(To listen to an audio version of this blog, click on link …. Bison, elephants and whales part 2
Humpback whale. Photo by Wwells14, Wikimedia Commons
Last week, we were looking at large consumer species that have been extirpated from their ecosystems. This week we’ll look at the largest consumers in the largest of all ecosystems and explore some of the consequences of their absence!
Whales:
It is difficult to come up with a reliable estimate of the size of the Earth’s whale community prior to the onset of the widespread whale-hunting in the 18th, 19th and 20th Centuries. We do, however, have solid data on the numbers and kinds of whales that were killed during the two or three hundred years of commercial hunting, and we can look at the various whale populations in our present day oceans (especially at features of their genetic compositions) in order to try to reconstruct their original population numbers. We can only generate, though, very rough estimations of the pre-hunting abundance and diversity of these whale species.
Most of us visualize the heyday of whale hunting in 19th Century images: wooden, sailing ships with rowed whaling boats and hand-thrown harpoons. Lots of bearded men with lots of exotic tattoos! Moby Dick both as a book that so many of us had to read in high school and as a film and then as a TV mini-series is deeply etched into our collective consciousnesses. It was, however, in the 20th Century that the intensity of whale hunting and killing reached its peak. Huge hunting and carcass-processing ships with high powered pursuit boats and mechanically launched, explosive harpoons scoured the Earth’s oceans and swept up whales from the Northern Atlantic and Pacific down to the Southern Ocean.
Sperm whale pod. Photo by G. Barathieu, Wikimedia Commons
In the 20th Century over two million whales (fin whales, sperm whales, blue whales, sei whales and humpback whales) were killed in the Southern Ocean alone. For many of these species, these numbers represent 99% of their estimated, pre-hunting populations! Similar data are available for the impacts of commercial whaling in the North Atlantic and the North Pacific where another million whales were killed in the 20th Century. Consequently, only a tiny fraction of the pre-hunting whale community survived this great, systematic slaughter.
Commercial whale hunting continued into the 1980’s and was stopped not so much because of public outrage and growing ecological awareness but more because of economics and the Law of Diminishing Returns: there were so few whales left in the ocean it was hard to profitably “harvest” them! This economic reality along with subsequent conservation programs and regulations, have given many whale species the opportunity to slowly recover even though many still teeter on the edge of extinction.
Blue whale. Photo by NPS.
It has been postulated that this human-directed destruction of whales represents the greatest single erasure of animal biomass from the ecosystems of the Earth in modern history. The millions of American bison or the hundreds of thousands of African elephants killed in their respective slaughters do not come close to matching the lost biomass of these massive, sea-dwelling giants. We should remember, after all, that the blue whale is the largest animal EVER to exist on Earth! It is larger than the largest elephant and larger than the largest dinosaur!
We’ve talked about some of the impacts of losing keystone, consumer species from terrestrial ecosystems: the decline in productivity of the prairie grasslands, the shrinkage and decline of the tropical savannahs and the diminished robustness of the tropical rainforests. There were also profound consequences in the marine ecosystems when these huge cetaceans are eliminated, but it took a long period of time for humans to notice them! Many of these impacts centered around a common feature of both the terrestrial and marine ecosystems: the feces produced by their large consumers, and its role in the cycling of important nutrients to sustain the respective ecosystems’ trophic webs.
Humpback whale with her calf. Photo by National Marine Sanctuaries. Wikimedia Commons
There are two big categories of whales: “toothed” whales and “baleen” whales. The toothed whales include sperm whales, beluga whales, narwhals, orcas, pilot whales and the oceanic dolphins. The baleen whales include blue whales, fin whales, right whales, bowhead whales and gray whales. Toothed whales eat primarily fish and squid (and maybe the odd penguin or seal), while the baleen whales primarily eat krill (small crustaceans that are a major component of the zooplankton of the ocean and which may be the most numerically abundant animal on Earth!).
All of these marine food sources are rich in nitrogen, and much of this ingested nitrogen passes through the digestive tract of the whale and is excreted out, typically, in vast, liquid plumes of feces. Often the air-breathing whales feed on their fish and squid and/or krill deep under water but then release their liquid feces near the surface when they come up for a breath. These fresh fecal foams then float on the water’s surface until they break up.
The consequences of the nitrogen richness of the whale feces and also its location in the surface water are profound. Nitrogen is a key limiting nutrient in marine ecosystems for the growth of photosynthetic phytoplankton that forms the energy base for almost all marine ecosystems. The whale delivered nitrogen, then, stimulates great blooms of phytoplankton upon which zooplankton (like krill) feed. This energy is then passed up through all of levels of the marine food chain.
The Whale Pump. Figure by R. J. McCarthy. Wikimedia Commons
This whale delivery of nitrogen from the depths moves nutrients in the opposite direction from the typical, downward flow of detritus and debris (the “marine snow” that sends materials down into the deep sediments of the sea). This upward, biological movement of nitrogen is often referred to as the “whale pump,” and it is vital for the sustainable productivity of the marine ecosystem!
As we said, baleen whales feed almost exclusively on krill and, consequently, also accumulate and move another important limiting marine nutrient: iron. Krill are rich in iron and a baleen whale passes huge amount of iron through its digestive tract and out into its feces. The baleen whale’s feces then delivers this iron to the surface waters where it stimulates even more growth of phytoplankton and, especially, new krill.
The nitrogen and iron-stimulated phytoplankton fixes many thousands of tons of carbon dioxide-carbon from the atmosphere as it carries out its photosynthesis, and the growth of the krill more than replaces the krill eaten by the baleen whales. So, a consequence of whales’ feeding on the krill, is that the krill population actually increases!
Some researchers have proposed adding iron to the waters of the Southern Ocean to stimulate the growth of plankton and reap the benefits of removing carbon dioxide carbon from the atmosphere. These climate change reversing events, though, could be carried out more efficiently and less expensively by supporting whale conservation programs and encouraging the growth and reproduction of these vital keystone species!
Large consumers in ecosystems are vital agents that control the nutrient cycles of those systems. They are also critical to the maintenance of the stability and structure of those systems. If we want to have prairies, or savannahs, or tropical rain forests or productive oceans, we need to leave room for the bison, the elephants and the whales!