To better understand how ecosystems work you must know the terms:
Energy provides power for life and moves through communities within ecosystems through a one-way flow. Nutrients on the other hand pass as atoms and molecules derived from the environment and cycle within and between ecosystems.
Energy flows through a process called "photosynthesis", and passes nutrients through food chains of decomposers, primary and secondary producers. Light energy comes from the sun, which is our main source, a reaction of thermonuclear conversion of matter such as hydrogen into helium.
Photosynthesis ables energy from light to convert to chemical energy from the 1% of radition from the sun that reaches the earth. This conversion is depicted in the picture below.
Finally, a small amount reaches the earth in waves of energy: heat (low energy), visible light (usable energy), and UV light (high/dangerous energy). Made up of atoms and molecules from the environmental, nutrients pass through organisms that require them for survival. Nutrients cycle through ecosystems - land, water, air, and make their way through the food chain.
Harnessed energy into biological matter - invertibrates, mammals, fish, and birds. Dead organisms, or "decomposers", carry energy and nutrients just like consumers that is passed on to future organisms.
Producers are the bottom of the food chain, they are our plants that harness energy from the sun, water, and air through a process calleed "photosynthesis". Our primary producers are capable of producing complex organic compounds from simple substances by consuming plants. Most common primary producers are herbivores. Secondary producers cannot harness energy from light or other sources, so much "feed" on primary producers. These are omnivores and carnivores. Lastly, we have our tertiary consumers, such as larger mammels, fish, and birds.
There are many interconnected food webs, in the picture below you can see the lion and the hawk at the top of the food chain as tertiary consumers, while producers, primary producers, and secondary producers underneath share energy for other creatures.
Trophic levels describe the position of energy in food chains and webs. To better understand the percentage of energy distribution will help better understand the issues of populations of animals (average number of animals per species) in an ecosystem, and the seriousness of population changes. Energy is transfered in the form of nutrients, and 90% is actually lost during the conversion pyramid. The image below shows the distinct percentages per stage and the changes as energy converts and transfers.
Trophic cascades is the changes in population that happen so quickly that it makes a large change on an ecosystem. This is a domino effect that causes serious changes in other populations. An example would be killer whale populations failing because of salmon populations depeleting due to over-fishing. Tracing population changes is extremely difficult and requires ongoing assessments to determine ecosystem changes.
The picture below depicts life distribution across the globe. Find your current location and do some simple research of the ecosystems in your area so that you have a better understanding of what creatures live in it, how it has changed over time, and what you can do to help conserve it.