What primary factor likely maintained widespread *I. pacificus* populations across the West over the last century?

Rapid recovery and sustained geographic presence of deer populations, specifically *Odocoileus hemionus* subspecies.

The sustained, widespread support of *Ixodes pacificus* populations across western North America appears directly linked to the status of their primary reproductive hosts for adult ticks, which are deer, specifically subspecies of *Odocoileus hemionus*. Unlike in the Eastern US where deforestation and hunting drastically reduced deer populations in the early 1900s, these pressures were reportedly less severe or managed differently in the West. Following early 1900s conservation efforts, deer populations in the West recovered more quickly and maintained sufficient numbers and geographic coverage across the tick’s core range. This continuous and widespread host availability acted as an evolutionary constraint, preventing the large-scale range shifts observed in the East.