This graph shows the sightings of raccoons over the years. The reasoning for the rapid decline in raccoon sightings in 2013 could be from the spike in population, or birth increase, that occurred in 2010. Their resources available were not enough to keep the population at that rate so many died. Once the amount of raccoons began to decrease the resources went up and the up and down cycle repeats.
Raccoons are primarily found in North America. Let's say If large numbers of raccoons were to migrate from the United States to Canada, the size of the population could be affected. If too many are crowed in one area resources would become scarce and the population would exceed carrying capacity, causing a decline in population. Another possible cause of population decline could be if raccoons were to emigrate into urban areas. Food would be much more difficult to find and people could become a new predator.
The majority of the raccoon species live in wooded areas. If their habitat were to be affected by some kind of natural disaster, for example a fire, that would cause mass migration to a new location. Another environmental factor would be if prey populations decreased in their habitat. They would also then have to migrate to a new location in order to survive. Both of these would obviously result in a population incline in the specific area where they moved to.