Non-Indigenous Species Thematic Assessment

Climate change effects on non-indigenous species

Climate change, primarily ocean temperature increases, may facilitate the introduction and establishment of non-indigenous species (NIS). In addition, climate change may increase the magnitude of impacts associated with invasive NIS, for example by reducing the resilience of native ecosystems and habitats. In addition to the impacts from climate change, higher rates of intercontinental dispersal events, associated with increasing levels of international trade and human travelling, are expected (Hewitt et al., 2018; Sardain et al., 2019).

The combined effects of climate and rapid transport are likely to bring worldwide changes to the earth’s biota, including largescale biotic homogenisation, which could easily exceed the impact of either climate change or of NIS invasion considered individually. Proper management of NIS introductions is therefore essential, considering the risk associated with the continually changing climate. Despite the hypothesis that global climate change can lead to the degradation of marine ecosystems through invasive NIS (Dukes and Mooney, 1999), climate change and invasive species are most commonly treated as independent issues (Pyke et al., 2008).

The effects of ocean climate change and acidification on NIS introductions and impacts are frequently discussed in the literature (Occhipinti-Ambrogi, 2021). However, there seems to be limited documentation of the causal effects. The primary stressor appears to be warming, whereas less emphasis is placed on altered ocean pH, wind speed and precipitation and climate-related changes in salinity. A general poleward expansion of marine species due to the effect of seawater warming is expected (Pinsky et al., 2013; Poloczanska et al., 2016), with some evidence and indications of climate-related changes on increasing abundances of NIS in marine systems (García-Gómez et al., 2020; Sorte et al., 2010; Staehr et al., 2020). Considering the different pathways of NIS introduction, poleward expansion related to ocean warming would be most relevant in the case of secondary introductions. The reasoning for this is that hotspots of NIS introduction in southern seas would provide source populations for further introductions of NIS in northerly regions as temperature conditions gradually become favourable there.

In the Greater North Sea (Region II), Celtic Seas (Region III) and Bay of Biscay and Iberian Coast (Region IV), the rate of new NIS introductions has been significantly decreasing during the last three assessment periods (2003 to 2020) (see:  pressure chapter ). However, due to time lags in data reporting, sparse monitoring of NIS, and lack of a thorough analysis of the importance of environmental conditions, it is likely that the NIS introduction rate, especially in the most recent assessment period, is underestimated, and thus temperature-mediated effects on selected NIS species cannot be ruled out. Considering the expectation of a climate-driven poleward expansion of NIS, the observed trends in NIS development among OSPAR Regions do not support the hypothesis of southern regions acting as a donor for northerly regions. Among the 426 NIS records provided by the 11 Contracting Parties from 2003 to 2020, there were a total of 250 non-duplicated NIS records across the three OSPAR Regions, with a tendency for more NIS to be observed in southern countries (see Figure 3 in the NIS trend indicator assessment ).

This broad trend gives support to the previously published records of higher NIS arrivals in southern seas (Tsiamis et al., 2019; Tsiamis et al., 2018), with up to 76% of all NIS primary introductions in Europe having been reported first from the Mediterranean Sea. This could indicate the importance of secondary non-assisted dispersal for many of the northerly observed NIS species. However, differences in monitoring efforts between regions makes it difficult to conclude on the importance of secondary-assisted dispersal of NIS. Secondary introductions (i.e. the spread) accounted for only 5% of NIS introductions in the OSPAR Regions (see Figure d in the NIS trend indicator assessment ). The influence of climate-related warming on NIS introductions is therefore not strongly supported by the data collected for this assessment and this issue will require a more targeted species-specific analysis.