That's right, it's time to talk about relationships.  Not the birds and the bees conversation you may be thinking about, but the biological interactions that make up how all organisms can interact with each other!

In ecology, we observe the ways that plants and animals interact with each other within their respective communities.  Depending on the resources available and the biodiversity present, the range of these interactions can vary, but they all have potential to appear in ecosystems around the world.  A number of these relationships even play directly into the flow of energy throughout our planet.

Today, we'll be going over 6 of these types of relationships from across the spectrum of positive and negative effects on the organisms involved in the interaction.  I'm sure some of these will jump right to the front of your mind, but here's hoping some of these will be a learning experience.  At the very least, I'm hopeful some of the examples will open your mind to the fascinating things nature is capable of supporting!
Let's dive in!

Predation

We'll start off with one that most people are likely aware of: predation.  At least within the realm of animals killing and consuming other animals, this is a predator-prey relationship.  Animal-plant interactions in this category go by the term herbivory, but still fall squarely into this camp.  Regardless of the makeup of the interaction, predation is a positive-negative relationship, being a boon to the predator and a detriment to the prey.
Predation is a short-term interaction where the predator in the scenario is reliant on the prey for energy.  An example of this relationship would be that of a bear and salmon.  The bear actively hunts the salmon for sustenance.  Bear diets aren't completely animal-based, however, and they often eat fruits and plants, which would be considered herbivory.  Bears also go on the offensive in self-defense, but this interaction would not be considered predation in most cases.  The bear would not likely consume the animal it was defending against, making the scenario not an inherently energy-dependent action.

Parasitism

While researching this topic, it seemed that some folks consider parasitism an act of predation.  At face value, this makes some sense as the relationship is a positive-negative scenario similar to predation.  However, the interaction is long-term, depending on the host in the scenario to be alive in order for the parasite to gain benefit from the relationship.
Predation is a means to move energy around the food web. When animals consume plants, this predation is known as herbivory
This long-term trait of the relationship is key to identifying the difference.  There are other identifiers to look for such as a metabolic dependency on the host, the parasite typically being smaller than the host, or the negative act on the host being passive in nature.  Most of these indicators exist on a spectrum rather than always being true or false, though.
Parasites can do some astounding things to their hosts, from spreading disease all the way to extremes such as brainwashing!

Yikes!
One of my favorite, somewhat gruesome, examples is a type of hairworm (Spinochordodes tellinii) and grasshopper.  This hairworm will embed itself within the grasshopper and entirely sustain itself on the nutrients the grasshopper consumes.  All the while, the hairworm is slowly altering the grasshopper's brain chemistry to a point that it can actually control the grasshopper.  When the hairworm is fully grown and capable of reproducing, it will trigger a chemical attack on the grasshopper's brain.  This causes the grasshopper to leap into a body of water where it will drown, allowing the hairworm to eject itself and find a space to reproduce.  Truly fascinating (and terrifying) stuff!

Commensalism

​Okay, so let's walk away a bit from the gruesomeness of nature and look at a slightly more positive relationship.  Tiny steps first, though.  Commensalism is a positive-zero relationship where one party gets a benefit while the other gets seemingly nothing, positive or negative.  These relationships are hard to pinpoint in nature since most organisms enter relationships with a goal of a positive outcome, a net gain of sorts.  Most would wager that cases of commensalism may very well be a positive-positive relationship, but we haven't discovered both positive sides yet.
One such example is lichen and trees.  Lichen are organisms that use trees as a substrate to grow on, so the tree offers them a space above the ground to thrive.  Interestingly, trees seem to get nothing from this relationship and have not developed any means of shedding the lichen from their bark as with other unwanted visitors.  Curiously, lichen are also an oddball as far as organisms go.  They are actually a symbiotic relationship between moss and fungi, whose behavior changes to something uniquely its own from its component organisms.  Coincidentally, this type of relationship can be categorized as...

Mutualism

That's right: mutualism!  This is the positive-positive type of relationship between organisms, where all parties enjoy some kind of net benefit.  These relationships are likely what catch peoples' attention in nature videos or documentaries.  Clownfish and sea anemone, the aforementioned moss and fungi lichen, and crocodiles and plovers are a few more commonly known examples where each party works together for benefits they can't achieve alone in their communities.
One example that I'd learned of recently is called myrmecochory, a term specific to a relationship plants and ants share with one another.  Plants that live in particularly harsh or competitive environments need to find a safe or nutrient-rich place to disperse their seeds.  However, right beside themselves may not be a great opportunity.  Enter the ant, who lives alongside the plant in question.  Some plants have developed sugary coatings around their seed casings that attract ants to them.  The ants take the seeds back to their colonies and eat the pleasant sugary bits before dropping the remainder of the seed in a chamber specific to...well...refuse.  Or, to the ants it's refuse, but to the seed, it's a nutrient-rich safe space underground for it to grow.  The plant provides food to the ants and the ants provide a seed delivery service.  Really neat!

Amensalism

​Taking a step back from all the positivity, we have the negative-zero interaction known as amensalism.  This relationship is typically more accidental in nature, a side effect of something existing analogous to something else in a community.  Often these interactions are unintentional due to the nature of one of the organisms, usually the one getting zero benefit from the interaction.
Some plants pay ants for seed delivery services by coating their seeds in sweet, sugary coatings that the ants love to eat. 

​This arrangement is termed myrmecochory.
Most examples of these appear in our microverse, where chemicals released from organisms have an adverse effect on bacteria in the area.  A more macro example of this relationship could be that of the hippopotamus and ground cover foliage.  Hippos tend to travel in straight lines when they move as a group, but their underbellies are so large that they drag along the ground.  Now, on its own, one hippo isn't going to do much damage, but the repeated disturbance for an entire bloat of hippos (yes, that's the actual term for a collection of hippos) can do irrecoverable harm to that stretch of plants.  In a way, all creatures who make these sorts of "cow paths" (or hippo paths in this case) are guilty of participating in these negative-zero interactions.
Plants and animals will find or adapt to niches, or specialties, in their ecosystems to avoid needless competition with each other

Competition

Now let's look at competition, the negative-negative interaction between organisms.  In a competitive environment, multiple plants or animals may be vying for a shared resource or space.  When this happens, there are a few choices that the creatures have: they duke it out, one of them migrates, or one of them adapts.  In the case of actually competing or driving the other out, this is seen as a negative-negative for all parties since there is no clear winner of the resource.  Any time that resource is needed, and both parties are there, neither is certain of which will come out ahead, be that being able to eat for the day or having a place to sleep.
This gets into a whole topic of discussion in ecology called niches.  These are the things that plants and animals do to survive in a community that only they do.  This is a means of avoiding conflict that comes about of competition in a community.  This deserves a page all its own, but suffice to say finding and adapting to a niche is a big part of how organisms find their way out of competition without dying out or being forced out of their environment.
One popular example of competition is the case study of "Darwin's Finches."  These were birds that Charles Darwin brought back from the Galapagos Islands while he was traveling the world.  Which, by the by, were not actually finches.  They happened to be misidentified while traveling the world, but even after that was known, the study still reflects the misnomer in its common name.  At any rate, Darwin thought that the differences in these birds' beaks were quite stark considering the smallness of the islands they inhabited.  If they both required the same food source, how did they come to look and act so different?
​
Well, the answer is found in niches!  The birds were, for a long time, in competition with one another.  However, over time, they split off from each other, evolving traits that were conducive to different modes of finding and eating food.  In this way, the original species panned out to fill certain niches on the island in order to continue living together without causing the population to die out.  I guess not even negative-negative interactions have to stay negative forever!
4 examples of Darwin's "finches"
​And that about covers the broad spectrum of ecological relationships on Earth.  In each of these interaction types, you can dig in to find even more nuanced examples that really explore just how creative and diverse our planet is.  Hopefully this primer helped serve as either a refresher or even a learning experience to ecological newcomers.  As with all of these topics, we hope this works as a springboard for you into understanding our planet even just a little bit better!
~ And, as always, don't forget to keep wondering ~
Sources:
* Begon, M., Townsend, C., & Harper, J. (2006). Ecology: From Individuals to Ecosystems (4th ed.). Wiley-Blackwell.
* Scoville, H. (2019). How Finches Helped Darwin Develop His Theory of Evolution. ThoughtCo. Retrieved 15 August 2020, from https://www.thoughtco.com/charles-darwins-finches-1224472.​
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