An ecosystem composes of an environment and the organisms that interact with said environment. Changing temperature leads to an unstable ecosystem because of two things; failure of adaptation for the environment, or failure of adaptations for the animals.
If the environment fails to adapt to a change in temperature, plants will die, thus causing herbivores to die and the whole food web/chain to disrupt. (Im not really going in to deep detail, but plants can die from the soil failing to fertilize.)
For animals, it may also disrupt the food chain, but natural resources such as water may be needed more in the heat. If said animal doesnt have the adaptations to compete in such an event, they die. Also, especially if the environment was previously a desert, all adaptations will be useless, and other animals will start to move in. More competition. Not only that, if the ecosystems temperature decreases, for example, many animals might die from the cold and such. They do not hibernate/ have the adaptations of an animal that lives in the cold.
All in all, changing temperature can cause an unstable ecosystem because it can disrupt the food chain/web, kill plants, kill animals, cause an increase of animals, cause a decrees of natural resources (such as water) and much more.
(Sorry for grammar, I'm on mobile and not really paying attention to it. im not really double checking my spelling because im trying to give you the information, not really write it for you as that would be too much work)
