Climate change is reshuffling Earth’s biota as species ranges shift to track increasing habitat temperatures. While redistribution may be necessary for species persistence, there can also be impacts on existing communities upon arrival of novel, range-shifting species. Anticipating the beneficial versus deleterious impacts of range-shifting species is essential for determining whether active management is needed, which could include employing strategies from facilitation (eg managed relocation) to suppression (eg prevention/control). We employ an impact assessment protocol developed for invasive species to evaluate potential consequences of range shifts in coastal marine ecosystems of North America. Our review demonstrates how invasion impact assessment combined with species vulnerability assessment could support decisions about management of range shifts. We found that ~50% of these shifting coastal species have had negative impacts in their expanded range. Thus, the importance of proactive management is likely to increase as the number and extent of range shifts accelerates.

Identification of study species

We identified 40 marine species with documented shifts in range limits along the coastline (<15 km from shore) of North America, including plants, invertebrates, fish, a protist, and a bird. Of these, 26 species were compiled by Sorte et al. (2010), and we added 14 species from an updated literature review. We searched Google Scholar (on 08/20/2019) using this search string: marine "range expansion" species "range shift". We reviewed titles and, when appropriate, abstracts and text of the first 600 results, identifying 12 additional species from eight papers. We added two species (Brachidontes adamsianus and Mexacanthina lugubris) from our literature files and personal observations. We excluded migratory or pelagic species with large biogeographic ranges, for which it was difficult to confirm historical native ranges. 

Review of published impacts

Evidence of species’ impacts was compiled from online database searches and literature review. We conducted individual Web of Science searches for the 40 shifting species using each species’ scientific name (and synonyms). Papers reporting species impacts were identified by reviewing titles and abstracts. For species with >800 Web of Science results, the first 400 results were scanned and remaining results were filtered using this search string: “ecology” OR “invas*” OR “impact”. For species with <100 Web of Science results, we also performed Google Scholar searches, and relevant papers were identified from the first 400 results. Additional impact studies were added opportunistically from citations within papers found in database searches). In total, we reviewed 11,508 papers for this impact assessment of 40 range-shifting species.

Impact assessment

We evaluated environmental and socioeconomic impacts using modified versions of the Environmental Impact Classification of Alien Taxa (EICAT; Hawkins et al. 2015) and Socio-economic Impact Classification of Alien Taxa (SEICAT; Bacher et al. 2017) protocols. The EICAT and SEICAT protocols focus on impacts on native, non-human populations and human activities, respectively. Primary modifications were the inclusion of beneficial (rather than only detrimental) impacts and use of studies in species’ native and expanded ranges to estimate impacts (rather than only non-native ranges). 

Impacts were classified by mechanism. The following are mechanisms that we identified as responsible for negative impacts by shifting species on native (non-human) species: competition, predation, herbivory, parasitism, disease transmission, interaction with other invaders, biofouling, bioturbation, physical disturbance, poisoning/toxicity, and “other” negative impacts (including those with an unknown mechanism). We also found evidence of positive ecological impacts by the following mechanisms: food provisioning, habitat provisioning, and “other” positive impacts. Our SEICAT analysis revealed socioeconomic impacts associated with alterations in health, material and immaterial assets, and social, spiritual or cultural relations.We assigned levels of impacts based on categories described in the EICAT and SEICAT protocols (Hawkins et al. 2015, Bacher et al. 2017). Impacts range across a semi-quantitative gradient from 1 (lowest) to 5 (highest). For each published study, we scored impacts of shifting species based on the highest level response from the categories. Impact scores, thus, represent the maximum impact that has been observed. Both EICAT and SEICAT protocols were modified to incorporate positive impacts, essentially switching the direction or sign of negative impacts (Table 1). Species for which we found no published papers on impacts were categorized as “data deficient”.

For both EICAT and SEICAT assessments, we collected additional information about the shifting species and study. These characteristics included taxonomic classifications, study location, and whether the study was conducted in the shifting species’ “native” or non-native, “expanded” range. Ranges were defined as "native"or "expanded" based primarily on documentation within the source reporting the range shift. "Expanded" ranges were designated as such conservatively, acknowledging potential lack of benchmark data for species ranges, with most range shifts documented after 1985 (Sorte et al. 2010). (Virnstein and Hall 2009, Canning-Clode 2011, Goddard et al. 2011, Poloczanska et al. 2013, Gericke et al. 2014, Heck et al. 2015, Cannizzo and Griffen 2016, Timbs et al. 2019)

The uploaded data includes an Excel spreadsheet with formatting that contains the database and the metadata on separate sheets. In addition, there are four .csv files. First, the impact assessment database. Second, metadata with description of columns. Third, the reference list of country names used, and fourth, the reference list for habitats. A second spreadsheet with meta-data information about columns. See README file for more information.