Plants found to travel by foot

21 October 2008 by Tom Marshall

Seeds stuck to people's feet may play an important role in helping plants spread into new areas, according to new research.

Seeds stuck to sandal

Mammals, birds, ants and many other animals are known to spread seeds. Scientists have long suspected human shoes could do the same, but nobody had measured the effect before.

Researchers at the Centre for Ecology & Hydrology (CEH) dipped their shoes in mud and then seeds, and walked for five kilometres, checking how many remained after given distances. Their results appear in Proceedings of the Royal Society.

Most seeds had already fallen off within five metres, but around 2 per cent still clung to the shoes at the end of the walk. This suggests that what the researchers call 'human-mediated dispersal' could be an important factor in helping plants colonise new areas.

The scientists have created a mathematical model that closely matched their results, relying on just two inputs - how many seeds were on the shoe at the start, and how quickly they fell off.

While such long-distance dispersal by walking might be rare, it might have a profound effect on the spread of certain species, particularly invasive ones.

Compared to other methods of dispersing seeds, such as relying on the wind, dispersal on people's feet can take seeds very long distances in a short period.

"We've never looked at quantifying how many seeds get transported over what distances," says Dr Matthias Wichmann, a population ecologist at CEH. "While such long-distance dispersal by walking might be rare, it might have a profound effect on the spread of certain species, particularly invasive ones that are new to a region."

He adds that their results have profound implications for the speed at which plants may colonise new areas. Wind dispersal may result in spread rates of only a few hundred metres per year, whereas shoe dispersal may allow the species to cover several kilometres of new habitat per year with the effect building up over several years.

Tracking alien invaders

Earlier work by Wichmann and colleagues, led by Marc Niggemann, a PhD student visiting CEH, had already suggested the potential importance of humans in spreading plant seeds. The team looked at surveys of the distribution of plants within a 50 kilometre by 50 kilometre plot in 1970 and 2003.

Collecting seeds from shoes

Marc Niggemann gets his shoes checked for seeds

The team collaborated with social scientists, who interviewed residents in settlements in the plot about how often they travelled between particular points. This produced a matrix of the connections between different places.

Taking the earlier survey as their starting-point, the team then used mathematical models to estimate how plant distribution would have changed between 1970 and 2003 under two different scenarios - one in which a plant's chance of spreading to a new location was mostly determined by distance alone, and another in which humans carried seeds, with the likelihood of a plant spreading from one place to another being determined by how often people made the same journey.

They then compared these scenarios with the changes that had actually taken place by 2003. The results were mixed. Some plants seemed to spread in a way closer to the first scenario; others, including the invasive Himalayan Balsam, seemed to appear in new locations in a way consistent with being carried by people.

Motor transport

The team are now following up with work on the possibility that cars help disperse seeds. In this case the seeds aren't thought to be stuck to the cars themselves - the hypothesis is that the seeds are picked up off the road surface and blown along for a few metres by the gust of wind caused by a passing car.

Each car may not take a seed far, but on a busy highway tens of thousands of cars can pass in a day. Wichmann thinks this could end up moving seeds comparatively quickly over long distances.

Seeds on the road

Dr Moritz von der Lippe and Tatjana Knopp placing coloured seeds on CEH's study road.

"With the increasing number of cars on the road, there is very high potential for seeds to be dispersed in this way," he says. Seeds could travel to new areas along highways and then spread into the countryside on peoples' feet.

The team is experimenting on a section of disused road, covering the road surface in colour-coded seeds of different species, driving a car over them and measuring how far they move. The researchers are also looking at other factors, such as the effects of seeds starting on the road surface itself compared to on the verge next to it.

The work could shed light on how invasive species spread - and not just invasive plants. Wichmann cites the example of the leaf miner moth, whose larvae harm horse chestnut trees by feeding on their leaves, causing them to turn brown prematurely in midsummer.

The first case in Britain was found in Wimbledon in 2002 - a common destination for foreign visitors to Britain because of its tennis tournament - and since then the moth has rapidly spread across the country, often appearing suddenly in the middle of hitherto untouched countryside.

This year it has turned up in west Wales. Adult leaf miners can fly, but not far or fast enough to easily explain their lightning spread across Britain.

Wichmann thinks people may be inadvertently spreading the insects or their larvae, perhaps when pieces of horse chestnut leaf containing leaf miner larvae get stuck to cars.

"In the long term I'd like to work more closely with scientists interested in invasive species," says Wichmann. "It isn't well understood how these species spread so quickly, and I don't think the possibility of human-mediated dispersal has been given enough attention."