The answer is "an atom."
It's important to note that atoms, while being the fundamental building blocks of matter, are not ecosystems. They don't have the characteristics of a biological community nor do they interact with their environment in the same way living organisms do. Atoms are microscopic particles that make up everything in the universe, including living organisms and their environment. However, they don't form ecosystems on their own.
On the other hand, an ecosystem is a biological community of interacting organisms and their physical environment. It consists of living organisms, such as plants, animals, and microorganisms, as well as non-living components, such as soil, water, and air. Examples of ecosystems include a forest, a lake, and an ocean. Each of these has its own unique characteristics and supports a variety of life, from tiny microorganisms to large mammals.
By understanding the differences between atoms and ecosystems, we can better understand the natural world around us and appreciate the complexity and diversity of life on Earth.
hope this helps :)