These days, 60 to 70% of the plastic waste discarded into the sea is floating on the ocean because its specific gravity is lighter than that of seawater (Engler 2012).
Plastics are light and durable. Because they basically cannot be decomposed by microorganisms, they can keep floating on the ocean for a long time. Therefore, they create a unique habitat, just like a small island, for various kinds of creature (Harrison et al. 2011, Zettler et al. 2013).
A lot of species are living on the marine debris that is floating and drifting on the ocean. The most frequently seen of these is the barnacle. Since records have begun, more than 387 species have been found on drifting marine debris (Kiessling et al. 2015).
It doesn’t have to be a big piece of debris to become a habitat for marine animals.
For example, one type of seawater strider living in the North Pacific Ocean has been found to lay its eggs on the tiny drifting microplastics (Goldstein et al. 2012).
The number of seawater striders has been increasing in the North Pacific Ocean as large quantities of plastic waste is being found. The reason for this phenomenon is most likely because the seawater strider can find more places to lay its eggs due to the increase in microplastics (Goldstein et al. 2012).
Before the huge amount of plastic waste came to the North Pacific Ocean, the flotsam on which seawater striders could lay eggs was limited to natural small floating debris, such as driftwood.
This is not only valid for the seawater strider. Many animals relied on natural floating debris, such as drifting coconuts, driftwood, pumice, and grass, before the sea became overwhelmed by plastic debris.
Since such natural floating debris cannot be found easily, the number of animals living or relying on them must have been limited. However, the drifting plastic debris became tiny islands, supplying a habitat for a lot of species.
“Island Biology” is a famous and long-studied theory. It explains that the larger the island is, the more creatures can inhabit it and vice versa.
Some scientists are considering that the same things are happening on drifting plastic debris.
One of the concerns caused by drifting plastic waste is the problem of alien species. Scientists worry that alien species, that is, those which do not originally live in a given location, can arrive by hitchhiking on plastic debris.
For example, the tsunami caused by the Great East Japan Earthquake in 2011 carried lots and various kinds of marine debris to the western coast of North America, including rather large objects such as boats and docks. Scientists who researched this marine debris in the states of Oregon and Washington found 14 species of hydroids on the debris, whereby at least 5 of these had not been recorded in Northern America before (Calder et al. 2014).
It has already been confirmed that many alien species have been found on marine debris. It is worrying that the increase in marine debris may allow these invasive alien species to naturalize, affecting the ecosystem and human activities (Kiessling et al. 2015).