Squirrels easily kill some trees because

Step 1: Analyze the question The question asks how squirrels are able to kill trees. We need to identify the feeding or physical behavior that causes fatal damage.

Step 2: Explain the concept Squirrels are rodents. Rodents have chisel-shaped incisor teeth that grow continuously, so they must gnaw on hard materials (bark, wood, nuts) to wear their teeth down and keep them sharp.

Under the bark of a tree is a ring of living tissue made up of the phloem (which carries manufactured food from the leaves to the rest of the plant) and the cambium (which produces new tissue). When an animal gnaws away the bark all the way around the trunk or branch, this is called girdling (or ringing).

Step 3: Work through the options

• A. They inject poisonous substances into the trees. Squirrels are not venomous and do not inject poison. Wrong.
• B. Most trees are susceptible to squirrel bites. This is not a biological explanation. Trees are not killed by "bites" in the way an animal would be. Wrong.
• C. They feed voraciously on the roots. Squirrels are mainly tree-dwelling animals that eat nuts, seeds, fruits and bark. They are not known for feeding on roots. Wrong.
• D. They gnaw the bark and destroy it. Correct. Removing the bark destroys the phloem, so food cannot move down to the roots. The roots starve, and the tree eventually dies.

Step 4: Answer The correct option is D.

Step 5: Exam tip Remember the link: Rodent → gnawing → bark removal → phloem destroyed → tree starves. Whenever you see "squirrels" or "rodents" and "trees" in a question, think of gnawing and girdling.