Answer:
a.) The garden plants provide energy to all the other organisms. While not every organism consumes garden plants directly, every organism's food lineage can be traced to garden plants. For instance, while ground beetles do not directly consume garden plants, they do eat snails, and snails eat garden plants.
b.) The slug population might increase is the aphid population decreased because there would be less competition for food resources. Both populations consume garden plants, and there is only a finite number of garden plants. Therefore, there is a limit that each population can consume. If there were less aphids eating the garden plants, there would be more left for the slugs. Less starving slugs would lead to greater reproduction and thus a higher slug population.