Introduced species in a tidal ecosystem of mud and sand: curse or blessing?

Marine Biodiversity (2023) 53:5 https://doi.org/10.1007/s12526-022-01302-3 REVIEW Introduced species in a tidal ecosystem of mud and sand: curse or blessing? Karsten Reise 1 & Christian Buschbaum 1 K. Mathias Wegner 1 1 & Dagmar Lackschewitz & David W. Thieltges 2,3 & Andreas M. Waser 1 & Received: 30 March 2022 / Revised: 1 August 2022 / Accepted: 22 August 2022 # The Author(s) 2022 Abstract For about a century, biodiversity in the tidal Wadden Sea (North Sea, European Atlantic) has increased by more than one hundred introduced species from overseas. Most originate from warmer waters and could facilitate the transformation of this coastal ecosystem to comply with climate warming. Some introduced species promote sediment stabilization and mud accretion. This could help tidal flats to keep up with sea level rise. Although some introduced species also entail negative effects, introductions have diversified lower food web levels, and may benefit foraging birds. So far, no resident populations have gone extinct because an introduced species had established. Rather than degrading the ecosystem, the establishment of introduced species seems to have raised the capacity to follow environmental change. We support increasing efforts against introductions to avoid risk. However, once species are integrated, the common condemnation attitude against “non-natives” or “aliens” ought to be reconsidered for tidal ecosystems of low biodiversity. Keywords Biodiversity . Climate change . Species introductions . Macrobenthos . Wadden Sea Introduction Coastal ecosystems are susceptible recipients for species introduced from oceans beyond their natural dispersal range (Hedgpeth 1980; Anton et al. 2019; Bailey et al. 2020). These introductions are blending human history with natural history, being part of our cultural heritage while also changing the species composition and functioning of recipient coastal waters. Introduced species have been conventionally condemned as a kind of baneful “biological pollution”, “biocontamination”, “biosecurity risk” and as “threatening” or “degrading” ecosystem services (i.e., Elliott 2003; Olenin et al. 2007, 2011; Arbačiauskas et al. 2008; Essl et al. 2017; Ojaveer et al. 2018; Anton et al. 2019; Pyšek et al. 2020; Tarkan et al. 2021; Collin and Shucksmith 2022) or as “guilty until proven innocent” (Ruesink et al. 1995). In addition to ecological change, health problems and economic costs may arise (i.e., Cuthbert et al. 2021). Voices demanding more modesty and differentiation with regard to the effects of species introductions (i.e., Davis et al. 2011; Cassini 2020) met fierce opposition (i.e., Simberloff 2011; Ricciardi and Ryan 2018). This conflict is still lingering on, although more and more voices now argue for considering negative effects of introductions in conjunction with positive ones when advising nature Communicated by H. Hillebrand This article is a contribution to the Topical Collection Biodiversity and Ecology of the Wadden Sea under changing environments * Karsten Reise 1 Alfred Wegener Institute, Helmholtz Centre for Polar- and Marine Research, Wadden Sea Station Sylt, 25992 List, Germany 2 Department of Coastal Systems, NIOZ Royal Netherlands Institute for Sea Research, 1790 ABDen Burg, Texel, The Netherlands 3 Groningen Institute for Evolutionary Life-Sciences, GELIFES, University of Groningen, Nijenborgh 7, 9747 AG Groningen, The Netherlands 5 Page 2 of 24 management (i.e., Schlaepfer et al. 2011; Katsanevakis et al. 2014; Vimercati et al. 2020; García-Díaz et al. 2021; Kourantidou et al. 2022). A similar situation exists in the European Wadden Sea, an extensive tidal flat ecosystem at the south-eastern coast of the North Sea with a UNESCO world heritage status. For the Danish-German-Dutch Wadden Sea, regular trilateral quality status reports included chapters on introduced species (Reise et al. 2005; Nehring et al. 2009; Buettger et al. 2017, Buettger et al. 2022), and overviews on introductions for sub-regions of the Wadden Sea are given in Nehring and Leuchs (1999), Wolff (2005), Gittenberger et al. (2010) and Lackschewitz et al. (2015, 2022). In a first review on the entire Wadden Sea, Buschbaum et al. (2012) compiled a list of 66 introduced marine-to-brackish macrobenthic species (known until 2010; not included were pelagic and microscopic taxa) which has increased to over 100 benthic and planktonic species recently (Buettger et al. 2022). Twelve percent had arrived in the Wadden Sea directly from overseas, while the majority of introductions arrived in adjacent regions further south, and from there, they spread by secondary introduction and natural dispersal towards the Wadden Sea. In particular, artificial hard substrates attract introduced species, and the share of introduced species is highest in the low salinity, estuarine areas of the Wadden Sea. Although no evidence so far suggests that invading species had expelled resident populations (Buschbaum et al. 2012), the rise of introduced species has sparked numerous discussions in management and conservation circles about the presumably accumulating negative impacts (see Bouma et al. 2011). This has resulted in the request by the UNESCO to develop a trilateral strategic framework for dealing with alien species in the Wadden Sea world heritage site (Buettger et al. 2017). The major aim of this study is to revisit the discussion around introduced species in the tidal ecosystem of the Wadden Sea by adopting a neutral perspective without prior decision upon whether changes caused by introductions might be good or bad for resident species, the environment or human interests. Instead, we employ species introductions as ‘unplanned experiments’ leading to a better understanding of the functioning of the ecological web, and to wiser nature protection strategies in the Wadden Sea World Heritage Site. Change is an inherent property of coastal ecosystems and is inevitable when newcomers interact with residents and their new environment. We specifically ask (1) Why is the Wadden Sea so susceptible to introductions? (2) Which species have been introduced at what time? (3) Is an end to species introductions in sight? (4) To what extent have introductions transformed food web and habitats? (5) May introductions comply with rapid climate warming? (6) Will introductions homogenize the Wadden Sea ecosystem with those of other sedimentary coasts in the temperate climate zone? Based on the answers to these questions, we finally ask (7) whether introduced Marine Biodiversity (2023) 53:5 species might be better than their reputation for a tidal ecosystem such as the Wadden Sea. Scope and terminology In this review, we deliberately focus on introduced macrobenthic species in the Wadden Sea ecosystem, and neglect introductions to its vast but insufficiently known “dark biodiversity” of viral, unicellular, fungal, and meiobenthic organisms. Where appropriate, we consider paras (...truncated)