Gulls can spread weeds over large distances and between habitats

Authors, Víctor Martín-Vélez and Andy J. Green, discuss their recent study which highlights the importance of non‐frugivorous waterbirds as vectors for long‐distance plant dispersal: Spatial patterns of weed dispersal by wintering gulls within and beyond an agricultural landscape.

You can also read the Press Release for this article here.

Weeds are plants that spontaneously grow on land modified by humans, and typically have small, dry fruits associated with abiotic dispersal. Indeed, 80% of the world’s agricultural weeds have been assigned to “unassisted” syndromes, because they lack the characters used to define other dispersal syndromes associated with wind, frugivory, or attachment to fur. This seems contradictory, since weeds are renowned for their ability to spread, although this can largely be explained by the way humans spread weeds around.

Representation of the four model weed species with their seeds (Polypogon monspeliensis, Cyperus difformis, Amaranthus retroflexus and Juncus bufonius) used for the experimental seed dispersal modelling. Photograph: Víctor Martín-Vélez.

Our research group focuses on the role that waterbirds play in seed dispersal. It is now clear that they disperse a broad range of angiosperm species through endozoochory, the majority of which are traditionally assumed to depend entirely on abiotic dispersal. Our new study focuses on the role waterbirds have as dispersers of weeds, which has economic implications for agriculture. Many weeds are also alien species, and can endanger native flora in natural habitats.

Specifically, we focused on gulls, which have been increasing in inland anthropogenic habitats worldwide, and are a common sight in agricultural fields. Our study considers how gulls facilitate weed expansion between different fields, and also different habitats, including different crops as well as natural habitats. We centred our study in the largest area of ricefields in Spain, sandwiched between Seville and the Doñana World Heritage Site. Here, we could take advantage of high quality bird movement data, and extensive empirical data on what plant seeds occur in gull droppings, showing that >10,000 seeds can be dispersed each day by gulls using the ricefield area of 370 km2.

Close-up picture of the weed Cyperus difformis along the dykes from the rice tables. Photograph: Víctor Martín-Vélez.

Tens of thousands of gulls feed in these ricefields during the harvest period, especially Lesser Black-Backed Gulls Larus fuscus. Some of these gulls are fitted with GPS tags by colleagues working in breeding colonies in the UK, Belgium and the Netherlands, so we could use GPS data for 19 Larus fuscus individuals between 2010 and 2017. We also had data from a captive experiment to work out the retention time curve, i.e. how long seeds spend in the gut between ingestion and egestion. We had these data for four weeds regularly found in gull excreta: toad rush (Juncus bufonius), small-flowered nut sedge (Cyperus difformis), annual beard grass (Polypogon monspeliensis) and common amaranth (Amaranthus retroflexus) (see figure). The nut sedge is one of the world’s 40st worst weeds, and the amaranth is a highly invasive alien. The experimental data were also contrasted with a theoretical retention time curve based on the scaling relationship between body mass and retention time for birds.

Our spatial models based on the movement and retention time data suggested that 92% of seeds carried by gulls are dispersed within the ricefields, with 25% of these moved >1.8 km (and up to 40km). Owing to the high mobility of gulls between different places in Andalusia, our models indicate that the remaining 8% of seeds were dispersed into other habitats outside the ricefields, up to maximum distances exceeding 150 km. Of these seeds, 42% reached other moist environments likely to be suitable for establishment, such as other irrigated agriculture (e.g. cotton or wheat fields), and protected wetlands such as Fuente de Piedra lagoon in Malaga, or Doñana Natural Space. Until now, these four weed species were thought unable to disperse over distances exceeding 100 m, except when moved by humans or their machinery.

GIF showing gull movements within a single day in the ricefields. Andy J. Green.

These results suggest that waterbirds can be important, overlooked dispersal vectors for weeds between agricultural habitats and natural areas. A better understanding of this role for birds is vital when predicting the spread of weeds, and planning control measures. For example, herbicide-resistant genes will spread much faster within weed populations if seeds are spread between different fields, or even different crops, inside birds. This study also underlines how classical dispersal syndromes have limited value for predicting dispersal mechanisms.

Víctor Martín‐Vélez & Andy J. Green Department of Wetland Ecology, Estación Biológica de Doñana, Spain

You can read the full article by Martín-Vélez et al in Journal of Ecology: Spatial patterns of weed dispersal by wintering gulls within and beyond an agricultural landscape.

Here you can read the Press Release for this paper on the BES News site!

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