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Exploring the open spaces of the world where grasses are the main type of plant life.
Imagine standing in a place where the earth meets the sky in every direction, and the only thing moving is a sea of waving green. How do these massive open spaces survive without the protection of a forest canopy?
Grasslands are 'in-between' biomes. They get more rain than deserts but not enough to grow thick forests. There are two main types: Tropical Savannas and Temperate Prairies. Savannas, like those in Africa, stay warm all year and have two main seasons: wet and dry. They often have scattered trees like the flat-topped acacia. Prairies, like the Great Plains of North America, have four distinct seasons with freezing winters and scorching summers. Because of the deep roots of the grass, the soil in prairies is some of the richest on Earth, often reaching depths of over feet!
Quick Check
If you are standing in a grassland that has freezing cold winters and very few trees, which type are you likely in?
Answer
A temperate prairie.
Why don't trees just grow and take over the grasslands? Two 'gardeners' keep the grass in charge: wildfires and grazing animals. Periodic fires might look scary, but they clear out dead plants and kill small tree saplings while leaving the deep grass roots unharmed. Meanwhile, grazers like bison and zebras act like living lawnmowers. By eating the tops of the plants, they encourage the grass to grow thicker and prevent any one plant from crowding out the others. Without these two forces, many grasslands would eventually turn into scrublands or forests.
Imagine a field where a small oak tree starts to grow. 1. A herd of bison passes through and nibbles on the young tree's leaves. 2. Later, a lightning strike starts a quick-moving grass fire. 3. The fire burns the young tree, but the grass roots survive underground. 4. Result: The field stays a grassland instead of becoming a forest.
Quick Check
How do grass roots survive a wildfire while a small tree might not?
Answer
Grass roots grow very deep underground, protecting them from the heat of the fire on the surface.
Grasslands are home to some of the largest land animals on the planet. In the North American prairies, the American Bison is the king. These massive herbivores have flat teeth designed for grinding tough, silica-rich grass. In the African Savanna, you find a huge variety of life, including zebras, giraffes, and elephants. Because there are no trees to hide behind, many of these animals have evolved to be very fast runners or to live in large groups for protection. A zebra's stripes, for example, make it hard for a lion to pick out just one animal from a moving herd!
Suppose a fence is built that stops bison from moving across a large prairie. 1. The grass in one area grows very tall and thick. 2. Dead grass piles up because it isn't being eaten or stepped on. 3. New seeds can't reach the soil through the 'thatch' of dead grass. 4. The biodiversity (variety of plants) decreases because the 'lawnmower' effect is gone.
During the dry season in a savanna, water becomes scarce. 1. Animals must migrate long distances to find water. 2. Predators like lions wait near the few remaining water holes. 3. This creates a high-pressure environment where only the strongest or fastest survive. 4. If the dry season lasts longer than usual, the population of grazers may drop significantly, affecting the entire food chain.
Which of these is a characteristic of a Temperate Prairie?
What role do bison and zebras play in the grassland ecosystem?
Wildfires are always bad for grasslands and should be prevented at all costs.
Review Tomorrow
Tomorrow morning, try to explain to someone the two main reasons why trees don't take over the Great Plains.
Practice Activity
Look at a map of the world and try to find one major grassland on three different continents (Hint: Look at North America, Africa, and South America).