A series of recently published maps of the global distribution of small organisms in the world’s oceans turns its back on taxonomic identity and instead describes nature in a radically different way—based on shared traits.
Since Darwin wrote his famous On the Origin of Species, looking at different species of organisms has been an ingrained way for natural scientists to understand nature, biology, and ecosystems. However, there is a risk that the taxonomic approach turns out to be a dead-end, according to Professor Thomas Kiørboe from the Centre for Ocean Life at DTU Aqua, DTU Technical University of Denmark.
“The entire concept of grouping according to species is something that we have all been brought up with, and is therefore hard to break away from. However, in many ways the species concept is a trap. For marine creatures do not live where and in the way they do because of the name they have been given by man, but because of how they fit into and make the most of their surrounding environment. Therefore, we are looking directly at the traits—for example how an organism feeds—which tell us something about adaption to the environment across various species, and using this as a way of grouping instead of taxonomic identity in these biogeographical maps. It also simplifies our description of marine ecosystems, because instead of describing the distribution of all the hundreds of thousands of species in the sea, we describe the distribution of the most important functional properties of the organisms, which are actually very few.”
The maps that the professor refers to are so-called trait-based biogeographies of the distribution of copepods, an important group of zooplankton that is found globally in the world’s oceans. The map is the first of its kind, and has just been published in the scientific journal Ecology Letters.
To produce the map, PhD student Philipp Brun from the Centre for Ocean Life at DTU Aqua processed millions of measurements of zooplankton taken from the world’s oceans over the past 100 years.
"In many ways the species concept is a trap. For marine creatures do not live where and in the way they do because of the name they have been given by man."
The mapping does not look at taxonomic identity; instead, the organisms are distributed according to four different traits or characteristics: body size, reproduction strategy, feeding strategy, and self-protection (defence) strategy. This makes it possible to understand nature’s processes more directly – and thus predict how distributions will change as a result of, for example, fishing impacts, temperature increases, or other factors.
Copepods play a very important role in the oceans as a food for fish. They are also important ‘carbon dioxide dustmen’, that remove carbon from the atmosphere and transport it down to the bottom of the sea, thereby reducing the impact of carbon-related climate change. And in relation to this function, size—which is one of the parameters in the mapping—is a good proxy for their effect, explains Philipp Brun, Centre for Ocean Life at DTU Aqua:
“Copepods remove carbon from the atmosphere via the so-called ‘carbon pump’ in three ways—and body size is a factor in all of them. They remove carbon by excreting it with their faecal pellets that sinks to the bottom of the sea; by migrating down in the water column during daytime and expire carbon dioxide at depth; and by hibernating at great depth during winter where the transform fat (lipid) reserves into expired carbon dioxide, which is absorbed by the deep water nd removed from the atmosphere for more than thousand years. Larger organisms produce larger and faster sinking faecal pellets, they migrate deeper down and hibernate for longer, whereby their effect is greater.”
The mapping has both confirmed and challenged what is already known about the role and distribution of copepods in the sea. As expected, it shows that the biggest copepods are found in the temperate and especially the arctic regions, and that the copepods in sub-tropical areas are smaller than copepods near the Equator. The arctic copepods are large because they need to be able to store fat for their winter hibernation, while copepods in more nutrient poor areas have adapted to the food supply there, which typically consists of much smaller plankton algae.
On the other hand, the researchers have not been able to explain why the plankton communities in the large oceanic basins appear to differ so much, for example between the south Atlantic Ocean and the Indian Ocean.
“The open ocean is still one of the least explored areas on Earth. The mapping indicates that there are a number of processes which are not yet fully understood, but which are worth studying more closely,” says Philipp Brun, Centre for Ocean Life, DTU Aqua.
Read the scientific paper
Brun, P. Payne M. R., and Kiørboe, T. (2016) Trait biogeography of marine copepods - an analysis across scales. Ecolology Letters, 2016