An example of a food chain has been provided in this question. The components of this food chain are seeds, mice and fox. Here, seeds are the primary food, which is eaten by the mice, who in turn are eaten by the foxes. In a given ecosystem, various components of a food chain are connected to each other and a change at one level will affect the other levels as well.
For example, if the fox population is suddenly increased, the extra foxes would need food, and this will cause a reduction in the mice population (due to sudden increase in predation levels). With sudden reduction in the mice population, there would be very few predators of seeds, and the seed content in the ecosystem will increase.
However, this is not a very sustainable scenario since the higher fox population cannot be indefinitely supported by ever declining mice population, hence at some point in time, a new equilibrium (between seeds, mice and fox populations) will be established.
Hope this helps.