Food Chains

• Although forests and oceans are easily differentiated, some basic interactions remain true across all ecosystems. The most important of these is the concept of a food chain, also called the trophic structure. Whether animals live in forests or in oceans, the ways in which they receive energy share a basic structure.
  
  It starts with the primary producers, then moves up through consumer levels to the top consumers. It doesn't end there. Every ecosystem has decomposers that recycle the remains of the top consumer. The decomposers release all the nutrients that have accumulated along the food chain back into the environment, so that the primary producers can utilize them and start the process all over again. Animals are vital to this because they can move far and wide, distributing and mixing nutrients or plant seeds that could never spread across such distances on their own.
  
  

Oceans

• Because of their size and coverage on the earth, oceans are extremely productive ecosystems. However, the nutrient pathways are often very long and in some cases involve animals that migrate from high Arctic waters all the way to the South Pole. High-level consumers such as whales, sharks and swordfish make this long-distance mixing possible, because they don't always die in the places where they gathered a localized nutrient.
  
  

Ocean Primary Producers

• Ultimately, ocean-dwelling animals are dependent on just the top few inches of water. This is where the ocean's primary producers, microscopic plants called phytoplankton, receive the maximum amount of sunlight and thus where they are the most dense. Ocean currents mix these tiny plants into deeper water, where they are eaten by the primary consumers, then secondary, then tertiary and so on, all the way up to animals such as great white sharks.
  
  

Forests

• The cycle from birth to death in a forest system is easier to understand simply because the results are more visible. Trees are the most recognizable staples in a forest, but they aren't there just as scenery. Tree species provide both habitat and food for diverse ranges of animals.
  
  Many smaller animals such as birds and squirrels make their homes in trees, as well as eating the seeds and fruits produced in their habitat. Larger animals prey on the smallest animals. Many omnivorous species will also eat the seeds and fruits available.
  
  

Forest Primary Producers

• Just as most ocean dwelling species never leave the water, many forest dwellers live their entire lives in the tops of trees, known as the canopy. The greater availability of sunlight in the tops of trees makes the canopy a more productive part of the food chain. Green leaves are widely available for insect species, while fruits and seeds make up the diets of small vertebrate species. The canopy layer is an important food source for the majority of primary and secondary consumer species.