In this post, Ben Stewart-Koster talks about his recent paper with Julian D. Olden & Pieter T.J. Johnson “Integrating landscape connectivity and habitat suitability to guide offensive and defensive invasive species management

Also, see the Associate Editor, Shelly Arnott’s blog post about this article “A practical guide to prioritize efforts to prevent the spread of invasive species

Invasive species – these days they’re almost everywhere, and spreading quickly. Natural resource managers are frequently charged with one of the most challenging tasks, protecting uninvaded habitats and stopping invaders spreading beyond their current illicit places of residence. Ecologists have previously divided this task into “offensive” and “defensive” management strategies.

Managers can tackle the problem offensively by working to prevent invasive species leaving places they’ve already settled. Alternatively managers can set their defences around places yet to know the disturbance of an unwelcome arrival. Figuring out which road to take when there are thousands of habitats across a landscape, many of which are already invaded, is a tricky prospect. But a common four letter word may provide a way forward – Risk.

Zebra mussels line the shore on Green Bay at Red River County Park in Kewaunee County Wisconsin. Picture courtesy of Royalbroil Wikimedia Commons
Zebra mussels line the shore on Green Bay at Red River County Park in Kewaunee County Wisconsin. Picture courtesy of Royalbroil, Wikimedia Commons.

Estimating invasion risk provides a robust avenue of ranking, or prioritizing for management, habitats that are vulnerable to invasion or probable invasion sources. This can be used to allocate effort into offensive or defensive strategies, which is particularly useful in these days of declining budgets coupled with increasing needs for ecosystem management. There are many ways researchers can estimate risk, and in a new paper in Journal of Applied Ecology we developed a metric of invasion risk that integrated an estimate of habitat suitability from a spatial statistical model, with an estimate of probable connectivity derived from graph theoretic methods.

We developed our approach in the Great Lakes Region of the state of Wisconsin and part of Michigan in the USA, using over 5000 lakes and reservoirs as potential habitats for zebra mussel and Eurasian watermilfoil. These two species are highly invasive and very problematic once established, consequently there is a great deal of interest in reducing their secondary spread across this and many other regions.

Eurasian watermilfoil warning sign. Picture courtesy of Fungus Guy Wikimedia Commons.
Eurasian watermilfoil warning sign. Picture courtesy of Fungus Guy, Wikimedia Commons.

Both species can be transported from invaded water bodies to new habitats by unsuspecting recreational boaters who can inadvertently pick up viable propagules tangled on propellers or in live bait buckets. These “aquatic hitchhikers” are subsequently transported to the boater’s next destination potentially introducing the species to a new habitat.

Using an empirical distribution of boaters’ travel habits across the region, we estimated the probability of connectivity of every pair of waterbodies in the study, thereby building a spatial graph of the habitat patches in the region. Knowing the invasion status of several hundred waterbodies, we combined the estimated connectivity with invasion status or estimates of habitat suitability based on environmental conditions. Integrating these indices into a metric of invasion risk provided a way to prioritize each habitat for management, either offensively if already invaded, or defensively if it was uninvaded or unknown to be.

To assist with the prioritization further, we identified a threshold to define a pair of waterbodies as either “connected” or “disconnected” and used this to identify a set of components in the spatial graph. Each component comprises a collection of water bodies in which there is a path, though not necessarily a direct link, between all pairs. As such, all components are effectively disconnected from each other and can be used as discrete management units to guide invasive species interventions at a more local level.

Dave Herasimtschuk, Freshwaters Illustrated
A reservoir of the type in the paper from Stewart-Koster, Olden & Johnson. Picture courtesy of Dave Herasimtschuk and Freshwaters Illustrated.

Ultimately, the problem of managing a seemingly never ending problem, across large regions with thousands of potential habitat patches will persist. Integrating all available data with appropriate statistical and mathematical techniques, provides an avenue to target ecosystem management to slow, and perhaps even stop, the spread of invasive species.