Incorporation of colonization pressure into the propagule pressure-based global ballast water standard

In 2024 cargo vessels must meet the International Maritime Organization's global ballast water discharge standards (IMO D-2) that limit the concentration of living organisms. D-2 focuses on reducing invasion risk by reducing 'community propagule pressure (CPP)', though it does not consider colonization pressure (CP). We modeled risk differences in IMO D-2-compliant discharges (10 ind. m-3) for communities that had inverse patterns of CP and species' individual propagule pressures (IPP). Secondly, we determined the effect on risk of varying CPP and CP. As part of this we tested whether the IMO D-2 standard for zooplankton-sized organisms of <10 individuals m-3 was an optimal choice. Risk was defined as probability of at least one species invading using four risk-release models. Risk differed strongly at the D-2 limit based on community composition. At low CPP (