Marine Research Findings of the VECTORS Project

This website provides access to the research results of the VECTORS project, which can be used to support marine management decisions, policies and governance as well as future research and investment. VECTORS was a large scale project that brought together more than 200 expert researchers from 16 different countries. It examined the significant changes taking place in European seas, their causes, and the impacts they will have on society.

Effects of invading seaweeds on resident assemblages - variability between trophic levels

Biological invasions are globally acknowledged among the major threats to biodiversity . Despite a large research effort, a comprehensive framework for understanding the impacts of non-native plants is still lacking. Some syntheses suggest a tendency for the effects on resident plant communities to be consistently negative, while effects on animal communities are more variable12. An assessment of how effects of non-native plants vary according to a finer categorisation of trophic levels of residents, as well as to the recipient habitat type, should refine current invasion theories. The spread of non-native seaweeds represent an ideal system were to investigate these topics. Anthropogenic activities have caused more than 400 cases of introduction of seaweeds to non-native locations worldwide. Importantly, there are indications that these non-native plants have notable effects on resident organisms throughout the food chain3 and references therein.


Effects of non-native seaweeds are generally negative on resident primary producers (seaweeds, plants), while more heterogeneous on different categories of consumers and dependent upon habitat type. An ecological indicator for potential impact of non-native seaweeds should rely on effects on resident primary producers in different habitats.

We performed a global scale literature search and used a meta-analytical approach to summarise and compare the effects of non-native seaweeds on resident primary producers to those on different categories of consumers (herbivores, predators or other consumers) in both rocky and soft bottom habitats. Non-native seaweeds caused a significant decrease in the abundance, biomass, diversity and evenness of primary producer communities (i.e. resident seaweeds and plants), likely reflecting the negative effects on the growth and survival of individual species. Effects on consumers were less pronounced, with high heterogeneity in responses of some consumer categories. Results suggest that impacts on consumers are more species-specific in comparison to the generalized competitive effect at the same trophic level.

Features of invaded habitat (rocky versus soft bottom) may further increase variability in non-native seaweeds’ impacts. More experimental data, especially on consumer’ response, are needed to disentangle the effects of non-native seaweeds from those of other environmental stressors. A negative impact on native primary producers might result in the lessening of important ecosystems services, such as nutrient cycling, carbon storage, mitigation of coastal erosion through the dampening of wave action, reduced amenity and recreational value of coastal areas. A better understanding of the changes caused by biological invasions is crucial for exploring current and future ecological, social and economic consequences of human pressures in the marine environment, a key objective of VECTORS.


Major threats are currently experienced and expected in the Mediterranean as a consequence of the spread of the invading Caulerpa racemosa. Being a biodiversity hotspot, the Mediterranean should be considered as a priority area for conservation. Sounded monitoring programmes will be key to prevent the further spread of non-native species.

We performed additional analyses to separate the effects of different non-native species as well as to investigate the overall effects of non-native seaweeds in different regions. Of the non-native species investigated, Caulerpa racemosa generated the largest negative changes on primary producer communities. There was considerable variation in the effects of non-native seaweeds among regions, but negative effects were particularly severe in the Western Mediterranean, an area where a considerable proportion of the economic income is produced by tourism activities, tightly connected to the integrity of a coastal and marine environment characterized by a high level of endemism4. Our results suggest that the presence of two species of Caulerpa would be a major cause of alteration of benthic communities. Although C. taxifolia likely play an important role in shaping the results for the Western Mediterranean, its presence and abundance has declined in the last decade or so, probably due to replacement by its congener, C. racemosa5. As a consequence, at present, C. racemosa seems to be among the most noxious invaders and a serious threat to the marine biodiversity of this region, with potential negative effects also on economically valued fish species6. These findings are relevant for regional case studies investigating the effects of major invading seaweeds and, in particular, to experimental work carried out in the Western Mediterranean within the VECTORS project.

Relevance for Policy:
  • Alien Invasive Species Directive
  • Convention on Biological Diversity
  • Environmental Impact Assessment Directive
  • EU Biodiversity Strategy
  • Guidelines for the Control and Management of Ships’ Biofouling to Minimize the Transfer of Invasive Aquatic Species
  • International Convention on the Control of Harmful Anti-Fouling Systems on Ships
  • International Convention for the Control and Management of Ship's Ballast Water and Sediments
  • Marine Strategy Framework Directive
Data availability:

Data used: Data from invaded and non-invaded units, extracted from relevant literature at global scale

Where it is held: Dipartimento di Biologia, University of Pisa

Contact: Dr. Elena Maggi,

Availability: Password protected


Lead Author:

Elena Maggi
University of Pisa (UPISA)
Date of research: September 2014

Related articles:

Non-native seaweeds and anthropogenic disturbance

Ecosystem impacts of non-indigenous species

Food web change along a nearshore-offshore gradient

Food webs along the European Atlantic coast

Growth model for jellyfish Pelagia noctiluca 

Impact of invasive mussels on carbon flow 

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This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 266445
© Vectors 2015. Coordinated by Plymouth Marine Laboratory.