Agrassland area has many different populations that all live together and interact. what is the name for a group of interacting populations?

The group of interacting populations living in a defined area is called Community.  

In biology, an ecosystem consists of living things interacting with one another and with non-living things. It starts with a single organism, an individual living thing. When there's a group of organisms that interbreed in the same area, it will then be called a population.  This collection of populations is called community as mentioned above. The interactions among a community make up the ecosystem.

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