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, 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).
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.