Endozoochory of the same community of plants lacking fleshy fruits by storks and gulls

By Víctor Martín-Vélez, Ádám Lovas-Kiss, Marta I. Sánchez and Andy J. Green

Sampling bird faeces and regurgitates from a dyke alongside recently harvested ricefields in Doñana, south-west Spain. Photo credit: Andy J. Green.

We teach our children that seeds disperse in different manners according to their morphology, and how only those inside berries and other fleshy fruits are able to disperse inside birds, a process known as “endozoochory”. In reality, nature is more complex, and many other plants can disperse in a similar manner. This is especially true for plants with relatively small, hard seeds more likely to get through a bird’s digestive system intact. For example, ducks swallow many seeds deliberately, but only digest some of them. Other birds may swallow seeds that are attached to, or inside, their prey.

We did our study in the largest area (370 km2) of ricefields in Spain, near Seville, an area hugely important for wintering waterbirds and adjacent to the Doñana World Heritage Site. We had two main research questions. Firstly, what kinds of plants are dispersed by waterbirds feeding in newly harvested ricefields? Secondly, what differences are there between birds of different size and form in the plant species they disperse? We compared lesser black-backed gulls (visiting from their breeding areas in northern Europe) with white storks (which breed both locally and in central Europe). We collected their droppings and regurgitated pellets from dykes where they rest after feeding, then extracted and germinated intact seeds. These birds dispersed seeds from 21 different plants, eight of which are weeds, four are alien species not-native to Spain, and only two of which have a fleshy fruit. Half of all seeds were from the toad rush Juncus bufonius, which is usually assumed to be incapable of dispersing more than 100 m, whereas these birds can move them over distances exceeding 100 km at a time.

Pellet regurgitated by a lesser-black backed-gull that had been feeding on crayfish in Doñana ricefields. Photo credit: Andy J. Green.

Remarkably, we found no differences in the relative abundance of different plants among those spread by storks or gulls, reflecting a similar feeding strategy. Both birds concentrate on alien crayfish exposed as the rice is harvested, and which have seeds stuck on their shell. Roughly 100,000 intact seeds are dispersed each day within and beyond ricefields by these birds, and their tremendous mobility makes them ideal for spreading weeds between different fields, and into new habitat types. The overall lesson is that the dispersal mechanisms of plants cannot reliably be predicted by inspecting the morphology of their seeds, and that we must pay more attention to birds and their ability to spread a broad variety of plants around. Much more research is needed on what plants are carried inside waterbirds. Too often, it has been assumed that waterbirds only carry seeds on their feet or feathers. Only by taking full account of waterbirds can we understand how quickly new weeds or alien species can spread, and where to, or how native plants can move to compensate for climate change.

White storks (in the foreground), gulls and other birds feeding in Doñana ricefields as a tractor tills the straw. Photo credit: Andy J. Green.

This is a plain language summary for the paper of Martín-Vélez et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12967). This post was prepared by Andy J. Green.