David M. Larson
Black-footed Albatross chick with plastics. Photograph by Dan Clark/USFWS. CC BY-NC 2.0
Everyone knows that plastic debris is a blight on the landscape throughout the world. But the situation is also dire in the oceans where micro and macro plastic debris abound. One of the more grim end products of this marine plastic debris is the mortality of adult and young seabirds due to ingestion of plastic. Seeing the remains of an albatross chick filled with indigestible plastic is wrenching. But why do the birds consume plastic debris, and why has this problem been increasing? The increase may be due to more and more small plastic debris in the marine environment, some of which may look like food and some of which may not. But what if it actually smells like food?
Plastic particles make excellent substrates for growth of marine organisms. When phytoplankton grow on the plastic, they attract zooplankton. When zooplankton graze on phytoplankton, volatile dimethyl sulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) are released, and those odorant compounds act as infochemicals, triggering foraging in marine organisms. Many species from zooplankton to whales react to these infochemicals in a trophic cascade. One of the most sensitive groups is the tubenoses—seabirds in the order Procellariiformes. Procellariiforms such as albatrosses, shearwaters, and petrels use volatile compounds to locate food in the open ocean, and many have been shown to use DMS to find feeding hotspots. If plastic bits in the ocean provide substrate for growth of phytoplankton and emit DMS and DMSP, then that might explain the attraction of indigestible plastics to these seabirds.
Savoca and coworkers (Savoca, et al., 2016) tested this potential trophic cascade by a series of experiments. First, they exposed virgin plastic spheres to an oceanic environment in the photic zone for three weeks, and then they tested the spheres for DMS. Using gas chromatography, there was no detectable DMS coming from samples of virgin high-density polyethylene, low-density polyethylene, or polypropylene beads. However, the marine-exposed bead samples of all three plastic types emitted DMS at concentrations detectable by procellariiforms.
The authors then did a meta-analysis of 55 studies on 25 species of procellariiforms and found a statistically strong relationship between DMS sensitivity and consumption of plastic by these birds. They went on to mathematically model this relationship, which suggested that DMS-sensitive species ingest plastic five times as frequently as nonsensitive species. There is a strong positive relationship between DMS-sensitivity and birds that nest in burrows, as opposed to those that nest on the surface, so the authors used burrow-nesting as a proxy for DMS-sensitivity. This expanded their analysis to 62 procellariiform species. Within this expanded species set, model analysis suggested burrow-nesting tubenoses ingested plastic three times more frequently than surface-nesting species, roughly consistent with the meta-analysis.
This study suggests that DMS-sensitive, tubenosed seabirds are particularly susceptible to ingesting plastic in the marine environment because of the aroma of DMS from the phytoplankton that grows on the plastic. Mediation efforts might be aimed at reducing the plastic load in the oceans—estimated in 2014 at over 250 million metric tons—or at producing plastics that do not support phytoplankton growth. Clearly the problem of plastic ingestion is not limited to seabirds, since many studies have raised the alarm over the increased plastic load in marine fish, marine mammals, and sea turtles. Other studies have demonstrated the importance of DMS in foraging cascades in marine organisms from zooplankton to cetaceans. Seabirds are perhaps the most at risk, with clear negative consequences due to gastrointestinal obstruction and chemical toxicity.
Finally, I found the collaborations in this study to be perfect, pairing biologists with the Department of Viticulture and Enology, all at University of California Davis. It all comes down to the volatiles and the bouquet.
- Savoca, M.S., M.E. Wohlfeil, S.E. Ebeler, and G.A. Nevitt. 2016. Marine Plastic Debris Emits a Keystone Infochemical for Olfactory Foraging Seabirds. Science Advances 2: e1600395. Accessed November 24, 2016.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon’s Joppa Flats Education Center in Newburyport, the Director of Mass Audubon’s Birder’s Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, Vice President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer..