Can Native Plant Diversity Limit the Survival of Invasive Species?

Phragmites australis, the common reed, is a perennial grass that can be found on most continents except Antarctica. During the last century Phragmites has rapidly expanded its habitat, invading many salt marsh communities along the Atlantic coast in North America. This degrades healthy ecosystems by replacing short stalk native grasses with long stands of reed. Peter and Burdick (2010) postulated that native plant competition would reduce the success of Phragmites to invade a tidal salt marsh. They tested whether native plant diversity and composition changed the invasive plant’s ability to survive, and analyzed the growth of Phragmites for a single growing season when planted among different native plant communities. In general, they found that in the presence of any native grass, Phragmites’ ability to thrive was reduced as a result of resource competition. Acadia Tucker
Peter, C., Burdick D., 2010. Can plant competition and diversity reduce the growth and survival of exotic Phragmites australis invading a tidal marsh? Estuaries and Coasts 33, 1225–1236

Species in diverse and productive salt marshes are highly competitive in regards to resource partitioning. Different native plants inhabit different spatial niches and capture all the available nutrients, space, and light for each area within an ecosystem. However once an ecosystem is disrupted the balance of resources can shift to allow new species to colonize. Phragmites easily conquers a disturbed landscape, creating a homogenous distribution of plants that offers fewer ecosystem services than a landscape scattered with a multitude of native grasses. Restricted tides blocked by jetties, increased runoff, and eutrophication all reduce the health of a salt marsh but promote the invasion of Phragmites. This reed has a strong competitive ability, with shoots up to 3 meters tall, that block the sun from smaller native plants once it has taken root. It can also survive in hyper-saline and waterlogged soils more competitively than native grasses. The invasion of Phragmites has decreased the numbers of birds, reduced nekton diversity, and diminished benthic invertebrate colonization for many salt marsh communities.

 Peter and Burdick tested the effect of native grass competition on the growth of Phragmites in Meadow Marsh Pond in Hampton, New Hampshire, because it has a history of human obstruction. The irregular flooding in the north section caused by restricted tides has led to the invasion of Phragmites. Peter and Burdick planted 30 cm x 30 cm plots of densely vegetated Phragmites among two distinct communities of native plants. Phragmites was planted with one native competitor (S. patens in the high marsh or S. alterniflora in the low marsh) or a mixture of several native competitors. The plots with multiple competitors represented the effects of species richness (the number of different species in a given area), species evenness (a measure of biodiversity) and species composition on the success of Phragmites. Results were quantified by the amount of dried aboveground biomass, shoot length and density, shoot survival, and aphid damage (aphids only feed on healthy plants).
Phragmites planted alone always outgrew Phragmites planted with native competitors. Shoot length, density, and survival as well as the presence of aphids were all significantly diminished when Phragmites was planted with native plants, with the most dramatic decline occurring among multiple competitors. Above ground biomass decreased 83.7%, shoot length decreased 60.0%, shoot density decreased 48.3%, shoot survival decreased 58.8% and the amount of aphid damage decreased 74.2% in the presence of one native competitor. The greater the species richness and evenness, the less Phragmite’s ability to thrive. For example, shoot density decreased an additional 31.5% in the presence of multiple competitors compared to a single competitor.
Native plants inhibit the expansion of Phragmites by competing for space, nutrients, and light. In a well-established salt mash ecosystem, native plants have a monopoly on all of the resources within a particular niche. For example, native grasses prevent the proliferation of Phragmites roots limiting the amount of nutrients a given plant can absorb. Native canopies also severely reduce the amount of light germinating shoots receive. Native grasses have the capacity to inhibit invasion as long as the ecosystem remains healthy. Therefore if the impacts of climate change or human disturbances remove native plants, it is far easier for invasive species to colonize an area
In this study, it was inconclusive if species composition had an overall effect on the growth of Phragmites. Competitive relationships can reverse with excessive nutrient loading or along resource gradients by changing the interspecific competitive relationships within a salt marsh. S. alterniflora had a greater capability of reducing the growth of Phragmites than S. patens. Peter and Burdick predicted that species with taller canopies and that are normally weaker competitors for nutrients may be the best opponent against invasive species in disturbed salt marshes.
Past restoration techniques have focused on the removal of specific species by controlled burning or mowing but physical plant damage is only temporary when environmental conditions are ripe for invasive species. Restoration should focus on and mitigate the human disturbances and impacts of climate change that create the favorable conditions for invasive species to thrive. It is more effective to reestablish routine flooding and maintain dense populations of native plants after the removal of Phragmites to help prevent reinvasion by increasing native plant competition for available nutrients. 

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