Interactions between organisms

All living organisms interact with their environment and each other. Actually, this fact comes so easy to us, that we quite often forget about it (which is most probably the reason behind many alterations of an environment and high pollution rates). We define the interactions of organisms as competitors, predators, symbionts, and detritivores. Some scientists argue, that we should consider parasitism as another form of interaction, but I will talk about it under the paragraph about predation.

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Two different kinds of competition exists: the exploitation competition and interference competition. When talking about the exploitation competition, we speak of the indirect competition. This means that the organisms compete because they all need the same (limited) resources (food, mates, habitat, etc.) in order to survive. These organisms do not necessarily belong to the same species or to the same trophic level. The relationship can also be cryptic (not clearly visible) and therefore hard to detect.  

Example: a bear catches a fish and eats it and therefore outcompetes other bears even though there are other fish available in the river.

When talking about interference competition, we speak of direct competition between the organisms. The competing organisms can belong to the same species (intraspecific competition) or to different species (interspecific competition). The organisms compete for limited resources (food, mates, habitat etc.) and their interactions are often aggressive towards one another.

Example: the Rainbow trout and marble trout compete for the same breeding grounds. The former is more competitive and more aggressive and therefore takes all the best breeding grounds. This enables it to establish big populations and makes it one of the most common invasive species in Europe.

Predation (and parasitism)

Predation and parasitism are both an example of an interaction, where one organism has the benefit and the other loss. Predators are divided into three groups: herbivores (actually, they somehow prey on plants), carnivores (including scavengers) and omnivores. Herbivores are animals, that are anatomically and physiologically adapted to eat and get the nutrients from the plant material. The same is true for omnivores, but their digestive system is also adapted to obtain the nutrients from the animal-based diet. The third group are the carnivores, which get the nutrients solely out of the animal-based diet. At this point, you maybe also thought about the carnivorous plants. Well, they do get some nutrients from the soil but the amount they get is very low, and because of that, they developed such a cool adaptation.

Examples: The biggest herbivore alive is the African elephant, which also plays the role of a keystone species and an ecosystem engineer. A similar role is also played by carnivorous (also known to be scavengers) wolves and omnivorous crayfish.

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As you see, predators are divided into groups depending on what they eat, but we also divide them by their predatory technique. The true predators kill the prey and eat it whole or only a part of it. We know many herbivorous groups by the name of grazers. They tend to graze on grass, shrubs, trees or any other type of plant. When grazing, they do not kill the plant but only bite off the tip or a part of the plant, leaving the root system intact.

The last group here are parasites and parasitoids. The latter are organisms, that find a host, lay their eggs in them and leave or die. When the eggs develop and hatch, the baby parasitoids feed on the host, killing it at the end (sometimes, they also influence the behavior of the host along the process). Parasites, on the other hand, can be endoparasites (live inside of the host’s body) or ectoparasites (live on the outside of the host). The endoparasites usually try their best to keep the host happy, up and running, because if the host dies, the parasite dies with it. There are three types of parasites depending on what they prefer to eat: monophagous (one type of food), oligophages (several types) and polyphages (many types/ whatever they stumble across).

Examples: A good example of a true predator are the big cats. They have many hunting techniques, which is also true for the grazers. Examples of grazers are the sea turtles and a hippopotamus. Parasitoism occurs in many wasp species, but we also know of some flies and beetles that do this. When speaking about the parasites, bedbugs or ticks present a good example of an ectoparasite and a tapeworm is an example of an endoparasite.  


We know three types of symbiosis – mutualism (both organisms benefit), commensalism (one benefits and the other one is not significantly helped or harmed) and parasitism (where one benefits and the other is harmed). All of these relationships have evolved through a long process of coevolution. We have already talked about the parasites in the paragraph before and there is not really anything else to add in the case of commensalism. When speaking of a mutualistic relationship, we know facultative and obligatory mutualists. Facultative mutualists can survive on their own but perform a lot better, when in a mutualistic relationship. On the other hand, the organisms in obligatory mutualism cannot survive (or reproduce) without one another.

Examples: The facultative symbiosis is known among fruit trees and birds. If the bird eats the seed it will help with the dispersion of the seeds, but both species can survive without this interaction. An example of an obligatory mutualism is the relationship between corals and zooxanthellae.

Related article: Coral reefs 


This is an interaction where one organism benefits and the other couldn’t care less (because it’s dead). Detritivores (and scavengers (even though we think of them as predators)) feed on dead plant (or animal) matter and feces and present an important link in nutrient and energy flow. They are also very important in cleaning the ecosystem and preventing the spreading of diseases (rotten organic matter is paradise for bacteria and viruses that can cause deadly diseases).

Examples: Earthworms aerate the soil and relocate the organic matter and nutrients to deeper parts of the soil for the plants to use.

As you see, everything in the ecosystem has its own, important role to play and because of this, the conservation of all ecosystems is important.

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