Ecology – Rules for Living on Earth: Crash Course Biology #40


For the last 38 episodes
of Crash Course Biology, we’ve talked about how
to make an organism. And you know what I’ve
learned in those 38 weeks? Putting a living
thing together is hard! There are molecules that make
up organelles that run cells, which come together
to form tissues, which make up organs
that make up systems. And knowing this stuff
is incredibly important, because it shows us the ground
rules for being a living thing, on this particular planet anyway. But still, there’s so much
more to biology than that! I mean, understanding how an
organism goes about its internal business is great,
but it doesn’t tell us much about its place in our world. For that, we need ecology, the study of the rules of engagement
for all of us Earthlings. Ecology seeks to explain why the
world looks and acts the way it does. Why the South Pole looks
different from the Congo, and why there are mosquitos
all over the place while black rhinos are
practically extinct. The short answer to
this question is: because the world is crammed
with things, both animate and not, that have been interacting with
each other all the time, every day, since life on this planet began. The even shorter answer is that
all life and all of these things interacting with each other
depend on just two things. Try to guess what they are. In the meantime, get ready, because
Crash Course Biology is taking its final voyage outside the
body and into the entire world! In a way, you can think of all
living things, great white sharks, pond scum, potato plants, as
molecules that react with each other. Each one of us organisms is pretty
piddling in the scheme of things, just like a single oxygen molecule,
which we need to make ATP to fuel our bodies. But it can’t get
much done by itself. But if you get a million
oxygen molecules together with some other types of molecules,
suddenly they’re unleashing a googlejillion
megawatts of ATP power to animate the bag
of meat that is you. This same principle
applies to organisms: As you put individual
organisms together, they can interact with each
other and their environments, to create something larger
than the sum of its parts. And just as every organism has
a hierarchy of biological systems, from molecules to organelles,
cells, tissues and organs, so too does Earth have
tiers of ecological order. Like, when a bunch of members of
a species are together in a certain area, and they interact pretty
often, you’ve got a population. Population ecologists study why
populations grow or shrink over time, depending on where they are. When two or more populations of
different species live together, we call that a community. Think of an ecological community
as Mr. Roger’s Neighborhood, but with the people
in the neighborhood eating each other sometimes. Because that’s what
species do when they live together, they interact. Sometimes that means predation,
sometimes cooperation, and sometimes competition for resources like
food, water and living space. So, a community ecologist studies
how the interactions between community members and their
environment affect how many of each species there are within a community. One level up from
communities are ecosytems, which are made up of groups
of organisms in a specific area and the nonliving parts
of their environment, like soil, water, and air. If you take a bunch of living things
and plop them down in one place that has a specific mix of climate,
soil chemistry, and topography, that’s going to make up
one kind of ecosystem. But if you put them down in
a completely different place, they’re going to work in
completely different ways to form a completely
different ecosystem. Ecosystem ecology specifically
explores how energy and materials flow through an ecosystem,
and how the physical environment impacts the stuff living there. Now, a lot of people
get ecosystems confused with the next step
up: which is biomes. A biome, however, is where
organisms have evolved similar techniques to adapt to a
general set of conditions. For example, a grassland is
a kind of biome, there are scores of different grassland
ecosystems all over the globe, but the organisms in each one
have made similar evolutionary concessions to all the
conditions that grasslands share, like hot summers, cold winters,
and not too much rain but more rain than you’d
find in a desert biome. Other biomes include
tropical rainforest, tundra, deserts, and oceans. The only level above the
biome is the biosphere, which includes the atmosphere,
the whole earth and everything that gets used by
anything that’s alive. So, why do all of these many
levels of ecological activity look the way that they do? Like, why do some organisms like to
live in one place but not another? And what makes Earth’s
various populations, communities, ecosystems and biomes
different from each other? Well, factors that determine
what a place is gonna look like fall into two different
categories: biotic, or living, and abiotic, not living. Biotic factors include
stuff like predators, as well as animals or plants
that provide either competition or some benefit,
like food or shelter. Abiotic factors, on the
other hand, include temperature, moisture, sunlight, elevation, elements that have nothing to
do with organisms in the ecosystem, but which influence them just as
much as other living things do. Now, from these two categories,
the most influential factors are the ones that living
things are most particular about. That is, the things they need
most, but only at certain levels. And these preferences all
come down to chemistry. For example, almost all chemical
reactions that happen inside living things are
governed by enzymes. They’re the catalysts
for pretty much all the action
going on inside you. And these enzymes are most effective
within a set of temperatures: Chemical reactions within the body
slow way down when it’s really cold, and very high temperatures
change the shape of enzymes, making them less effective. So temperature is one of the
major factors that determines why animals live in certain places. And if you look at the places
on the earth with the most biodiversity, or different
kinds of living things, you’ll find that it’s in the
places where the temperature’s within the ideal range
for enzyme function. What else? Well, everybody’s
got to eat, at least if you’re an animal or a fungus or
some other kind of heterotroph, so you’d think that food
would also be way up on the list. But actually, it’s plants and
other autotrophs like cyanobacteria and protists that are the
base of nearly every food chain, and they have to be fed, too. So again, it comes down to chemistry. The key ingredient plants need
for photosynthesis is water, which is also what
we need to burn ATP, maintain homeostasis in our
bodies, and all that jazz. So the quest for food ultimately
comes down for a need for water. So, yeah: surprise! Water and
temperature are the two things that organisms care about the most. Ergo, they’re what ecologists
focus on when determining why certain organisms hang out
in one place over another. Together, these two factors define
every biome on the planet. For instance, a Saguaro cactus
has evolved to live in the Sonoran Desert of North
America, which is super hot and gets very little precipitation. So, the Sonoran Desert is full
of animals and plants that can, just like the Saguaro, take the
heat and also the extreme, face-crumbling dryness. But if you put these animals
in the Amazon rainforest, even though it’s hot enough
for them, it’s just too wet. So, yeah, the things that
live in a biome are ultimately determined by how much water
is there and the temperature. And in turn, these
inhabitants determine how the biome looks,
called its physiognomy. So now, we are going to take
a look at all the different types of biomes out there. There are places on the planet
that get lots and lots of rain, around 300 centimeters a
year, and are pretty warm, around 25-30 degrees C on average,
which is Speedo-wearin’ weather, as far as I’m concerned. These biomes are the tropical
rainforests, which generally hug the equator and have unbelievably
high biodiversity because everybody’s wanting to get a piece
of that sweet tropical action. And then on the complete
opposite side of that scale, we have the tundra, most of which
is above the Arctic Circle, in Antarctica, or way up at
the top of some mountains. Tundra gets little precipitation
and some well-below-zero temperatures. And what lives there? Not much.
A couple of mosses and liverworts, maybe a few species of grasses,
some birds and a handful of mammals. The same goes for the desert biome,
where there’s very little rainfall and very high temperatures. Like the tundra, without much water,
there can’t be very many large plants. And where there aren’t a lot of
plants, there aren’t a lot of other organisms,
even when temperatures are close to what makes living things happiest. Between these three extremes,
we’ve got biomes that require more or less water, combined with
high-ish or lowish temperatures. These are your moderate
or temperate biomes, and they include temperate
grasslands, like what you find in the North American prairie,
or temperate deciduous forests, found over much of
Europe and North America, and taigas or coniferous
forests, found across Canada, much of northern
Russia, and Scandinavia. So, if all these biomes in the
middle experience pretty moderate temperatures most of the time,
the availability of water must be what makes them
different from each other. Some of these biomes have a
lot of trees, and as we know, trees need a lot of water. So if you find yourself
in a temperate forest, it’s a pretty safe bet that
that particular ecosystem gets a fair amount of precipitation. And if the Carboniferous
forests taught us anything, it’s that having a bunch of trees
around changes the landscape, the climate, and even
the geology of a biome. If you don’t have a lot
of trees in a biome, it means you probably don’t get
enough rainfall for their liking. And without trees, more sunlight
reaches the ground and gets to grasses and other small plants, leading to more of a
temperate grassland ecosystem. And where you get grass, you get
animals like bison and pronghorn and other ungulates whose digestive
systems are big fermentation vats that process
cellulose all day long. And then when you’ve got
ungulates, you also get predators. All these animals are way different than what you’d find
in a temperate forest. So, biomes are different, because
the plants are different, because the rainfall and
temperatures are different. But, of course, there are also
biomes entirely underwater. We can’t forget that the surface of
the planet is three-fourths water. And since water availability
isn’t an issue in the ocean, marine biomes differ in things
like temperature, pressure, oxygen content, how much light
is available, and stuff like that. So, thanks to the science of ecology,
we know that the way the world works can be explained mostly
by temperature and water. But this is just the beginning
my friends. Oh yes! The end of Biology 101, maybe. And we’ll always have that time we
spent learning and loving, won’t we? But there’s so much more
to find out together! How do living things affect the
climate, the chemical makeup of the atmosphere, even the
geology of our planet? How do they affect each other? And maybe more importantly,
how are we humans affecting all of these things, and what can we
do differently to ensure that we all get to keep existing? Join me as we get
to know our planet on a whole new level,
starting next week! Thanks for watching this final
episode of Crash Course Biology and if you’ve been
with us the whole time thank you for participating and
learning with us here at Crash Course. And of course, thank you to all
the people who helped write these episodes, who helped do
these awesome animations, the people who filmed
and edited them. It really is a team effort
here at Crash Course. If you want to review anything
that we talked about in this episode there’s a table
of contents over there. And if you have any questions,
we’re on Facebook, Twitter, and of course, in
the comments below.

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