What happens if a new organism is added to an ecosystem or if the temperature was to change suddenly

Each organism is suited to its own environmental niche. So if the temperatures were to suddenly change, then the organism of that species with the maximum or at least a few variations will be able to survive while the others perish, thus resulting in evolution of the species. This ensures that the species is not wiped out if the environmental conditions of a place or region were to suddenly change,
For example, imagine a particular type of bacteria lives in a pond. Some of these bacteria have variations that allows them to survive in harsh weather conditions. Now, imagine the temperature of the water were to drastically increase due to global warming. Then the bacteria with the variations will be able to supervise and carry on the species.