Robustness in life history of the brown seaweed Ascophyllum nodosum (Fucales, Phaeophyceae) across large scales: effects of spatially and temporally induced variability on population growth
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Understanding the demography and function of biotope-forming seaweed species is of great importance for the conservation of the target species itself, as well as its associated organisms. The brown seaweed Ascophyllum nodosum is fundamental for the functioning of coastal marine ecosystems in the North Atlantic. In this study, we use a data-based size-classified matrix model to investigate the temporal and spatial variability in demography, and the environment-specific stochastic sensitivity and elasticity, of two A. nodosum populations, one in western Sweden and one on the Isle of Man in the Irish Sea. A significant difference between the two populations was that the Swedish population had comparably low and more variable stochastic population growth rate (λs). This pattern was partly explained by the relatively high and varying mortality rates during extreme ice-years in Sweden, and by the lower survival of small individuals during all years. There were also fewer large individuals in Sweden due to lower transitions to the larger size-classes and higher probability of shrinkage. Sensitivities were analogous in the two populations, and showed a high selection pressure for increased individual growth. Elasticities were also similar, with the exception that survival of the smallest individuals (i.e., transition a1,1), had a higher elasticity on the Isle of Man. Overall, the stochastic growth rate (λs) was most sensitive to proportional changes in loop- (i.e., survival within size-class) and, to some extent, growth-transitions in both study areas. These results show that structurally and demographically diverging A. nodosum populations may be similarly sensitive to changes in vital rates. This, in turn, indicates a plastic life history of A. nodosum that may cope with large environmental variability. The results further suggest that environmental change affecting the survival or growth of the larger, reproductive A. nodosum individuals could have severe and regional effects on the abundance and biomass of this species, with potential negative effects on the biodiversity of the associated communities.
KeywordsVital Rate Primary Shoot Ascophyllum Nodosum Swedish West Coast Stochastic Sensitivity
We are grateful to the staff and students at Port Erin Marine Laboratory and Tjärnö Marine Biological Laboratory for their help and hospitality. This study was supported by the Swedish Natural Science Research Council through contract B-BU 9949-302, by the Swedish Research Council through contract 621-2007-5779, by Formas through contract 21.0/2004-0550, and by the Swedish Environmental Protection Agency through the project MARBIPP (Marine Biodiversity Patterns and Processes). MARICE (an interdisciplinary research platform at the Faculty of Sciences, University of Gothenburg), the Royal Swedish Academy of Science, through the foundation of Hierta-Retzius, and the foundations of Helge Ax:son Johnson, Knut and Alice Wallenberg, Wilhelm and Martina Lundgren and Kapten Carl Stenholm provided additional support.
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