Eurasian Watermilfoil: Where did it go?

Eurasian Watermilfoil Outbreak Crashes: Really? 

Restoring a Moving Target

Milfoil Slide

As many of you may have noticed, this last winter the Fall River saw a die-back of the aquatic invasive species Eurasian Watermilfoil, as well as, native aquatic vegetation. Although it is unclear what may have caused this die-back, it is unlikely that it can be attributed to any single cause.

The Fall River is not the first watershed to see an invasion and dominance of Eurasian watermilfoil for a few years, and then an unexplained decline and die-back. Despite numerous studies across North America, researchers have not identified any one mechanism responsible; however it is likely a combination of factors at play.

The good news is that the native vegetation, especially Z-grass, appears to be recovering vigorously, whereas, there have only been patchy pockets of Eurasian watermilfoil observed that have returned to the lower sections of the river. If this trend continues, the Z-grass and other native aquatic vegetation may be able to establish itself and develop a competitive advantage for resources over the Eurasian watermilfoil. Although we are in the early stages of observing and recording these positive changes to the aquatic vegetation community, there is still work we can do to ensure Eurasian watermilfoil or other invasive species do not establish themselves in the Fall River.

But to be clear, Eurasian watermilfoil in the Fall River is likely not gone for good: especially in the lower river. We’ll be monitoring it closely and ready to act if it comes back. Thus is the nature of trying to restore a complex, cold-water, spring-creek fishery in Northern California: it’s a moving target. Long ago, Aristotle suggested that nature has a reason for everything. If only he were here to explain.  Unfortunately, as with so many ecological mysteries, we can only shake our heads and wonder.  How can this happen? What now? Is it really gone?

Summary of What We Know:

  1. No single factor caused the decline
  2. Dense milfoil infestations in lakes sometimes die back after approximately a decade of growth
  3. In other locations across the United States, Eurasian watermilfoil populations increased to high levels of dominance, maintained dominance for a few years, and then declined

Why do dense milfoil populations suddenly decline?

Thanks to our science partners at the US Department of Agriculture and the UC Davis Graduate Group of Entomology, specifically Ray Carruthers (USDA) and Erin Donley (USDA/UCD), we do have some understanding of factors that can affect the growth and decline of Eurasian watermilfoil.  Donley and Carruthers are quick to point out that no single cause or mechanism has been identified to explain the recent collapse.

More likely, the synergistic effects of several different BIOTIC (biologically-based) and ABIOTIC causes contributed to the decline. Donley identified two existing scientific papers that are particularly relevant to the Fall River Eurasian watermilfoil issue.

  1. Carpenter, SR. 1980. The decline of Myriophyllum spicatum in a eutrophic Wisconsin Lake. Canadian Journal of Botany. 58: 527-535.
  2. Smith, CS and JW Barko. 1990. Ecology of Eurasian watermilfoil. Journal of Aquatic Plant Management. 28:55-64.

Summary of Key Points in Existing Scientific Literature


Late arrival of predators and/or pathogens: When an exotic plant becomes established in a new environment, it can sometimes invade quickly because it arrives without the predators and/or pathogens that would normally check its population in its native habitat.  However, over time, these predators and/or pathogens may also show up in the newly infested environment (often using the same pathways the exotic plants were initially introduced through — i.e., ballast water, boat props, etc.). Lake Venice Disease (most likely a bacterial agent) and Northeast disease for instance, have been documented at many infested locations and appear to reduce fitness in Eurasian watermilfoil.

Native plants can adapt over time to compete with exotic invaders: An additional consequence of an exotic plant being transplanted to a new environment is that the initial invasion usually begins with just a few individual plants (which represent only a small fraction of the assortment of genes [genetic diversity] one would find in the populations these plants originally came from). Whereas, the native plants at the site of invasion generally have relatively high levels of genetic diversity relative to the exotics. Although the exotic plant may dominate initially, the native individuals that survive in spite of the impact of the exotic plant become the dominant breeding individuals in the native populations. As a consequence, the offspring of the hearty native strains are also more likely to have traits that allow them to tolerate or compete with the exotics. However, this process of breeding hearty natives may take many years. It is possible that after a decade, the natives have become better competitors.

Dense epiphytes preventing photosynthesis: When epiphytes become too dense, they can cover so much of the macrophyte’s surface area that it can reduce the amount of photosynthesis that can take place. In some cases when a reduction of milfoil was observed, dense epiphyte coverage was also observed. I think this is a case where it would be extremely important to use experiments to better understand this relationship between milfoil and epiphytes because happen-chance observations (which I believe are what is being reported in these papers) only tell us about correlations (not causes).

Other factors that may play a role in milfoil’s decline

According to USDA, the following ABIOTIC factors were also considered in existing literature. However, their impacts were not found to show consistent trends within and/or between sites (in some cases these factors played a role, but in other cases they did not):

  1. Toxins
  2. Mechanical removal of Eurasian watermilfoil
  3. Herbicide application
  4. Climate
  5. Temporal or geographic factors


Slide 3

According to Donley, “The bottom line is the pattern of proliferation and subsequent decline in Eurasian watermilfoil has been observed in many different places across the country. However, there is no consistent explanation for why this occurs.

On the Fall River last winter, we saw die-back of milfoil AND native vegetation. Ray Carruthers, a senior scientist at USDA and leading Fall River researcher, noted that die-back of vegetation across the board is not something he has observed in previous years. Clearly, the natives (especially Z-grass) have recovered beautifully and last season we really only saw patchy pockets of Eurasian watermilfoil in the lower sections of the river.

It is possible that z-grass has adapted to be a better competitor to milfoil by becoming established more quickly as it recovers. In the academic world, there is a term called “priority effects,” which is essentially the idea that if an organism can become established before any other organisms and occupy the limiting factors (be they space, nutrients, light, etc.), that organisms will be dominant. This may be one possible explanation for what we are observing.”

Next Steps

The Fall River Conservancy is committed to working with researchers, agencies, land-owners and the community in the next stages of the fight against Eurasian watermilfoil. Given that the future of Eurasian watermilfoil is uncertain in the Fall River, we are at a critical stage to combat against it by observing and recording the changes that take place in the river over the next few months. The Fall River Conservancy is working closely with CalTrout and other landowners to establish an education campaign focused on preventing the spread and introduction of invasive species. Through informative signage and brochures located at key launch sites, we will introduce some basic steps that all of us as river-users can take to prevent spreading harmful invasive plants and species to our regions cherished rivers and lakes.  With your help we can ensure the Fall River remains a cold-water oasis dominated by native species for years to come.

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