Alien Invasion: Is the Eradication of Certain Non-Native Plants Hurting Duck Populations?
March 09, 2015
When Jon Hart heads to one of his favorite lakes for a morning hunt, he knows exactly where to toss out a decoy spread. He simply looks for hydrilla, an aquatic plant that provides a smorgasbord of food for the lake's diving and puddle ducks.
These days, finding one of those grass beds on Virginia's Lake Gaston is becoming more difficult. So is killing a few birds. Lake managers have stocked tens of thousands of grass carp in the 20,000-acre reservoir since 2003 in an effort to rid the lake of the invasive aquatic plant. They also hire contractors to spray the grass with herbicide every year. Their goal is to eliminate the plant, ducks be damned.
"As the grass goes, the ducks go," says Hart, a veteran game warden and long-time duck hunter from southern Virginia. "I talk to a lot of other duck hunters when I'm on patrol, so I'm not the only one who sees a pattern. The less grass we have on these lakes, the less ducks that hunters see. It's really a shame to see so much effort put into wiping out the grass."
It's no secret ducks flock to aquatic vegetation. A study conducted on the tidal Potomac River examined waterfowl abundance in relation to the presence of hydrilla, an invasive, non-native aquatic plant first brought to Florida in 1959 for the aquarium trade. It found a direct and strong link.
"We examined data from the Audubon Society's annual Christmas Bird Count and compared it to aquatic vegetation coverage going back to 1959," says study author Nancy Rybicki, a U.S. Geological Survey hydrologist.
Observers counted 1.6 Canada geese per hour when hydrilla and other aquatic grass was virtually non-existent in the Potomac in the 1960s, and 20 per hour after aquatic vegetation became abundant around 1982. Bluebill counts also skyrocketed.
In fact, waterfowl numbers overall quadrupled after large areas of the river became covered with hydrilla, milfoil and native aquatic vegetation. In fact, Rybicki says native aquatic vegetation rebounded after hydrilla become abundant.
"The water quality improved and we saw no evidence of a decline in native vegetation as a result of increased coverage of hydrilla," she adds.
Hydrilla may be good for water and ducks, but there's no question a number of other aquatic invasive plants are a major nuisance. There's no better example than on Louisiana's Lake Maurepas. It used to be loaded with birds. Not anymore.
"It's one giant 70,000-acre carpet of salvinia. Ducks don't use it anymore," says Louisiana Department of Wildlife and Fisheries waterfowl study leader Larry Reynolds.
Giant salvinia is native to Brazil and was likely imported to the U.S. for the aquarium trade. Although it can be controlled with herbicides, it is difficult to contain and will re-establish quickly given the chance, forming massive floating mats that shade out native vegetation.
"I honestly don't know how to solve this problem," adds Reynolds. "It's so prolific it covers entire lakes and swamps. It is one of the most overwhelming problems I have as a waterfowl manager. It completely blocked an access canal at Lacassine National Wildlife Refuge last year."
His agency is also dealing with other invasives like water hyacinth, a significant problem in Florida and other southern states, and phragmites, a 15-foot-high non-native reed that grows in dense colonies and overtakes native emergent vegetation. Phragmites are a major problem along the northeastern coast, Chesapeake Bay, Great Lakes, Gulf Coast and in some inland states like Utah.
Phragmites, also known as common reed, provide no benefit to waterfowl and other wildlife. It grows in fresh and salt water and is difficult to eradicate. Worse, it encroaches on shallow marshes, squeezing ducks and duck hunters into smaller and smaller areas. Phragmites has choked out at least 27,000 acres of wetlands on the Great Salt Lake and Utah Lake, and covers as much as 10,000 acres on various wildlife management areas in Utah.
"We are finally starting to win now that we are putting resources toward controlling phragmites, although it is a very slow process," says Utah Division of Wildlife wetlands manager Randy Berger. "Duck hunters spoke up and the state legislature listened. We are allocated about $200,000 per year for phragmites control and eradication efforts, so we are seeing some success thanks to the hunting community. Other state agencies are also working on the problem. I think we are turning the tide."
Hart, however, wonders why some state, federal and local agencies are waging an all-out war on those invasive species that are clearly beneficial to fish and wildlife. Instead of eradicating hydrilla and other aquatic vegetation, he would like to see a different approach, one that considers all user groups, not just recreational boaters and lakefront property owners, who don't like seeing large mats of hydrilla in front of their homes. He's fighting an uphill battle.
The U.S. Army Corps of Engineers released 17,000 triploid, or sterile, grass carp into Kerr Reservoir, one of Hart's favorite lakes, within the last two years. The grass was just starting to become established in part of the lake. The duck hunting was also improving.
Known as lawn mowers with fins, grass carp can grow to 60 pounds and devour as much as half their weight in vegetation in a single day. They have been used in reservoirs throughout much of the country to control unwanted submerged aquatic vegetation, including South Carolina's Santee-Cooper Reservoir system. At its peak in the mid-1990s, hydrilla was present on nearly one-quarter of the waterway's 170,000 acres. It can grow up to eight inches per day.
Just as Rybicki found a direct link between hydrilla and duck numbers on the Potomac River, a U.S. Army Corps of Engineers research project found a similar pattern on Santee-Cooper. Duck hunters killed an estimated 1,500 birds in 1995 when hydrilla coverage was near its peak. However, officials started releasing grass carp in the lake in 1989, stocking 768,500 by 1996. The lake's aquatic vegetation was nearly gone by 1997 and the duck harvest plummeted to fewer than 500 birds.
Hart understands the dilemma natural resource managers are facing. Despite the obvious benefit to waterfowl and hunters, vegetation like hydrilla can become a costly nuisance. A huge mat of it clogged a water intake at Santee-Cooper in 1991, forcing managers to temporarily shut down the dam's generators. It's also a major headache for recreational boaters. What concerns him and other waterfowl hunters is what seems to be a total disregard for user groups that benefit from the abundant grass. It doesn't have to be an all-or-nothing approach.
"It's starting to take hold parts of Kerr Reservoir in places where there are no lakefront homes. Let's not mess this up like we did at Lake Gaston and just attempt to kill it all off for the sake of killing it," says Hart. "Control it where it needs to be controlled but leave it alone in places where it is not creating any problems."