“There is symbolic as well as actual beauty in the migration of the birds, the ebb and flow of the tides, the folded bud ready for the spring,” wrote American conservationist Rachel Carson. “There is something infinitely healing in the repeated refrains of nature.”

For many species, however, the repeated refrains of migration are more than just symbolic or healing. The journeys, often long and arduous, are undertaken for sheer survival. The assurance of better food or water supply, escape from inhospitable conditions, or the promise of mates and safe breeding grounds, stirs wildlife to undertake seasonal migrations across the globe.

As species migrate, they run the gauntlet of increased predatory pressures, unfavourable or extreme weather conditions, some of which are linked to climate change, diminishing food supplies, habitat loss and anthropogenic development that poses barriers to well-established routes.

In recent years, plastic pollution has been added to this list of threats, as it is not just a burgeoning global problem, but one that disproportionately affects migratory species.

When species migrate across the globe, their chances of encountering and interacting with plastic increase and they are, therefore, more vulnerable. As migrations are undertaken across continents, via land journeys, riverine routes, oceanic currents, or flyways, the effects of plastic pollution are far-reaching. Migratory species also play an unwitting role in transporting plastic across trophic levels and ecosystems – definitely a cause for global concern and action.

These concerns were highlighted in a recent report by the Convention on Migratory Species and the United Nations Environment Programme. Published in August, as part of the CounterMEASURE II project, the report documents the threats faced by terrestrial, freshwater and avian migratory species in habitats across Asia and the Pacific.

Effect on biodiversity

Based on the size, plastic waste is divided into two types: macroplastics and microplastics. Macroplastics are large-sized materials that cannot be easily reused or recycled and are discarded or accidentally lost from waste management cycles. Microplastics are 1 µm-5 mm sized particles designed for cosmetic, domestic or industrial use, or formed when larger plastics break down.

Macroplastics often entangle organisms making it difficult for animals to feed and in some cases, lead to smothering, or injuries such as the loss of a vital limb. Discarded fishing gear or ghost nets in rivers and oceans may also ensnare aquatic animals causing them to drown.

Ingestion of plastics is another serious issue, resulting in gut blockages and eventual starvation, in both terrestrial or aquatic species. When plastics are ingested, chemicals leaking from them accumulate inside the tissues resulting in a toxic build-up, a process known as bioaccumulation.

If the contamination travels up the food chain, these toxins get concentrated at successively higher trophic levels in a process known as biomagnification. In aquatic systems, both riverine and marine, plastics form durable islands that organisms colonise. These islands facilitate the hitchhiking of non-native, invasive or pathogenic species to new regions, where they pose a serious threat to native biodiversity.

Microplastics, on the other hand, pose a different set of problems. This past decade, research into the impact of microplastics dispersed by human activities, weather phenomena, ocean currents and planetary winds has yielded disturbing results. Microplastics have been found in the most remote of areas like the Mariana Trench, the deepest point in the world’s ocean, within Arctic ice, in the freshwater channels at Antarctica and even on desolate mountainous ridges.

For organisms, the implications are more severe – microplastic ingestion can lead to particle toxicity, gut blockages, internal abrasions, tissue build-up or bioaccumulation and inflammation affecting the fitness of an individual. At a population level, the effects may cause massive die-offs, however, no long-term studies of their impact on overall fertility rates have been undertaken yet.

Both macroplastics and microplastics are relatively new contaminants, yet as a worldwide issue, their impact on migratory species is undeniable.

Migration hub

Every year, India receives 457 migratory species: 380 birds, 46 mammals, many of which are bats, 24 fish and seven reptiles. Their journeys represent the country’s tenuous, seasonal connection with almost all parts of the globe. Most migrants, both flighted and non-flighted, arrive in India via Central Asia over the Himalayas.

In the Himalayan stretches of Nepal, Bhutan and Sikkim, certain communities collect yarshagomba – a ghost moth caterpillar infected by the parasitic Cordyceps fungus, also known as Himalayan gold or Himalayan viagra. The yarshagomba is considered an aphrodisiac and hallucinogenic drug and commands a price equivalent to its weight in gold.

The local communities who seasonally gather these caterpillars from their high-altitude habitats, often leave waste such as tarpaulin wrappers and plastic sheets behind. The plastic footprint of tourism is also writ large in these remote parts. These communities share their habitat with the elusive snow leopard, among other biodiversity.

Until recently, the migratory route of snow leopards (Uncia uncia) was not well established. Radio-collar studies determined that snow leopards are long-distance nomads and stake a territory across 12 countries in Central and Southeast Asia.

They prey upon species like marmot, pika, hare, Himalayan ibex, blue sheep, as well as migratory species like Argali sheep (Ovis ammon), urial (Ovis vignei) and the wild yak (Bos grunniens). Snow leopards have been photographed near garbage dumps, or in the wild, encountering plastics and some studies have found plastic in their scat.

Their prey inhabits the same polluted territory. Grazing animals are indiscriminate feeders and could ingest plastic that makes its way into the Himalayan and trans-Himalayan food chains, and can have serious impacts on prey and predator alike. Yet, as per the Convention on the Conservation of Migratory Species of Wild Animals-United Nations Environment Programme report, the impact of plastic pollution on migratory terrestrial mammals is far lesser than that on avian and aquatic species.

Bar-tailed godwit (Limosa lapponica) migrate over the Pacific to winter, among other destinations, along India’s western and eastern coasts. Photo credit: P Subraty via Pixabay

The bar-tailed godwit (Limosa lapponica) charts one of the longest, most arduous journeys. It breeds in Alaska and flies across the Pacific to winter along the coasts of Africa, the Persian Gulf, Southeast Asia, Australia and New Zealand. Through winter, wetlands along the west and east coasts of India host bar-tailed godwits in huge numbers.

Along the water’s edge or in the shallows, these birds feed on a wide range of prey, like earthworms, polychaete worms, insects and insect larvae, spiders, molluscs, crustaceans, tadpoles and small fish. For the bar-tailed godwit and other wading birds, the threat of plastic pollution in wetlands includes entanglement, ingestion, and bioaccumulation.

“Earthworms are vital prey for wetland wintering birds,” said Reuben Gergan, the Biodiversity and Plastic Pollution Consultant with the UN Environment Programme. “There is some research on microplastic pollution in soil and its impact on earthworms.”

“Yet more research on the toxic build-up from soils into earthworms and other prey is needed if we are to understand the role of wetlands as sanctuaries for migratory species,” he adds.

Beyond earthworms, fish is another link through which toxins accumulate in the food web. In September 2021, a synthesis of published literature on microplastics found that 49% of all fish sampled globally, had ingested plastic, with an average of 3.5 pieces per fish. Such contamination can have widespread consequences across marine, riverine and estuarine habitats.

Fish form an important linkage between different food chains, terrestrial, aquatic and avian, and plastic pollution along waterways, can have an impact on entire ecosystems. Photo credit: Joel Saucedo via Pixabay

Fish are the preferred prey for many migratory species of birds, reptiles like turtles and crocodiles, and cetaceans – including whales, dolphins, and porpoises. India has over 18 species of cetaceans, eight species of turtles and two species of crocodiles, that migrate in both marine and riverine habitats.

“While many global studies have been undertaken to study the impact of plastics in freshwater and marine fish, few studies have been published from India”, said VS Basheer, Principal Scientist at ICAR-National Bureau of Fish Genetic Resources.

The Convention on Migratory Species report showed that aquatic creatures are most prone to ingestion – and India’s visitors are no exception. Across India, reports of whales, dolphins, porpoises, among others, washing up dead on shores abound. Autopsies reveal that their stomachs were full of plastic, and point to starvation as a cause of death.

Recent studies have largely focused on plastic pollution in marine systems, yet there is a need for more research on its impact on riverine species. This was highlighted at the 13th Conference of the Parties to the Convention on the Conservation of Migratory Species of Wild Animals.

River ecosystems are affected when lesser sediment travels from upstream channels and disrupts the formation of sandbanks for breeding, disturbances caused by large-scale dams, indiscriminate fishing activities, illegal sand mining, and the dumping of industrial effluents. Plastic waste is a recent addition to this list and migratory riverine species pay the price.

The critically endangered gharial is a keystone species of riverine ecosystems, both in India and Nepal. Photo by Joachim Schnürle via Pixabay

For instance, the critically endangered gharial (Gavialis gangeticus), was once common along the Ganges, Indus and Mahanadi basins. Today, it only inhabits a few rivers in India and Nepal and is nearly extinct in Bangladesh.

As a freshwater species, the gharial is intrinsically linked to the health and vitality of our rivers and it is telling that the relatively unpolluted Chambal river, is among the last of its breeding grounds. Gharials are also very likely to get entangled in ghost nets – an often fatal encounter, as their snouts are rather sensitive. Exposure to toxic chemicals leached from plastics that enter the food chain via contaminated fish is another problem shared by creatures who undertake riverine migrations each year.

Effect on humans

These stories do not just represent the seasonal journeys of wildlife, but also the routes along which toxic plastic migrates across geographies. Migratory species carry plastic within their systems back to their native habitats, which adversely affects the local flora and fauna.

In 2014, turtles were recorded ingesting plastic as they migrate across open oceans, only to succumb to death closer to their breeding grounds. In 2015, a study of 50 years’ worth of literature, compared litter distribution and the range covered by 186 species of seabirds to assess the risk of plastic migration.

The study found that seabirds carried plastic back from global hotspots to areas with low anthropogenic pressures and debris concentrations like the Southern Ocean boundary in the Tasman Sea between Australia and New Zealand. Imagine the risk posed by thousands of migratory species, the geographies covered, and the distances plastics travel.

These repeated refrains of nature bear one last grim truth, all this migration of plastic has one final destination. Humans.

Humans are the top consumers in any food chain and inhabit almost every ecosystem in the world. The effects of bioaccumulation and biomagnification of plastic and its toxins will be felt most acutely by our species. What goes around, comes around. The Food and Agriculture Organization estimates that over 3.3 billion of the world’s population relies on fish for their protein intake and India is among the 3rd largest capture producers, which means it contributes significantly to global fisheries harvests (as of 2018). Thereby contamination in India’s water channels, and fish stocks will have implications across the globe.

How far can such plastic migrate? It is difficult to say, but in December 2020, a study found microplastics in the placenta of live pregnancies, and a more recent study in September 2021, demonstrated that the first faeces of newborn infants may contain up to ten times more polyester than in an adult’s sample.

Research is constantly unravelling the consequences of plastic pollution, more so microplastics, on human health. Apathy is not an option, nor is inaction. “Perhaps understanding these crucial linkages of plastic pollution in the food web, and their cascading effects on all organisms, including humans, would be the only way to influence behavioural change,” laments UNEP’s Reuben Gergan.

This article first appeared on Mongabay.