Science Interviews


Sun, 27th Nov 2011

Alien Hikers - Planet Earth Online

Professor James Bullock, Wallingford’s Centre for Ecology and Hydrology

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Dave - Alien, or non-native species, can have serious effects on the landscape and indigenous wildlife.  Recent research in Australia has found that hikers could be partly to blame…During just one season in a national park there, hikers were found to carry up to two million plant seeds just on their socks!

James Bullock, from Wallingford’s Centre for Ecology and Hydrology, was one of the authors of the study, which was based on some of his earlier work in Dorset, Planet Earth podcast presenter Sue Nelson joined him at a picturesque section of the River Thames in Oxfordshire to see what they could find….

James -   Riverbanks are actually one of the most invaded habitats we have in Britain, so the sort of species you might see along here are Himalayan Balsam or Japanese Knot Weed, all of which cause problems with riverbank stability affecting native biodiversity.  What you will see also this time of year is the chestnut leaf miner, a lot of chestnut trees their leaves will be removed by mining of this moth caterpillar.  Actually, we can see over here, just next to us, are Canada geese which have been established in this country for a long time and cause a lot of problems, not just affecting native bird species but by pooing in waterways cause a fertilisation effect which can effect what's growing in the water as well.japanese knot weed

Sue -   So the effects then of invasive species are quite wide and varied, it's not just a threat to biodiversity in some cases?

James -   No, they have quite a wide range of effects.  They can effect biodiversity directly, but they can also affect aspects of the natural world which are of more direct impact on humans, such as bank erosion, pollution of waterways, causing problems with grazing lands, so you have invasive species spreading across grazing areas which affects the ability of animals to graze on them.

Sue -   Now the Australian study found that different parts of a hiker’s clothing could spread different numbers and types of seeds.

James -   There are some seeds which have hairs or bristles on them which are probably evolved to allow their dispersal by animals; we come along with our socks or our trousers, almost like an animal skin and it forms a nice material for these seeds to attach to.  So, the more woolly the clothing you're wearing and the more bristly the seed the more seeds will get dispersed by people.

Sue -   How did this come out of research from Dorset?

James -   We were interested in quite a different aspect of dispersal in Dorset.  We were working on a species called Wild Cabbage there, which is quite a rare species restricted to our coastline and it forms quite discreet populations along the coastline which have been sitting there for probably hundreds of years.  So, we were interested in the reverse side of the question - what limits this species to grow where it grows and what are the possibilities for its dispersal? 

Along the coastline of Britain we have loads and loads of footpaths and one possibility we thought of was that hikers could take the seeds of the cabbage around.  So we did an experiment where we found that seeds rather than socks or trousers would stick into the mud on hikers boots and could be transported very long distances, the natural dispersal by wind could take seeds a maximum of about 200 metres, but hikers could take the seeds over five kilometres.

Sue -   Is that why you want to know?  Is that why you do this sort of research in order to predict possibly, or can you even predict when you've got millions of seeds capable of sticking to somebody's hiking socks what species are going to be transported where?

James -   Yes.  The prediction is the key here.  What we want to understand for alien species or non-native species is what species is transported, how far they're transported and what are the mechanisms of their transportation.  And the reason we do that is not simply understanding, but then we can use this sort of information in models of spread of these species to work out how we might limit that spread, so a lot of work on alien species so far has been saying, ok, we find out where the populations are growing and we go and try and kill them in some way. 

A much more efficient and effective method would be to prevent the movement of these species in the first place and so that's what we're working towards - understanding the whole process of spread so we can look at the crunch points and limit that spread by especially targeting dispersal.

Sue -   How would you advise hikers, how on earth are they going to limit the spread of seeds?

James -   Yes, that is a big question and it could sound a bit over the top telling people to be very careful about what they're transporting.  But certainly in the study in Australia, this is a highly protected area which is under threat from these European plant species coming in, so in that case, the recommendation has been to put signs up for walkers, to educate walkers, to say before you walk into these remote areas just clean your trousers and socks off, just pick the seeds off before you spread them into these pristine areas.  So, I think in very specific circumstances, where we're trying to protect particular areas, there is something that can be done.




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I'm sure ramblers take great care not to disturb the countryside when they set off on a hike. But I am not so sure they understand that they can be the instrument of spreading invasive species and even transferring plants which might only grow at one end of the country to the other.

Doubtless many a rambler has been the unwitting accomplice in these long distance moves and the spread of Japanese Knotweed etc.

The problem is, how do you prevent this without spoiling the ramblers enjoyment of our beautiful countryside. Ensuring that boots, clothing, rucksacks etc are thoroughly cleaned between ventures may help, but some seeds can be very resilient. Don_1, Tue, 29th Nov 2011

Once species jump continents or mountain ranges, isn't it a losing battle? Does making hikers wipe their feet really help? The deer and the raccoons are not going to cooperate. Does spraying down your boat prevent zebra mussel infestation of lake, when there are thousands of ducks landing and taking off. Maybe slowing down the spread gives an eco system time to adjust, but I doubt you can stop invasive species if their a niche to fill. cheryl j, Wed, 30th Nov 2011

Hikers often drive hundreds of miles to get to the prime hiking spot.  Sometimes they will travel from country to country, so it certainly is possible that they could bring unwanted pests with them. 

Usually my socks are clean going into the mountains.  Sometimes I wonder if the greater risk is to bring the invasive species back home before stuff gets into the wash.

It might be reasonable to design boots to minimize the risk of picking up seeds. 
Less aggressive soles (also minimizes tracking mud into the house)
Less fabric to pick up grass seeds.  Leather and plastics pick up fewer seeds
Attention to crevices in the upper portion of the boot.
CliffordK, Wed, 30th Nov 2011

What you say here is, of course, absolutely true, but migration of species by way of hitching a ride on another species, would have been taking place since species began to migrate. As you suggest, such migration would have been successful only where there was a niche to fill and nature would have had time to adjust accordingly.

But I would speculate that such migrations would probably have occurred many thousands of years ago and are today very unlikely. For example, it is likely that migrating Swallows from the UK to South Africa would probably have transferred viable UK species to S.Africa and S.African species to the UK many thousands of years ago. It is unlikely that they would now transfer any further species and extremely unlikely that they would change their destinations.

In contrast to these long defined natural migration habits, man moves in all directions at any time of the year, so has the potential to transport all manner of species all over the world. An early example of such being rat infested ships.

But we also transport species deliberately, such as the Romans bringing Rabbits to the British Isles and horticulturalists bringing Rhododendron and the accursed Japanese Knotweed. I'll grant you that these man-migrated species have the potential to now hitch a lift on the long established migrators and as such, we have probably put the cat among the pigeons well and truly. But this is no reason to just sit back and say 'Ah well, such is life'. Efforts need to be made to try to lessen the spread of these unwanted and often damaging invasive species. Don_1, Thu, 1st Dec 2011

There are, of course, invasive animal species, including invasive migratory species.

For example, Canadian Geese are considered an invasive species in Europe, so they would not have long traditional migratory patterns.  And, even in North America, their migration patterns are also slowly altering with climate changes (keeping in mind that the climate has been changing since the beginning of the Holocene).

In the USA, there may be a sharp distinction between Rocky Mountain species and Appalachian Mountain species due to the separation of the Great Plains.  However, if there is ever a jump between the ranges, or between continents, then local wildlife could spread the invasive species all over.

Anyway, animals can spread some flora...
But, humans are much better at doing it!!!

And some animals such as rats and mice have adapted to being human hitchhikers!!! CliffordK, Thu, 1st Dec 2011

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