Not everyone gets along. One pair of friends might always be mean to each other, while another pair has one nice person and one that takes advantage of the nice person. A mutually beneficial relationship between people would involve them being kind to each other and getting along. 

Similar relationships exist amongst other living things. These interactions, called symbiosis, range from mutually beneficial to parasitic. Besides the different types based on who benefits, symbiotic relationships are also classified by necessity, physical attachment, and whether one organism lives inside or outside of another.

Types of symbiosis

If at least one of the species in a symbiotic relationship requires that interaction to survive, the relationship is said to be obligate. If the interaction is unnecessary, the symbiosis is facultative. One example is our relationship with food crops: it is obligatory to grow crops to eat them, but farmers facilitate that process so that each person doesn’t have to grow their own food. 

Note that obligate and facultative relationships might not be reciprocal. That means interactions might be obligate for both parties, obligate for one and facultative for the other, or facultative for both. Head lice and humans have an obligate-facultative relationship, and the Nile crocodile and Egyptian plover a facultative-facultative one. Obligate-obligate relationships are rare in nature but have been found among a number of bacteria species.

The symbiotic relationship divisions dealing with physical location and attachment are more of a dichotomy than the obligate/facultative combinations. 

If the organisms involved in an interaction are physically connected, they are in conjunctive symbiosis. If they are not attached, they are in disjunctive symbiosis. Spanish moss and trees are disjunctive in nature; arguably, most symbiotic relationships are disjunctive. Lichens, made of an algae or cyanobacteria and a fungus – we’ll come back to these organisms later – are an example of a composite of conjunctive symbionts

Some species in a symbiotic relationship live inside of the other species, in endosymbiosis. In ectosymbiosis, neither organism is inside of or merged with the other. Barnacles live on the surface of baleen whales as ectosymbionts, while hookworms are endosymbionts that live within humans, cats, or dogs.

Now, we move onto symbioses relating to which organisms in an interaction benefit and which are harmed. After reading the following paragraph, label the relationships listed earlier in this section by which type of symbiosis they involve. 

Mutualism is a relationship where both organisms benefit. Commensalism is a symbiosis in which one member benefits and the other is unharmed. Predatory relationships involve one organism (a predator) hunting another (its prey). Parasitic relationships are also harmful to one partner, but the other member lives on or in it (though the parasite may still kill its host over time). Competition occurs when organisms of the same (intraspecific) or different (interspecific) species must contend for resources.

Complex symbiotic connections

Back to lichens. Lichens are an organism consisting of an algae or cyanobacteria and a fungi. Due to their unique characteristics, lichens inhabit many biomes and geographical areas. More than 20,000 species of lichens are known. Lichens are in a conjunctive endosymbiotic obligate-obligate mutualistic relationship. Phew! Every symbiotic relationship can be described using a combination of terms similar to that phrase. 

Let’s consider other animal interactions. How would you classify the symbioses between ocellaris clownfish and anemones? How about those between a snake and a mouse?

Symbiotic relationships are also discussed in sociology. Every relationship between a human and another human can be classified using the same terms we refer to animals with. 

Think of someone you know and your interactions with them. Are they positive or negative? Who benefits from the relationship? Do you compete with each other over anything? 

What symbiotic relationships do you find most interesting? Do some research to find out about another organism-organism interaction. Here are some ideas if you’re stumped:

  • Escherichia coli and the human intestines
  • Tobacco mosaic virus (TMV) and tomatoes
  • Lions and antelopes
  • Remoras and sharks
  • Bobcats and coyotes

symbiosis worksheet

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