Picture this: the sun rises over the African savannah. A giraffe stretches to nibble the acacia tree's tiny leaves, carefully avoiding the stinging ants that live within the tree's long thorns. Wildebeests and gazelle graze on grass that flourishes in volcanic soils set down long ago by the pulling apart of Earth's tectonic plates. A cheetah sits high on a rocky outcrop—searching for a meal that she will share with her cubs. Vultures watch from above, ready to pounce on the remains of the cheetah's catch. A family of warthogs scurries around a termite mound, disturbing an ostrich as she flutters her feathers in the dust to shake off unwanted parasites.

Now put yourself in that scene. Are you the predator or are you prey? How will you survive? What will you need? Where are the threats? What are the dangers? How will you get what you need without being eaten?

On the African plains, Earth's soil provides nutrients to the acacia tree. Roots quickly absorb the water that falls from the sky as rain. The tree transports the water to the leaves where, using sunlight, it produces food that the giraffe devours. When the giraffe dies, its body feeds the vultures. Insects and microbes consume the decaying flesh and release nutrients into the soil. It is a continuous cycle of life, giving to and taking from the planet. Even you are part of that cycle.

This interaction between organisms and their environment is called ecology. The word ecology comes from the Greek words, oikos, meaning "house", and logos, meaning "study of." In this case, house refers to something much larger than the building in which you live. It is our planet and everything on it is connected.

Each of these animals struggles to survive in its niche in the African savannah, and limiting factors in that environment control how many of them survive.

Russell Cuhel observes how and when organisms connect with the living and nonliving components in Lake Michigan. He uses the data to make predictions about how a seemingly small change—a tiny new mussel in the lake—can have such a significant effect on the environment and people.

Just a thin layer of Earth—20 km (12.4 mi) thick—is where life can live. When compared to the volume of our planet and its atmosphere, Earth's biosphere is as thin as the skin on an apple. The biosphere is divided into smaller units called ecosystems. Ecosystems have two parts. The abiotic factors include nonliving parts like rocks, moisture, the amount of sunlight, and temperature. The biotic factors include all the organisms.

In Lake Michigan, for example, the abiotic factors include the amount of dissolved oxygen in the water, temperature, wind, currents, and how much light penetrates the water. These support the biotic factors like mussels, fish, turtles, and algae.

Dr. Cuhel began his ecological investigation of Lake Michigan by studying algae, but he soon ran into a problem. Algae were disappearing from the lake. Russell used scientific detective work to solve the mystery and shifted his focus to understanding the ecology of the alien invaders who ate the algae—the zebra and quagga mussels.

A group of one species, like all the zebra mussels in the lake, makes up a population. The different populations of organisms that interact with each other form a community. The mussels, fish, submerged plants, algae, and even a fisherman sitting on a boat in the lake make up the community of Lake Michigan.

Organisms depend on both biotic and abiotic factors for energy and matter. To stay alive, all life requires liquid water, energy, and nutrients. The place where an organism can find what it needs to live is called its habitat. The combination of behaviors and resources that an organism is adapted to exploit is its niche. In Lake Michigan, there is competition between the two invading mussel species because their niches overlap. However, their habitats in the lake are different. For example, quaggas can live on most submerged substrates, whereas zebras can live only on hard submerged surfaces like rocks or pipes. 

Biodiversity is the number and variety of organisms that live in an ecosystem and is dependent upon the abiotic and biotic resources available in that environment. Land ecosystems, grouped by common amounts of precipitation, temperature ranges, types of soil, and the plants that grow in that area, are called biomes. The tropical rain forest biome has the greatest biodiversity on land.

Over time, members of a community adapt to each other and adjust to the amount of available resources. Thus, the ecosystem becomes stable.

The introduction of a new species can disrupt the stability of an ecosystem. The increased competition for resources causes a redistribution of those resources which can change the population of each species in the ecosystem.

The largest population that an environment can support over a long time is called the carrying capacity. Limiting factors control the size of populations and the variety of organisms that can live in an ecosystem. A limiting factor can be food, shelter, temperature, sunlight, predators or even parasites. One limiting factor in Lake Michigan is algae. Algae are primary food for many organisms in the lake. If that resource decreases, the lake will not be able to support as many algae-eaters and the organisms that eat them. In the case of the zebra mussels, the limiting factor is substrate, which gives the quaggas a competitive advantage.