Ducks and seeds

Seeds from aquatic plants have been known to successfully germinate even after passing through the digestive tract of a bird – this is one method by which plants can spread from one body of water to another.

Sometimes, though, seeds don’t make the full, daunting journey through a bird’s bowls.

“Regurgitation, or vomiting, is a common behavior in the daily life of many bird species,” write the authors of a paper recently published in the journal Aquatic Botany. If a plant seed is thrown up before passing all the way through a bird’s body, thereby “circumvent[ing] many of the damaging digestive processes,” so much the better for the seed.

The scientists, interested in whether or not the seeds of aquatic plants might spread via bird vomit, fed ducks seeds collected from 10 species of wetland plants. The ducks regurgitated seeds in about half of the 64 feeding trials the researchers conducted, seemingly in response to one of two conditions: overfeeding, or eating large, indigestible seeds.

If the birds ate a lot of food in a short time (as they sometimes do in the wild, when they happen upon an abundant food source), they threw up some of the seeds they had recently eaten, regardless of seed size, within three hours of eating. Large (more than 10 millimeters, or a little bit less than half an inch), tough seeds were sometimes regurgitated early on in response to overeating, but also between 11 and 24 hours post feeding, apparently after the seeds were rejected from the birds’ gizzards due to their size.

“As regurgitation in birds requires a suffocate movement which is impossible during flight,” the scientists note, “regurgitation most likely occurs after landing in wetland habitat,” meaning that bird vomit may be an important way for aquatic plants to spread.

Seeds from the aquatic plant Iris pseudacorus were among the largest seeds fed to ducks (and later regurgitated by them) during feeding trials. 

(Original image by Paul van de Velde via Flickr/Creative Commons license)

Successful germination

Invasive aquatic plants can cause real harm in lakes and ponds. They can spread between water bodies via many different pathways – they’ve been known to hitch rides on boat trailers, for instance, and can also spread via floods, wind, or even gardening.

Aquatic vegetation can also stow away on more mobile organisms – either when plant fragments cling to fur or feathers (a form of locomotion that crayfish may also take advantage of), or when animals or birds eat their seeds.

As a team of researchers recently reported in the journal Freshwater Biology, aquatic plant seeds eaten by waterfowl fare differently depending on what type of plant they come from. The scientists fed mallard ducks and greylag geese seeds from four different aquatic plants, two of which are invasive in Europe (where the scientists are based).

The researchers found that the seeds of one of the invasive species were particularly successful in making the trip through the gut of the birds intact – they recovered more than a third of the Ludwigia grandiflora (or water primrose) seeds from the birds’ waste, but less than a tenth of each of the other types of seeds they fed to the birds.

The scientists also attempted to grow the seeds they retrieved – after all, if seeds can’t develop once they’ve gone through digestion, it doesn’t matter if birds spread them around. They found that “[f]or mallards, 9.14% of the tested seeds germinated successfully, compared to 24.18% for the greylag geese.” Some of those seeds were retained in the birds’ guts for 72 or even 96 hours before they were excreted, though the seeds of one plant species, the other invasive, Spartina densiflora (or cordgrass), only sprouted if they spent eight hours or less inside the birds.

Ducks and geese can travel much farther than plants can on their own over three or four days. “Ducks and geese evidently have the potential for long-distance transport of alien and native plant seeds,” the authors write, “with maximal dispersal distances of well over 1,000 km,” or 620 miles, about 20 miles farther than the drive from Chicago to Chattanooga, Tenn. That’s a long way for an aquatic plant to hitch a ride.

Seeds from Ludwigia grandiflora, or water primrose, were better able to survive waterfowl digestion than the seeds of other aquatic plants. 

(Image by bathyporeia via Flickr)

Plants hitching a ride

Growing up, I spent my summers exploring the lakes of New York’s Adirondack Mountains (I recently wrote about one Adirondack lake in particular here). I loved to swim, but I did not love swimming through patches of plants growing up from a lakebed – the feeling of their tendrils swaying in the wake of my passing, clinging to my skin as if they wanted to grab my body and pull me down into the depths of the water, was enough to send me thrashing back to the lakeshore.

Eurasian water-milfoil, an aquatic plant native to Eurasia and northern Africa that has spread across much of North America, grew in such abundance in Upper Saranac Lake (a lake that I swam in many times as a child) that a local foundation raised $1.5 million to begin removing the plant from the lake in the early 2000s. Invasive aquatic plants often thrive and proliferate in their new environments so much that they crowd out native plants, degrade fish habitat, and clog waterways, preventing them from being used for boating or swimming.

The control effort in Upper Saranac Lake – which involves sending divers into the water to pluck milfoil by hand – has been largely successful at reducing the amount of the plant in the lake (in recent years, divers have collected roughly one-fortieth the amount of milfoil harvested at the beginning of the project), but if the removal stops, Eurasian water-milfoil could quickly rebound – meaning maintenance dives will have to continue, and someone will have to keep paying for them, indefinitely.

Dealing with invasive species in aquatic environments is expensive and time-consuming, and, as is the case with Eurasian water-milfoil in Upper Saranac Lake, they often cannot be completely eliminated. Perhaps the best offense, then, is a good defense – if invasive aquatic plants aren’t allowed into lakes to begin with, then no one has to dive down to the lakebed to remove them later.

One way for aquatic plants to spread among lakes is to hitch a ride on boats or trailers. If the bits of plant matter that get wrapped around a boat propeller or caught in the wheel wells of a trailer can survive out of water long enough to reach the next lake a hapless boat-owner visits, the plant might be able to spread to that lake. (This is why many states require boaters to wash their boats between lake visits.)

Research recently reported in the journal Hydrobiologia shows that some plant-parts are particularly adept at surviving dry spells. A team of scientists working in northern Wisconsin collected stems from two aquatic plants, Eurasian water-milfoil and curly-leaf pondweed, as well as buds from the pondweed, allowed them to dry outdoors (in order to simulate the conditions plants caught on a boat or trailer might experience), then placed them back in tubs of water to see if they were still capable of growth and, presumably, establishing themselves in a new lake.

Single plant stems were able to grow after up to 12 to 18 hours of drying, and stems that were coiled, as if twirled around a boat propeller, grew after up to 48 hours out of the water. The curly-leaf pondweed buds were able to survive for the longest – some sprouted after 28 days on land.

“The high cost and difficulty of eradicating introduced invasive species makes preventing secondary spread a management priority,” the authors write. Knowledge of how aquatic plants are able to spread between lakes, and how long they can survive out of the water, can help lake managers develop guidelines for cleaning boats and, hopefully, eliminate the need to send divers down to weed invasive plants from the beds of any more lakes.

Native aquatic plants are an important part of lake ecosystems, but invasive species, like Eurasian water-milfoil in North American lakes, can grow so much that they take over, killing other plants and preventing boating and swimming. 

(Image by dhobern via Flickr)

Edible flowers

Flowers exist in most terrestrial ecosystems, and though humans have appropriated flowers for our own decorative and aromatic purposes, the plants that produce them use them for one thing – reproduction. When flowers are pollinated, they produce fertilized seeds that, given the right circumstances, develop into the next generation of the plant.

Though most Valentines Day bouquets are composed of terrestrial flowers, many aquatic plants produce flowers, too. These flowers typically grow at the tip of a long stem stretching to the surface of the water, so that pollination via the normal vectors (wind, insects, other animals) can occur. (Plants, including submerged vegetation, also have a few other options for reproduction, including fragmentation and root branching [pdf].)

In a study recently published in the journal Aquatic Botany, a team of Spanish scientists investigated the seasonal dynamics of waterfowl feeding on aquatic plants in a coastal lagoon in the northwestern Mediterranean Sea; they found that flowers appeared to be a particularly appealing meal for the birds.

The researchers suspected that waterfowl – ducks and coots – might consume more aquatic vegetation as a group during the autumn and winter because, due to their migratory patterns, they are much more abundant during those seasons than in the summer in the coastal lagoon the scientists studied. (During the year the study occurred, there were about five to six times as many individual birds present during the autumn and winter than in the summer.)

To test their idea, the scientists created “exclusion cages,” to protect aquatic plants from waterfowl, then compared vegetation height and mass between the exclusion plots and other plots where birds could feed freely. Contrary to their expectations, the scientists found that there were no differences in vegetation between the two types of plots during the autumn and winter months; the ducks and coots did not appear to be eating more vegetation during those seasons.

During the summer, however, the waterfowl did eat a significant amount of one species of aquatic plant, Ruppia cirrhosa, or spiral ditch grass (the plants were shorter, and their biomass smaller, in the plots were waterfowl were present). The birds also ate the flowers of the plant – there were approximately eight times fewer Ruppia cirrhosa flowers in the plots where ducks and coots were present and able to eat them than in the exclusion cages.

The scientists suggest that ducks and coots in the lagoon may have focused their feeding on other food resources, like algae, insects, and seeds, during the autumn and winter, when aquatic plants stopped growing. “[T]he strongest waterfowl impacts on the submerged vegetation within brackish Mediterranean lagoons do not occur when abundance of individuals is higher,” they write, “but in summer when plants and flowers are largely available.”

For plants, flowers are a practical way to reproduce; for humans, they’re a way to send a message of love or congratulations, or a fragrant way to decorate a counter; for waterfowl, they appear to be a convenient way to make a meal.

In the plots free of waterfowl, Ruppia cirrhosa produced about 10 times more flowers than Potamogeton pectinous (the plant shown here). 

(Image by Ruppia2000 via Wikimedia Commons)