字幕列表 影片播放 列印英文字幕 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 space… or 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 leaves… or 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.