Mechanisms that structure marine benthic communities at the Semichi Islands, Alaska.

摘要

The purpose of this dissertation was twofold. First, long-term (1987--1997) monitoring at the Semichi Islands, Alaska explored the effects that a keystone species (Enhydra lutris) had on the structure of the nearshore ecosystem. Second, the mechanisms that maintain borders between two alternate subtidal community states (urchin dominated and foliose algal dominated) were examined.; The long-term monitoring showed that otters had limited keystone effects on the nearshore ecosystem. As otter densities fluctuated, so did sea urchin biomass, mean maximum test diameter, and kelp cover. At peak otter densities, a transformation from an urchin dominated community to a kelp dominated community never occurred. After five years of otter residence, urchins were still abundant at shallower water depths. This study demonstrated the need for a minimum density of otters or a minimum residence time to change the community structure from urchin dominated to kelp dominated. It also illustrated the importance of spatial and temporal patterns in the role of keystone species.; The examination of borders between alternate state subtidal communities showed that plant-animal interactions and physical disturbance worked together to maintain the interface. Natural and manipulative experiments evaluated these interactions in two systems at Shemya Island, one of the Semichi Islands. One system consisted of annual algal dominated pinnacles, while the other consisted of perennial algal dominated flat benches. In both systems, mechanical abrasion by algae excluded urchins from entering the kelp beds. In the annual algal system, the abrasive action was seasonal and worked in conjunction with storms to inhibit urchin encroachment into the kelp. Low annual algal recruitment and calm seas would allow the urchins to move to the tops of the pinnacles and remove the remaining foliose algal cover. Also in this system, interspecific competition assisted in structuring the community. In the perennial algal system, the perennial algal cover alone was capable of inhibiting urchin movement. This inhibition has allowed these perennial algal kelp beds to persist over the years regardless of any variation in algal recruitment or seasonal storms.