Key terms
| Term | Meaning |
|---|---|
| Community | All the populations of all the different species that live together in a particular area |
| Interspecific interaction | Relationship between different species in a community |
| Carrying capacity | The maximum population size of a species that the environment can sustain |
| Competition | Organisms of two species use the same limited resource and have a negative impact on each other |
| Predation | A member of one species, predator, eats all or part of the body of a member of another species, prey |
| Herbivory | A special case of predation in which the prey species is a plant |
| Symbiosis | Interspecific interaction in which two species live together in a long-term, intimate association |
| Mutualism | A symbiotic relationship between two species in which both partners benefit |
| Commensalism | A symbiotic relationship between two species in which one benefits and the other is unaffected |
| Parasitism | A symbiotic relationship between two species in which one benefits and the other is harmed |
Ecological relationships
The populations of all the different species that live together in an area make up an ecological community.
Community ecologists examine how different species in a community interact with each other. Interactions between two or more species are called interspecific interactions.
Different types of interspecific interactions have different effects on the two participants, which may be positive (+), negative (-), or neutral (0).
Competition
In interspecific competition, members of two different species use the same limited resource and therefore compete for it. Competition negatively affects both participants (-/- interaction), as either species would have higher survival and reproduction if the other was absent.
Species compete when they have overlapping niches, that is, overlapping ecological roles and requirements for survival and reproduction.
Predation
Photograph of a leopard killing a bushbuck
In predation, a member of one species (the predator) eats part or all of the living, or recently living, body of another organism (the prey). This interaction is beneficial for the predator, but harmful for the prey (+/- interaction). Predation may involve two animal species, but it can also involve an animal, such as a mammal, bird, or insect consuming part of a plant, a special case of predation known as herbivory.
Symbiosis
Symbiosis is a general term for interspecific interactions in which two species live together in a long-term, intimate association.
In mutualism, two species have a long-term interaction that is beneficial to both of them (+/+ interaction).
Mutualism example: Starlings benefit because they can eat ticks for free, while the buffalo gets the benefit because itching caused by ticks can be slightly reduced.
In commensalism, two species have a long-term interaction that is beneficial to one and has no positive or negative effect on the other (+/0 interaction).
In parasitism, two species have a close, lasting interaction that is beneficial to one, the parasite, and harmful to the other, the host (+/- interaction).
Ecological relationship summary
| Interspecific interaction | Effect on species 1 | Effect on species 2 |
|---|---|---|
| Competition | - | - |
| Predation/herbivory | + | - |
| Mutualism | + | + |
| Commensalism | + | 0 |
| Parasitism | + | - |
Population regulation
The maximum population size of a species that the environment can sustain is its carrying capacity.
Basically, any kind of resource important to a species’ survival can act as a limit. For plants, the water, sunlight, nutrients, and the space to grow are some key resources. For animals, important resources include food, water, shelter, and nesting space.
Limited quantities of these resources results in competition.Competition for resources may not affect populations that are well below their carrying capacity, assuming resources are plentiful and all individuals can obtain what they need. However, as population size increases, the competition also increases.
The graph above shows the carrying capacity of a seal population. Often, population size may bounce around a bit when it gets to carrying capacity, dipping below or jumping above this value. It’s common for real populations to oscillate (bounce back and forth) continually around carrying capacity, rather than forming a perfectly flat line.
Things to remember
In everyday life, we sometimes use the term symbiosis to mean a relationship that benefits both parties. However, in ecologist-speak, symbiosis is a broader concept and can include close, lasting relationships with a variety of positive or negative effects on the participants.
There is not always a clear division between symbiotic relationships. It's worth noting that many apparent commensalistic relationships actually turn out to be slightly mutualistic or slightly parasitic when we look at them more closely.
For instance, biologists are finding more and more evidence that our normal microbial inhabitants play a key role in health, whereas we used to think there was no benefit to humans at all!
Although parasitism and predation both have a +/- effect, they are not the same. In predation (or herbivory), the prey is consumed by the predator. Parasitism is different from predation in that the negatively affected species is not eaten or killed during a single feeding. In addition, parasites tend to be smaller than their hosts.
However, as in symbiotic relationships, the line is often blurred. Organisms may fall within a gradient between the two.
