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 (<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. 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 is required to determine whether vessels can consistently reduce CP as it lower CPP in order to meet these requirements.

Data was collected through a mechanistic model. The model considered real-world Ballast water volumes from Bradie, Rolla, Bailey & MacIsaac (2022), which consisted of 10000 recorded ballast water discharges that ranged from 20 to 1060620 m3 (13871 ± 25738, mean ± standard deviation). Using these volumes we then set community propagule pressure and colonization pressure while varying individual propagule pressure to determine the risk of the establishment (1 - risk of extinction). Next, we set individual propagule pressure, colonization pressure, and community propagule pressure and determined the risk of establishment. Each combination of community propagule pressure, individual propagule pressure, and colonization pressure was run for 100 iterations to simulate 100 trips. 

The risk of establishments per species across 100 trips was used to calculate the probability of at least one species establishing following Wonham, Byers, Grosholz & Leung (2013). Additionally, the probability of exactly one species establishing by comparing the establishment probability per species to a uniform distribution. This comparison determined if a single species established or became extinct in a single trip. From the establishment or extinct classifications, we determined how many species established on a single trip. We then calculated the probability of ‘exactly one species establishing’ by dividing the number of trips in which exactly one species established by the total number of trips per community propagule pressure and colonization pressure. Lastly, we fit the risk-release relationship by linear, hyperbolic (Michaelis-Menten), exponential, and logistic.

For more details, please see the corresponding paper.

Bradie, J. N., Rolla, M., Bailey, S. A., MacIsaac, H. J. (2022). Managing risk of non-indigenous species establishment associated with ballast water discharges from ships with inoperable or bypassed ballast water management systems. Journal of Applied Ecology, 60, 193-204. doi.org/10.1111/1365-2664.1432

Wonham, M. J., Byers, J. E., Grosholz, E. D., Leung, B. (2013). Modeling the relationship between propagule pressure and invasion risk to inform policy and management. Ecological Applications, 23, 1691-1706. doi.org/10.1890/12-1985.1

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