I love this simulation. There are few if any better representations of Next Generation Science Standard MS-LS2-4. Students can absolutely work on the simulation without a worksheet however I like having one available to keep them focused on the learning objective and I feel that this helps.
Here is a link to the simulation, if you would like proof of student learning then download the worksheet.
MS-LS2-4.Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.