a decrease is a kind of change & cutting down trees in a forest causes the number of trees to decrease in that forest ->  cutting down trees in a forest causes that forest to change  humans build roads & building usually requires cutting down trees ->  humans building roads usually requires cutting down trees  cutting down trees in a forest causes that forest to change & humans building roads usually requires cutting down trees ->  humans building roads by cutting down trees in a forest causes that forest to change  humans building roads by cutting down trees in a forest causes that forest to change & a trail is a kind of road ->  humans building trails by cutting down trees in a forest causes that forest to change  humans building trails by cutting down trees in a forest causes that forest to change & humans changing ecosystems / environments usually has a negative impact on an ecosystem / organisms living in an ecosystem ->  humans building trails by cutting down trees in a forest usually has a negative impact on that forest  a forest is a kind of ecosystem & the earth contains many ecosystems ->  the earth contains forests  humans building trails by cutting down trees in a forest usually has a negative impact on that forest & the earth contains forests -> humans building trails by cutting down trees has a negative impact on that forest on earth