Answer:
1. Disease
2. If a new animal species were to be introduced
3. If a new plant species were to be introduced
Step-by-step explanation:
1. Because of the closeness between plants and animals some disease that affect one species can affect the other.
2. If a new species was introduced it would affect the vegetation (by eating it or by ruining it when walking over it), which can create a problem for animals that rely on those plants for food, mainly herbivores.
3. If a new species of plant was introduced it could be poisonous to animals which could kill them or it could grow larger than small ground shrubs and cover them from the sun, therefore depriving them of sunlight for photosynthesis.