In the context of a food web, a keystone species is one that has a disproportionately large impact on its environment relative to its abundance. Considering the animals mentioned (frog, mouse, grasshopper, and deer), we can analyze their roles in the ecosystem.
- Frog: Frogs may control insect populations and serve as prey for larger animals, but they are not typically considered foundational.
- Mouse: Mice can serve as prey for many predators and contribute to seed dispersion but do not have a significant impact on the ecosystem as a whole.
- Grasshopper: Grasshoppers can be an important food source for many animals, but they are primarily herbivores and do not significantly shape the ecosystem.
- Deer: Deer can influence vegetation through grazing and serve as prey for large predators, making them a more impactful part of the food web.
Based on these considerations, the deer is the most likely candidate to be a keystone species in this context, as they can significantly affect plant communities and the structure of the whole ecosystem through their foraging habits and as a key prey item for larger predators.