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  • On the island of Madagascar, there's a kind of moth that drinks tears from the eyes of

  • sleeping birds.

  • When I first heard this, I just sat with that weird idea: there's a moth that gets most

  • of the nutrients it needs to survive by drinking bird tears!

  • Welcome to the biosphere -- the sphere of life that extends from the depths of the ocean

  • all the way up to 8 kilometers above Earth.

  • A lot of incredible things live here, so of course, as geographers, we want to know why

  • bananas and bacteria and tear-drinking moths show up in some spaces but not others.

  • And to do that, we have to zoom out a little.

  • For example, that moth gets its nutrients from birds, while birds rely on seeds and

  • berries from the surrounding plants, which grow with the help of the Sun.

  • So the moth and the birds and the plants and the Sun are all part of an ecosystem -- a

  • community of living organisms in an area interacting with their environment.

  • Ecosystems are built on relationships -- even strange ones that involve tear-theft.

  • And the relationship between the amount of energy a place receives and the movement of

  • nutrients is what makes the incredible diversity of life possible.

  • I'm Alizé Carrère and this is Crash Course Geography.

  • INTRO

  • The biosphere is a complex web of interconnected ecosystems.

  • And all ecosystems depend on two key things: the one-way movement of energy and the cyclic

  • movement of nutrients.

  • Energy flows are the paths energy can take through an ecosystem.

  • Energy generally enters ecosystems from the Sun but doesn't return to the Sun -- so

  • energy flows are one-way relationships.

  • Plants absorb the Sun's energy during photosynthesis, adding carbon dioxide and water to make carbohydrates

  • and grow bigger.

  • So the Sun's energy is converted into chemical energy, which is stored in biomass -- any

  • plant or other living thing.

  • If a bit of biomass is eaten, it passes on its chemical energy to continue the energy flow.

  • The rate photosynthesis makes energy across an entire ecosystem, minus the rate that energy

  • is used is its net primary production -- or the amount of stored chemical energy in an

  • ecosystem over a certain amount of time.

  • For example, on a really small scale, think of a fish tank ecosystem that you can hold

  • in your hands.

  • There's water, a fish, soil, rocks, air, light, food, and one little plant all in a glass bowl.

  • In this fish tank ecosystem, the net primary production is pretty low because only that

  • one little plant is absorbing energy from the Sun (along with any photosynthetic bacteria

  • or algae that grows when I forget to clean the bowl).

  • Globally, net primary production on land generally changes with latitude.

  • Productivity is highest between the tropics and decreases towards higher latitudes and elevations.

  • Biogeographers and ecologists who study how life is distributed on Earth probably figured

  • that calling regions of the world "very productive ecosystem" or "extremely not productive ecosystem"

  • would be boring.

  • Instead, we classify ecosystems into biomes, or habitats with similar characteristics,

  • including productivity!

  • The names are much more descriptive and fun.

  • The equator gets the most direct sunlight and a lot of precipitation, so there's a

  • lot of photosynthesis happening here.

  • These highly productive ecosystems are all tropical rainforest biomes, which are some

  • of the most diverse and complex areas of the planet -- so it's no wonder the tear-drinking

  • moth lives here.

  • Similar patterns happen on either side of the equator, but we're going to turn north

  • because there's more land in the northern hemisphere.

  • There's also less and less precipitation as we move out from the equator, and less

  • and less productivity because photosynthesis can't happen without water.

  • The biomes gradually shift from tropical rainforests to tropical savanna to desert.

  • Further north, in temperate and high latitudes, the net primary production varies seasonally.

  • Like one biome is the broadleaf deciduous forests with oak, beech, hickory, maple, elm

  • and chestnut trees.

  • These trees have increased productivity in the sunny spring and summer, and shed their

  • leaves in the cooler fall and winter seasons.

  • Up here in the middle of continents, there are temperate grassland biomes with rich soils

  • that produce the tall grass of prairies and the shortgrass of steppe climates.

  • Further north where there are poorer soils and colder climates, we meet the boreal forest

  • biomes, which have mainly evergreen pine, spruce, fir and larch trees.

  • At even higher latitudes, the decreasing temperatures give us the icy tundra biome with no trees

  • and very little productivity.

  • So the amount of energy flow through different ecosystems varies wildly, which limits which

  • type of plants can thrive there.

  • And because plants feed more consumers than any other food source, more plants means more

  • biodiversity, or the number of different plants and animals in an ecosystem.

  • And we can't talk about biodiversity without the other key component of all ecosystems: nutrients.

  • Nutrients are chemical elements like carbon, oxygen, nitrogen, sulfur, and phosphorus -- stored

  • in both the living and nonliving parts of an ecosystem.

  • And we actually have technical terms for those too.

  • The living things like plants and animals and bacteria (or their dead bodies) are the

  • biotic parts of an ecosystem.

  • And the nonliving things like the soil, atmosphere, and groundwater are the abiotic parts.

  • Unlike how energy flows in one direction, the paths that nutrients take through the

  • ecosystem are nutrient cycles between the biotic and abiotic parts.

  • And unlike energy from the Sun, all the nutrients we have right now on Earth are all we'll ever have.

  • It's like how nitrogen moves from being a gas in the atmosphere to a solid in the soil.

  • [Instead of a one-way system like...aliens dropping gift-wrapped boxes of nitrogen from

  • spaceor at least not that we know of].

  • The biotic parts of ecosystems really help facilitate these nutrient cycles.

  • Like, let's look at our fish tank ecosystem again!

  • Producers like our little plant capture nutrients from the abiotic parts, turning carbon dioxide

  • into carbohydrates through photosynthesis or absorbing nitrogen compounds through its roots.

  • Consumers like the fish take nutrients from other organisms, munching on fish food or

  • the plant's leaves.

  • And decomposers break down dead plant leavesor our fish eventually... and return the nutrients,

  • like nitrogen gas, to the abiotic parts of the tank.

  • Ultimately, nutrients cycling through ecosystems depend on biological, geological, and chemical

  • processes operating within the atmosphere, hydrosphere and lithosphere, and make up Earth's

  • biogeochemical cycles.

  • We can compare nutrients across the Earth's biosphere just like we compared net primary

  • production across different latitudes and biomes.

  • Like let's look at three biomes we met before: the tropical rainforest, deciduous forest,

  • and boreal forests.

  • We know that there's less and less productivity as we move up in latitude, so there's less

  • and less biomass, and there's also less and less nutrients.

  • Fewer nutrients isn't necessarily a death sentence for the trees, though.

  • It just means that the ecosystem is structured differently.

  • Like boreal forests have a lot of nutrient filled litter because the cold keeps material

  • from decomposing.

  • But deciduous forests have a lot of nutrient-rich soil because it's warm enough for material

  • to decompose, but not warm enough for a lot of biomass to grow.

  • So a tree that's adapted to life in a cold boreal forest might not make it in a tropical

  • rainforest because of the different energy availability and nutrient stores.

  • Let's consider the tropical rainforests, which are the most diverse biomes with lush

  • vegetation and a lot of biodiversity.

  • But that decadence hides the fragile balance of all the complex energy flows and nutrient cycles.

  • Let's go to the Thought Bubble!

  • Within the tropical rainforests, broadleaf evergreen trees form a canopy at different

  • heights, and little or no sunlight reaches the shady forest floor.

  • These huge trees absorb most of the soil nutrients, which doesn't leave a lot for other organisms.

  • And they have a shallow root system to grab as many of the minerals as possible from biogeochemical

  • processes near the surface.

  • And as the large amounts of rain filter down through the soil, the minerals that dissolve

  • in water are leached away to inaccessible deeper levels.

  • To survive, the rainforest has to rapidly cycle nutrients.

  • The canopy trees are producers, along with understory plants that work together to keep

  • vital nutrients moving through the ecosystem.

  • Herbivores like gorillas and caterpillars take in those nutrients and move them around

  • through their excrement and by being eaten themselves, like by jaguars or geckos.

  • And the warmth and humidity helps decomposers and their chemical reactions, so any dead

  • plants or animals decay quickly.

  • Because nutrients get sucked from the soils so quickly, when those huge trees are cut

  • down, the energy flows and nutrient cycles break.

  • Those big producers aren't there to sustain consumers or shed leaves to recycle nutrients.

  • So deforestation, or removing trees to use the land for something else, can be especially

  • destructive in tropical regions if you don't consider the biogeochemical cycles.

  • Thanks, Thought Bubble.

  • We have negative associations with the word "deforestation" for good reason -- a lot of

  • tree removal has caused immense damage to ecosystems.

  • But indigenous communities have figured out a type of calculated clearing that allows

  • them to work with the rapid nutrient recycling of tropical rainforest biomes.

  • In parts of Asia, Africa, and South America with dense tropical forests, many farmers

  • have to rely on a kind of subsistence agricultural practice, which means they only grow enough

  • food for their families.

  • Staples like rice are grown in southeast Asia, maize and cassava in South America, and sorghum in Africa.

  • Yams, sugarcane, plantains, and vegetables are also planted to supplement staples and

  • to provide fuel and fodder for animals.

  • This practice goes by many names, like swidden, shifting cultivation, and slash-and-burn agriculture.

  • The farmers begin by cutting small areas of tropical forests into slash, or cut vegetation,

  • that's then dried and burned.

  • The ash gets mixed with the poor soil to provide needed minerals and nutrients -- basically

  • using all the good stuff stored up in the vegetation biomass to help new crop plants grow.

  • Of course, these crop plants use minerals and nutrients from the soil as they grow,

  • and we eat them to get those minerals and nutrients in our bodies.

  • So after a few years, and before the soil is completely exhausted, the farmers move

  • on to another part of land and repeat the clearing, burning, and planting process.

  • The previous plot is left unplanted, and eventually the forest will naturally expand to start

  • using that soil as part of its carefully balanced nutrient cycling.

  • This land rotation is a key part of why humans have been able to keep farming like this for

  • thousands of years.

  • But when widespread clear-cutting happens, ecosystems can collapse.

  • For example, we've seen this destruction in the Amazon when rice, soy, and corn have been

  • commercially cultivated and sold in domestic and international markets.

  • The soil is exhausted after 3-5 years, so crops can't really grow anymore, and then

  • large cattle operations move in.

  • As cattle feed and trample the ground, the soils are exposed to plenty of UV radiation

  • from sunlight, as well as cycles of wetting and drying from precipitation.