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)

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)

Lab work, field work

Sometimes scientists work in a lab, under carefully coordinated conditions finely tuned to elicit certain reactions from whatever they’re studying. Sometimes, they work in the field, under whatever circumstances nature serves up when they happen to be outside, observing, measuring, recording, and sweating or shivering, depending on the day.

Ecologists are interested in how the natural world works – lab experiments can fill in the details, but they (by definition) don’t include all of the variables that are actually at play in the real world. Field observations can flesh out the big picture, but without control over environmental dynamics, researchers often can’t be sure which factors are responsible for observed changes.

One way to balance the strengths and weaknesses of these two approaches is to move the lab outside – which is just what two researchers from the Netherlands Institute of Ecology did when they wanted to investigate the way nutrients and plant-eating ducks affect aquatic vegetation, as reported recently in the journal Oecologia.

The scientists combined the benefits of a controlled setting – they used 20 man-made ponds in their experiment, all the same size, shape, and depth – with the advantages of working in a natural system: they filled the ponds with lake water and exposed them to the daily rhythms of weather and sunlight by working outside.

The researchers fertilized half of the experimental ponds with extra nutrients, and introduced ducks – which eat mainly plants, though they also consume insects – to half of each type of pond (fertilized and unfertilized). By the end of the experiment, the fertilized ponds with ducks had 50% less plant material than the fertilized ponds without ducks, but in the unfertilized ponds, the ducks didn’t appear to have eaten any of the aquatic plants. The scientists suspect that this may have been due to plant nutrient levels – the plants in the fertilized ponds contained more nutrients than the ones in the unfertilized ponds, which could have made them more appetizing to the ducks. The fertilized ponds also developed a different plant community than the unfertilized ponds – perhaps the most prevalent plant in the fertilized ponds simply tasted better, from the ducks’ perspective, than the plant that came to dominate the unfertilized ponds.

“In our study,” the scientists write, “the effect of plant species and pond nutrient status cannot by fully separated and the relative importance of plant species and plant nutrient concentrations in determining grazing pressure thus remains to be investigated in more detail.” Therein lies the challenge of experiments conducted in natural settings – when all factors cannot be controlled, it’s difficult to tease apart which ones are most important.

Field and lab studies both have advantages and drawbacks, but, luckily for science, not the same ones – fieldwork and lab work are two sides of the same coin, complementary tools that scientists can use to puzzle out the mysteries of the natural world.

Mallard ducks can live in almost any kind of slow-moving aquatic habitat, including ponds, marshes, floodplains, and many other places. 

(Image by Carsten Niehaus via Wikimedia Commons)