access for the other.
Key features:
- The resource must be limited.
- Interacting organisms have overlapping ecological niches (they need similar things in similar ways).
- Both parties experience reduced growth, reproduction, or survival compared with a situation without the competitor.
Types of competition
- Intraspecific competition: between individuals of the same species
- Example: seedlings of the same tree species competing for light and nutrients under a dense canopy.
- Interspecific competition: between individuals of different species
- Example: grasses and nearby shrubs competing for water in semi-arid habitats.
You may also see a distinction between:
- Exploitative (indirect) competition: organisms do not interact directly but reduce the resource availability for others by consuming it (e.g., two plankton species using the same dissolved nutrients).
- Interference (direct) competition: organisms physically prevent others from accessing resources (e.g., territorial behavior in animals, plants releasing root toxins that inhibit neighbors).
Ecological consequences of competition
Competition is a major evolutionary and ecological force because it can:
- Limit distribution and abundance: one species may be excluded from part of its potential range where a superior competitor is present.
- Drive niche differentiation: species may evolve to use different parts of the habitat or different resources (resource partitioning).
- Influence character traits: body size, beak shape, root depth, etc., can diverge where closely related competitors co-occur (character displacement).
In the context of symbiogenesis, persistent competitive pressure can create conditions favoring close, stable associations—some competitive interactions may be “resolved” over evolutionary time into cooperative or exploitative relationships.
Symbiosis (Mutualism)
Symbiosis as mutual benefit
In this course, we use “symbiosis” in the narrower sense of mutualism: a close, long-term interaction in which both partners gain a net benefit ($+$ / $+$). The relationship can be obligate (necessary for survival or reproduction) or facultative (beneficial but not absolutely necessary).
Typical benefits include:
- Access to nutrients or energy sources
- Protection from predators, pathogens, or environmental stress
- Assistance in reproduction or dispersal (e.g., pollination, seed dispersal)
- Improved habitat structure or microclimate
Degrees of dependence
- Obligate mutualism: partners cannot survive or reproduce effectively without each other.
- Example: many intracellular bacteria that live only inside animal or insect cells, providing essential nutrients while receiving shelter and resources.
- Facultative mutualism: partners can live independently but do better together.
- Example: some plant–mycorrhizal fungi associations in nutrient-rich soils, where the plant can survive without fungal partners but grows better with them.
Symbiosis, specialization, and stability
Persistent mutualistic symbioses tend to lead to:
- Specialization: each partner evolves specific structures, behaviors, or biochemical pathways that make the relationship more efficient (e.g., specialized root structures for hosting mycorrhizal fungi).
- Increased interdependence: over evolutionary time, each partner may lose the ability to perform tasks that the other now provides, deepening the symbiosis.
- Potential vulnerability: strong dependence can make both partners sensitive to changes that affect either one (e.g., loss of a particular pollinator species).
These trends are central to symbiogenesis: when endosymbiotic partners become so integrated that they eventually form a new kind of organism (for example, the origin of mitochondria and chloroplasts from free-living bacteria, covered in another chapter).
Mutualism is not “altruism”
Even though both partners benefit, natural selection favors mutualism because it increases the fitness of each partner individually, not because organisms “want to help.” Mutualisms can shift to exploitation or conflict if the balance of costs and benefits changes.
Commensalism
One-sided benefit, no clear cost
Commensalism describes an interaction in which:
- One organism gains a benefit (e.g., food, shelter, transport), and
- The other is neither significantly harmed nor helped ($+$ / $0$).
Examples often involve using another organism simply as a habitat or support:
- Epiphytic plants (such as many orchids and bromeliads) growing on tree branches may gain more light and better air circulation, while the tree experiences little or no measurable effect.
- Certain barnacles attached to whales gain transport to nutrient-rich waters and better feeding opportunities; the whale is usually not significantly affected.
Why “no effect” is hard to prove
In practice, it is difficult to demonstrate that there is truly no effect on one partner, because:
- Very small positive or negative effects may be hard to detect.
- Effects can change with environmental conditions (e.g., in times of stress, even small additional loads might become harmful).
Therefore, commensalism is often a working category for interactions where any costs or benefits for one partner are negligible under typical conditions.
Commensalism and evolutionary transitions
Commensal relationships can serve as starting points for more intense interactions:
- A commensal using another species as habitat may evolve traits that begin to affect its host (e.g., mild resource use, slight protection).
- Over time, this can shift to mutualism (if the partner begins to confer a benefit, such as predator defense) or to parasitism/antibiosis (if the partner begins to cause harm).
Thus, commensalism can be a “gateway” state in the evolutionary dynamics of interspecific relationships.
Antibiosis (Amensalism)
Harm without clear benefit
Antibiosis (often called amensalism) describes an interaction where:
- One organism is harmed (reduced growth, survival, or reproduction),
- The other is apparently unaffected in terms of fitness ($-$ / $0$).
Classic cases involve one species producing substances or physical conditions that inhibit another species, without gaining a direct, measurable benefit from doing so.
Examples:
- Microorganisms or fungi that secrete antibiotics into their surroundings, killing or inhibiting other microbes, but doing so as a byproduct of normal metabolism.
- Plants that release toxic compounds into the soil (allelopathy) that suppress the germination or growth of neighboring plants, without clear evidence that the producer individually benefits under all conditions.
Distinguishing antibiosis from competition and defense
Antibiosis can resemble other interaction types:
- Competition: if inhibitory chemicals reduce a competitor’s success and thereby increase resource availability for the producer, the interaction may be better classified as interference competition ($-$ / $+$).
- Defense: if a toxic substance reduces herbivory or infection, the effect is harmful to consumers or pathogens but beneficial to the producer ($-$ / $+$ again).
True antibiosis is used for situations where the harmful effect appears to be a side-effect with no clear adaptive benefit to the producer. However, as with commensalism, this can be difficult to prove.
Ecological and evolutionary role
Even if the producer does not directly benefit:
- Antibiosis can shape community composition by suppressing sensitive species and allowing tolerant ones to dominate.
- It can create selection pressure for resistance in the affected species, leading to coevolutionary changes (e.g., evolution of antibiotic resistance in bacteria).
In the context of symbiogenesis, antibiotic-producing microbes and antibiotic-resistant hosts can enter new forms of association (for example, antibiotic-producing bacteria living in or on animals, protecting them against pathogens). Over long periods, this may stabilize into specialized symbiotic partnerships.
Transitions and Context Dependence
The four interaction types described here are not rigid boxes; they represent idealized end points of a spectrum. The same pair of species can shift along this spectrum with changing conditions such as:
- Resource availability
- Population densities
- Environmental stress (e.g., drought, toxins, temperature extremes)
- Presence or absence of predators, parasites, or competitors
Examples of potential shifts:
- A mutualistic relationship (symbiosis) can become parasitic if one partner’s costs increase or benefits decrease (e.g., a fungus that normally aids a plant’s nutrient uptake may draw too many resources under nutrient-poor conditions).
- A commensal relationship can evolve into mutualism if the host begins to gain a defensive or cleaning benefit from the associate.
- A seemingly neutral association can become competitive or antagonistic when a resource becomes scarce.
From an evolutionary perspective, these dynamic shifts provide opportunities for:
- Novel partnerships to arise (transition from competition or commensalism to mutualism),
- Increased integration (leading to the forms of endosymbiosis discussed in symbiogenesis),
- Breakdown of associations when the net effect becomes too costly for one partner.
Understanding competition, symbiosis, commensalism, and antibiosis thus provides a framework for analyzing how organisms influence each other’s survival and evolution, and how complex, integrated life forms can emerge from initially loose or even hostile interactions.