Biotic factors are the living factors that can affect a community. They are the availability of food, new predators arriving, new pathogens, and one species outcompeting another so that the numbers are no longer sufficient to breed.
If the food supply falls, animals that eat it have less to eat, so their population falls. A new predator arriving in a habitat kills more of its prey, so the prey population may drop sharply. A new pathogen, such as a disease-causing bacterium or virus, can spread through a population and kill many individuals, because they have never met it before.
Competition can also change a community. If one species is better at getting food or space, it can outcompete another species. The weaker species may be left with too few individuals to breed, so it can die out in that area.
When you are given data, link each change in numbers to the biotic factor that causes it. A fall in one population often causes changes in the species that depend on it.