"More ticks means more tick-borne diseases" There’s no single reason that explains why these blood-suckers are on the move. But one fascinating culprit is an increase in forest cover in the eastern US, which has helped facilitate the spread of white-tailed deer and other animals that ticks love to feed on. It’s a rare, counterintuitive example of how reforestation can carry unwanted consequences — or “ecosystem disservices,” as some researchers have put it.