UA01-Teses
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- Phenotypic traits of macroalgal populations at their southern marginsPublication . Araújo, Rita Micaela dos Santos Fernandes; Serrão, Ester; Åberg, Per; Pinto, Isabel Sousa; Valero, MyriamUnderstanding the factors that determine species geographical ranges is a fundamental issue in ecology and evolutionary biology. Species geographical distribution reflects the range with suitable conditions for their persistence beyond which organisms’ tolerances and capacities are presumably constrained. At peripheral locations, where habitats are often fragmented, population persistence frequently relies on the differentiation of lifehistory traits either by phenotypic plasticity or local adaptation. This capability is particularly relevant for species with limited dispersal of propagules and living in variable physical environments as is the case of many marine macrophytes. In this study the variables population structure, dynamics, demography and investment in fitnessrelated phenotypic traits were investigated at the margin and center of the distribution of Ascophyllum nodosum and/or Fucus serratus. These are two ecologically similar and phylogenetically related brown seaweed species with very different life-spans, growth rates and phylogeographical histories. Additionally, the effects of different sources of disturbance on marginal populations of both species were experimentally evaluated. The population structure and dynamics of F. serratus was similar at central and southern border locations but these marginal populations were more constrained in habitat, restricted to a narrower vertical range on the lower intertidal. On the contrary, marginal A. nodosum populations occupied a narrower fringe within the same vertical range as central populations, but were denser, with smaller individuals and had higher reproductive output than central ones. Population stochastic growth rate (λs) was lower and more variable in central than edge populations for A. nodosum, but in F. serratus it was much lower and more variable at the southern limit than in the central range. Elasticity analysis showed that fertility elements had a small contribution to λs; this was more sensitive to changes in survival, particularly for loop in A. nodosum and for growth in F. serratus. However, fertility transitions were more important for population growth rate for F. serratus than for A. nodosum. Surprisingly, the reproductive effort and investment of marginal populations of A. nodosum was higher than in central ones, although similar growth rates were found at both locations. The production of defence compounds was higher at central than at marginal populations suggesting a trade-off in the allocation of resources to reproduction and defence dependent on the local habitat conditions. In this context, the results obtained on the effects of human trampling on A. nodosum populations are of great concern given the levels of human frequentation in the northern intertidal zone in Portugal, its southernmost population, and the importance of individuals´ shrinkage for population growth in these marginal populations. Human trampling negatively affected the A. nodosum communities by significantly reducing the cover of A. nodosum at all the experimental trampling intensities tested but also by affecting the abundance of the associated seaweeds. The long-term recovery of these communities was slow for the two highest trampling intensities, and after approximately 4 years the cover of A. nodosum, as well as the structure of the whole community, were still significantly different from control plots. The interactive effects of abrasion, sediment deposition and herbivory in the survival and growth of F. serratus recruits in marginal populations and in its spatially contiguous species F. vesiculosus were also studied. These two species showed different sensibilities to these tested sources of disturbance. The survival of F. serratus recruits was negatively affected by herbivory whereas F. vesiculosus showed significantly lower survival of recruits under the interactive effects of abrasion, sediment deposition and herbivory. Overall, the results show that A. nodosum marginal populations have high capability for differentiation of life-history traits and seem to be able to cope with the environmental conditions experienced at its southern edge location. On the contrary, F. serratus, although exhibiting a similar population structure in central and marginal locations (at more benign intertidal levels), shows very variable population growth rate which might be indicative of its higher sensibility to environmental variation. These differences between marginal populations of both species are likely related to their different lifehistory characteristics and distinct genetic backgrounds. Furthermore, the results of this study show that perturbations negatively affecting both species at their southern limits, like human trampling for A. nodosum and grazing for F. serratus, might disrupt population dynamics and threaten the persistence of populations at these locations.