[Explainer] What is the effect of plastic on ecosystems?

The largest division on the geological time scale (GTS), which depicts distinct time periods in geological history, is called eons. Eons are further divided into eras, which are divided further into periods, epochs, and ages. The Anthropocene – a proposed, but still to be accepted term, represents the epoch when human activity began to leave an indelible, irrefutable mark on the planet.

Plastic is poised to be the most perceivable, persistent and pervasive indicator of the Anthropocene era. How has plastic become a hyperobject that is an indicator of industrial legacy and what is the butterfly effect of the presence of plastic?

Since when has human activity shaped the processes on Earth? 

The answer lies in rocks, seafloor sediments, and glaciers. For over a decade, scientists from around the world have been collecting core samples from rocks, sediments, and ice in search of signatures from human activity. Such signatures include concentrations of mercury or lead pollution, heavy metals, fly ash, nuclear debris from the atomic age, isotopes in the rocks, trees, atmosphere, or humans, and plastics, especially microplastics. This is what author Timothy Moton defines as ‘hyperobjects’ or a group of objects massively distributed in time and space, in his book Hyperobjects: Philosophy and Ecology after the End of the World. As none of these occur naturally, concentrations preserved in the geological record can be attributed to human industrial activity. This will help researchers, both in the present and the future, to identify the tell-tale, dramatic spike in the range of human activities, a period known as the ‘Great Acceleration’.

Of all the evidence, plastic is poised to be the most perceivable, persistent, and pervasive indicator of the Anthropocene. There is a lot of recent research on the impact of plastics in different ecosystems; terrestrial, freshwater including wetlands, lakes, and rivers, and marine systems from continental shelves to deep ocean trenches. Yet it is not widely understood how interlinked these ecosystems are, how plastic travels through them, its effect on biodiversity and how it will impact the entire biosphere.

What is the origin and journey of plastic?

Most plastic originates on land, but is often dumped into wetlands, lakes, and rivers contaminating the shores, banks and beds. These waterways serve as conduits for plastic to travel further out past the estuaries, into seas where it settles on continental shelves or is carried across the open ocean. Within each waterbody, different organisms interact with plastics in different ways; some inhabit these new, long-lasting surfaces; a few others eat it and break it down for energy; but most ingest it, get entangled in it or are poisoned by the toxins with dangerous, often fatal results. The impact of plastics is overwhelmingly negative, and humans are just beginning to realise the scale of the problem. Sometimes, the scales can be minuscule, as with microplastics, yet can have cascading effects on ecosystems.

What is the butterfly effect of microplastic in the ecosystem? 

In chaos theory, the butterfly effect describes how small changes can have non-linear impacts on complex systems – often compared to a butterfly flapping its wings and causing a typhoon. Microplastics have a similar result in ecosystems, as per a recent research paper titled Are microplastics destabilizing the global network of terrestrial and aquatic ecosystem services?.  “Most research focuses on the effect of microplastics on biodiversity and human health, yet there was a marked absence of documentation on how different ecosystems are affected,” explained Srinidhi Sridharan, a senior research fellow at CSIR-National Environmental Engineering Research Institute (NEERI), and first author of the paper. “Hence, via an extensive literature review, we tried to understand the impact of different microplastics on terrestrial and aquatic ecosystems, as well as how they affect the ecosystem services rendered by different keystone species or bioindicators,” she continued. Keystone species are those that play a crucial role in an ecosystem, and thereby ‘hold it together’. Bioindicators are organisms that are susceptible to small, adverse changes, and are used to study the environmental health.

In terrestrial ecosystems, earthworms, springtails, mites, and some land snails are important denizens that facilitate soil nutrient cycling via decomposition, burrowing, tilling, and aeration. Soil can be contaminated with microplastics through sources like composting, sewage waste, agricultural run-off, and mulching. Studies on microplastic contamination in soil bioindicators documents poor reproductive success, retarded nutrient absorption and growth, increased antibiotic resistance, behavioural changes, and neurotoxicity. This will in turn have consequences on agriculture productivity or what is known as the green economy.

Terrestrial mammals such as the polar bear — a keystone predator in the northern hemisphere and the Asiatic elephant –key to maintaining grasslands across the Indian subcontinent, also interact with microplastics and its chemical toxins, yet no physiological and ecological effects have yet been documented.

How does terrestrial contamination spill over into aquatic ecosystems?

Terrestrial ecosystems are intrinsically linked to aquatic systems, as water, sediments and organisms move from one to the other, as do the contaminants. Bioaccumulation – the toxic build-up of chemicals as they travel up the food chain, begins here, and can have far-reaching implications across the biosphere. Yet research on the terrestrial impact of plastics falls far short of those in aquatic habitats. More documentation is crucial to understanding how contamination can spill over into other ecosystems.

Bioindicators in freshwater and brackish water such as aquatic plants, various plankton and crustacean species, zebrafish and other fish, also interact with microplastics, much to their detriment. Deformed larvae, reduced mobility due to neurotoxicity, impaired immunity, and increased death rates, are among some of the effects studied. Microplastic ingestion has also been recorded in some swamp eels, pond loaches and freshwater crayfish – key species for nutrient cycling in aquatic systems, though the long-term impacts remain to be studied.

While freshwater organisms do not migrate to the sea, microplastics can travel via water currents, ever-shifting sediments along lake and river bottoms, airways, and even precipitation, to reach other aquatic or marine ecosystems.

Studies on marine Prochlorococcus bacteria exposed to plastic leachates showed poorer photosynthesis efficiency, and productivity. If other autotrophs are similarly affected, this could have staggering consequences across marine food webs. Microplastic ingestion by filter feeders such as larvaceans, oysters and mussels hinder their ability to filter water of organic and inorganic pollutants and causes plastics to be inadvertently transferred through marine ecosystems. Contaminated or dwindling fish stocks will cripple global fisheries or the blue economy, and lead to bioaccumulation further up the food chain. The list goes on.

How does plastic interact with marine snowfall?

Plankton are the invisible heroes of many aquatic and marine ecosystems, as they provide food for a variety of species. Microalgae or phytoplankton, are unicellular, photosynthetic organisms that occur as individuals, in chains, or clusters through aquatic ecosystems.  Phytoplankton are eaten by zooplankton, and are in turn consumed by molluscs, small crustaceans like shrimp and krill, sardines, herrings, manta rays, and even baleen and blue whales and form crucial links in aquatic food webs.

A study on aquatic microalgae determined that its growth, shape, and photosynthetic activity are adversely affected by the chemicals in microplastics. Another paper highlights how zooplankton stray from their usual diet of phytoplankton, and graze on microplastics instead. These changes can disrupt the entire food chain, hamper the nutrient cycling of phosphorus, nitrogen and carbon, and may cause plastics to accumulate further along trophic levels.

Plankton interact with plastics in other ways too. Along with other microorganisms, they inhabit plastic surfaces, creating a plastisphere – an ecosystem of discarded waste in open waters. The plastisphere, as defined in the book Mare Plasticum – The Plastic Sea, is a mini-ecosystem in itself, with primary producers, grazers, predators, parasites, symbionts, and nutrient recyclers. Plastispheres may increase productivity in the otherwise unproductive ocean ecosystem, yet they serve as islands for harmful invasive microbes to travel across wide ranges, emitting greenhouse gases and ferrying antibiotic resistance genes along the way. Studying these man-made ecosystems will be crucial in the years ahead.

Interestingly, plankton play a vital role even in death. When plankton die or are consumed, they cause particles of carbon to sink from the surface to the deep ocean in a process known as marine snowfall. Some of this carbon is consumed by sea organisms along the way, some gets chemically broken down, but most of it reaches the deep ocean where it settles for hundreds or thousands of years. The ocean bed is an essential carbon sink for anthropogenic carbon emissions. When the plankton cycles are affected, they could have consequences for the oceanic carbon sinks, and also disrupt global biogeochemical cycles. If plastic travels along with sinking plankton, it will find its way into the ocean sediments and the geological record, and bear testimony to the Anthropocene.

What lies ahead?

Microplastics are ubiquitous, and their effects undeniable. “We have enough data to conclude that microplastics are hazardous to the functioning of keystone species and ecosystem services,” Sridharan adds. Yet most papers end on inconclusive notes. Researchers are unwilling to comment on the large-scale, or ecosystem-level impact of plastics, and call for more studies to be conducted. This might be in part due to the caution scientists naturally exhibit, as hard evidence is what good science is founded on, or partly due to some studies with no discernible, measurable impacts on biodiversity. Yet as Ritesh Kumar, director of the Wetlands International South Asia (WISA) cautions, “with research analyses, the absence of evidence, should not be mistaken as evidence of absence”.

To add to the evidence, the CounterMEASURES II project, implemented by UNEP, hopes to generate new knowledge for policy changes at the local, national, and global levels. The recent report by the Convention on Migratory Species (CMS) and the United Nations Environment Programme (UNEP) highlighted the disproportionate impact that plastics have on migratory species. “Early next year, the Wetlands International will release a wetlands management plan with a species risk assessment tool,” reveals Kumar.

“Future research should focus on documenting the microplastic cycle, with standardised frameworks to quantify the generation, segregation, recycling, disposal, leaching of waste, to better understand the scale of the problem,” suggests Manish Kumar, a project scientist at CSIR-NEERI, and a co-author on the paper on microplastics. “Research cannot keep generalising plastics and microplastics; the latter needs to be categorised as a pollutant in its own right,” interjects Sridharan. “Integrated, interdisciplinary research correlating the abundance of plastics with the degradation of habitats, the biotic and abiotic disruptions, and the long-term implications for the biodiversity and ecosystems at large, with ground assessments wherever possible, will be key to finding effective solutions,” the authors concur.

Read more: [Explainer] The life of plastic: born a hero, dying an outcast, resurrected a monster

Banner image: Segregation of plastics at a site in Bangladesh – one among many developing countries where trash from developing countries is shipped for recycling, though most of it ends up in landfills or waterways. Photo by Maruf Rahman/Pixabay.