a tree is a kind of plant & a plant is a kind of organism ->  a tree is a kind of organism  a tree is a kind of organism & trees require a certain thickness of soil to grow ->  trees are a kind of organism that require a certain thickness of soil to grow  trees are a kind of organism that require a certain thickness of soil to grow & the decrease of something required by an organism has a negative impact on that organism 's survival ->  a decrease in soil thickness can cause harm to trees  a decrease in soil thickness can cause harm to trees & drought decreases the thickness of soil -> drought can cause harm to trees by decreasing the thickness of soil