Post by Jenny Seifert
Among the challenges to the resilience of ecosystems are invasions by non-native plants and animals, which can arrive in myriad ways—from hitchhiking on an unknowing person’s shoe to being intentionally introduced, often without a full understanding of the unintended consequences of doing so.
Invasive species can alter native ecosystems in many ways, sometimes disrupting the natural benefits they provide us.
For example, ecosystems play a role helping us combat climate change by regulating greenhouse gas emissions from the terrestrial world. They can control the volume of greenhouse gases emitted by either storing them in plants and soils or releasing them through activities such as fire and respiration (even the soil respires!).
Invasive species can affect how well ecosystems perform this function. But there hasn’t been a good big-picture view of just how they do so, until now.
WSC researcher Jiangxiao Qiu has combed through the research and recently published a study in Global Ecology and Biogeography showing that, on the whole, invasive species have both good and bad impacts.
The good news is invasive species can promote carbon sequestration, or the capturing and storing of carbon dioxide, a greenhouse gas, in soils and plant tissues. The bad news is invasive species can also increase emissions of nitrous oxide, a more powerful climate-warming gas.
While Qiu was unable to determine the net impact invasive species have on greenhouse gas emissions due to insufficient research on a third greenhouse gas, methane, he says his study implies that invasive species are not always bad, at least from an emissions perspective.
Qiu provides more detail about his study below.
Why do invasive species generally increase nitrous oxide emissions, but promote carbon sequestration?
JQ: Invasive species sometimes possess characteristics that can accelerate nitrogen cycling in ecosystems, meaning they add extra nitrogen to the system than would otherwise be there and cause nitrogen to be more easily released into the atmosphere.
For example, some invasive species, such as Morella faya [a.k.a. fire tree], have the ability to convert atmospheric nitrogen (N) into the inorganic N compounds ammonium (NH4+) and nitrate (NO3–), which then become available in the soil for plants and microbes to use.
Similar to adding nitrogen fertilizer in agricultural systems, invasive plants will take up inorganic N and thus increase the amount of nitrogen in their leaves. When the plants die and decompose, the increased nitrogen gets into the soil and becomes ammonium and nitrate again. With this increased inorganic N in the system, microbes could potentially transform the extra nitrogen into nitrous oxide, which gets emitted into the atmosphere.
Yet, these same traits also allow these invasive species to outgrow native species, which means they can absorb and store more carbon.
Did you find any cases of these effects in Wisconsin or the Upper Midwest?
JQ: Yes. I found studies on invasive species commonly found in the Midwest, including the emerald ash borer, European invasive earthworms and reed canary grass. Emerald ash borer can cause tree mortality and thus reduce the amount of carbon forests can store. European invasive earthworms often increase nitrous oxide and carbon dioxide emissions through their feeding, burrowing and production of casts [i.e., worm manure]. In contrast, reed canary grass, which invades wetlands, may store more carbon in soils as compared to their native counterparts; although, on the flip side, it outcompetes other wetland species that provide other benefits.
In your study, you were also interested in why the effects of invasive species on emissions vary. What factors explain this variation?
JQ: The effects vary among ecosystems, species and local climatic conditions. For example, the effects on nitrous oxide emissions are generally greater in forests, and the effects on carbon storage are greater in grasslands.
Animal invaders tend to decrease carbon storage, whereas invasive plants enhance carbon storage overall. Even more specifically, woody plants, such as trees and shrubs, tend to promote more carbon storage than non-woody plants.
Finally, in climates where temperature and precipitation are either extremely high or extremely low, invasive species had more pronounced effects on carbon storage, meaning they increased storage relative to their native neighbors.
How does your study affect how we might think about the role of invasive species as climate change continues?
JQ: Climatic conditions influence the distribution and impacts of invasive species. Thus, if the climate continues to warm, as is projected, and precipitation becomes more variable, we may expect shifts in the effects of invasive plants from what we currently observe.
How does your study connect with your work on the Water Sustainability and Climate Project?
JQ: One component of my PhD research is an examination of how an emergent invasive earthworm in the Yahara Watershed, the Asian jumping worm, affects forest and prairie soils, which could have important implications for ecosystem health and water quality. This study and the survey of the current literature provided me a better understanding of the conditions under which invasive species can affect our ecosystems and in what ways they do so.