Oh, crap! Worrying evidence of weed dispersal in horse dung

Horse dung has been shown to be a worrying cause of weed dispersal.
Horse dung has been shown to be a worrying cause of weed dispersal.

An Australian researcher has uncovered worrying evidence about the dispersal of weed seeds in horse dung.

Associate Professor Catherine Pickering, of Griffith University in Queensland, carried out a global review of weeds that can germinate from horse dung.

Pickering, with fellow researcher Michael Ansong, reviewed 15 studies on seed germination from horse dung – six from Europe, four from North America, three from Australia and one study each from Africa and Central America.

Seed from 249 species from 43 families were identified as germinating from horse dung.

“The diversity of species with seed that can germinate from horse dung highlights the potential of horses to disperse a range of seed over long distances,” the pair wrote in their findings, published in the journal, Ecological Management & Restoration.

“Whether such dispersal is beneficial or harmful depends on the plant and the context in which it germinates,” they wrote. “To maintain the conservation value of protected areas, it is important to understand and manage the different potential weed dispersal vectors, including horses.”

“Of the 2739 non-native plants that are naturalised in Australia,” Pickering added, “156 have been shown to germinate in horse dung.

“What is very concerning is this includes 16 of the 429 listed noxious weeds in Australia and two weeds of national significance,” she said.

The study found a similar threat was emerging overseas, with seeds from 105 of the 1596 invasive/noxious plant species in North America also germinating in horse dung.

“Not only are the seeds dispersed through dung but the manure provides the means by which the introduced plant takes hold,” Pickering said.

“Habitat disturbance from trampling has been demonstrated to further facilitate the germination of seedlings from dung in both natural and experimental studies.”

The study highlighted the range of plant species with the potential to be dispersed over long distances, but the extent to which this dispersal is harmful depended on the individual plant species, Pickering said.

Some plants germinated from dung and went on to reach maturity and flower, while others germinated but did not survive due to unfavourable growing conditions in the field.

Pickering said there were other factors to consider. “Additional threats come in the form of trampled soils and vegetation, nutrient addition via dung and urine, and changing hydrology via damage to riverine systems.

“Legislators everywhere should take these into consideration before opening parks to this recreational activity,” she said.

The study, entitled “A global review of weeds that can germinate from horse dung”, found that almost two-thirds of the species that germinated in horse dung were forbs (herbaceous flowering plants) and 33 percent were graminoids (grasses, sedges and rushes), with over half being perennials and 32 percent annuals.

Nearly every species – 99 percent of those reviewed – was considered a weed somewhere, with 47 percent recorded as invasive and 19 percent international environmental weeds.

Ansong, M. and Pickering, C. (2013), A global review of weeds that can germinate from horse dung. Ecological Management & Restoration, 14: 216–223. doi: 10.1111/emr.12057

The abstract can be read here.




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