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

Aim: 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).Location: Global.Methods: We modelled 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.Results: Risk differed strongly at the D-2 limit based on community composition. At low CPP (<25 ind. m(-3)), risk was strongly affected by CP for hyperbolic and linear risk-release models and weakly for exponential and logistic models, while CPP affected only the former two model types. Across a much wider range of CPP values, risk was affected by CP, CPP and by their interaction for all models.Main Conclusion: The IMO D-2 standard for zooplankton-sized organisms requires very low CPP and even lower IPPs in mixed-species releases, which will impede successful colonization. Species-abundance theory predicts that discharges meeting the D-2 standard (low CPP) will also have low CP. Much more empirical data are required to determine whether vessels can consistently reduce CP as it lowers CPP in order to meet these requirements.

Automated summary

This study models the risk differences in cargo vessel discharges that meet the IMO D-2 standards and analyzes the impact on invasion risk. The results indicate that community composition strongly affects the risk, and colonization pressure has a significant influence at low community propagule pressure. The IMO D-2 standards require very low individual propagule pressures and community propagule pressures, and more empirical research is needed to determine if vessels can meet these requirements.