Because predator species often will switch its prey as one prey species becomes less abundant.
Why this is so should be obvious - because it's not in the interest of predators for the prey species to completely die out as it would effectively also mean that they would die out due to not having enough food to eat.
A stationary front is depicted by an alternating red and blue line with a triangle on the blue portion and half-moon on the opposite side of the red portion of the line. A cold front (or warm front) that stops moving becomes a stationary front.
