Study Guide

Ecology: Organisms and Their Environments - Symbiosis

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The final species interaction we will discuss is symbiosis, or the interaction where two species live in the same location, and one or both receive a benefit from the other. Under this definition, predation could also be considered a type of symbiosis. In fact, as you will see below, one of the types of symbiosis—parasitism—is actually just a special form of predation. Despite this, ecologists generally classify predation and symbiosis as two different kinds of interactions. Don’t ask us why.

There are three main type of symbiotic interactions:

  1. Mutualism
  2. Parasitism
  3. Commensalism

Each of these types differ in how each of the participating species benefits or harms the other. In mutualistic relationships, both species benefit each other. In parasitic interactions, one species is harmed and the other is benefited. In the last form, commensalism, one species is benefited and the other receives no harm or benefit. When people hear the word "symbiosis" they often think of its most famous form, mutualism. You now know, of course, that symbiosis is not just mutualism, but that it also consists of other types of close living relationships. Just for kicks, let’s take a brief look at a couple examples of each type of symbiotic relationship.

One of the most famous, and most ecologically important, mutualistic relationships is the one enjoyed by insects and plants everywhere: pollination. In pollinator-plant relationships, the pollinator—usually an insect, but some birds and mammals can get in on the action as well—receives food in the form of nectar. The plant, on the other hand, receives the great reproductive benefit of having its pollen, equivalent to sperm in animals, transferred to another plant. Since plants don’t often find themselves at the singles bar, having a reproductive "go-between" is vitally important, in every sense of the word. Without pollinators, many plants would quickly become extinct. On the flipside, without pollinating plants, the pollinators themselves would be in deep trouble. It’s truly a win-win situation.

Another fascinating mutualistic relationship is seen among certain species of ants and aphids. Aphids are tiny, soft insects that feed on plant sap. Aphids only need part of the sap and end up excreting most of the sugar and waters they extract from the plant as waste. Some species of ants have found it productive to feed on the sugar and water excreted by aphids. In fact, they often treat the aphids like humans treat cattle, moving them from one feeding source to the next. Nice, huh? Aphids get the benefit of protection from predators and free transport from plant to plant while the ants get easy access to yummy plant sap. Perhaps, if you keep your eyes peeled, you will see an aphid dude ranch run by some young, gun-slinging ants.

Parasitic relationships, in contrast to mutualistic ones, are not nearly as warm and fuzzy. In fact, these relationships are always win-lose, with the loser facing anything from discomfort to death. Abusive relationship, much? There are lots of different kinds of parasites, but they fall into two main types.

Some establish their homes inside their hosts, or the organism they reside in, while others feed at the surface.

The first type, called endoparasites, includes the protozoan Giardia lamblia, which causes diarrhea and extreme pain, and the nematode Wuchereria bancrofti, which clogs up lymph nodes and causes limbs to swell in grotesque ways in mammals. The disease is appropriately called elephantiasis.

The second type, called ectoparasites, include head lice, disease-transmitting mosquitoes, and Dermacentor andersoni, the carrier of Rocky Mountain spotted fever. In each of these relationships, the parasite gains the benefit of a shelter, food, or both. At the same time, the host receives varying degrees of harm.

The last symbiotic relationship, commensalism, is the most difficult to detect in nature. However, a few examples will help you clarify the concept. Many commensalistic relationships are found in the ocean. The remora, for example, has a sticky disc on its head that allows it to physically hitchhike on larger creatures, like whales and sharks (and scuba divers), and feed on any food their unwary traveling companions might miss.

A non-marine example of commensalism would include the "squatter" behavior exhibited by some desert lizards that take residence in abandoned rat or snake holes. The lizards receive shelter while the other animal receives nothing in return. Not even a thank you card.

It turns out that some ecologists believe that truly commensalisitic relationships are extremely rare. They claim that we just haven’t discovered the harm or benefit being received by the other party in most of the commensalisitic relationships we’ve found. In the first example above, for instance, it would be difficult to measure the harm or benefit received by the whale from the remora. In the second example, it is even questionable whether a symbiotic relationship is occurring at all, since one of the interacting species is not actually present in the environment.

When thinking about symbiotic relationships, it's important to recognize that such interactions can have great influences on population and community dynamics. It would be silly to study ant populations by themselves, for instance, without taking into account the aphids they "manage." Therefore, a community perspective is just as important to understanding populations as a population perspective is to understanding communities And you thought human relationships were complicated.

Brain Snack

Humans are covered by symbiotic microscopic organisms known as our microflora; they outnumber your cells 10:1. These organisms are involved in protecting us from disease, training our immune system, and providing us with nutrients. Read more about them here.

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