Litchen growing on rocks mechanical or chemical?

They cause chemical weathering, releasing small quantities of organic acids which can dissolve limestone. This is sometimes referred to as biological weathering.

Humans can affect weathering and erosion in several ways.  humans cause  increases in acid rain and pollution, which increase the amount of  weathering  agents in the air and water, and then on land.for an easy answer: 1) acid rain2) pollution

Since the black bears eat both fruit and meat that makes them an omnivore