Once students understand how plants capture the sun’s energy and turn it into usable energy, students analyze the phenomena of how energy moves through a forest food web as organisms eat one another.
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Storing Food
Eastern gray squirrels love acorns. Acorns are the seeds of oak trees. Right before a squirrel eats an acorn, it shakes it.
The shaking happens so quickly it can be hard to see. But Michael Steele has videoed squirrels eating. When he watched the videos in slow-motion, he saw squirrels shaking the acorns.
Michael is a researcher who has studied squirrel behaviors. He says there is a simple explanation for why the squirrels shake acorns. They want to know if the quality of the seed is good. Depending on what they sense, they will either eat the seed right away or bury it to eat later.
Why Squirrels Bury Seeds
Squirrels bury seeds so they will have enough food to last them through the winter. Squirrels need to eat acorns and other plant materials because they are animals. They cannot make their own food, as plants can. Instead, they have to eat other organisms for energy and nutrients. When squirrels eat acorns, they access some of the energy that the oak tree has produced through photosynthesis and stored in glucose molecules.
Ecosystem Relationships
Michael is particularly interested in relationships between squirrels and the trees that produce squirrels’ food. Both the squirrels and the trees are part of an ecosystem. An ecosystem is a community of different species that depend on interacting with each other and their physical environment for survival. All ecosystems include living things that must eat one another for energy and nutrients. They also include oxygen and carbon dioxide from the atmosphere, water, and energy from the sun.
An ecosystem can be as large as a forest or as small as an oak tree. Regardless of its size, all the parts of an ecosystem work together to make a balanced system.
Mutually Beneficial Relationships
Michael has found that squirrels and oak trees have a mutually beneficial relationship. The squirrels benefit because they need the seeds’ energy and nutrients. They also need oxygen to breathe in. The oak trees benefit because acorns don’t grow well if they are right beneath the parent tree. The branches of the parent tree will block the sun.
The squirrels move acorns to bury them. Gray squirrels are known as scatter hoarders. This is because they bury acorns and other seeds in many places. Researchers have found that gray squirrels have a system when they bury acorns from an oak tree. They bury less-desirable acorns closest to the tree. They bury more-desirable acorns farther away from the tree.
If the squirrel doesn’t make it back to its acorns, those acorns might grow into new trees. This benefits the oak tree because it has passed along its genes. It also benefits the squirrels because it means more food sources.
Squirrels have a competitive relationship with one another. Competition between organisms occurs whenever two or more organisms require the same limited resource. Water and food are both resources. Shelter is another resource. Squirrels will often take another squirrel’s supply of buried seeds if they find them. Because of this, gray squirrels will bury thousands of seeds each season.
There are competitive interactions in every ecosystem. There are also mutually beneficial interactions in every ecosystem.
There are also predatory interactions in every ecosystem. Predation is an interaction that occurs when one organism (a predator) eats another organism (prey). Organisms can be both predators and prey. For example, squirrels are prey to hawks and foxes. Squirrels are also predators of acorns and other plants.
In a balanced ecosystem, predators and prey act as checks on one another so that no one population of organisms becomes too large. A population is all of the members of a species within a particular area.
Food Chains and Food Webs
Oak trees, squirrels, and hawks are all connected together in a food web. A food web is a visual that shows the network of food chains in an ecosystem. Food chains show specific paths that energy travels as one organism eats another. Each level of a food web or food chain is called a trophic level.
Scientists study how energy flows through ecosystems to better understand how different organisms are connected together. They group organisms within an ecosystem according to how they obtain energy in a food web.
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