If you asked your neighbor to describe their community, they might talk about the other folks on the street, or at their church, or maybe the Breakfast Club-esque group of lovable misfits that they call friends.

But you can be pretty sure that their answer will focus on a single species: humans. Ask an ecologist to describe their community, though, and they might tell you about the birds that build a nest in their gutters, or the gophers swiping carrots from their garden, or even the fleas that their dog is always scratching.

That’s because in ecology, a group of different species living in the same area is called a community. Studying community ecology helps us to better understand the good, the bad, and the ugly – when it comes to interactions between species in an ecosystem. Hi, I'm Dr.

Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology. [THEME MUSIC] A community is sort of like an ecosystem, minus all the non-living, abiotic, stuff like rocks, water, or weather. Science is all about asking questions. So, an ecologist might ask a big question like "How do these animals coexist in the African savanna, or for that matter, why did Mufasa think it was a good idea to put an African hornbill in charge of babysitting a lion?" And a community ecologist might look for answers by studying the interactions between the groups of a species within a particular environment.

Like how a predator hunts its prey or how a parasite can sap nutrients from other species. Now that might seem a little bleak, but the interactions between species in a community are not always violent or one-sided. Just like your community is not only the guy honking behind you at a red light or the neighbor that steals your wi-fi.

A community can be huge, with lots of interactions among populations, or members of the same species, and tons of interactions among different species including animals, plants, and even microbes. By the way, if you're wondering how we draw the line between species, the simple answer is that if they can't make babies together, they’re usually different species. And having different species in a community is really beneficial.

Because diversity contributes to community stability, making the community more resistant to disturbance. For example, if a harmful parasite is introduced, diversity could prevent the whole community from becoming infected, because some species might be resistant. Other examples of disturbance include fires, landslides, flooding, and hurricanes.

And some disturbances can wipe an area clean, destroying much of the life present. But the awesome thing about communities is that they rebuild! To find out how, let’s head to the Thought Bubble.

Following a couple months of tremors, Mount St. Helens, in the Pacific Northwest region of the United States, erupted in May of 1980, blasting the top clean off the volcano. Killing trees in a 42-square-mile scorch zone, and spreading ash for hundreds of miles, it was a spectacular display that made news worldwide.

But not everything died. Bigger animals like elk perished, but mice survived. Large trees were blasted, but seedlings of the same species managed to make it.

The frozen lakes and snow still covering the ground shielded some aquatic life and bacteria from the blast, too. In a phenomenon called ecological succession, the organisms that survived and grew back fastest repopulated the area, these pioneer species paved the way for slower-growing species to eventually move in and out-populate them. The community continued to shift until it became stable in what is known as a climax community.

In other words, ugh, you’re gonna make me say it aren’t you, [Sammy chuckles] fine! “Life finds a way.” Thanks, Thought Bubble! You always find a way too. But let’s move away from cataclysms and talk about life on an ordinary day in a community, where members of different species are frequently connecting through what’s called interspecific interactions.

These aren’t just one-off, random encounters —like, when you’re riding your bike really fast and accidentally swallow a fly. We wouldn’t call you a fly predator. Flies just need to be more careful.

Instead, with interspecific interactions, we’re talking about ongoing, long-term relationships that are categorized by how the relationships affect each species involved. Positive, if the relationship benefits an organism, negative if it harms it, and neutral if it doesn’t really have an effect in either direction. When one species eats another in an act of predation, a positive-negative scenario, it doesn’t exactly conjure up the warm fuzzies, but that predator-prey relationship can play an important part in maintaining the balance in a community.

Without enough predators, communities can get all out of whack. Like, in 1859 this wealthy Australian guy had 13 live European rabbits shipped to him. Sounds adorable, right?

Yeah - only this guy had them shipped in for sport. Well, if I learned anything from 80’s movies, it’s that the scrappy underdogs will always beat the rich preps. The rabbit outsmarted the hunters and, well, they did what rabbits do.

And today, because they don’t have a lot of natural predators to keep their population size in check, the European rabbit is an invasive species in Australia, contributing to the decline of native plants and animals. Predation is not just animals eating other animals; animals eat plants too, we call that herbivory. Of course, when there are plenty of predators, the prey need a way to help maintain the balance on their side – enter our good friend, evolution.

Some animals respond to predation by evolving defense mechanisms, like the ginormous super sharp quills of a porcupine. But what do plants do when danger is near? Let’s set the stage and find out, over in the Theater of Life… So, Dr.

Anurag Agrawal, an ecologist, showed that animals are not the only ones with defense mechanisms. Some plants develop techniques for responding to predation as well. Agrawal and his colleagues were studying a population of radishes commonly eaten by caterpillars to figure out if the plants could fight back – and how.

When he tested some plant samples, he found the caterpillar-muched plants were producing ten times more of a defensive chemical compared to un-munched plants! But Agrawal isn’t just an ecologist; he’s also an evolutionary biologist who wondered if this defensive mechanism might get passed on to new generations. So, in a later experiment, he compared seedlings from the damaged plants to the undamaged ones.

And he compared caterpillars that munched on each. He found that the caterpillars who snacked on seedlings whose parents had also been nibbled gained 20 percent less weight. This tracked with his hypothesis that the plants changed from generation to generation to better protect their offspring from hungry caterpillars.

Woah, they really upped the production value today. So, Dr. Agrawal’s discovery helps us understand how plants can fight back, but what about those species that don’t use protective chemicals?

Sometimes they evolve to look like other, more threatening species. Like the Owl Butterfly, which gets its name from the prominent yellow spots on its wings, which resemble the eyes of an owl. This mimicry can provide them an evolutionary advantage by scaring off predators.

And then you have your lose-lose scenarios. When two species are in competition for the same resources, the situation kind of stinks for everyone involved — it negatively impacts both species. Neither has enough of what it wants and it limits the survival and reproductive ability of both.

This happens because the organisms have an overlapping ecological niche, which includes where and how they live. Take Virginia’s Warbler for example, which contrary to their name lives in the American Southwest, not in Virginia. In central Arizona, the Orange-Crowned Warbler occupies the same niche as Virginia’s Warbler.

Ecologists removed one species to see if the other would thrive and sure enough, it did, indicating that the two were in direct competition with each other – kind of like a roommate that just eats anything in the fridge without asking who it belongs to. Oh wait, that was me. So some organisms that share the same niche use a technique called resource partitioning, which is like drawing a line down the middle of their habitat.

It alleviates the mutually negative competitive relationship. Take the golden spiny mouse and the common spiny mouse, whose niches overlap in rocky habitats in the Middle East. They both like to stay up all night —they’re nocturnal— but the golden spiny mouse adjusts its biological clock when it has to share space with the common spiny mouse, becoming active in the daytime instead.

But sometimes, a species is just really bad at getting along with the other species around it and it kicks out its competitors entirely. This competitive exclusion happens when two species actually share the same niche and one species has an advantage over the other, like the single-celled Paramecium aurelia and Paramecium caudatum. Independently, they grow quickly until they exhaust their resources.

But put them together, and you have got the Paramecium equivalent of an old-time Western showdown. And then there are parasites. Unlike competition, which is negative-negative for both parties, parasitism is a positive-negative relationship where one species benefits.

A parasite lives in or on another species, called a host, and benefits by taking nutrients or resources from the host. Of course, not all interactions between species are antagonistic. Take the bacteria in our guts.

They help us break down complex carbohydrates, and get a tasty snack and a cozy place to live in return. That’s a mutualistic relationship where we both benefit. And even some bacteria that aren’t actively helping with digestion, usually aren’t hurting us, so it’s a form of commensalism, meaning a positive-neutral relationship.

So, ecological communities — much like the human ones we’re a part of — consist of many different types of relationships. And some of those relationships exert more influence than others. For example, elephants and beavers change the availability of resources for other species by altering the biotic and abiotic environment.

Elephants knock down trees and stomp giant mud holes; beavers build dams, changing the environment and making them what’s called ecosystem engineers. The influence of ecosystem engineers on other species in the community is just one example of how keystone species disproportionately impact their community relative to their population size. Without keystone species, the community in its current state wouldn’t be able to exist quite the same way anymore.

And even something as small as a bee — which pollinates flowers, helping plants reproduce in an act of mutualism — can serve as a keystone species. That’s because all of the species that feed off the plants, and the animals that in turn feed off of them, rely on those bees. And of course, big, strong predators, like wolves, and lions, and bears, can serve as keystone species as well by reducing the number of plant-eating herbivores, in turn increasing the number of primary producers, like plants or algae.

When these predators on higher trophic levels affect the population size of organisms below them, we call this top-down regulation. Regulation can also start from the bottom since the energy available from primary producers limits the number of predators at the top of the food chain. This type of control is called bottom-up regulation.

This type of ecological regulation isn’t an intentional thing. A wolf doesn’t wake up one day and decide there should be more plants in their community, and then gorge itself on a buffet of local deer. That’s because top-down regulation is an emergent property, meaning it arises organically, and collectively, from organisms all surviving in their own unique ways.

And through that, whole complicated systems of relationship and energy transfer emerge. Think about a food chain, which follows a single path of energy through a community — showing how energy passes from the sun to plants, to animals, and then to bigger animals, and so forth. But it’s actually way more interesting than that.

Because it’s rare for the energy sources near the bottom of the food chain to only have one predator. Like, the same plant may be eaten by a rabbit, and a tortoise, and a sheep. So, trophic structures emerge with many interconnected food chains that end up looking more like webs, really.

And these food webs are also an emergent property of a community — meaning they arise in response to many little actions from members of the community, acting all at once. Community ecology helps us understand how species interact in direct and indirect ways. It helps us to come to terms with violent acts like predation, but also see the beauty in the mutual relationship between a bee and a flower.

And, like in Dr. Agrawal’s radish plant, it can reveal ways that interactions between species can lead to evolutionary changes through generations. And of course, we humans are a part of these communities as well.

We might not always realize it, but every day, we interact with different species in our communities. At times, our behavior can drastically alter living conditions for all of us. Humans might be one of the most consequential keystone species around.

And that’s a big responsibility. But the more we understand our relationships to, and reliance on, other species, the better roommates we’ll become. This series was produced in collaboration with HHMI BioInteractive.

If you’re an educator, visit BioInteractive.org/CrashCourse for classroom resources and professional development related to the topics covered in this course. Thanks for watching this episode of Crash Course Biology, which was made with the help of all these nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.