Hitching a ride on plastic: alien invasions and the plastisphere

As if directly injuring and killing animals wasn't enough, plastic poses a whole other host of threats to biodiversity and marine ecological systems.

Marine plastic debris is resilient, non-biodegradable and abundant, making it the perfect substrate for species wanting to hitch a ride across the oceans. There are a number of natural substrates which usually fulfil this role, such as wood, feathers and macroalgae. But plastic is outnumbering all the natural substrates and vastly increasing the potential for species transport and dispersal. This enables species to travel further and in greater numbers. Marine plastic is another example of anthropogenic activities causing the spread of invasive species, which have serious consequences for biodiversity.

A wide variety of animals use marine debris as a mode of transportation and dispersal, especially bryozoans, barnacles, polychaete worms and molluscs. In 2002 Barnes studied 30 remote islands across all the worlds oceans, on which over 200 items were found washed ashore. Of these items, anywhere between 20 and 80% were anthropogenic.

Map of study sites, with inset debris on beach. Source: Barnes, 2002.

It was found that the proportion of anthropogenic debris increases with latitude (fig. 2 a). The increase in anthropogenic debris represents an increase in potential for transporting organisms. Figure 2 b shows the effects of remoteness of the island on colonisation of debris. Distance from the mainland is given as  hundreds of kilometres (circles), tens of kilometres (triangles) or less than 10 kilometres (squares). The distance from the mainland doesn't have a significant effect on the proportion of colonised debris. It can also be seen that there were no samples recorded as colonised beyond 60 degrees, most likely due to the persistent low temperatures found at such high latitudes. Global warming will exacerbate the issue, enabling plastic colonisers to travel further poleward. Figure 2 c shows the ratio of colonisation on non-anthropogenic debris to colonisation on anthropogenic debris, with the amount on anthropogenic debris rapidly increasing up to 60 degrees. From this Barnes has concluded that anthropogenic debris in the oceans has approximately doubled the spread of fauna in the sub-tropics, and increased it more than 3 times at latitudes over 50 degrees!

Figure 2: comparison of anthropogenic and colonised debris with latitude. Note: white symbols are in the Northern hemisphere; black symbols are in the Southern hemisphere. Source: Barnes, 2002.

Plastic is doing more than just transporting species: delving down to a much smaller scale we find the 'plastisphere', a new ecological realm. Zettler et al. (2013) carried out an comprehensive study classifying the microbial communities found on polypropylene and polyethylene fragments (used in packaging and single-use plastic items) in the North Atlantic sub-tropical gyre. Using SEM micrographs they found over 50 distinct morphotypes and over 1000 species equivalents of operational taxonomic units.

Figure 2: examples of different morphotypes of bacteria. Source: Zettler et al., 2013.

Their key finding is that the 'plastisphere' microbial communities are distinct from those in surrounding seawater. What this means is that anthropogenic plastic pollution into the oceans has created a novel ecological habitat! I won't go into detailing every single microbe they found, but figure 4 is a bar chart of all different operational taxonomic units, and it is obvious how different the seawater communities are from those found on plastic fragments.

Figure 4: bar chart of different microbial operational taxonomic units illustrating the difference between plastisphere communities and surrounding seawater communities. Source: Zettler et al., 2013.  

Figure 5: Venn diagram illustrating the significant differences between plastisphere communities and surrounding seawater communities. Source: Zettler et al., 2013. 

The Venn diagram shows the disparity between plastisphere communities and seawater communities - there is only a limited overlap. Also, different types of plastic appear to have largely different communities too. What this all means is that we have created a new plastic ecosystem, further evidence for the significant effects of anthropogenic activities on Earth!