A recent study in the journal, Ecosystems, says that, when it comes to invasive species, it’s time to think bigger. While the movement of species from their native ranges to exotic lands is a major component of global ecosystem change, the authors contend, too often the scientific study of these invasive species is, well, local.
We sat down with Center for Limnology professor and lead author of the study, Jake Vander Zanden, to ask him why science needs broader views on invasive species and how researchers can start building the framework needed to truly understand their impacts.
CFL: What was the inspiration for this study?
JVZ: Ultimately, it’s about digging into these areas of invasive species ecology that are not well explored. When we think about invasive species impacts, what do we really mean? Often we mean the impact in a single lake. It’s sort of like looking at economic growth only in terms of your own household and how your income grew from last year to this year. But the real big picture only comes into focus when you say, “Let’s think about it at the scale of the whole economy.” And that’s what this study is saying in regards to non-native species in aquatic ecosystems.
Scientists often study what’s happening in a single lake, rather than the species’ impact across the landscape. So, if we want to know about the impact of invasive species in Wisconsin, what we want to know is “What’s the overall summed impact across all 15,000 lakes?” And, when you try to do that, there are a lot of things you need to know, and there are a lot of gaps in our knowledge. So we were trying to highlight these gaps in knowledge and think about this more broadly.
CFL: What are some of these things we need to know or gaps in knowledge?
JVZ: So some of the things we need to know for invasive species are questions like, “Where are they? Where aren’t they? Where they are, how abundant are they? And how does that abundance translate to impact? Once we start answering those questions, we can build an overall picture of impact and describe what’s going on at a landscape level.
But that’s hard to do. For starters, we don’t even have good data how many of Wisconsin’s lakes even have invasive species in them. One of my co-authors, Alex Latzka, is leading a study that is trying to answer this question and he’s found that we’re severely underestimating the number of Wisconsin lakes that are home to some of these species. And it’s largely driven by unknown populations of species we don’t pay much attention to, like the Chinese mystery snail, which is actually way more widespread than species we all know about like zebra mussels or Eurasian water milfoil. While Chinese mystery snails may not have the dramatic single-lake impacts of zebra mussels, maybe because they are so widespread, they have more combined impact when you sum up across the landscape. We don’t really know what’s going on out there. That means that a lot more study is needed on many fronts.
CFL: Is there an example of how knowing more might help us better understand broad-scale impacts?
JVZ: Well, in the paper, one species we focused on was rusty crayfish, which has invaded a lot of northern Wisconsin lakes and had, in some cases, markedly negative impacts on native species of plants, fish and invertebrates. But, when we look at all of our data for rusty crayfish in Wisconsin lakes, in most lakes where they are or, at least, were we know they are, they’re at pretty low [population] densities. So you could say that they’re mostly rare and rarely common. (Editor’s Note: For more on this phenomenon, read this write-up of a previous Vander Zanden study).
And, looking at that data, we were able to ask, “Okay where is that breaking point in population density where they really begin to have impact? What’s the relationship between abundance and impact?” There’s a threshold density above which you see impact.
So, then you can ask, across all the lakes in Wisconsin, how is that impact distributed? And we found that, for most lakes in Wisconsin that have rusty crayfish, the population size is low enough that you don’t really see these big negative impacts. So the landscape-level picture of the invasive rusty crayfish in Wisconsin is much different than the picture when you look at only a single highly invaded lake.
CFL: What are some of the more practical applications the paper can provide either resource managers or other ecologists?
JVZ: We want to get at landscape-level impacts. How many lakes is an invasive species in? How abundant is it? Is it having an impact? And then you package it all up and that’s what some of the equations in the paper are trying to do, to offer a way to see this. And that could help us develop a sort of a triage approach to invasive species management and guide how we prioritize our responses. But it’s also just thinking about invasive species impact at a broader scale. It’s emphasizing a different way of thinking about invasions than we normally do.
CFL alumni Gretchen Hansen and Alex Latzka were also authors of the report. You can download it – “A Framework for Evaluating Heterogeneity and Landscape-Level Impacts of Non-native Aquatic Species” – here.