The Dalmatian toadflax may not strike terror in your heart, but many farmers cringe when they hear the name. The invasive species has crept through 32 US states, leading to a perpetual battle with farmers trying to stay ahead of the weed.1
Climate change won’t help matters. An eight-year study in Wyoming predicted the effect of warming temperatures on the survival of such invasive plants and found that the coverage of toadflax would spread to 13 times its current size.2 Invasive species live up to their name by being so adaptive to change, making them particularly dangerous to plants finding more comfortable latitudes over the next century of global warming .
But just how dangerous? Most experimental research thus far has focused on how climate change directly impacts plant survival, but we know little about how climate change interacts with the introduction of new competitors fighting for resources. Global warming forces plants to adapt by migrating north or to higher elevations, but what happens when this migration bumps them into a new species? Or what if a species fails to migrate and an invasive variety wanders into its neighborhood?
Such questions have been difficult to answer to date, mainly because of an uncertainty of which species would meet and the experimental challenge of creating large, hypothetical communities of plants in particular climatic conditions But a new study3 published in Nature has taken on this open question and found that it is not just climate change or competitors alone that matters, but an interaction of the two that creates significant stress that impacts plant survival.
Mountains provide the perfect playground to understand the effect of changing temperatures on plant survival: just slide a species up or down the mountain slope to find colder or warmer climates. Using this idea, researchers in Switzerland looked at four different species in the Swiss Alps, with such evocative names as the alpine kidney vetch and glossy scabious, moving them up and down the mountain slope while either keeping other plant species the same or introducing new competitors to their region. In particular, elevations were chosen to mimic a 3 degrees Celsius temperature change, matching climate model predictions for the next 100 years.
Researchers examined four specific scenarios shown in the figure below. The plant species of interest (the one whose survival we care about) is called the focal species (green plant in the figure). Due to climate change, the focal species could fail to migrate, in which case it could (1) only compete with species that also didn’t migrate or (2) have to compete with a new species that has migrated to a higher elevation. On the other hand, if the focal species does migrate to higher elevation to adapt to global warming, it could either (3) face the same competition as before due to similar migration by other species or (4) face new competition from other species that have failed to migrate (4).
That’s a mouthful! But the basic idea is that there are two possible stressors: changing temperature and changing competitors, and these two variables give four possible conditions that the focal species must face. So how do each of these combinations affect survival?
It turns out that if the focal species tracks the climate, moving to a higher elevation to adapt to warming, then it has a high probability of survival with or without a new competitive species. However, if the plant does not adapt to climate change, staying at its current elevation and now in warmer temperatures, then its survival dramatically depends on whether a competitive species migrates to the same region. In this case, focal species competing with new plants demonstrate a 50-80% decrease in survival, a 50-60% decrease in biomass, and a 70% decrease in flowering. Thus, it is not just climatic stress but the combined challenges of adapting to new temperatures and competitors that will make survival so difficult in a future CO2-packed world.
Other factors could be affecting these different survival rates, such as changing chemistry or new organic biomass in the soil, but the researchers controlled for these variables and still found the significant competitor-climate interaction.
One artificial aspect of the study is the pace at which the focal species must adapt. True climate change and species migration will occur over a period of decades to centuries, whereas this study forced an immediate transplantation to a new location and looked at survival over two years. Such a dramatic shift will likely make species more sensitive to new competitors than in reality, however it is equally impressive that the species could adapt so well in this same time frame to increased temperatures alone. Therefore, the central finding that competition may be the strongest driver of plant survival still likely holds.
These results provide an important picture of what challenges plants will face in a warming world. Even if most species do migrate in response to warming, trailing edges of biomass will always reside in the old, hotter climate less conducive to survival. If these trailing edges meet other migrating species, a battle for survival will likely ensue.
- Runyon, L. “In ranchers vs. weeds, climate change gives weeds an edge.” NPR, accessed September 18, 2015.
- USDA. “The prairie heating and carbon dioxide enrichment experiment.” Accessed on September 18, 2015.
- Alexander, JM et al. “Novel competitors shape species’ responses to climate change.” Nature, published online September 16, 2015.
Diagram of elevation and migration changes courtesy of Reference 3