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Interactions in communities

AP.BIO:
ENE‑4 (EU)
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ENE‑4.B.4 (EK)
Overview of competition, predation, herbivory, mutualism, commensalism, and parasitism.

Key points:

  • An ecological community consists of all the populations of all the different species that live together in a particular area.
  • Interactions between different species in a community are called interspecific interactionsinter- means "between."
  • Different types of interspecific interactions have different effects on the two participants, which may be positive (+), negative (-), or neutral (0).
  • The main types of interspecific interactions include competition (-/-), predation (+/-), mutualism, (+/+), commensalism (+/0), and parasitism (+/-).

Introduction

When we took a tour through population ecology, we mostly looked at populations of individual species in isolation. In reality, though, populations of one species are rarely—if ever!—isolated from populations of other species.
In most cases, many species share a habitat, and the interactions between them play a major role in regulating population growth and abundance.
Together, the populations of all the different species that live together in an area make up what's called an ecological community. For instance, if we wanted to describe the ecological community of a coral reef, we would include the populations of every single type of organism we could find, from coral species to fish species to the single-celled, photosynthetic algae living in the corals. For a healthy reef, that comes out to a whole lot of different species!
Image of a coral reef, showing many diverse species of fishes and corals living together and interacting with each other
Image credit: Fish aquarium sea fish tank by visavietnam, public domain
Community ecologists seek to understand what drives the patterns of species coexistence, diversity, and distribution that we see in nature. A core part of how they address these questions is by examining how different species in a community interact with each other. Interactions between two or more species are called interspecific interactionsinter- means "between."
In the rest of this article, we'll take take a look at the main types of interspecies interactions seen in ecological communities. Here is a quick preview:
NameDescriptionEffect
CompetitionOrganisms of two species use the same limited resource and have a negative impact on each other.- / -
PredationA member of one species, predator, eats all or part of the body of a member of another species, prey.+ / -
HerbivoryA special case of predation in which the prey species is a plant+ / -
MutualismA long-term, close association between two species in which both partners benefit+ / +
CommensalismA long-term, close association between two species in which one benefits and the other is unaffected+ / 0
ParasitismA long-term, close association between two species in which one benefits and the other is harmed+ / -

Overview: interspecies interactions

Interspecies interactions can be broken into three main categories: competition, predation, and symbiosis. Let's take a closer look at each.

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. Competition can be minimized if two species with overlapping niches evolve by natural selection to utilize less similar resources, resulting in resource partitioning.

Predation

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 or insect consuming part of a plant, a special case of predation known as herbivory.
Photograph of a leopard killing a bushbuck
Predators and prey regulate each other's population dynamics. Also, many species in predator-prey relationships have evolved adaptations—beneficial features arising by natural selection—related to their interaction. On the prey end, these include mechanical, chemical, and behavioral defenses. Some species also have warning coloration that alerts potential predators to their defenses; other harmless species may mimic this warning coloration.

Symbiosis

Symbiosis is a general term for interspecific interactions in which two species live together in a long-term, intimate association. 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.

Mutualism

In a mutualism, two species have a long-term interaction that is beneficial to both of them (+/+ interaction). For example, some types of fungi form mutualistic associations with plant roots. The plant can photosynthesize, and it provides the fungus with fixed carbon in the form of sugars and other organic molecules. The fungus has a network of threadlike structures called hyphae, which allow it to capture water and nutrients from the soil and provide them to the plant.
Photograph of an adult tapeworm. Based on the ruler provided for scale, the tapeworm appears to be over 25 feet long!
The brown structures are roots of Picea glauca, white spruce. The fuzzy white threads are the hyphae of a mutualistic fungus that interacts with the roots. Image credit: Mycorhizes by André Picard, Ph. D., CC BY-SA 3.0

Commensalism

In a 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). For instance, many of the bacteria that inhabit our bodies seem to have a commensal relationship with us. They benefit by getting shelter and nutrients and have no obvious helpful or harmful effect on us.
It's worth noting that many apparent commensalisms actually turn out to be slightly mutualistic or slightly parasitic (harmful to one party, see below) 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.

Parasitism

In a parasitism, two species have a close, lasting interaction that is beneficial to one, the parasite, and harmful to the other, the host (+/- interaction).
Some parasites cause familiar human diseases. For instance, if there is a tapeworm living in your intestine, you are the host and the tapeworm is the parasite—your presence enhances the tapeworm's quality of life, but not vice versa!
Image of a mature
A mature tapeworm. For a sense of scale, see the ruler at the bottom of the image—this is a long tapeworm! Image credit: Taenia saginata adult by CDC Public Health Library, public domain

Summary of interspecific interactions

NameDescriptionEffect
CompetitionOrganisms of two species use the same limited resource and have a negative impact on each other.- / -
PredationA member of one species, predator, eats all or part of the body of a member of another species, prey.+ / -
HerbivoryA special case of predation in which the prey species is a plant+ / -
MutualismA long-term, close association between two species in which both partners benefit+ / +
CommensalismA long-term, close association between two species in which one benefits and the other is unaffected+ / 0
ParasitismA long-term, close association between two species in which one benefits and the other is harmed+ / -

Want to join the conversation?

  • starky ultimate style avatar for user Aivan Cuway
    In the overview, they made mention of herbivory. is there any such ting as carnivory or omnivory? and if there was, what would have been the effect, considering the predator and prey relationship.
    (5 votes)
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    • female robot grace style avatar for user Chiara
      Yes, there are such things as carnivory and omnivory, the predator-prey relationship will still be +/-, because the predator benefits from eating the other organism but it harms/kills the prey. I hope you find this useful! :)
      (14 votes)
  • piceratops ultimate style avatar for user Joshua
    Is there a type of interaction for a plant eating an animal, like a Venus flytrap eating a fly? It doesn't seem like a predatory interaction, but it also doesn't seem like a herbivorous interaction.
    (3 votes)
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  • blobby green style avatar for user Areeba Sajjad
    What are some examples of Intra and Interspecific Interactions? Please response quickly, urgently required. Thanks.
    (3 votes)
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  • blobby green style avatar for user idchambers
    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 + / -
    Mutualism A long-term, close association between two species in which both partners benefit + / +
    Commensalism A long-term, close association between two species in which one benefits and the other is unaffected + / 0
    Parasitism A long-term, close association between two species in which one benefits and the other is harmed. what are examples of these
    (2 votes)
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  • piceratops ultimate style avatar for user WarmSausageTea‭
    What about 0/0 relationships, where both species live in the same environment, but do not affect each other?
    (2 votes)
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  • blobby green style avatar for user usdiane
    Isn't Amensalism among interspecific interaction?
    (2 votes)
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  • blobby green style avatar for user S20026786
    Where do we draw the line between parasitism and predation. Would a mosquito sucking on our blood or lice eating our scalp be parasitic or predatory?
    (2 votes)
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  • male robot hal style avatar for user 0m0gbaatar
    is (-/0) type of interaction not a thing?
    (0 votes)
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  • blobby green style avatar for user Coryonna Harris
    How come Mutualism cant be a +/-?
    (1 vote)
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  • blobby green style avatar for user fouziasultana34
    why does the secondary consumer still depend on the producer despite consuming the primary consumer?
    (0 votes)
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    • leaf grey style avatar for user Alex
      It's a chain of interactions, where all levels can affect all other levels. For example, if all the producers died off due to a disaster, the primary consumer population would drop drastically, which of course would affect the secondary consumers.
      (2 votes)