Sporobolus alterniflorus: The Ecological Impact of an Invasive Cordgrass
Sporobolus alterniflorus, a species of cordgrass, poses a significant threat to coastal ecosystems when introduced outside its native range. Whether acting alone or through hybridization with native species, this resilient grass can fundamentally alter the environments it inhabits, disrupting both biological diversity and human infrastructure.
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The Environmental Consequences of Invasion
When S. alterniflorus becomes invasive, it often crowds out native vegetation, leading to a decline in local biodiversity. This shift in flora creates a ripple effect throughout the food chain. For instance, as the grass overgrows mud flats, the invertebrates that typically dwell there lose their habitat. Consequently, birds that rely on these invertebrates as a primary food source face shrinking resources.
Beyond biological impacts, the physical presence of dense meadows can obstruct water circulation and drainage, and in some cases, block critical boating channels.
Hybridization and the Rise of S. anglicus
One of the most concerning aspects of S. alterniflorus is its ability to hybridize. A notable example is Sporobolus anglicus, a fertile polyploid (an organism with more than two complete sets of chromosomes) resulting from the cross between S. alterniflorus and S. maritimus (also known as S. townsendii).
First appearing in southern England around 1870, S. anglicus possesses several competitive advantages over native plants:
- High saline tolerance: Ability to thrive in salty environments.
- Efficient photosynthesis: More productive at lower temperatures than similar plants.
- Versatile growth: Capable of growing on a wider variety of sediments.
- Water resilience: Ability to survive longer periods of saltwater inundation.
These traits have allowed S. anglicus to spread across northwest Europe and into eastern North America, where it was originally introduced for erosion control.
Global Case Studies of Infestation
China: The Largest Invasion
The most extensive invasion of S. alterniflorus is found in China. Starting in 1979, plants from various North American locations were intentionally introduced to capture sediment and protect shorelines. This effort backfired, resulting in an invasion covering over 34,000 hectares across ten provinces and Hong Kong.
Washington State: Willapa Bay
In Willapa Bay, the grass likely arrived accidentally during 19th-century oyster transplants. By 2003, the infestation peaked, covering approximately 3,000 hectares and spreading across an area of 8,000 hectares. However, aggressive management reduced this to less than 3 solid hectares by 2016.
California: San Francisco Bay
Introduced in 1973 by the Army Corps of Engineers for marshland reclamation, S. alterniflorus became well established in the San Francisco Bay. It outcompeted the native S. foliosa and eventually hybridized with it.
The resulting hybrid, S. alterniflorus × S. foliosa, is particularly dangerous. It produces massive amounts of pollen that swamp the stigmas of native flowers, preventing the production of unhybridized offspring. These hybrids also produce more fertile seeds and may be capable of self-fertilization. Interestingly, because the hybrid is taller than its parents, it provides shelter for the Ridgway's rail, which has complicated eradication efforts. Despite this, infestations were reduced by 96% between their peak and 2014.
Control and Eradication Methods
Managing S. alterniflorus is challenging due to its biological resilience. Hand pulling is generally ineffective because small fragments of rhizomes (underground stems) left in the soil can sprout new shoots.
Various strategies have been employed to combat the spread:
- Chemical Control: Imazapyr, an herbicide approved for aquatic use, has been used effectively in California and Washington.
- Biological Control: In Willapa Bay, leafhopper bugs (Prokelisia marginata) were introduced to protect the oyster industry, though this method failed to contain the invasion.
- Monitoring: Authorities near San Francisco utilize air, land, and sea surveys to track the spread of the species.
Summary of Regional Impacts
| Region | Introduction Method | Peak/Current Scale | Primary Impact |
|---|---|---|---|
| China | Intentional (Shore protection) | >34,000 hectares | Massive landscape alteration |
| Willapa Bay, WA | Accidental (Oyster transplants) | 3,000 hectares (Peak) | Threat to oyster industry |
| San Francisco Bay, CA | Intentional (Marsh reclamation) | 323 net hectares (Peak) | Hybridization with S. foliosa |
| Northwest Europe | Introduction of S. alterniflorus | Widespread (S. anglicus) | Outcompeting native flora |
Key Facts
- Hybridization: S. alterniflorus can create new, more aggressive species like S. anglicus and S. alterniflorus × S. foliosa.
- Biodiversity Loss: Overgrowth destroys mud flat habitats for invertebrates, reducing food for birds.
- Resilience: The plant can survive long saltwater inundation and regrow from tiny rhizome fragments.
- Global Reach: Major invasions have occurred in China, Europe, and the United States.
- Control: Imazapyr is an effective chemical control, while hand pulling is ineffective.
Frequently Asked Questions
Why is hand pulling ineffective for removing this grass?
Hand pulling often leaves behind small fragments of rhizomes in the soil. These fragments are capable of regenerating and sending up new shoots, meaning the plant often returns.
What is Sporobolus anglicus?
It is a fertile polyploid hybrid derived from S. alterniflorus and S. maritimus. It is highly competitive due to its salt tolerance and ability to photosynthesize efficiently at low temperatures.
How does the hybrid in San Francisco Bay threaten native species?
The hybrid produces vast amounts of pollen that overwhelm the native S. foliosa, preventing the native species from producing pure offspring. It also produces more fertile seeds and can potentially self-fertilize.
What was the purpose of planting this grass in China?
Starting in 1979, it was intentionally planted to provide shore protection and to help capture sediment.
Which herbicide is used to control Sporobolus alterniflorus?
Imazapyr is the herbicide approved for aquatic use that has been effectively employed in Washington and California.