A few would be maybe a change in mating? Like if the animal isn't leaving it's habitat to go and get nutrition and mate then something might be off with the animal, or the seasons are messing them up due to weather.
Another would be possibly be animals who usually don't go into competition, going into it with animals that are usually in a good zone, most likely due to food dropping or habitat population increase.
And then there's invasive species, they come in and kill off animal species causing a whole environmental habitat of animals to die off, eat their food and take in all the nutrients, and then possibly animals having to go off and find a new habitat causing them to become invasive species.
Answer:
Should there be a picture or something?
Explanation:
