Organisms depend on each other for nourishment to survive, and this joins them together in something called a food chain. The food chain describes how organisms in an ecosystem are linked together by what they eat. 
Are plants always at the bottom of the food chain? 
What would happen if one set of organism disappears from the food chain? 

Food chains often combine to form complex food webs, containing many interconnected and overlapping chains. 

 What is an energy pyramid?
The energy pyramid is a model for how energy spreads through an ecosystem.
How are the animals linked together in an ecosystem?
Where is most of the energy used up by an individual?
Why is the energy transfer in a food chain represented as a pyramid?
How much does the energy decrease as we move up in an energy pyramid?
Does the biomass remain the same in a food chain? 

 What is an ecosystem?
Ecosystem: A group of living things interacting with each other and their environment. 
Resources like space and water are limited. Ecosystems force species to make the most of what’s available. Their success depends on how effectively they can use the living and nonliving things in their habitat. This activity defines the species’ role, or niche.

What happens in an ecosystem when the resources become limited?
What happens to an ecosystem when humans use more resources? 

 What is a symbiotic relationship in an ecosystem?
A symbiotic relationship, or symbiosis, occurs when two species live in close contact with one another. 

What are the different types of symbiosis in an ecosystem?
There are three main categories of symbiosis: distinguished by whether the relationship is harmful or helpful to one or both of the species.