Convergence in the time-space continuum:a predator-prey interaction

Abstract

Predation is a key structuring mechanism for some marine communities. Prey abundances may fluctuate with strength of predator recruitment and persistence, except in cases where some of the prey population has a refuge in space or time from predation. The sponge Halichondria panicea is patchily distributed in the rocky intertidal on the south shore of Kachemak Bay, southcentral Alaska, and in certain locations is the spatial dominant. This long-lived sponge is dispersed by planktonic larvae. At one site H. panicea has dominated the mid-intertidal for at least 10 yr. Percent cover estimates show that H. panicea averaged 53.4 ± 9.9% cover from August 1994 through August 1996. A major predator on H. panicea is the nudibranch Archidoris montereyensis, which is also planktonically dispersed and has an annual life cycle. Predators with larval dispersal have the same obstacles to and potential for recruitment in suitable habitats as planktonically dispersed prey with the added constraint of locating within-habitat prey patches. Total numbers of A. montereyensis at the study site (550 m 2 ) ranged from 12 to 42 from 1994 to 1996. In the spring of 1997, strong recruitment resulted in an average population of 156 A. montereyensis on site from May to July. Percent cover of H. panicea declined from visual estimates of 40% in May to 15% in July. By August 1997, sponge was absent at the study site and the number of nudibranchs declined to 7 individuals by September. Even though H. panicea is abundant in the region and potential recruits should be numerous, as of June 1999, the site once dominated by H. panicea is open rock with heavy recruitment of annual macroalgae occurring. The predator-prey relationship of A. montereyensis and H. panicea is an example of a chase through space and time, with convergence resulting in extreme population fluctuations and an unstable community.