How does the range stability of *I. pacificus* over the last half-century contrast with *I. scapularis*?

*I. pacificus* shows little dramatic distributional change, whereas *I. scapularis* has expanded markedly.

A striking disparity exists between the recent distributional histories of the two North American blacklegged ticks. Over the past fifty years, *Ixodes pacificus* has exhibited remarkable stability, maintaining a geographic outline highly similar to maps drawn from the late 1800s through the 1940s, stretching along the Pacific Coast from Southern California to Northern Washington, including some western Utah regions. In stark contrast, the eastern blacklegged tick, *I. scapularis*, has undergone a noticeable and marked range expansion across the eastern United States during the same fifty-year period. This difference is attributed to ecological pressures affecting host availability, as severe host reduction in the East pushed *I. scapularis* into isolated refugia, while stable deer populations in the West continuously supported widespread *I. pacificus* populations.