An ecosystem is a community of organisms interacting with one another and with the chemical and physical factors making up their environment combined with the physical environment. A few of the chemical and physical factors include sunlight, rainfall, soil nutrients, climate, etc. An ecosystem is inherently "leaky" - energy and nutrients move in and out. Individual organisms (such as seeds, spiders, and sparrows) move in and out as well. .
Ecosystem is a "complex" word .it does not describe just one system; and yet it does. All parts of the planet, from the highest microbe drifting in the atmosphere, to cockroaches, to the most exotic species in the deepest ocean trenches are all parts of ecosystems. All of these species can be considered parts of other ecosystems climbing up and down the ladder of species of the entire planet - each defined to serve a purpose. Even though you may look at the whole picture as one entire ecosystem it is in reality comprised of countless separate entities which are in fact their own ecosystems. .
Ecosystems are not necessarily stable, nor are all of their component species necessarily native to the area. Some ecosystems may be threatened by new species and it might even pose a risk the continued existence of some resident species. The brown tree snake of Guam and the zebra mussel in the Great Lakes are examples of damaging species whose introduction is destabilizing their new ecosystems; the voracious snake has already caused the extinction of some bird species, and the mussel is clogging streams and crowding out native species.
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Why are these systems important? .
Ecosystems are very important because each and every part of a system impacts the other. This occurs from the tiniest single celled ameba up to the highest form of life - they are all linked. For instance, if one part of a system is destroyed other parts will have to find a substitute or even a replacement.