This world has multitudes of ecosystems, encompassing large and small areas. An ecosystem is composed of the diverse living and nonliving things in an area which interact with one another and live or exist under the same environmental conditions like, but not limited to, rainfall, amount of sunlight, and rich or poor soil. Ecosystems do not exist within clear boundaries. What happens in one ecosystem affects conditions in another. In this way, ecosystems can be interrelated.
Living inhabitants can and do cross over from one ecosystem to another. Last winter, great northern owls ventured south from their normal feeding grounds because of a decrease in their food supply. Consider migrating birds like Canadian geese which have summer and winter grounds in different ecosystems.
Sometimes species are introduced to an area and their interaction with native species can either cause the native inhabitants to adapt or be decimated in numbers. A good example is the lamprey that appeared in Lake Superior and began to attach themselves to lake trout, affecting the population of that species and the larger fish that feed on lake trout.
Sometimes a natural disaster like a hurricane, a tsunami, volcano, or earthquake can spread its influence over several ecosystems. When Mount Saint Helens erupted, it affected not only the surrounding area. The ash from its eruption was carried by the wind to the eastern United States in three days and volcanic particles got into the jet stream. A greater than normal snowfall can affect mountaintops and, during the spring thaw, affect other ecosystems in the way of any flooding that occurs.
Sometimes several ecosystems are influenced by things man does. Think about this scenario. Mercury is emitted into the atmosphere by certain human activities. The mercury pollutants fall into lakes and seeps into groundwater. Groundwater has no boundaries. The plant life and bacteria in a lake begin to die because of the pollution. A fish, nibbling on algae, concentrates mercury in its system. A bald eagle swoops down to capture the fish in its talons and carries the fish back to its nest. The bald eagle shares the meal with its young, which as they mature, lay eggs that have thinner shells and greater risk of breakage.
Granted, the destruction of a stand of trees to build a parking lot in northern Illinois may not affect a coral reef in the Philippines in any large way, but ecosystems in close proximity have great influence on each other.