symbiotic relationships

Relationships Among Organisms : 

Relationships Among Organisms

Making Connections : 

Making Connections If you look closely at the organisms that live in your neighborhood or your yard or a nearby park, you can probably spot examples of relationships among organisms.  That is, you will see organisms interacting with one another in various ways. Let's take an example with which you're no doubt familiar—the relationship between squirrels and oak trees.

Making Connections (cont.): 

Making Connections (cont.) The benefits to squirrels may be more immediately clear.  Squirrels, of course, get nourishment from oak trees by eating acorns.  They may also gain protection from predators by climbing these trees' tall branches. Do the trees get anything in return?  Yes, they do.  Acorns contain the seeds of oak trees.  When a squirrel eats an acorn, there is no benefit to the tree.  However, squirrels are known to store acorns away in multiple hiding places for later use.  In doing this, they benefit oak trees in two ways.  First, they scatter the trees' seeds.  Acorns are so big and clunky that they don't spread easily by other means such as wind.  Second, squirrels store acorns underground, which means that, more or less, they plant the seeds. Of course, squirrels return later in time to eat many of these acorns, but some acorns are lost or forgotten.  Other times, a squirrel may die before it has a chance to return to its acorn stash.  Thus, some seeds get the chance to grow into new oak trees.

Symbiosis: 

Symbiosis The relationship between squirrels and oak trees is mutually beneficial, meaning both organisms benefit.  Thus, it's an example of what we call mutualism . Mutualism is an example of a symbiotic relationship .  The other three types we'll be discussing are commensalism , parasitism , and amensalism . Although often used by people to refer to mutualism specifically, symbiosis simply refers to two otherwise unrelated species living together closely.  Thus, symbiotic relationships vary in nature: they're not all mutually beneficial. You'll learn about the other three types of symbiotic relationships soon.  First, we'll look at a few more examples of mutualism.

Mutualism: 

Mutualism As you just learned, mutualism is a symbiotic relationship in which both species benefit. Let's examine a few more examples.  Have you ever heard of probiotics ? Probiotics are live bacteria that people consume intentionally. Probiotics may come in the form of juices, yogurts, or pills. Humans are believed to reap a variety of rewards from consuming these so-called healthful bacteria, including improved digestion and immune function.

Mutualism (cont.): 

Mutualism (cont.) The bacteria benefit by being provided food and a good place to live. Another example of mutualism exists in lichens.  Lichens are actually made up of two different organisms: fungi and algae.  Algae conduct photosynthesis, providing sugars and oxygen to fungi.  In return, fungi provide water, salts, and protection to algae. While there are some that argue that algae actually have a higher rate of survival and reproduction on their own than in their partnership with fungi, one thing is clear.  As a result of their partnership, the two organisms are able to survive in harsh conditions that alone neither would be able to tolerate.  Lichens are found in the harshest deserts, as well as the Arctic tundra.  In this respect, their relationship can indeed be considered mutualistic .

Mutualism (cont.): 

Mutualism (cont.) One more example of mutualism exists between sea anemones and clownfishes .  Sea anemones are equipped with stingers that are fatal to many organisms, but clownfishes produce a mucus that protects them from sea anemone stings.  Thus, clownfishes are able to cozy up to sea anemones for shelter and protection. What do sea anemones get out of it?  A meal.  As predators hunt clownfishes , sea anemones make prey out of them, capturing them in their tentacles.

Commensalism: 

Commensalism Next, let's look at commensalism .  It is defined as a relationship between species in which one species benefits, while the other is unaffected (neither helped nor harmed).  This relationship is considered rare since in almost any relationship between two species, both are affected in some way or another.  Sometimes scientists consider a relationship commensalistic even when both species benefit if the benefits to one far outweigh the benefits to the other. One commonly cited example of commensalism is the relationship between sharks and remora.  Remora are very small fish that cling onto or hang around sharks, eating scraps of food leftover from shark meals.  The sharks do not appear to benefit from the relationship, yet they also don't appear to be interested in eating the remora.

Parasitism: 

Parasitism Parasitism is a symbiotic relationship in which one species benefits, while the other is harmed.  As with mutualism, there are many examples of parasitism we could examine. For example, many diseases are caused by parasites .  Malaria is one such disease.  It's caused by a protist , which is injected into humans via the bites of mosquitoes. Or consider tapeworms.  These parasites attach themselves inside the intestinal walls of humans and other animals.  There they absorb food that their host has already broken down for them.  Tapeworms can grow to be enormously long.  Some have been discovered to be as long as 18 meters!  While tapeworms do not tend to be deadly, they can weaken their hosts severely since they steal so much of the host's nutrients.

Parasitism (cont.): 

Parasitism (cont.) While you may know mistletoe as the holiday kissing plant, it has a darker side.  Mistletoes are parasites.  They grow onto the branches of trees, penetrating the trees' tissues to steal water and nutrients.  Sometimes the trees are simply weakened and their growth stunted.  Other times, if an infestation is particularly severe, trees may die as a result of this parasite.

Amensalism: 

Amensalism Amensalism is a symbiotic relationship in which one species is harmed, while the other is unaffected.  Like commensalism, cases of amensalism may be rare because it's rather unlikely that a species is unaffected by its interactions with another species. One example of amensalism , however, is the relationship between grass and larger animals that may trample across it.  When larger animals trample across grass, or lie down on it, they may kill or injure some of it.  This action is unlikely to hurt the animals because even if they are grazers, they're likely to trample on grass only if it's plentiful.

Other Interactions Among Organisms: Competition: 

Other Interactions Among Organisms: Competition Now that you've learned about the various types of symbiosis, let's talk about two other ways in which organisms interact: competition and predation. Competition occurs when species compete for the same resources.  For example, some trees in forests grow so tall that they block sunlight from other plants.  Some predators, such as coyotes and red foxes, are considered competitors if they share the same habitat and food resource.  Introducing non-native species into an environment can often result in competition because if the species is well-suited to the environment, it may threaten to take over the niche of a native species. Competition typically occurs when a resource is not plentiful enough for all.  It's also unlikely that both species are equally fit to compete for the resource.  Usually, one prevails over the other.  The other population may dwindle, or the species may even become extinct. Oftentimes, competition leads to evolution by natural selection.  If a resource is limited, then individuals among the two populations that exhibit traits that allow them to out-compete for the resource, or exploit other resources, are more likely to pass on their genes.  Over time, the two populations may evolve to occupy different niches.

Other Interactions Among Organisms: Predation: 

Other Interactions Among Organisms: Predation Predation occurs when one species is a resource for another species. For example, baleen whales prey upon plankton.  Lions prey upon deer and other mammals. While the individual prey is eaten by the predator, the prey species typically has developed adaptations that help it recover to some extent from predation.  That is, a balance is often reached.  Rather than destroy a population, predation may help control it, so that it doesn't grow out of control.