A grassland 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.