Whales are the engineers of their ecosystems

What do baleen and sperm whales have to do with climate change? More than you may initially think! Long-lived species help to enhance the predictability of marine environments. This helps to keep the ecosystem stable. However, after an extensive period of whaling, humans have disrupted this natural process. The number of great whales has declined between 66% and 90% from pre-whaling numbers. This has impacted the structure of oceanic ecosystems in various ways, genuinely showing these “ecosystem engineers” make huge changes in the marine environment. In a 2014 paper led by Joe Roman of the University of Vermont, the four mechanisms by which whales act as ecosystem engineers are defined as 1) prey, 2) consumers, 3) nutrient vectors (the whale pump), and 4) detritus.

The collapse of large whales

Due to their massive size, baleen and sperm whales are not prey to many marine animals, with the exception historically of now-extinct large raptorial cetaceans, and more recently, killer whales. The drastic decline in larger whales in the past century was caused by industrial whaling, which in turn disrupted the feeding dynamics of their predators, the killer whales. In fact, some experts believe that marine-mammal-eating killer whales had to shift their diets from large whales to sea otters. Consequently, killer whales have indirectly become responsible for the decline of some sea otter populations. In the North Pacific, increased killer whale predation of sea otters allowed the otter’s sea urchin prey to multiply, causing a decline in coastal kelp forests, which influenced coastal fish populations and marine carbon sequestration. Therefore, the absence of large whales may have impacted entire ecosystems.

The link between whales and their prey

Whales are highly active in their marine communities as direct prey consumers and as influences on other food-web interactions. In one example, Arctic crustaceans may have had their evolution shaped by whales. Indeed, whales have a preference for large individuals which caused them to evolve to be smaller and shorter-lived than they are currently – natural selection. The absence of whales allowed selection for longer-lived and larger species to dominate. 

Human fishers may see fish-eating whales as competition, but the opposite is also true in these ecosystems. When whale populations decrease, so do the fish stocks. The reason for this association between whales and fish is a phenomenon called the “whale pump”. Indeed, whales release nutrients into the ecosystem, through nutrient-rich feces and bottom-feeding behaviors, which release some of the sediment into the water column, bringing crustaceans to the surface. The crustaceans can in turn be eaten by fish at the surface, which increases the number of fish available to fishers and other predators. This shows how important whales can be as ocean engineers, not just to the aquatic food web but also to humans.

Pumping nutrients throughout the oceans

Vertical transfer of nutrients

Whales collect nutrients from the foods that they ingest. These nutrients go back to surface waters when great whales release fecal fumes and urine during resting and respiration. This vertical movement of nutrients, the “whale pump,” can be essential for nitrogen dispersal. In fact, nitrogen can be a scarce but essential nutrient but is critical to plankton productivity. The whale pump works similarly for iron – with sperm whale fecal plumes having an iron concentration 10 million times greater than the surrounding water! 

These increased plankton blooms associated with the whale pump will eventually sink once the conditions are no longer profitable. When they sink, in the form of “marine snow”, the plankton and all its carbon are removed from the atmosphere. Thus, whales could lead to at least 200,000 tons of carbon a year being removed from the atmosphere and deposited in the deep ocean. This is certainly a factor to consider with increasing atmospheric carbon dioxide levels!

Horizontal transfer of nutrients

Whales circulate nutrients vertically throughout the ocean and horizontally during their migrations, which, for baleen whales, is among the farthest annual movements of mammals on the entire planet. Humpbacks in the Northern Hemisphere may move from coastal Alaska down to Mexico and in the Southern Hemisphere from Antarctica to Australia, South America, and Africa. These movements bring the whales close to the Equator, to the calving grounds, where they usually fast but still release nitrogen-rich urea. If we consider pre-whaling numbers of blue whales alone, the increased nitrogen concentrations could have allowed tropical phytoplankton to absorb 15% more carbon per year on average. With other whale species having similar migration patterns, the effects of nitrogen transfer to the lower latitudes by baleen whales are not to be underestimated.

When carbon returns to the sea

When a whale dies, the carcasses fall from the surface to the sediment at the bottom of the ocean. This adds a considerable amount of nutritious organic material to the seabed. Sharks and hagfish can consume soft tissues, while the decomposition of the skeleton can support bacterial and animal growth. 

A whale carcass at the bottom of the sea – Credit: NOAA

More than 60 species rely on whale falls to survive, and therefore their populations suffered during industrial whaling. Indeed, carcasses were rare on the bottom of the ocean since whalers were taking the whales’ bodies to the shore. In areas where species like the bowhead and right whales became functionally extinct, the whale-fall species likely did.

While whales can move nitrogen up the water column, they can also move the carbon stored in their bodies down. Global whale fall is estimated to transfer 190,000 tons of carbon per year from the atmosphere to the ocean floor. The restoration of whale populations to pre-whaling numbers is expected to lead to a carbon removal from the atmosphere on the same level of magnitude as climate-engineering projects designed to mitigate climate change. Go whales!

Conclusion

Historically, whales have been viewed as valuable due to their commercial value as oil, meat, and baleen that could be woven into baskets and corsets. However, the role of whales as ecosystem engineers throughout their natural lifecycle is far more valuable to us in the long term! Their ability to cycle nutrients allows ecosystem maintenance and contributes to climate mitigation measures by removing carbon dioxide from the atmosphere. As whale populations begin to recover from an intensive period of whaling, we hope the positive effects of their presence will also increase. 

References

  • T. Branch and T. Williams, in Whales, whaling and ocean ecosystems, University of California Press, Berkeley, 2006.
  • J. Roman, J. A. Estes, L. Morissette, C. Smith, D. Costa, J. McCarthy, J. B. Nation, S. Nicol, A. Pershing and V. Smetacek, Front. Ecol. Environ., 2014, 12, 377–385.
  • T. J. Lavery, B. Roudnew, J. Seymour and J. G. Mitchell, Mar. MAMMAL Sci., 2014, 30, 888–904.
  • A. J. Pershing, L. B. Christensen, N. R. Record, G. D. Sherwood and P. B. Stetson, PLoS One, , DOI:10.1371/JOURNAL.PONE.0012444.

Thank you for reading! Did you enjoy this post? Check out our other post on what happens to whales once they die:

What happens to dead whales?
MSc Student at University of St Andrews | Website

Miranda is a current postgraduate student on the Marine Mammal Science MSc at the University of St Andrews and recently graduated from a Chemistry BSc at University College London. After the MSc, she hopes to combine her passions of chemistry and cetaceans to work in the field of ecotoxicology.

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