Economic effects and the efficacy of intervention: exploring unintended effects of management and policy on the spread of non-indigenous species

Abstract

Models have been well developed describing human movements as vectors of the spread of non-indigenous species (NIS). However, to be maximally useful, predictions need to be integrated with management models of how different policies change human behaviour and lead to concurrent changes in invasion risk. Using the dispersal of freshwater organisms by recreational boaters as our study system and mandatory boat washing as our management strategy, we conducted a survey of recreational boaters (n = 580 respondents, t = 2354 boating trips) in Ontario, Canada, and performed counterfactual analysis of boater behavior across different management options. We developed a model to quantify three responses to mandatory boat washing policies: (1) the continued use of a policy lake; (2) switching to a non-policy lake (“trip redistribution”); or, (3) a reduction in boating trips (“trip loss”). We found that boater and locational traits did not have a significant effect, but even modest user fees at washing stations greatly influenced trip redistribution and loss, explaining 87% of the variation in boater choices. These results indicate that user fees can strongly reduce the effectiveness of boat washing programs to mitigate invasion risk and could have unintended local economic effects, supporting the need to minimize boater expense as a program goal. In contrast, only minor redistribution and loss occurred if users washed but did not pay, and when taken together with the lack of effect for boater and locational traits, suggest that simple human-mediated dispersal models would be sufficient to prioritize management actions under “zero fee” scenarios. Simulating management scenarios using an existing spread model for 10 aquatic NIS in Ontario further emphasized the benefit of zero fees. Although averted invasions increased monotonically with effort (number of lakes with washing stations), the relative effectiveness (number of invasions averted per unit effort) was high even with management of a single lake, given zero fees, but required washing stations at far more lakes to maximize relative effectiveness when user fees were imposed.