MEPS 268:69-80 (2004)  -  doi:10.3354/meps268069

Population ecology of the invasive kelp Undaria pinnatifida in California: environmental and biological controls on demography

Carol S. Thornber1,*, Brian P. Kinlan2, Michael H. Graham3, John J. Stachowicz1

1Center for Population Biology, University of California, One Shields Avenue, Davis, California 95616, USA
2Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California 93106, USA
3Moss Landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing, California 95039, USA

ABSTRACT: We combined field monitoring and laboratory experiments to examine the population ecology of both the microscopic and macroscopic stages of a new invasion of Undaria pinnatifida in California. Over the course of 1 yr, we observed 2 distinct recruitment pulses of individuals in the Santa Barbara harbor; the appearance of these pulses was strongly correlated with a 4°C drop in ocean temperature approximately 2 mo prior to recruitment. Cultures of zoospores and successive microscopic stages revealed thermal tolerances consistent with field recruitment data; individuals grown at 13°C had significantly higher survivorship than individuals grown at higher temperatures (harbor temperatures annually ranged from 12 to 21°C). The 2 cohorts also differed greatly in individual size, growth rate, and survival to maturity. Grazing by herbivores, predominately the native kelp crab Pugettia producta, effectively prevented nearly all individuals in the second cohort from reaching reproductive maturity. Grazer control was effective despite far higher rates of recruitment during the second recruitment pulse. Our results highlight the potential for extreme variability in U. pinnatifida demography mediated by local oceanographic and biotic factors. Understanding controls on U. pinnatifida demography helps to explain variation in the spread and impact of this invader worldwide, and allows better prediction of when and where U. pinnatifida may continue its invasion along the west coast of North America.

KEY WORDS: Invasive species · Undaria pinnatifida · Macroalgae · Kelp · Sporophyte · Gametophyte · Life cycle · California · Herbivory · Pugettia producta

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