Study raises troubling questions over plastic ingestion by animals

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Anecdotally, horse owners and farmers seem to be finding increasing quantities of waste materials being blown into, or dropped, in their pastures.
Photo by Ivan Radic

Researchers have painted a worrying picture of plastic ingestion by small mammals in England and Wales, with traces found in the droppings of more than half the species examined.

The findings by researchers from the University of Sussex, the Mammal Society and the University of Exeter highlight the ubiquitous nature of plastic waste, and raise wider questions about the risk it poses to horses and farm livestock.

Anecdotally, horse owners and farmers seem to be finding increasing quantities of waste materials being blown into, or dropped, in their pastures.

The scientists, writing in the journal, Science of the Total Environment, reported that the densities of plastic excreted in the small mammals at the centre of their study were comparable with those reported in human studies.

Fiona Mathews, professor of environmental biology at the University of Sussex, said much is known about the impact of plastic on aquatic ecosystems, but little is known about the impact on land-based systems.

“By analysing the droppings of some of our most widespread small mammals, we’ve been able to provide a glimpse of the potential impact plastic is having on our wildlife – and the most commonly found plastics leaking into our environment,” she says.

They identified plastic polymers in four out of the seven species for which they had faecal samples. The European hedgehog, wood mouse, field vole and brown rat were all found to be plastic positive.

While expecting to see higher plastic concentrations in samples from urban locations and less plastic in herbivorous species, researchers actually discovered that ingestion of plastics was occurring across locations, as well as across differing dietary habits – from herbivores, insectivores and omnivores.

University of Sussex Master of Science graduate Emily Thrift, one of the authors of the study, says it is worrying that the traces of plastic were so widely distributed across locations and species of different dietary habits.

“This suggests that plastics could be seeping into all areas of our environment in different ways.

“We’re also concerned that the European hedgehog and field vole are both species suffering declines in numbers in the United Kingdom.”

Using equipment at the Greenpeace labs at the University of Exeter, the team analysed 261 faecal samples, with 16.5% containing plastic.

The most common types identified were polyester, polyethylene (widely used in single-use packaging), and polynorbornene (used mainly in the rubber industry).

Polyester accounted for 27% of the fragments identified, and was found in all the plastic positive species, except the wood mouse. Widely used in textiles and the fashion industry, the journal paper explains that microfibres can enter the wastewater system through household washing and subsequently end up on the land through the use of sewage sludge as fertiliser.

Over a quarter of the plastics found in the study were also “biodegradable”, or bioplastics. The authors warn that while these types of plastics may degrade faster than polymers, they can still be ingested by small mammals and further research is needed to investigate their true biological impacts.

The authors believe that the microplastics found in the study are likely to have entered species’ guts as a result of the consumption of contaminated prey or through direct ingestion. With ingestion, researchers believe species could be mistaking plastics for food or chewing macroplastics used as nesting material or to escape entanglement.

The potential impact of plastics on the food chain is another issue raised as a concern by the authors, who urge further study on the matter.

Professor Mathews said a deeper understanding of the implications of plastic ingestion on land mammals is required, as well as the potential impacts on their conservation status.

“In our study, droppings from European hedgehogs carried the highest quantity of plastic polymers. As a species, they are already in decline in the UK for reasons that are largely unknown, and they are classified as Vulnerable to Extinction on the IUCN-compliant regional Red List.

“European hedgehogs consume earthworms and previous studies have found these to contain microplastics. So we really need further research to establish the scale and route of exposure more precisely, and to assess prevalence in predatory species that consume small mammals, so that we can take adequate steps to try to protect our declining wildlife from plastics.”

Dr Adam Porter, a post-doctoral research fellow at the University of Exeter who also worked on the study, said: “In the UK, plastic pollution can often seem like a problem somewhere else when most images are of polluted shorelines of tropical landscapes, or charismatic organisms like turtles or sea lions.

“This study brings the focus home, into our lands and in some of our much-beloved mammal species. Further, it demonstrates that the amount of plastic waste we produce is having an impact.

“We must change our relationship with plastic altogether; moving away from disposable items and moving towards replacing plastic for better alternatives and establishing truly circular economies.”

The chief executive of the Mammal Society, Andy Bool, said the organisation was proud to have helped and part-funded the research.

“It represents an important step into the study of the impact of plastics on terrestrial mammals. With a number of small mammal species experiencing worrying declines in numbers it highlights one of the challenges they face.

“We can all make a difference to help protect them from this threat by reducing the amount of single-use plastic we use and reusing and recycling what we do use properly.”

The full study team comprised Thrift, Mathews and Dr Frazer Coomber of the University of Sussex and the Mammal Society; with Porter and Professor Tamara Galloway, of the University of Exeter.

Ifan Jâms and his fellow researchers, in a paper published in Nature Communications in 2020, said the ingestion of plastics appears to be widespread throughout the animal kingdom, with risks to individuals, ecosystems and human health.

In their study, they found a clear relationship between body size and the size of plastic waste ingested.

Based on more than 2000 gut content analyses from animals ranging in length from 9 millimetres to 10 metres, they found that body length alone accounted for 42% of the variance in the length of plastic an animal may ingest.

Their findings pointed to a size ratio of roughly 20:1 between animal body length and the largest plastic the animal may ingest.

Thrift E, Porter A, Galloway T, Coomber F, Mathews F (2022) Ingestion of plastics by terrestrial small mammals. Science of the total environment, https://doi.org/10.1016/j.scitotenv.2022.156679

Jâms, I.B., Windsor, F.M., Poudevigne-Durance, T. et al. Estimating the size distribution of plastics ingested by animals. Nat Commun 11, 1594 (2020). https://doi.org/10.1038/s41467-020-15406-6

 

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