Ranjan Muthukrishnan, St. Olaf College in the USA, discusses his article: Warming conditions reduce the impacts of an aquatic invasive macrophyte across a latitudinal gradient
The challenge
Climate change and the spread of invasive species are two major drivers of global ecological change. As with many ecological stressors, an important question is: what is the relationship between multiple stresses? For example, if the impacts of each stress will simply add up with each other, so the total effect is the sum of the two individual stresses. Alternatively, it’s possible that the two stressors interact with each other in a way that multiplies their effects, making their impacts even bigger. Trying to understand these types of interactions is particularly challenging with global change stressors like climate change and biological invasions, because it’s impossible to run field experiments at realistic scales, and lab experiments or modeling studies tend to miss important ways that species interact with and influence each other.
Our approach
To better understand how the climate influences biological invasions, we took advantage of multiple invasions by the freshwater macroalga, starry stonewort (Nitellopsis obtusa), across a latitudinal gradient of lakes across the upper Midwest of the US. We tracked invasions over multiple years in lakes that spanned approximately 1000km and differed by about 6°C in annual average daily temperatures (from 3.3°C in the northernmost site to 9.2°C in the southernmost). In fact, areas near our more southern sites are predicted to be climate analogues for where northern parts of our gradient will be by around 2080. By comparing invader populations and spread rates between lakes along the gradient, and between years where conditions were warmer or cooler, we tried to understand how starry stonewort invasions might change under a warmer climate.
Members of the research team pulling a sample of starry stonewort out of a lake to measure biomass (left, photo by Ranjan Muthukrishnan). Ranjan Muthukrishnan sorting aquatic plants collected from a monitoring site (right, photo by Carolyn Kalinowski).
An unexpected result
We generally expect invasions to continually expand and grow faster as they get bigger, but we found that starry stonewort populations varied quite a bit and even got smaller, in some cases. This seemed to be related to temperature, with starry stonewort populations declining in warmer years, while native aquatic plant species expanded in years with warm winters. This meant that in warm years, the native plant community was able to recover and take back some of the space that starry stonewort had previously invaded. This ran counter to our expectation that climate conditions and the invasion would interact in a way that made the impacts worse. Because starry stonewort did poorly under warmer conditions, its invasions may actually be less severe under some conditions that may occur with climate change.
Why did this happen?
It’s possible that the diversity of plants in the native community meant that at least some of the native species can tolerate warmer conditions relatively well. So those species were able to take advantage in warmer conditions, while starry stonewort may be living near the edge of the environmental conditions it can tolerate and is negatively impacted by warming. Even if starry stonewort could persist under these warmer conditions, native species might outcompete it.
Where do we go from here?
It remains unclear if native species will be able to hold off starry stonewort in the long run or if warm years just slow the eventual progression of the invasion. We hope to return to our monitoring sites in the future and see what happened after 5 or 10 years. In the meantime, it is useful to know that native communities can recover if starry stonewort abundances can be limited by warm conditions, and this might help managers identify better strategies or opportunities to intervene for optimal recovery. It is also still important to recognize that this may be an uncommon scenario where the invader is near its environmental limits but the native species have broader tolerances to manage warming. In many cases invasive species themselves tend to have broad environmental tolerances, which can be a trait that helps them be invasive. Overall, having good empirical examples of interactions between global change stressors is the foundation of building up a more general understanding of how ecosystems will respond to continuing human impacts across the globe.
