At this point, you already have a group of animals from one of the location ranges. This group will usually include three producers and ten animals. It is recommended to add the producers automatically into your ecosystem, since they can be eaten but don't need to eat, which makes them easier to work with.
This leaves the need to choose five animals out of the ten animals in this range. Since all animals need to eat, it is usually wise to prioritize animals with higher 'Calories Provided' and, when possible, low 'Calories Needed'. Overall, you want the 'Calories Provided' in the ecosystem to exceed the 'Calories Needed', so keep this in mind while choosing your animals.
A helpful approach is to start with the animal with the highest 'Calories Provided', and then select the best option among its food sources (prioritizing high 'Calories Provided'), and continue this chain until you've chosen all five animals.
Finally, try to keep the food sources diverse. For example, try to avoid choosing three animals that all have the same producer as their food source. Spreading consumption across different producers and animals generally leads to a more stable ecosystem.