Individual, population, community and ecosystem are some of the levels of organization within an ecosystem. An individual is an organism of a particular species, say a deer or tiger, etc. A number of individuals of a particular species that live in the same area form a population. A community is composed of populations of various species living in a given geographic region at a given time. An ecosystem is composed of communities (biotic component) and their environment (abiotic component).
In a given ecosystem, each species is dependent on another species, either as a prey or predator or competitor. Thus, a change in the population of an organism is bound to affect the community and the ecosystem. For example, deer is a competitor of cows (for food), is a prey for wolf (which is a predator) and is a predator of grass (its food). If the population of deer were to decrease, its competitor will have access to more food and population of cows will grow. Deer's predator will find less food and will decrease in number (unless they find another prey). This will cause a shift in community towards more cows, less deer and less wolves. At ecosystem level, this will probably change the population of herbivores and carnivores and also affect the level of grass.
Thus, a change in the population of one organism will affect the community and the ecosystem.
Hope this helps.