Neighbourhood competition in coexisting species: The native Cystoseira humilis vs the invasive Sargassum muticum

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. (C) 2014 Elsevier B.V. All rights reserved.