Shorebirds are important plant vectors via endozoochory

Figure 1. A flock of black-tailed godwit in one of our study sites in England, shortly after returning from breeding grounds in Iceland. Photo by A. J. Green​.

By Andy J. Green and Ádám Lovas-Kiss

The potential importance of shorebirds (in Europe, often known as “waders”) as vectors of dispersal for plants and other organisms was recognized by Darwin and Wallace because of the amazing migrations they make and their frequent visits to oceanic islands. They believed shorebirds are influential in plant biogeography, but their focus was on rare events when propagules become attached to feathers, or to muddy feet (i.e. “epizoochory”). It is well known that frugivorous birds disperse seeds by gut passage (“endozoochory”) after ingesting them inside fleshy fruits, but other seeds are widely assumed to be destroyed and digested if swallowed by birds.

In our paper, we conduct the most extensive empirical study of endozoochory by shorebirds to date, covering six bird species along the Atlantic migratory flyway, with study areas in Spain, the UK, Ireland and Iceland. We even sampled flocks of Black-tailed Godwits immediately before or after migratory flights between the UK and Iceland. We recovered 164 intact seeds of flowering plants from 12 different families, plus the spores of stonewort algae. Crucially, only four of the 23 plant species had a fleshy fruit, the other 19 having a seed morphology which plant ecologists associate with other dispersal mechanisms. Most of the plant species have been assigned to wind, water or self-dispersal syndromes, these being mechanisms with a much lower maximum dispersal distance than that offered by migratory shorebirds. Endozoochory was not predictable for these species based on the current view of how to relate seed morphology to dispersal mechanisms, yet for six of them we germinated seeds in the laboratory to confirm viability. Although some of these seeds may occasionally be dispersed on the outside of birds (“epizoochory”), the rates of endozoochory events greatly exceed any epizoochory.

Figure 2. Seeds of a) alien buttonweed b) widgeon grass c) broadleaf plantain and d) creeping buttercup extracted from shorebird droppings.

All the plant species recovered have the extensive geographical distributions to be expected if dispersal by shorebirds is an important determinant of plant biogeography. Twenty of the 23 species were terrestrial plants, also contradicting the commonly expressed view that waterbirds are only important vectors for aquatic plants.

Many of the shorebirds we studied are declining quickly, yet the seed dispersal service they provide may now be crucial in allowing plants to adjust their distributions under climate change. Shorebirds may also play an overlooked role in the spread of some alien plant species, including the buttonweed Cotula coronopifolia. Our findings suggest that, in order to understand long-distance dispersal mechanisms, it is not enough to inspect seed morphology and to work in silico. It is vital to undertake further empirical work, including field studies of what plants shorebirds carry.

Figure 3. Dispersal syndromes previously assigned to the angiosperm seeds dispersed by shorebird gut-passage (endozoochory)​.

Original publication: Lovas-Kiss, Á., Sanchez, M.I., Wilkinson, D.M., Coughlan, N., Alves, J., Green, A.J. 2018. Shorebirds as important vectors for plant dispersal in Europe. Ecography doi:10.1111/ecog.04065

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