Previous: How Life is Organized: Crash Course Biology #4
Next: Why Don't These Plants Need Seeds? (Moss and Ferns): Crash Course Botany #8



View count:34,276
Last sync:2023-09-28 23:00
Ecology is the study of the interactions of living things with each other and their environment. It’s a field that not only lets us explore the interconnections between living things, but also how our environment affects us, and how we influence it in turn. In this episode of Crash Course Biology, we’ll get an overview of the field of ecology, see how matter and energy are conserved and transferred through ecosystems, and follow an ecological mystery surrounding the deaths of hundreds of elephants in Botswana, Africa.

An Ecological Mystery 00:00
Biotic & Abiotic Factors 1:13
Ecosystems 3:15
Fields of Ecology 5:54
Trophic Levels 7:04
Energy Flow through Ecosystems 9:25
Review & Credits 11:48

This series was produced in collaboration with HHMI BioInteractive, committed to empowering educators and inspiring students with engaging, accessible, and quality classroom resources. Visit for more information.


Crash Course is on Patreon! You can support us directly by signing up at

Thanks to the following patrons for their generous monthly contributions that help keep Crash Course free for everyone forever:
Tawny Whaley, Sean Saunders, Katie, Tori Thomas, DL Singfield, Ken Davidian, Stephen Akuffo, Toni Miles, Steve Segreto, Kyle & Katherine Callahan, Laurel Stevens, Burt Humburg, Aziz Y, Perry Joyce, Scott Harrison, Mark & Susan Billian, Alan Bridgeman, Rachel Creager, Breanna Bosso, Matt Curls, Jennifer Killen, Jon Allen, Sarah & Nathan Catchings, team dorsey, Trevin Beattie, Eric Koslow, Jennifer Dineen, Indija-ka Siriwardena, Jason Rostoker, Ken Penttinen, Nathan Taylor, Les Aker, William McGraw, ClareG, Rizwan Kassim, Constance Urist, Alex Hackman, Pineapples of Solidarity, Katie Dean, Stephen McCandless, Wai Jack Sin, Ian Dundore, Caleb Weeks

Want to find Crash Course elsewhere on the internet?
Facebook -
Twitter -
Instagram -

CC Kids:
Home to more than 130,000 elephants, Botswana, Africa has the world’s largest elephant population.

And in Botswana, elephants  are kind of a big deal. Just ask any of the dung beetles snacking on elephant poop, or the giraffe drinking from a watering hole dug by an elephant.

Elephants are the life of the party here, not because of their great dance moves, but because of their importance in the lives of so many other species, qualifying them as what’s known as a keystone species. Even some of the elephant’s behaviors that seem destructive are actually helping  sustain other living things. Like, when they’re knocking down trees, this can help smaller plants grow and thrive.

And you won’t hear the zebra —quite the foodie when it comes to these small plants— complaining about a few fallen trees. So in 2020 and 2021, when hundreds of elephants  in Botswana died suddenly, ecologists and conservationists became really concerned. Especially because the causes were a mystery.

So today, we’ll be the ecological  detectives on the case. Hi! I'm Dr.

Sammy, your friendly  neighborhood entomologist, and this is Crash Course Biology. [THEME MUSIC] But let’s back up. In order to be ecological detectives, we need to set the scene. And the scene is a bit more complicated than “in the library with the candlestick.” For ecologists, the scene is kind of everywhere, but more importantly, it’s the connections between everyone, and everything, everywhere.

Ecology is the study of how living things interact with each other and their environment. And when ecologists talk about the environment, they’re referring to all of the living and/or nonliving stuff that makes up the world around us. So, your dog is a part of your environment — and he’s a biotic factor because he’s living — and so are the fleas on your dog – even if ol’ Spike isn’t a fan; they’re a part of your dog’s environment.

And though we don’t always stop to think about them, nonliving elements — or abiotic factors — play a huge role in influencing our environment as well. Climate and water are two examples of abiotic factors. The climate of a frozen tundra is super different from a rainforest, and so, different organisms live in each of those ecosystems.

And a watering hole serves as a place of interaction between lots of different species. So, we’re not only connected to other life; we are deeply connected to our environment too. And, just like life, we can study ecology on so many scales, from the tiny to the tremendous.

Let’s start big. The biosphere is the part of our planet —from several kilometers above the ground to the deepest depths of the ocean— that all Earth-bound life calls home. Everywhere that stuff lives counts as part of the biosphere.

On this scale, abiotic factors include where the air is, where the water is, and where the land is. We call these the atmosphere, the hydrosphere, and the geosphere. OK, let’s zoom in a little.

Within the biosphere, there are many different ecosystems. Ecosystems are all of the organisms and non-living things that interact with each other in a specific area. And there’s typically no empty space between ecosystems.

They just bump right up against each other, like how the shoreline is home to  different types of critters than the ocean, and the ocean itself is home to different  types of marine life at different ocean depths. The ecosystem is where we can observe and study the circulation of the chemical elements that make up both the biotic factors, like elephants, and the abiotic factors, like rocks or soil. These are called biogeochemical cycles.

So, for example, a plant can absorb phosphorus minerals from the soil. Then, the phosphorus in the plant can end up becoming part of a grazing cow. And eventually, that same phosphorus returns to the soil again when the cow, um, poops.

Biogeochemical cycles are integral to a functioning biosphere. They’re responsible for everything from making it rain, through the water cycle, to producing the air we breathe, through the carbon cycle. And ecologists study this remarkably diverse set of interactions.

So it’s no wonder that people conflate ecology and environmental conservation. While ecology may lead to strategies to preserve and protect the environment, an ecologist focuses on relationships between organisms and their surroundings. So, an ecologist might examine which plants thrive in elephant-trampled areas and compare that to similar areas where elephants haven’t stomped their size-18 feet.

And a conservationist's goal is to protect or restore the environment. For example, poaching elephants for the ivory in their tusks is a major issue in Botswana, and conservationists there work to prevent it. And that brings us back to our mystery.

You might be thinking that poachers killed the elephants. And that’s not an unreasonable guess because poachers are responsible for the deaths of 20-to-30,000 African elephants every year. But the elephants in Botswana were found with their tusks intact.

So, if it wasn’t poachers, what was it? To look for more clues, we need to consider the  situation from a few different vantage points. One of the coolest things about ecosystems is that there’s only one size limitation; it needs to fit in the biosphere.

An ecosystem can be as enormous as the savanna in Botswana or as tiny as a puddle inside a muddy elephant footprint. And the different types of ecologists study the interactions of living things with their environment from a more or less zoomed-in perspective. Of course, we won’t be able to cover all of the specializations within the field of ecology.

But we’ll highlight five main fields. Landscape ecology has the biggest playground,  geographically speaking. It’s focused on multiple, connected ecosystems, and the interactions between them.

One step smaller is ecosystem ecology, which is the study of interactions and  dynamics of all of the organisms, plus all of the non-living things in a specific area. Even more zoomed in, there’s community ecology, which looks at interactions of two or more  species within a specific location. And relatedly, population ecology focuses on the dynamics surrounding just one species, a population, in a specific area.

Finally, like a parent watching their little kiddo navigating a new playground, behavioral ecology focuses on the way an organism acts in response to its environment. So, let’s use our knowledge of the fields of ecology to investigate the elephant deaths in Botswana. One place we might want to start  is with ecosystem ecology.

By studying where species sit on the food chain, we can find clues to what might have caused their deaths. When we’re talking about the food chain, we have to remember that the laws of physics apply. Energy is conserved, and so is mass.

In fact, one of the goals of ecosystem ecology is to track how mass and energy move through an ecosystem. To see how this breaks down in one particular example, I’ll introduce you to my favorite water-dwelling, buck-toothed rodent, the beaver. Let’s go to the Thought Bubble.

Beavers love to munch on lilies, grasses, and clover—so plants, really. Through the process of photosynthesis, plants use the energy from the Sun to assemble raw chemical ingredients (carbon dioxide and water) into energetically charged sugars. They pretty much make a sweet biochemical solar cell.

This photosynthetic organism is the  very first link on our food chain, what ecologists call a producer. And the producer doesn’t always have to be a plant; it could also include algae or bacteria that are capable of photosynthesis or chemosynthesis. The rest of our chain is made up of consumers, who take advantage of the energy stored in the sugars that the plant makes – the same way you take advantage of your friend’s power bank when your phone is on 4%.

Those links in the chain organize species into trophic levels, which are the groups of species in a food chain that share the same relationship to the flow of energy through the ecosystem. The beaver is the second rung in our food chain, a primary consumer of the clover. The fox that eats the beaver is a secondary consumer and the eagle that eats the fox is a tertiary consumer.

The top of the food chain. Now, the eagle doesn’t really have any natural predators, so it’s what’s called an apex predator. We’ve also got some action going  on at the edge of the food chain.

Decomposers, like fungi and detritivores, break down decaying organic matter. Fungi use chemical reactions to do the trick, and our friend the dung beetle is one of those detritivores, and he eats dung. Dung beetle, let’s not do lunch but um…thank you for all that you do!

And thank you, Thought Bubble! So, if we think like an ecologist and look at where elephants sit on the food chain, maybe we can see where the system breaks down. Elephants are voracious plant eaters — primary consumers that eat things like grass, shrubs, bark, and fruit.

To look closer, we can visualize data about how energy flows through the trophic system. This type of visualization is important in helping  ecologists recognize patterns in biology. And an ecological pyramid lets us  represent that energy flow graphically.  Energy transfer through a food chain  is a pretty inefficient process,   with the plant getting only about 1%  of the possible energy from sunlight,   and each trophic level harvesting only about 10% of the energy from the level below it.

This inefficient energy transfer limits  a food chain to four or five levels, max. By the time we reach a third or  fourth-level consumer, the ecosystem can support relatively few apex predators. As for predators… elephants are so large they don’t really have any.

Sure, maybe a baby gets snagged by a lion here and there, but very rarely do the adults. So, we can rule out predation as the cause of the deaths. So maybe it was starvation?

Hmm...let's look at the other end of the chain. Primary producers determine the energy budget for all other organisms in the food chain. The total available energy from all of the producers is called the gross production of an ecosystem.

But plants also need energy to fuel their own biological processes. The net production of the ecosystem is the remaining energy that is actually left over for the rest of the ecosystem to use. Another way to represent the flow through the ecosystem is using mass.

A biomass pyramid plots the total mass at each trophic level at any given time. So, it’s more like a quick snapshot of the available energy. In Botswana, the elephants were all relatively healthy and well-fed, and there didn’t appear to be a food shortage.

So it was clear that something else was causing their deaths. The elephants’ bodies were often found close to watering holes, and when scientists tested the water, they found high levels of cyanobacteria, a bacterium capable of photosynthesis. Cyanobacteria thrive in warm, stagnant water, and increased global temperatures due to  climate change have made them more prevalent.

And cyanobacteria can produce poisonous neurotoxins, so this led scientists to the conclusion that these toxins poisoned the elephants, causing the massive die-offs. Solving this mystery — as often happens in science — leads us to more questions. And ecology gives us the tools we need to answer these questions about life and the environment.

But don’t forget — we’re not just passive bystanders watching the journey! Well-functioning ecosystems also provide ecosystem services that can directly benefit humans. Food production is one of the major services that ecosystems provide us humans.

And thinking back to the burrito that I had for lunch, I’ve gotta recognize the primary producers and consumers that harvested the energy that I just took advantage of to record this video.   And ecosystems also helped to purify the  drinking water I used to wash down that burrito.  And there’s so much more! Tall trees in a forest buffer strong winds that might damage the places we live. And even the decomposers and detritivores perform an important service by not letting all that dead stuff and waste lie around for too long.

In fact, the nutrients that they return to the soil might just end up as an atom inside of you one day. Life, all of it is interconnected, and ecology helps us visualize these connections. Plus, ecology lets us see and study our connection to our environment.

Understanding these relationships can help us build the tools to address global issues that affect humankind, and really, almost all life on Earth. Whether you’ll use this  knowledge to do biological research, or better understand the world around you, or elect leaders that support science policies that you value, learning about ecology benefits all of us. In the coming episodes, we’ll look at interactions in communities and populations, and then zoom back out to see how ecology can help us understand climate change and guide conservation efforts.

And maybe, with the knowledge that we’re building about ecology, we can be better citizens of this amazing biosphere that we all call home. This series was produced in collaboration with HHMI BioInteractive. If you’re an educator, visit 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.