There are mice in the field which act as prey to snakes. Hawks eat the snakes. Some * dogs are brought into the field. What will be its effect?

Answer: (d) Mice and snakes will decrease

Dogs hunt and kill both mice and snakes, so both fall; with fewer snakes, hawks go hungry later.

The idea

The field has a simple chain: mice → snakes → hawks. A dog is a general hunter that goes after any small animal it can catch, so it presses on two links of the chain at once.

Dogs enter the field → They kill mice and snakes → Both fall in number → Later: hawks find fewer snakes to eat

Why the others are wrong

(a) Mice fall, but so do snakes — (d) is more complete.

(b) Hawks may fall later, but the direct effect is on mice and snakes.

(c) Dogs kill snakes, so snakes can’t increase.

The trap

You may think the dog helps the mice by killing snakes. Dogs chase mice too.

Also know

This is what happens when people bring a new predator to a place. Cats, dogs and mongooses taken to islands have wiped out many ground-nesting birds and reptiles.