Asystem is a group of related parts with specific roles that work together to achieve an observed result. seeds, soil, a plow, and a large field are parts of what type of system?

Seeds, soil, a plow, and a large field are parts of an Agricultural system, as these parts are all used for agriculture and all have a function that contributes to the result.

A system can be as simple as having just two components and as complex as having hundreds of components. Anything that has two or more parts that work together to serve to produce a specific result can be considered a system. 

Although i don't know what is the simulated system, starfish are predators that eat mostly  microalgae,  snails, sponges, bivalves, mussels and other small marine animals. removing starfish from any system would definitely increase this animals that they prey.