Characteristics of a Hybrid Zone Between Two Species of Short-Tailed Shrews (Blarina)

Abstract

Blarina brevicauda and B. hylophaga are distributed parapatrically in Nebraska and western Iowa, and the hybrid zone separating them is remarkably narrow. From 1992 to 1996, 1,324 shrews were captured at seven locations across eastern Nebraska and extreme southwestern Iowa. The zone of overlap between the two species averaged 1.48 km in width (range = 0.64-2.90 km), and only two individuals were captured >2 km inside the range of the other species. The hybrid zone follows an irregular course, is not associated with any obvious soil or vegetative ecotone, and is capable of rapid movement, having shifted ca. 2.4 km S in 22 months at one site. The zone of overlap does not appear to coincide with a region of decreased density but is often associated with partial barriers to dispersal; of seven sites mapped in detail, five coincided with streams and highways. Despite its potential for movement, the hybrid zone between B. brevicauda and B. hylophaga apparently has not changed greatly in position in the last 20-30 years.