Dermacentor reticulatus (Meadow tick) plays an important role in the spread of pathogens. Its typical hosts include most medium-sized sheep, cattle, dogs, horses, pigs, and humans. Currently two isolated populations of D. reticulatus are present in Poland. A species of D. reticulatus transmits the bacterium Rickettsia helvetica to any host that it infects.
Two populations of (host) sheep exist in Eastern and Western Poland, respectively. The Eastern, but not Western, variety is immune to the pathogen R. helvetica. The Meadow tick is the chief food for a particular kind of bird in the forest.
If all these birds were suddenly eliminated by hunters, which of the following predicts the MOST likely and immediately observable consequence?