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Ongoing changes in natural diversity due to anthropogenic activities can alter ecosystem functioning. Particular attention has been given to research on biodiversity loss and how those changes can affect the functioning of ecosystems, and, by extension, human welfare. Few studies, however, have addressed how increased diversity due to establishment of nonindigenous species ( NIS) may affect ecosystem function in the recipient communities. Marine algae have a highly important role in sustaining nearshore marine ecosystems and are considered a significant component of marine bioinvasions. Here, we examined the patterns of respiration and light-use efficiency across macroalgal assemblages with different levels of species richness and evenness. Additionally, we compared our results between native and invaded macroalgal assemblages, using the invasive brown macroalga Sargassum muticum (Yendo) Fensholt as a model species. Results showed that the presence of the invader increased the rates of respiration and production, most likely as a result of the high biomass of the invader. This effect disappeared when S. muticum lost most of its biomass after senescence. Moreover, predictability-diversity relationships of macroalgal assemblages varied between native and invaded assemblages. Hence, the introduction of high-impact invasive species may trigger major changes in ecosystem functioning. The impact of S. muticum may be related to its greater biomass in the invaded assemblages, although species interactions and seasonality influenced the magnitude of the impact. [ABSTRACT FROM AUTHOR]

Sargassum muticum (Yendo) Fensholt is an introduced brown seaweed with a very distinctive seasonal growth cycle on European shores. The present study links the dynamics of a population of S. muticum with the seasonal growth cycle of the species and the density-dependent processes operating throughout this cycle. Results indicate that both growth cycle and intraspecific competition influenced the structure and population dynamics. Size inequality increased during the slow growth phase (autumn–winter) of the 2-year study. Mechanisms generating inequality of size could be the existence of asymmetric competition and the inherent differences in growth rates between old (regenerated) and new thalli (recruits). Inequality of size distributions decreased progressively during the last months of the growth phase (spring–summer) and could be related to a process of self-thinning. There was a negative biomass–density relationship (as a measure of biomass accumulation-driven mortality) that confirms the importance of self-thinning as a major demographic factor in the S. muticum population. [ABSTRACT FROM AUTHOR]

Abstract: Despite the available information regarding the negative effects of non-indigenous species (NIS) on ecosystem structure and functioning, the mechanisms controlling NIS invasion remain poorly understood. Here, we investigated the relative roles of top-down and bottom-up control on the invasion of intertidal macroalgal assemblages by the macroalga Sargassum muticum (Yendo) Fensholt. Using a factorial experiment, nutrient availability and intensity of herbivory were manipulated along an intertidal rocky shore. We found that early recruitment of S. muticum was enhanced by low nutrient enrichment but no effect of grazers was observed. In contrast, at the end of the experiment (9-months after invasion) top-down control, together with the number of NIS and the percentage cover of ephemerals, was a significant predictor for the invasion success of S. muticum. In addition, both top-down and bottom-up forces played a significant role in structuring macroalgal assemblages, which indirectly could have influenced invasion success. Hence, by shaping community structure, main and interactive effects of bottom-up and top down forces may indirectly act on invasion. Our study highlights the importance of the recipient community structure on the invasion process and emphasizes the specific regulation of top-down and bottom-up forces in different stages of S. muticum invasion. [Copyright &y& Elsevier]

Abstract: The introduction of non-indigenous species (NIS) is expected to have negative effects on native competitors, particularly between functionally similar species. Nevertheless, the mechanisms underlying competitive relationships remain poorly studied. Here, a substitutive neighbourhood approach was used to examine the role of assemblage density and neighbour identity in the growth and nutritional strategy of a focal individual. We quantified intra- and interspecific competition between two similar adult macroalgae (Fucales: Sargassaceae), the invasive Sargassum muticum and the native Cystoseira humilis, known to co-occur on intertidal rocky shores from NE Atlantic. Using either S. muticum or C. humilis as the focal species, we monitored the focal individual growth responses and nutrient content in the field, with two different densities of neighbours. Additionally, we quantified the nutrient uptake rates for both species in the laboratory. Our results showed that C. humilis grew at a significantly faster rate showing N accumulation when surrounded by S. muticum, whereas the invader has only showed growth at low density conditions. In addition, C. humilis presented greater uptake rates of nitrate compared to S. muticum suggesting better competitive potential to exploit nitrogen transient pulses of high concentration. Our results suggest a dominant native alga vs the invasive, which is not supported by field observations. This research gives evidence that competition between adult macroalgal individuals may not be the key mechanism linked to the dominance of NIS in introduced habitats. [Copyright &y& Elsevier]