Studying ecosystem dynamics requires a holistic approach to the ecological unit. The physical limits, or boundaries, of an ecosystem are not always distinct, since ecosystems usually grade into each other. Normally, units such as a tropical rainforest, a lake, or an estuarine salt marsh would be considered ecosystems. An estuarine salt marsh, for instance, is influenced both by the ocean and its associated communities, and by a river and its associated communities. The physical factors in a salt marsh ecosystem are in a constant state of flux.
Ecosystems are dynamic.
They change over time. For example, if a volcanic island were newly formed out in the ocean, you would think the lava would probably not be the most hospitable place for life. Lichens would be the most likely pioneers. Since they absorb moisture and minerals from the air, they would be able to live in barren conditions like those on the new island. They might produce chemicals that slowly break down the minerals, freeing up small amounts from the substrate at first.