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Climate Change

Agricultural Stewardship Could Prevent Invasive Species Takeover Downstream


Phragmites australis, or Common Reed, is a marine grass likely introduced to North America from Eurasia in the late 1800s through the garden trade. It has since become one of the most aggressive invaders in history, replacing native grasses in wetlands and disrupting native ecosystem function.

A saltmarsh in Georgia (Trish Hartmann, Wikimedia Commons)

Wetlands are interesting study systems for invasive plants. Waterlogged and acidic, there are only so many plants that can live in these habitats. In the plant world, competition between species is almost always over space – and where in the wetland any individual species ends up depends on both the species’ tolerance for the local conditions and who is already occupying the area.

Like other invasive species, Common Reed grows fast, reproduces via several strategies, and has a broad range of tolerances to different stressors. Together, these traits make it good at both colonizing new space in a wetland and pushing other species out of their way.

It is considerably easier for a new species to take hold in an ecosystem when space is freed up by some disturbance, and coastal wetlands along the eastern seaboard have been dramatically altered by human activities since European settlement; some deliberately drained for farmland, some polluted by nutrients from farm runoff (manure and other fertilizers), and all of them exposed to the effects of global climate change. Common Reed takes advantage of its competitive traits to spread and thrive in these wetlands, colonizing disturbed patches and then kicking out neighbouring vegetation.

Invasive Phragmites australis, Common Reed (c1.staticflickr.com)

The native Spartina alterniflora, or Smooth Cordgrass, is particularly threatened by the spread of the Common Reed. Common Reed can form root beds twice the depth of Smooth Cordgrass, is tolerant of saltier conditions, and spreads quickly underneath the mud into new areas. How the invader and its native competitor react to local stressors (predominantly agricultural runoff and the effects of climate change) are important processes to understand if scientists hope to effectively manage the invading Common Reed.

Legault et al. from the University of New England investigated the dual effects of nutrient pollution and increased temperature on the competition between Common Reed and Smooth Cordgrass by monitoring their productivity (how well or fast they use their resources) in simulated laboratory environments. They exposed an equal number of Common Reed and Smooth Cordgrass plants in laboratory tubs to increased nutrients and temperature, with and without competition from the other species, and looked for differences in how the plants fared through time.

Plants like extra nutrients, as they live in nutrient-limited environments, so the addition of Nitrogen and Phosphorus can be expected to increase plant productivity. That’s why you see algae blooms when nutrient pollution is really bad. Common Reed is particularly nutrient-hungry and is known to increase productivity when its habitat is nutrient saturated. Plants generally do not like competition, so one would expect both of the grasses to be less productive in a competitive environment where they have to share their limited resources. Most of Legault’s findings are consistent with this – more nutrients means more growth, more competition means less. Interestingly, the team found that higher temperatures seemed to change the way the plants act in response to nutrient load and competition.

At normal temperatures, increasing nutrient levels resulted in greater productivity (size, number of new shoots) in both the invasive Common Reed and native Smooth Cordgrass. But at high temperatures, the beneficial effect of extra nutrients levels off and there is less of an increase in productivity, even though nutrient levels are high. This effect was particularly evident in the invasive Common Reed. In fact, Smooth Cordgrass was better able to maintain its productivity in the higher temperature system and was more productive even without the nutrient addition.

Runoff of nutrients from farm fields can harm coastal wetlands. (Lynn Betts/USDA)

Legault et. al.s’ results suggest that the native Smooth Cordgrass could have an advantage if temperatures continue to rise due to climate change and future nutrient pollution from land-based sources can be minimized. Because so much of the nutrient pollution along the east coast originates from farms, the widespread adoption of on-farm best practices for reducing runoff could go a long way towards preventing future invasion of the Common Reed. Considering the high cost of invasive species removals once they become established, preventative strategies like reducing runoff are likely to be the most cost effective way of saving our native wetlands.


Article: Legault R II, Zogg GP, Travis SE (2018) Competitive interactions between native Spartina alterniflora and non-native Phragmites australis depend on nutrient loading and temperature. PLoS ONE 13(2): e0192234. https://doi.org/10.1371/journal.pone.0192234







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