No. Ecosystems are defined by their having a stable food chain and being perfectly sustainable within themselves. If you were to seal off a true ecosystem, it could keep going forever. In most cities, all or most of the food consumed by the residents is imported from farms outside the city, so the city is not a self-sustaining ecosystem.
An ecosystem is really a system of life. Energy enters each ecosystem when plants absorb sunlight and turn it into energy. Then herbivores eat those plants and carnivores eat the herbivores and other carnivores. When an animal dies, it is used as food for other species in the ecosystem and anything not eaten by these animals is decomposed into the earth and incorporated into the soil for plants to use. As stated above, if an ecosystem was closed off, it could survive with no imports or exports. If a city were to be blocked off, it would not survive for long. However it is a different story when you think of small villages that live together with wild animals and plants.
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