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1. Seabird species worldwide are integral to both marine and terrestrial environments, connecting the two systems by transporting vast quantities of marine-derived nutrients and pollutants to terrestrial breeding, roosting, and nesting grounds via the deposition of guano and other allochthonous inputs (e.g., eggs, feathers). 2. We conducted a systematic review and meta-analysis and provide insight into what types of nutrients and pollutants seabirds are transporting, the influence these subsidies are having on recipient environments, with a particular focus on soil, and what may happen if seabird populations decline. 3. The addition of guano to colony soils substantially increased nutrient levels compared to control soils for all seabirds studied, with cascading positive effects observed across a range of habitats. Deposited guano sometimes led to negative impacts, such a guanotrophication, or guano-induced eutrophication, which was often observed where there was an excess of guano or in areas with high seabird densities. 4. While the literature describing nutrients transported by seabirds is extensive, literature regarding pollutant transfer is comparatively limited, with a focus on toxic and bioaccumulative metals. Research on persistent organic pollutants and plastics transported by seabirds is likely to increase in coming years. 5. Studies were limited geographically, with hotspots of research activity in a few locations, but data were lacking from large regions around the world. Studies were also limited to seabird species generally listed as Least Concern on the IUCN Red List. As seabird populations are impacted by multiple threats and steep declines have been observed for many species worldwide, gaps in the literature are particularly concerning. The loss of seabirds will impact nutrient cycling at localised levels and potentially on a global scale as well, yet it is unknown what may truly happen to areas that rely on seabirds if these populations disappear. The information in this record includes three spreadsheets and R code. Descriptions are included below: - The spreadsheets contain all information extracted from the publications that were critically reviewed (n = 181). The first spreadsheet contains information regarding each publication (1 publication per row), such as study location, sampling methods. The second spreadsheet contains information about the seabird species studied in each publication (1 row per seabird species per publication). The third spreadsheet contains data for the meta-analysis (1 row per publication, except if the publication studied multiple species, then it would be 1 row per species per publication). - The R code is for the meta-analyses that were undertaken. Comments are included within the code plus detailed information can be found in the Methods section of the paper.