Flowering plants remain viable after passing through the guts of waterfowl

We’ve long known that plant seeds can remain viable not only after floating a long time in fresh or salt water, but also when passing through the guts of birds, including waterfowl. In fact, I believe that at least half of the plants that arrive on oceanic islands, later to form new species, come from seeds pooped out by birds—as opposed to having floated across the ocean.

Now, however, there’s a new method of dispersal: instead of plant seeds being transported in bird guts, plants themselves can pass through a gut and remain able to reproduce (at least asexually). This is the first time it’s been shown for any flowering plant (“angiosperm”) in any bird, though there are previous reports of moss fragments and fern spores being able to pass through and remain viable in the guts of waterfowl. The study described here allows plants a new way of dispersal, and it happens to create a fortuitous mutualism between waterfowl and the plants.

The report, a short one, and free, is in Biology Letters of the Royal Society (click on screenshot, pdf here, and reference at bottom). 

The authors showed that the white-faced whistling duck (Dendrocygna viduata) and the coscoroba swan (Coscoroba coscoroba) eat a small duckweed-like plant called Wolffia columbiana, and that the plantlets—among the smallest of all angiosperms—can sometimes pass through the digestive tract of these species and remain able to divide (asexual reproduction) afterward. First, the players:

The white-faced whistling duck, found in the Southern hemisphere:

The coscoroba swan, endemic to southern South America:

And a mallard (not a whistling duck) eating duckweed, a similar plant, along with a close up of Wolffia columbiana (they’re tiny). Both regular duckweed (probably Lemna minor in the first photo below) and Wolffia are called “duckweed” because ducks nom the stuff with relish. (I’m kidding: they eat it plain!)

From a NYT story: Duckweed, and a duck, on a pond in Germany. CreditCreditKay Nietfeld/DPA, via Getty Images

W. columbiana on fingertip

 

A sarcastic tweet from the Twitter site #Wolffia:

⭐️WORST FLOWERING PLANT EVER! #Wolffia arrhiza makes no #flowers, no #roots, no stems, no leaves. It just swims stupidly in the water. I ordered an #Araceae but got this joke of #plant. Total rip-off! Never again! #ratemyspecies pic.twitter.com/Hp6KweAUea

— Norbert Holstein (@dr_norb) March 22, 2018

The authors collected duck and swan poop produced by both birds in five temporary wetlands in southern Brazil that harbored the plant. The poop was taken from near resting waterfowl, and was inspected to ensure that no plants adhered to the outside of the excrement.  Some samples were frozen and then defrosted for dissection to see if plants were in the poop, while plants taken from dissected unfrozen droppings were incubated in Petri dishes to see if they could reproduce asexually—by division.

The upshot is short and sweet: whole plants were found in frozen droppings of both swans and ducks, and intact plants found in three duck samples (they didn’t do this for the swan samples, and I don’t know why), and the plants began to divide in three out of the five Petri-dish cultures. Admittedly, that’s a small sample, but still meaningful. Half of the samples, fresh or frozen, contained whole plants. And remember that these tiny plants divide rapidly to produce exponential increase in numbers, so if a duck pooped out a plantlet into a pond, it wouldn’t take long until the pond had a tasty mat of Wolffia.

Here’s a figure from the paper showing live and dead plantlets; the caption is below:

(From paper): (a) Wetland where faeces of D. viduata with W. columbiana were collected. (b) Intact plantlets obtained from faeces, showing a healthy appearance (bright green colour and integral structure). (c) Seven plantlets observed after 7 experimental days, confirming asexual reproduction. (d) Plants that died during the experiment lost their colour.

Previously, biologists had thought that these plantlets, and similar plants, could disperse by epizoochory—adhering to the outside of the bird, like many seeds do. But this paper shows another method of dispersal for whole plants or plant parts, one called endozoochory (“gut dispersal”). The latter method is less likely to kill the plants by desiccation, though gut acids could do a number on the duckweeds.

How far can the plants disperse? The authors say that “the average retention time for waterfowl faeces is several hours”, so Wolffia could move several km per day via ducks who fly from pond to pond. The swans migrate over a thousand km, so the potential dispersal through them is greater (remember, though, that swan poop didn’t show viable and reproducing Wolffia, since they didn’t look for it.)

It also seems likely that the plants most likely to move this way are those that, like W. columbiana, reproduce clonally. All you have to do to colonize a new habitat this way is to divide asexually rather than grow and produce sexual parts (stamens and pistils), and it’s probable that the latter would be more sensitive to being killed in bird guts.

Finally, while this is a mutualism (the ducks help the plants colonize new habitat, and the plant growth helps feed the ducks), it’s not likely to be an coevolved mutualism. That is, perhaps the plant has evolved resistance to being passed through waterfowl guts, since those plants able to resist gut acids would be more likely to survive and colonize new habitats, but it’s unlikely that the duck has evolved so as to harbor and not kill the plants. There would be little advantage, I suspect, to a new mutation in a duck rendering it less likely to kill plants, as those mutant ducks, moving from pond to pond, wouldn’t benefit more than ducks that didn’t kill plants (all ducks benefit from a Wolffia harvest on a pond).

The results may sound trivial and unremarkable, but may not be. While plants that can survive passage through waterfowl guts may be rare, they’re still able to colonize distant habitats in a way that other plants can’t. And we have no idea yet how many plants can disperse in this way. This paper may stimulate more work along those lines. As the authors note:

 Given that production of asexual vegetative propagules and an ability to grow from fragments is widespread in plants, dispersal of such vegetative propagules (e.g. fragments of grasses or pondweeds, or whole floating plants) by endozoochory may be an important and overlooked process.

h/t: Nilou

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Silva, G. G. et al. 2018. Whole angiosperms Wolffia columbiana disperse by gut passage through wildfowl in South America. Biology Letters, 14 (12), online