What Is Symbiosis?

Stated Clearly presents: What is symbiosis in biology?

Normally when people talk about symbiosis they’re talking about two different types of organisms cooperating to help each other survive.

For example, clown fish hide from predators among the tentacles of sea anemones. In return, they feed the anemone with their own droppings. Yum! The enemone and the clown fish, enjoy a symbiotic relationship.

In biology however, symbiosis has a broader meaning than cooperation.

It’s classically defined as any “long-term living together of unlike organisms”

Mutualistic symbiosis, or mutualism, is when both partners benefit from the relationship – like clownfish and anemones.

When I say both partners benefit from the relationship, what I mean is that both organisms experience a significant increase in evolutionary fitness. They both end up being better at surviving and reproducing.

Parasitic symbiosis, or parasitism, is when one organism benefits while the other is harmed. Ticks are a good example of a parasite, they drink your blood and then sometimes repay with lyme disease. Total jerks.

Commensalistic symbiosis, or commensalism is when one organism benefits, while the other is not dramatically helped or harmed.

Squirrels live in oak trees, sometimes eat bark, leaf buds, and of course, acorns. They consume the flesh, of the oak trees unborn offspring. This is great for the squirrel of course but surely the squirrels are bad for their host’s evolutionary fitness, right?

Well, multiple studies have shown that the relationship between oak trees and squirrels is extremely complex and seems to stretch back millions of years, both organisms trapped in an evolutionary arms race against each other.

Through the ongoing process of descent with modification, acted upon by selection, Oak trees have evolved many tricks to control the behavior and population size of their ancient rivals, including the production of toxins in their seeds.

Squirrels, in return, have evolved a digestive system that can handle the toxins fairly well, but more importantly, squirrels have changed their behavior. When they find fresh acorns, instead of eating them, they stash them in shallow hiding places to let rainwater detoxify them over several weeks to several months. A single squirrel can make hundreds of stashes all over its territory each year.

If the squirrel dies before eating the acorns, or simply forgets where some of them were hidden, the squirrel has, in effect, planted new trees, often in places far enough from the parent tree that there will be no parent/offspring competition for sunlight when the saplings begin to grow. This is called seed dispersal, and actually helps a tree produce more successful offspring.

While it’s difficult to fully calculate evolutionary fitness, (there are so many unknown variables involved) it appears that right now in the middle of this crazy evolutionary arms race, many trees are either breaking even, or sometimes experiencing an overall fitness gain when squirrels move into their branches.

Here we see that there exists a symbiotic continuum from parasite to mutualist. Relationships can and do change dramatically over time.

Even ticks, when they’re not carrying diseases, sometimes act more like commensalists than parasites. Due to their small size, they take very little blood from their doners. This means that a single tick won’t usually cause a noteworthy fitness decline in its host. Results from a recent study show that possums may actually benefit from tick infestations.

This is because they snack on ticks, and a single possum may eat over 5,000 ticks per week during tick season!

Now, ticks are really small, but 5000 per week? That is a noteworthy addition to the possums regular diet.

Ticks are like little sack lunches that come directly to you – free delivery.

It might seem silly that biologists invented a word “symbiosis” to encompass everything from parasites to cooperators, but the reason for this is that parasites can evolve to become cooperators and vice versa.

Understanding these evolutionary transitions isn’t just fascinating, it is now helping us control diseases.

For example, many of the microbes living in your intestines help you digest your food. They eat the parts of it you can’t digest, and then excrete the waste, waste that you can digest. Yum!

Most of the microbes living inside us today, either have a mutualistic or commensalistic relationship with us but some of them started out as parasites.

In 1991, cholera, a deadly bacterial disease, broke out in South America. As you may know, cholera causes extreme diarrhea. It makes its host desperate to use the toilet and then spreads to new hosts either through dirty drinking water, or through person to person contact.

Biologist Paul Ewald studied its spread and the real-time evolution of cholera bacteria in different countries following the outbreak. In nations with bad water filtration, the bacteria remained deadly, year after year. This is because natural selection favoured strains of the bacteria that would make people use the toilet more frequently, even if it eventually killed them due to dehydration, because the bacteria could contaminate more water and spread faster to new hosts in the process.

In countries with good water filtration systems, strains of the bacteria evolved toward commensalism. In these environments, natural selection favored microbes that were “kinder” to people, because folks were healthy enough to go to school, work, and mingle with friends and family. This allows the bacteria spread slowly and non-violently, via human to human contact.

The take home message here is this: When cholera breaks out, not only should we immediately treat the sick, we should distribute clean drinking. Doing so will guide the bacteria’s evolution in a direction that is good for all of us.

These fascinating interactions and evolutionary transitions from enemy to friend, and sometimes friend to enemy, are all possible because of symbiosis: the long-term living together of unlike organisms.

While most people have no trouble understanding how evolution causes animals to adapt to their environments: a bear to the cold, for example, we often forget that a creature’s environment also includes the other organisms that share it. If two species live together and interact long enough, the slow process of evolution: descent with modification, acted upon by selection, can cause living things to adapt to each other.

I’m Jon Perry and that is symbiosis: the long-term living together of unlike organisms, stated clearly.