Abstract
Understanding the implications of different management strategies is necessary to identify best conservation trajectories for ecosystems exposed to anthropogenic stressors. For example, science-based risk assessments at large scales are needed to understand efficacy of different vector management approaches aimed at preventing biological invasions associated with commercial shipping. We conducted a landscape-scale analysis to examine the relative invasion risk of ballast water discharges among different shipping pathways (e.g., Transoceanic, Coastal or Domestic), ecosystems (e.g., freshwater, brackish and marine), and timescales (annual and per discharge event) under current and future management regimes. The arrival and survival potential of nonindigenous species (NIS) was estimated based on directional shipping networks and their associated propagule pressure, environmental similarity between donor-recipient ecosystems (based on salinity and temperature), and effects of current and future management strategies (i.e., ballast water exchange and treatment to meet proposed international biological discharge standards). Our findings show that current requirements for ballast water exchange effectively reduce invasion risk to freshwater ecosystems but are less protective of marine ecosystems because of greater environmental mismatch between source (oceanic) and recipient (freshwater) ecoregions. Future requirements for ballast water treatment are expected to reduce risk of zooplankton NIS introductions across ecosystem types but are expected to be less effective in reducing risk of phytoplankton NIS. This large-scale risk assessment across heterogeneous ecosystems represents a major step towards understanding the likelihood of invasion in relation to shipping networks, the relative efficacy of different invasion management regimes and seizing opportunities to reduce the ecological and economic implications of biological invasions.
Highlights
Aquatic invasive species are nonindigenous organisms that establish and spread in a new environment, frequently resulting in negative impacts on biodiversity [1], ecological structure and function [2], and imperilment of fauna [3]
The main requirement entails ballast water exchange (BWE), where vessels replace ballast water loaded at a foreign port with offshore oceanic water in an effort to reduce the abundance and diversity of coastal plankton transported between ports, thereby decreasing the likelihood of invasion because the number of individuals discharged in ballast water is an important determinant of invasion success [11]
The objective of this study is to evaluate the relative invasion risk to diverse ecosystems at landscape scales in relation to current and proposed vector management regimes
Summary
Aquatic invasive species are nonindigenous organisms that establish and spread in a new environment, frequently resulting in negative impacts on biodiversity [1], ecological structure and function [2], and imperilment of fauna [3]. As the severity of impacts is influenced by multiple interacting drivers that are difficult to predict, prevention through vector management has often been cited as the best conservation strategy [4, 5]. Despite these recommendations, little attention has been directed at understanding efficacy of different vector management approaches, at landscape scales, impeding the progress of science-based regulation and precluding assessments evaluating current and future ecological risk. Many countries have enacted ballast water management at landscape scales (i.e., at the scale of global or national shipping networks; [10]) to reduce the risk of species invasion and concomitant ecological change. The main requirement entails ballast water exchange (BWE), where vessels replace ballast water loaded at a foreign port with offshore oceanic water in an effort to reduce the abundance and diversity of coastal plankton transported between ports, thereby decreasing the likelihood of invasion because the number of individuals discharged in ballast water (propagule pressure) is an important determinant of invasion success [11]
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