In this post, Shelley Arnott discusses a paper she recently handled by Ben Stewart-Koster and colleagues “Integrating landscape connectivity and habitat suitability to guide offensive and defensive invasive species management

Also, see Ben Stewart-Koster’s post about his article “Lake Invaders! Can they get here and will they like it when they arrive?

Preventing the spread of invasive species to new habitats is critical for minimizing ecological and economic impact. Although the actual cost of mitigation and lost economic opportunities is difficult to pin down, it is clear that invasive species take an economic and ecological toll and that the most cost-effective management tactic is prevention. In response, legislation, public education, and management aimed at reducing the spread of non-native species have been developed at various jurisdictional scales (i.e. municipal, provincial/state, national). However, finite budgets for environmental protection and the vast number of habitats at risk of invasion make the containment of non-native species a daunting task. Managers are challenged with identifying strategies for prevention and prioritizing locales for investment of their limited resources.

Management strategies for reducing the spread of invasive species can be classified as offensive (containing invaders in their source site) and defensive (preventing their arrival to uninvaded sites). An example offensive strategy is the “Clean, Drain, and Dry” campaign by Michigan, Wisconsin, and Minnesota that encourages boaters to clean organisms off their boats, drain bilge water and live wells, and dry boats and equipment after leaving the lake. A defensive strategy employed by some lake associations in California requires boaters to provide certification that their boat is free of invasive species before being permitted on the lake.

Zebra mussels on a rock
Zebra mussels on a rock. Picture courtesy of Shelley Arnott

Stewart-Koster and his colleagues have addressed the need to prioritize efforts to prevent the spread of invasive species by developing a modelling framework that will allow managers to assess invasion risk across a heterogeneous landscape of invaded and uninvaded sites. They combine habitat suitability models with a graph and network theoretic approach that assesses habitat connectivity in order to identify sites or regions where offensive or defensive strategies would be most effective.

Using data on 5189 water bodies in Wisconsin and Michigan, Stewart-Koster and colleagues estimated the local risk of invasion by two nuisance aquatic invaders, water milfoil and zebra mussels, based on lake connectivity through boater movement and habitat suitability. Their modelling revealed that invasion risk varied throughout their study region and they were able to identify lakes and regions as high priorities for management efforts. For example, at relatively low investment of resources (i.e. 5% of lakes with targeted management to control the spread of zebra mussels), their analysis suggested that management efforts should focus on one region, with 71% of the effort for defensive strategies aimed at preventing the invader from leaving and 27% for offensive strategies to prevent the invader from arriving. Similarly, defensive strategies focused in the same region were primarily recommended for controlling water milfoil. In alternative scenarios with increased resources, their analyses were able to identify additional regions that should be targeted with management efforts.

This integrated modelling approach provides managers with an excellent tool to prioritize limited resources at locales most at risk of invasion, considering both habitat suitability and dispersal pathways.