Sea freshening may drive the ecological impacts of emerging and existing invasive non‐native species

James W E Dickey,Elizabeta Briski,Gregor T Steffen,Ross N Cuthbert,Jaimie T A Dick

doi:10.1111/ddi.13178

Abstract

AbstractAimThe spread of invasive non‐native species (INNS) will pose major threats to global biodiversity over the coming decades. However, predicting how key effects of climate change will influence the abilities of INNS to establish and exert ecological impact is a major challenge. One overlooked aspect of global change is the expected freshening of certain marine systems, which may interact with INNS and lead to drastic effects on community structure and stability.LocationBaltic Sea, Europe.MethodsHere, using three predatory amphipod crustaceans, we experimentally assessed how salinity reduction may affect the impacts of the emerging INNS, Pontogammarus maeoticus, relative to an existing INNS, Gammarus tigrinus and a trophically analogous native, Gammarus salinus. We quantified per capita impacts of the three species via the comparative functional response method (prey consumption over a range of prey densities) under a predicted seawater freshening scenario. We then combined amphipod functional responses with their life history traits to compare population‐level relative impact potential (RIP) on prey of the three amphipod species across salinities.ResultsFreshening substantially altered the predicted relative ecological impacts of both the INNS compared with the native. First, the functional responses of invasive P. maeoticus and G. tigrinus increased under freshening, while that of the native G. salinus decreased. Second, RIP became consistently higher for both the INNS compared to the native with increased freshening.Main conclusionsOur methods thus reveal potential for climate change via seawater freshening to drive large shifts in dominance and ecological impacts of INNS compared with natives. With the number of INNS introductions unlikely to saturate in the near future, we highlight the need to assess the impacts of potential future INNS, alongside established non‐natives and native species, in combination with abiotic changes associated with climate change.

Highlights

With consensus being reached that a sixth mass extinction event is underway (Ceballos et al, 2015; Turvey & Crees, 2019), global biodiversity faces an uncertain future (Johnson et al, 2017)
Two weeks prior to the experiments, 20 size-matched individuals of each of the three amphipod species were selected from their holding aquaria based on head to pleon length, and acclimated to a laboratory temperature of 17(±1)°C, which is currently common in shallow areas of the Baltic Sea and projected to become widespread by the end of the century (Holopainen et al, 2016)
For P. maeoticus and G. tigrinus, individuals were adapted to 16ppt from initial holding salinities of 10ppt, and for G. salinus, individuals were adapted from 16ppt to 10ppt

Summary

Introduction

With consensus being reached that a sixth mass extinction event is underway (Ceballos et al, 2015; Turvey & Crees, 2019), global biodiversity faces an uncertain future (Johnson et al, 2017). Ocean acidification and changing weather patterns are widely studied and documented (Harley et al, 2006), the ecological effects associated with salinity shifts of sea water remain understudied (Illing et al, 2016). This has been described as a “rich get richer” mechanism (Chou et al, 2009), whereby highly saline marine regions are getting saltier, and relatively fresh regions are getting fresher (Durack et al, 2012). How physiological stress resulting from salinity changes will differentially affect INNS and native species is currently unknown and requires urgent assessment and prediction to forecast the identities of likely future INNS, their potential impacts and any effective mitigation strategies

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