What is it called when two organisms are interacting together in an ecosystem and one benefit from the interaction while the other is harmed?

Commensalism is a type of relationship between two living organisms in which one organism benefits from the other without harming it. A commensal species benefits from another species by obtaining locomotion, shelter, food, or support from the host species, which (for the most part) neither benefits nor is harmed. Commensalism ranges from brief interactions between species to life-long symbiosis.

• Commensalism is a type of symbiotic relationship in which one species benefits, while the other species is neither harmed nor helped.
• The species that gains the benefit is called the commensal. The other species is termed the host species.
• An example is a golden jackal (the commensal) following a tiger (the host) to feed on leftovers from its kills.

The term was coined in 1876 by Belgian paleontologist and zoologist Pierre-Joseph van Beneden, along with the term mutualism. Beneden initially applied the word to describe the activity of carcass-eating animals that followed predators to eat their waste food. The word commensalism comes from the Latin word commensalis, which means "sharing a table." Commensalism is most often discussed in the fields of ecology and biology, although the term extends to other sciences.

Commensalism is often confused with related words:

Mutualism - Mutualism is a relationship in which two organisms benefit from each other.

Amensalism - A relationship in which one organism is harmed while the other is not affected.

Parasitism - A relationship in which one organism benefits and the other is harmed.

There's often debate about whether a particular relationship is an example of commensalism or another type of interaction. For example, some scientists consider the relationship between people and gut bacteria to be an example of commensalism, while others believe it is mutualistic because humans may gain a benefit from the relationship.

• Remora fish have a disk on their heads that makes them able to attach to larger animals, such as sharks, mantas, and whales. When the larger animal feeds, the remora detaches itself to eat the extra food.
• Nurse plants are larger plants that offer protection to seedlings from the weather and herbivores, giving them an opportunity to grow.
• Tree frogs use plants as protection.
• Golden jackals, once they have been expelled from a pack, will trail a tiger to feed on the remains of its kills.
• Goby fish live on other sea animals, changing color to blend in with the host, thus gaining protection from predators.
• Cattle egrets eat the insects stirred up by cattle when they are grazing. The cattle are unaffected, while the birds gain food.
• The burdock plant produces spiny seeds that cling to the fur of animals or clothing of humans. The plants rely on this method of seed dispersal for reproduction, while the animals are unaffected.

Inquilinism - In inquilinism, one organism uses another for permanent housing. An example is a bird that lives in a tree hole. Sometimes epiphytic plants growing on trees are considered iniquilism, while others might consider this to be a parasitic relationship because the epiphyte might weaken the tree or take nutrients that would otherwise go to the host.

Metabiosis - Metabiosis is a commensalistic relationship in which one organism forms a habitat for another. An example is a hermit crab, which uses a shell from a dead gastropod for protection. Another example would be maggots living on a dead organism.

Phoresy - In phoresy, one animal attaches to another for transport. This type of commensalism is most often seen in arthropods, such as mites living on insects. Other examples include anemone attachment to hermit crab shells, pseudoscorpions living on mammals, and millipedes traveling on birds. Phoresy may be either obligate or facultative.

Microbiota - Microbiota are commensal organisms that form communities within a host organism. An example is the bacterial flora found on human skin. Scientists disagree on whether microbiota is truly a type of commensalism. In the case of skin flora, for example, there is evidence the bacteria confer some protection on the host (which would be mutualism).

Domestic dogs, cats, and other animals appear to have started out with commensal relationships with humans. In the case of the dog, DNA evidence indicates dogs associated themselves with people before humans switched from hunting-gathering to agriculture. It's believed the ancestors of dogs followed hunters to eat remains of carcasses. Over time, the relationship became mutualistic, where humans also benefited from the relationship, gaining defense from other predators and assistance tracking and killing prey. As the relationship changed, so did the characteristics of dogs.

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Planet Earth is inhabited by millions of species—at least! Because different species often inhabit the same spaces and share—or compete for—the same resources, they interact in a variety of ways, known collectively as symbiosis. There are four main symbiotic relationships: mutualism, commensalism, parasitism, and competition.

To explore these relationships, let’s consider a natural ecosystem such as the ocean. Oceanic environments are known for their species diversity. Imagine you are on a diving expedition to explore the worlds beneath the waves. If we were in the warm waters of the Pacific or Indian Oceans, we’d likely spot an excellent example of mutualism: the relationship between clownfish and sea anemones. In a mutualistic relationship, both species benefit. Sea anemones live attached to the surface of coral reefs. They trap their prey with stinging cells called nematocysts, which are located on their tentacles. Nematocysts release toxins when a small animal contacts an anemone’s tentacle. This paralyzes the stung animal, allowing the anemone to easily bring the animal into its mouth for ingestion.

While other fish succumb to these toxic stings, clownfish secrete a substance in the mucus covering their bodies that suppresses the firing of nematocysts. This allows the clownfish to swim comfortably between the tentacles of anemones, creating a protected environment in which potential predators are killed off by anemone stings. This clearly benefits the clownfish, but how about the sea anemones? The brightly colored clownfish attract other fish looking for a meal. These unsuspecting would-be predators are then caught and eaten by the anemones.

As we continue in our imaginary deep-sea voyage, we may observe the commensalistic relationship that exists between barnacles and humpback whales (Megaptera novaeangliae). Commensalism happens when one species lives with, on, or in another species, known as the host. The host species neither benefits nor is harmed from the relationship. In our imagined example, various species of barnacles attach themselves to the skin of whales. Scientists have not discovered the exact mechanism by which barnacles are able to do this, but it does not appear to bother the whales. How do the barnacles benefit from this unlikely relationship? The huge whales transport the tiny barnacles to plankton-rich waters, where both species feast upon the abundant microorganisms that live there.

Of course, some symbiotic relationships do cause harm. In parasitism, one species (the parasite) lives with, on, or in a host species, at the expense of the host species. Unlike in predation, the host is not immediately killed by the parasite, though it may sicken and die over time. Examples of common parasites found in the ocean include nematodes, leeches, and barnacles. That’s right—though barnacles exist commensally with whales, they are parasites for swimming crabs. A barnacle may root itself within a crab’s reproductive system. While the crab does not die from this interaction, its reproductive capabilities are greatly diminished.

The last example of symbiosis we will explore on our imaginary dive is competition—the struggle among organisms for the same limited resources in an ecosystem. Competition can happen between members of the same species (intraspecific competition) and between different species (interspecific competition). An example of interspecific competition in the ocean is the relationship between corals and sponges. Sponges are very abundant in coral reefs. If they become too successful, however, they take needed food and other resources from the corals that make up the reef. Sponges may outcompete corals for resources in the short term, but if too many corals die, the reef itself becomes damaged. This is bad for the sponges, which may themselves begin to die off until the reef is balanced again.

Symbiotic relationships can be useful measures of an ecosystem’s health. For example, large tracts of coral reefs are severely damaged or dead because of recent increases in ocean temperature due to climate change. The temperature increase induces coral to expel the algae that live mutualistically within them. Without their algae, the coral turn white and die. This loss of symbiosis is an early sign of declining coral health and speaks to the importance not only of studying symbiosis within marine environments, but also of examining the negative impacts that humans can have on these interactions. In the words of National Geographic Explorer Sylvia Earle: “We need to respect the oceans and take care of them as if our lives depend on it. Because they do.”