Imagine that you are constantly eating, but slowly starving to death. Hundreds of species of marine mammals, fish, birds, and sea turtles face this risk every day when they mistake plastic debris for food.
Plastic debris can be found in oceans around the world. Scientists have estimated that there are over five trillion pieces of plastic weighing more than a quarter of a million tons floating at sea globally. Most of this plastic debris comes from sources on land and ends up in oceans and bays due largely to poor waste management.
Plastic does not biodegrade, but at sea large pieces of plastic break down into increasingly smaller fragments that are easy for animals to consume. Nothing good comes to animals that mistake plastic for a meal. They may suffer from malnutrition, intestinal blockage, or slow poisoning from chemicals in or attached to the plastic.
Despite the pervasiveness and severity of this problem, scientists still do not fully understand why so many marine animals make this mistake in the first place. It has been commonly assumed, but rarely tested, that seabirds eat plastic debris because it looks like the birds’ natural prey. However, in a study that my coauthors and I just published in Science Advances, we propose a new explanation: For many imperiled species, marine plastic debris also produces an odor that the birds associate with food.
A nose for sulfur
Perhaps the most severely impacted animals are tube-nosed seabirds, a group that includes albatrosses, shearwaters and petrels. These birds are pelagic: they often remain at sea for years at a time, searching for food over hundreds or thousands of square kilometers of open ocean, visiting land only to breed and rear their young. Many are also at risk of extinction. According to the International Union for the Conservation of Nature, nearly half of the approximately 120 species of tube-nosed seabirds are either threatened, endangered or critically endangered.
Although there are many fish in the sea, areas that reliably contain food are very patchy. In other words, tube-nosed seabirds are searching for a “needle in a haystack” when they forage. They may be searching for fish, squid, krill or other items, and it is possible that plastic debris visually resembles these prey. But we believe that tells only part of a more complex story.
Pioneering research by Dr. Thomas Grubb Jr. in the early 1970s showed that tube-nosed seabirds use their powerful sense of smell, or olfaction, to find food effectively, even when heavy fog obscures their vision. Two decades later, Dr. Gabrielle Nevitt and colleagues found that certain species of tube-nosed seabirds are attracted to dimethyl sulfide (DMS), a natural scented sulfur compound. DMS comes from marine algae, which produce a related chemical called DMSP inside their cells. When those cells are damaged – for example, when algae die, or when marine grazers like krill eat it – DMSP breaks down, producing DMS. The smell of DMS alerts seabirds that food is nearby – not the algae, but the krill that are consuming the algae.
Dr. Nevitt and I wondered whether these seabirds were being tricked into consuming marine plastic debris because of the way it smelled. To test this idea, my coauthors and I created a database collecting every study we could find that recorded plastic ingestion by tube-nosed seabirds over the past 50 years. This database contained information from over 20,000 birds of more than 70 species. It showed that species of birds that use DMS as a foraging cue eat plastic nearly six times as frequently as species that are not attracted to the smell of DMS while foraging.
To further test our theory, we needed to analyze how marine plastic debris smells. To do so, I took beads of the three most common types of floating plastic – polypropylene and low- and high-density polyethylene – and sewed them inside custom mesh bags, which we attached to two buoys off of California’s central coast. We hypothesized that algae would coat the plastic at sea, a process known as biofouling, and produce DMS.
After the plastic had been immersed for about a month at sea, I retrieved it and brought it to a lab that is not usually a stop for marine scientists: the Robert Mondavi Institute for Food and Wine Science at UC Davis. There we used a gas chromatograph, specifically built to detect sulfur odors in wine, beer and other food products, to measure the chemical signature of our experimental marine debris. Sulfur compounds have a very distinct odor; to humans they smell like rotten eggs or decaying seaweed on the beach, but to some species of seabirds DMS smells delicious!
Sure enough, every sample of plastic we collected was coated with algae and had substantial amounts of DMS associated with it. We found levels of DMS that were higher than normal background concentrations in the environment, and well above levels that tube-nosed seabirds can detect and use to find food. These results provide the first evidence that, in addition to looking like food, plastic debris may also confuse seabirds that hunt by smell.
When trash becomes bait
Our findings have important implications. First, they suggest that plastic debris may be a more insidious threat to marine life than we previously believed. If plastic looks and smells like food, it is more likely to be mistaken for prey than if it just looks like food.
Second, we found through data analysis that small, secretive burrow-nesting seabirds, such as prions, storm petrels, and shearwaters, are more likely to confuse plastic for food than their more charismatic, surface-nesting relatives such as albatrosses. This difference matters because populations of hard-to-observe burrow-nesting seabirds are more difficult to count than surface-nesting species, so they often are not surveyed as closely. Therefore, we recommend increased monitoring of these less charismatic species that may be at greater risk of plastic ingestion.
Finally, our results provide a deeper understanding for why certain marine organisms are inexorably trapped into mistaking plastic for food. The patterns we found in birds should also be investigated in other groups of species, like fish or sea turtles. Reducing marine plastic pollution is a long-term, large-scale challenge, but figuring out why some species continue to mistake plastic for food is the first step toward finding ways to protect them.
Last week, Grant Humphries told me about a fantastic new piece of research produced by the marine debris group from CSIRO's Ocean & Atmosphere flagship, based in Hobart, Tasmania. The researchers reported that they were able to detect a signature of plastic debris ingestion in several species of seabird by sampling the bird's preen gland oil and analyzing that sample on a GC-MS instrument. A GC-MS is able to detect and identify compounds at trace-level concentrations; the compounds that the researchers were interested in detecting were three common plasticizers, called phthalates.
This is awesome by itself, but the real breakthrough is that previously the only way to determine if a live seabird had ingested plastic was to lavage its stomach and sift through the regurgitated contents for plastic pieces. There are numerous problems with this approach; it would only show if the bird recently ingested plastic, data may be biased toward sick or weak individuals, and it's extremely invasive. This new GC-MS method could represent a paradigm shift in how wildlife managers and conservation biologists screen populations of marine animals - not only birds - for plastic ingestion.
My research on marine plastic debris is featured in the most-recent bulletin of the American Fisheries Society (Davis' subunit), put together by my good friend and superb sturgeon-surgeon, Emily Miller. Check out the bulletin describing the work (and other fish-related research going on at UC Davis) here. And if you're curious, stay tuned for more results from my summer's research coming up soon!
Two weeks ago, Deep Sea News had a fantastic post summarizing all the recent work done on marine nutrient recycling via excretion.
Believe it or not, this research topic is really gaining steam (I feel like I missed a poop-joke in there somewhere)! Read their write up of the recently-published research here.
Just yesterday, I heard a brilliant idea: economically incentivize people to clean up plastic waste from the world's beaches, oceans, and waterways.
The Canadian-based organization that is attempting to do this is called Plastic Bank. According to co-founder Davis Katz, plastic is more valuable than steel by weight and with literally billions of tons of plastics littering the world's beaches and waterways, there's trillions of dollars worth of plastic refuse out there just waiting to be recycled!
This is not the first time that economists have tried to mitigate a global environmental problem using a monetary solution. In fact, when I first heard of what Plastic Bank is trying to do, it made me think of carbon taxing, which has been implemented around the world with varying results.
With that being said, I have one word for you: plastics.
Are you intrigued by this market-based solution to a global environmental problem? Then follow Plastic Bank on twitter and like their facebook page!
The mini-symposium ran by eight awesome UC Davis Ecology PhD students and hosted by Folsom Lake College yesterday for Earth Day was a rousing success! Thanks to everyone who helped make it happen especially my fellow presenters and Dr. Steven Holzberg of Folsom Lake College.
I will have a longer post about the event next week, but for now, just know that the organizers and presenters had a great time!
Looking for something to do on the evening of Earth Day 2014?
Well, you're in luck because eight Ecology graduate students at UC Davis have teamed up with the Biology faculty at Folsom Lake College to run a FREE, public mini-symposium on the Ecological Effects of Global Anthropogenic Change.
Topics such as climate change, habitat fragmentation, invasive species, over-harvesting, and more will be discussed. The symposium (including panel discussion) will run from 4:00-7:30pm, next Tuesday (4/22). See the program flyer for more details.
We hope to see you there!
After roughly two years in the making, the first chapter of my PhD is in print, with full open access! This post summarizes that study.
A mutualism is a biological relationship where both parties benefit from the interaction. A common example is the relationship between Remora fish and sharks. The predatory shark can simply turn around and eat the Remora, but it doesn’t. Why? The reason is because the Remora provides the shark a service: it cleans ectoparasites off the shark’s body; in exchange, the shark gives the Remora a free meal and transport wherever the shark goes. That type of mutually beneficial relationship is what we have investigated in this paper.
Complicating matters some, we deem the mutualism in our paper a “tritrophic mutualism,” so what do we mean by “tritrophic”? A tritrophic interaction is one where two organisms, separated by at least one trophic level, interact in some biologically meaningful way. A common example of a tritrophic interaction is between lima bean plants and predatory insects. Herbivorous spider mites consume lima bean plants. As a result of being grazed, these lima bean plants release airborne compounds that alert predatory insects the plants are being attacked (Dicke 1986, Dicke et al. 2003). Olfactory-searching predatory insects detect odors emitted from damaged lima bean plants and use these odors to find the herbivorous spider mites, which are the preferred prey for many of these predatory insects. The interaction between lima bean plants (primary producer) and olfactory-searching predatory insects (secondary consumer) is the tritrophic interaction.
This is precisely the type of relationship we investigated in the marine environment between phytoplankton (primary producers) and procellariiform seabirds (top predators). To support this, we used diet data from 18 species of procellariiform seabirds collected over ~50 years. Dimethyl sulfide (DMS) is produced when phytoplankton cells burst, often due to predation, and DMS is known to be attractive to some species of procellariiform seabirds, but not all. We preformed a meta-analysis to show that those species of procellariiform seabirds that are attracted to DMS specialize in herbivorous crustacean prey. In other words, those seabird species attracted to DMS are using the phytoplankton-derived compound as an olfactory cue to find their preferred prey.
However, this alone isn’t enough to claim the interaction between phytoplankton and seabirds is mutualistic; we still needed to demonstrate a possible benefit the phytoplankton would receive from the attracted seabirds. One obvious answer is predatory release: seabirds are attracted to DMS, find the odor source, and depredate the herbivorous crustacea thereby “releasing” the phytoplankton from grazing pressure. While this is likely happening on some spatial scale, we instead chose to focus on the possible benefit foraging seabirds may provide phytoplankton through trace nutrient recycling via their defecation.
In the Southern Ocean, soluble iron limits primary production, and because iron is toxic to vertebrates if sequestered at high levels, most of ingested iron is excreted. It has recently been shown that whale and seabird feces are very high in iron content, relative to Southern Ocean seawater. Iron enrichment via defecation by top predators may therefore enhance primary productivity in portions of the iron-limited Southern Ocean.
Perhaps surprisingly, this is not the first time the concept of top predators fertilizing phytoplankton via their excrement has been considered. This concept has been tested recently with respect to whales in iron-limited (Lavery et al. 2010, Nicol et al. 2010) and nitrogen-limited (Roman and McCarthy 2010) systems. What makes our study unique is that we provide a mechanism – the chemoattraction of procellariiform seabirds to phytoplankton-produced DMS – linking the whole system together. It is possible that whales also use DMS as a foraging cue, although no data exists on the subject.
In summary, I’ll quote the last sentence of our paper, “results presented here illustrate a fundamental, albeit understudied, link between apex predators and the base of the pelagic food web, suggesting that a decline in seabird populations could negatively affect overall marine productivity.” (Savoca and Nevitt 2014).
Last weekend, I had the pleasure of presenting my research on marine plastic debris at the 7th Annual Ecology Graduate Student Symposium. Attendees included over 100 undergraduate and graduate students, professors from at least three different institutions, and members of the general public. It was a great day filled with interdisciplinary science, beautiful artwork, and stunning photographs. The symposium even got covered by the UC Davis newspaper, The California Aggie. Read the story here.
To top it off, I won the award for best talk! Considering the research I do on plastic debris is rather unusual, it was exciting to get a positive reception and encouraging feedback, especially since it was the first presentation I’ve given on this work.
Now I’m off to Honolulu, Hawaii for the 17th Ocean Sciences Meeting where I will present my preliminary findings to a much larger scientific audience. Whatever happens at the conference in Hawaii, I will be better off having presented at the EGSA Symposium first. Big thanks to James Farlin, Grace Ha, Katie Eskra, Matt Whalen, and many other volunteers too numerous to list for organizing such a fantastic event.
Visit UC Davis’ Graduate Group in Ecology’s website for more information on this powerhouse program.
Also, check out the Ecology Graduate Student Association’s webpage to learn about some of the great events and amazing work produced by members of my graduate group!
Great work by a good friend and colleague Andrea Townsend, demonstrating the negative impacts plastic debris has on a common land bird, the American Crow.
With so much research on plastic debris in marine ecosystems, it's good to see terrestrial communities garnering some attention.
About the photo: American Crow nestlings in Yolo County, California found tangled in plastic debris in their nests. Figure taken from Townsend and Barker 2014.
Read the full article here.
Heard back from the Society for Integrative and Comparative Biology (SICB) awards committee today and I am happy to announce I was selected as one of the 28 winners this year. First direct funding for my studies of marine plastic debris!
Find out more about SICB graduate student support options.
Evolutionary traps make the cover on this month's Trends in Ecology & Evolution (Robertson et al. 2013). Apparently, the prominence of one of my research topics is trending upwards!
Read the article here
And check out Bruce Robertson's website if you want to learn more about evolutionary trap research.
Awesome study (Amo et al. 2013) by a couple very famous ecologists, namely Marcel Visser and Marcel Dicke, demonstrating that terrestrial songbirds use induced plant volatiles to locate their herbivorous insect prey. Not too long ago, it was thought that most passerine birds did not have a sense of smell.
Read more here
Matthew Savoca holds a PhD in Ecology from the University of California, Davis. His research interests include sensory behavioral ecology, marine conservation biology, and seabird ecology.