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.
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.
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).