It has long been assumed that seed dispersal on the coats of animals has been an option for a very limited number of plants, but new research has turned that idea on its head. Transport of seeds, by sheep at least, is more commonplace than most botanists ever imagined. Not only do their woolly coats carry seeds with hooks, but they also give a ride to a considerable range of other seeds. What's more, they transport insects, grasshoppers especially, to places which they would not otherwise reach.
Sheep flocks, it seems, may play much more of a role in maintaining populations of wild plants and small animals than ecologists previously thought, especially in some of our richest habitats for flowering plants and insects, lowland grasslands.
The research, just published in the British Ecological Society's Journal of Applied Ecology and carried out by Sabine Fischer, Peter Poschlod and Burkhard Beinlick of Philipps - University Marburg in Germany, involved examinations of a sheep's fleece to see how many seeds - and animals - adhered to it. The sheep were being used to graze limestone grassland pastures rich in plant species in the Schwabische Alb mountains of southwest Ger-many. One of them was specially tamed to stand still while the researchers groomed it for seeds.
The number of seeds they found was astonishing. Sixteen searches, each covering about half of the fleece (because it proved difficult to search every part of the animal), produced 8,511 seeds from 85 plant species. Another 47 seeds proved impossible to identify.
While hooked seeds - of Sweet Vernal grass for instance - and bristly seeds - of the pink flowering Large Thyme for example - were, by far and away, in the majority (95 per cent), the fleece also held on to a wide array of plant species whose seeds had no clinging apparatus.
Smooth seeds of 15 plant species, such as the vivid, yellow-flowering Common Rockrose and the delicate white Fairy Flax (though making up barely 2 per cent by number of all the seeds recovered) had somehow become entangled in this woolly coat. Another 17 plant species with slightly coarse seed coatings were also found. They included plants such as Lady's Bedstraw and the purple-flowered Salad Burnet.
What's more, many of these seeds, whether hooked, bristled, or smooth, can stay attached for a long time, increasing the chances of them being transported over long distances as the sheep move about or are shepherded to other pastures. Experiments with marked seeds showed that perhaps half of them fall off within a few days but 17 per cent of the bristly seeds of a grass, Upright Brome, and 22 per cent of smooth Common Rockrose seeds, cling doggedly after two weeks on the fleece. After five weeks, the percentage of Rockrose seeds still hanging on had fallen only to 17 per cent. Even seven months later, by which time the sheep flock had covered a distance of over a 100 kilometres in their annual move between distant pastures, a few of the seeds held on. Such travel is far greater than many wind- borne seeds ever experience.
Some plants' seeds were notable for their abundance in the wool. Between them, the bristly seeds of Cocksfoot Grass and Upright Brome, together with the hooked seeds of the Agrimony flowers, accounted for 63 per cent of all the seeds discovered. Nearly 90 per cent of the seeds that hitched a ride were from plants over 60cm (2ft) in height while low-growing flowers often didn't make it.
By using a dummy "sheep" made of foam, covered with a sheepskin and fleece, and moved about, Monty Python-like, on the leg of one of the researchers, they found out how sheep movements and behaviour determined what seeds were picked up. For instance, low-growing plants were only found in the fleece if the (dummy) sheep had been wallowing on the ground. Walking around the pasture, without wallowing, the dummy collected fewer seeds of a much more restricted range of species.
But the seeds didn't have the fleeces all to themselves. Although some snails, spiders and one or two other creatures - including a lizard - were recorded occasionally hitching a ride, no less then 13 species of grasshopper were recorded on the sheep. They stayed put for between one minute and an hour, the average working out at 14 minutes, long enough for a sheep to move over 500m. Left to their own jumping, grasshoppers would be unlikely to disperse further than perhaps a fifth of this distance in their short life.
"Grasshoppers don't jump on to sheep in order to be transported," says Sabine Fischer, ruining any ideas of insects having a bit of fun. "Instead, they probably get on by chance while avoiding the trampling of the grazing flock. But, once on, they stay there to take advantage of the better sunning position."
What is the ecological consequence of all these free sheep rides, both for a wider range of plant seeds than anyone previously imagined, and for animals like grasshoppers?
Without this grazing or cutting, we would have virtually no grassland in the lowlands. "In western Europe," says Dr John Hopkins, a grasslands expert at the UK's Joint Nature Conservation Committee, "Truly natural grasslands which won't scrub over and slowly change to woodland in the absence of management, are confined to exposed coasts, mountains, or toxic soils where harsh environments prevent tree and shrub growth."
Getting the amount of grazing right on flower-rich grasslands is always a tricky business. Some species, like the corncrake, the Marbled White butterfly, and hay meadow plants such as the Autumn Crocus, need longer vegetation.
The gorgeous Adonis Blue butterfly and some other plants like Wild Thyme and the rare Early Spider Orchid need short grassland. If a mix of such species lives in one grassland, different intensities of grazing, or combinations of cutting and grazing, have to be part of its complex management.
But sheep are not just surrogate mowing machines. This research shows that they are responsible for transporting plants from place to place. Provided, of course, the seeds fall off in a suitable spot for them to germinate. Browsing and trampling, by any large animal, helps provide such places. So, too, does their manure; indirectly because fleece-carried seeds can drop into it and germinate; and, directly, because some seeds are swallowed as the animals graze and pass out again at the rear end. Sabine Fischer and her co-researchers also believe that sheep carrying a rich seed bank from a flower-rich grassland could help diversify the vegetation of poorer grasslands if they were regularly moved between both.
Conventional genetic wisdom has it that isolated populations of plants become increasingly restricted in the diversity of their genes, leaving them less able to respond to environmental change. "But", says Dr David Stevens, an expert on lowland grassland plants and their genetics at the Countryside Council for Wales, "Studies that have looked at plant population size and their genetic variation have been inconclusive. Most larger populations have more variation than small ones but it isn't always the case. We commissioned reports on grassland plants including one on Wood Bitter Vetch where we found that two small populations of less than 25 plants each had quite high levels of genetic diversity. But a large population of several hundred plants was genetically depleted."
Dr Stevens argues that some of these species might not have been isolated long enough for us to see the effect or that they exist across a range of environmental conditions in any one site, favouring diversity. "Mixing them with the same species from elsewhere shouldn't always be considered a benefit," he cautions. "It could be that some isolated species survive because they have become so well acclimatised to their particular conditions".
To what degree we can thank sheep for helping maintain what few flower- rich grasslands remain after centuries of use by man, we might never know. In the future they may help conservationists return agriculturally damaged pastures and downland to the cornucopia of flowers, grasses and insects many of them once were. !Reuse content