Nitrogen fixation is the process by which nitrogen gas from the atmosphere is converted into different compounds that can be used by plants and animals. There are three major ways in which this happens: first, by lightning; second, by industrial methods; finally, by bacteria living in the soil.
As the world evolves, there is a change in the ecosystem, and with that comes the fluctuation. Organisms have have changing populations as they adapt, slowly die out, or thrive in the new ecosystem.