Plant Succession Following the Mount St. Helens Volcanic Eruption: Facilitation by a Burrowing Rodent, Thomomys talpoides

Abstract

The 18 May 1980 eruption of Mount St. Helens in the Cascade Range (USA) initiated plant succession over a broad region. Northern pocket gophers Thomomys talpoides survived in widespread subalpine and montane sites where up to 25 cm of tephra had buried dormant or only recently snow-free vegetation. Burrowing and consequent construction of mounds by this herbivorous rodent has both modified the physical structure of the tephra and led to the placement of pre-eruption soil material on the tephra surface. We compared the composition of the plant community and the pattern of seedling establishment on and off gopher mounds. More plant species and individuals survived burial on mounds than at off-mound sites. Seedlings are also most diverse on mounds. We conclude that pocket gophers, through their soil-disturbing activities, can strongly affect plant population dynamics in volcanically disturbed areas, and thus may be an important agent in determining the pathway of succession. Limited observations suggest that differential herbivory may further affect plant community composition. Possible mechanisms that enhance plant survivorship and seedling establishment on mounds are discussed.