Ecosystems are unable to function without the existence of other supporting ecosystems around them. These ecosystems are all important to one another as each provides something to the other that is vital to it's continued existence.
One of the best ways to show that ecosystems are related is to look at the predator/prey relationship. While some predators hunt for their prey in the same general ecosystem that they themselves are in, many times they cross into other ecosystems to do this. Lets take a bird for example, hunting for fish. The bird will fly from it's normal forest system to a pond, which is a system by itself. The bird will take the fish from the pond in order to sustain itself. During the flight, the bird may defecate, which could be a mixture that includes seeds. By doing this, seeds are transferred to a different area.
Without water runoff from the mountains, many ponds would be empty. The fish require the runoff from these systems to go through a variety of other systems and ultimately fill up the aquifers which supply the pond. Also, with predators using the pond, they are able to support their own life in their own system, while regulating the population size within the pond. Without this regulation, fish would eventually die off from over population or from high PH levels within the pond itself.
With all of these different systems all interacting with one another, they are able to sustain within their own system, while managing the systems around them. But this isn't the only type of interrelation related to ecosystems. Within the ecosystem itself, you have to consider the number of relationships that are required in order to maintain life. For example, in a pond environment, you have the plants which feed the insect larvae which feed the fish, which feed, which die, and feed the bacteria, which release oxygen and nutrients into the soil as waste. During this process, they fertilize the ground so that more plants can grow, and the process continues on.
The reality is that without all of these different organisms all working together, whether intentionally or not, life would be unable to exist on the planet. Each and every organism has some for of dependence upon another for life, and because of that, it is incredibly important that we protect each and every creature as we never know how many other creatures we could wipe out in the process.