Bears have multiplied much faster than people in Pennsylvania, so it's not all that surprising that three black bears were roaming around Lancaster County this week.

The Pennsylvania Game Commission estimates the state's black bear population at around 20,000. That's up five-fold from the 1970s, when it was estimated at 4,000. The state's human population has grown about 8 percent over that time.

"A dramatic growth has provided more opportunities for people to see bears, which is an experience many treasure, and bear hunting has greatly improved," the Game Commission notes on its website.

Bear sightings are sporadic in Lancaster County, occurring every few years. Game Commission officials believe the bears spotted in Lancaster County this week are young males that came from mountainous areas to the north in search of new territory and a source of food.

Related: Black bears are expanding their range in Pennsylvania

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Black bears have been sighted in all Pennsylvania counties, according to the Game Commission. But most of the ones seen in Lancaster and other urbanized southeast counties are transient and show up in the summer months, long before the November bear-hunting season.

In 2015, hunters killed 3,748 black bears in Pennsylvania, but none in Lancaster County or six other southeast counties. The bear harvest was down from the record 4,350 in 2011, but was far greater than in earlier decades. From 1915 to 1983, fewer than 1,000 bears were killed each year, according to Game Commission records.

The harvest has averaged well over 3,000 a year since the year 2000, according to Game Commission records. Pennsylvania black bears are also known for their unusually large sizes, and the commission posts data on the size, age and gender of each bear killed by hunters.

Of the bears killed by hunters last year, 68 weighed 500 pounds or more and 18 topped 600 pounds, according to the Game Commission.

Here's a Game Commission map that shows the bear harvest in each county in 2014, when hunters bagged 3,371 bears. It is the most recent year for which a map is available on the Game Commission's website: