Monthly Archives: April 2014

Too many chefs: Smaller groups exhibit more accurate decision-making (Proceedings of the Royal Society B)

Flock behavior

Smaller groups actually tend to make more accurate decisions, according to a new study from Princeton University Professor Iain Couzin and graduate student Albert Kao. (Photo credit: Gabriel Miller)

By Morgan Kelly, Office of Communications

The trope that the likelihood of an accurate group decision increases with the abundance of brains involved might not hold up when a collective faces a variety of factors — as often happens in life and nature. Instead, Princeton University researchers report that smaller groups actually tend to make more accurate decisions, while larger assemblies may become excessively focused on only certain pieces of information.

The findings present a significant caveat to what is known about collective intelligence, or the “wisdom of crowds,” wherein individual observations — even if imperfect — coalesce into a single, accurate group decision. A classic example of crowd wisdom is English statistician Sir Francis Galton’s 1907 observation of a contest in which villagers attempted to guess the weight of an ox. Although not one of the 787 estimates was correct, the average of the guessed weights was a mere one-pound short of the animal’s recorded heft. Along those lines, the consensus has been that group decisions are enhanced as more individuals have input.

But collective decision-making has rarely been tested under complex, “realistic” circumstances where information comes from multiple sources, the Princeton researchers report in the journal Proceedings of the Royal Society B. In these scenarios, crowd wisdom peaks early then becomes less accurate as more individuals become involved, explained senior author Iain Couzin, a professor of ecology and evolutionary biology.

“This is an extension of the wisdom-of-crowds theory that allows us to relax the assumption that being in big groups is always the best way to make a decision,” Couzin said.

“It’s a starting point that opens up the possibility of capturing collective decision-making in a more realistic environment,” he said. “When we do see small groups of animals or organisms making decisions they are not necessarily compromising accuracy. They might actually do worse if more individuals were involved. I think that’s the new insight.”

Couzin and first author Albert Kao, a graduate student of ecology and evolutionary biology in Couzin’s group, created a theoretical model in which a “group” had to decide between two potential food sources. The group’s decision accuracy was determined by how well individuals could use two types of information: One that was known to all members of the group — known as correlated information — and another that was perceived by only some individuals, or uncorrelated information. The researchers found that the communal ability to pool both pieces of information into a correct, or accurate, decision was highest in a band of five to 20. After that, the accurate decision increasingly eluded the expanding group.

At work, Kao said, was the dynamic between correlated and uncorrelated cues. With more individuals, that which is known by all members comes to dominate the decision-making process. The uncorrelated information gets drowned out, even if individuals within the group are still well aware of it.

In smaller groups, on the other hand, the lesser-known cues nonetheless earn as much consideration as the more common information. This is due to the more random nature of small groups, which is known as “noise” and typically seen as an unwelcome distraction. Couzin and Kao, however, found that noise is surprisingly advantageous in these smaller arrangements.

“It’s surprising that noise can enhance the collective decision,” Kao said. “The typical assumption is that the larger the group, the greater the collective intelligence.

“We found that if you increase group size, you see the wisdom-of-crowds benefit, but if the group gets too large there is an over-reliance on high-correlation information,” he said. “You would find yourself in a situation where the group uses that information to the point that it dominates the group’s decision-making.”

None of this is to suggest that large groups would benefit from axing members, Couzin said. The size threshold he and Kao found corresponds with the number of individuals making the decisions, not the size of the group overall. The researchers cite numerous studies — including many from Couzin’s lab — showing that decisions in animal groups such as schools of fish can often fall to a select few members. Thusly, these organisms can exhibit highly coordinated movements despite vast numbers of individuals. (Such hierarchies could help animals realize a dual benefit of efficient decision-making and defense via strength-in-numbers, Kao said.)

“What’s important is the number of individuals making the decision,” Couzin said. “Just looking at group size per se is not necessarily relevant. It depends on the number of individuals making the decision.”

Read the abstract.

Kao, Albert B., Iain D. Couzin. 2014. Decision accuracy in complex environments is often maximized by small group sizes. Proceedings of the Royal Society B. Article published online April 23, 2014. DOI: 10.1098/rspb.2013.3305

This work was supported by a National Science Foundation Graduate Research Fellowship, National Science Foundation Doctoral Dissertation Improvement (grant no. 1210029), the National Science Foundation (grant no. PHY-0848755), the Office of Naval Research Award (no. N00014-09-1-1074), the Human Frontier Science Project (grant no. RGP0065/2012), the Army Research Office (grant no. W911NG-11-1-0385), and an NSF EAGER grant (no. IOS-1251585).

Study resolves controversy over nitrogen’s ocean “exit strategies” (Science)

By Catherine Zandonella, Office of the Dean for Research

Seawater collection device

Princeton University graduate student Andrew Babbin (left) prepares a seawater collection device known as a rosette. The team used samples of seawater to determine how nitrogen is removed from the oceans. (Research photos courtesy of A. Babbin)

A decades-long debate over how nitrogen is removed from the ocean may now be settled by new findings from researchers at Princeton University and their collaborators at the University of Washington.

The debate centers on how nitrogen — one of the most important food sources for ocean life and a controller of atmospheric carbon dioxide — becomes converted to a form that can exit the ocean and return to the atmosphere where it is reused in the global nitrogen cycle.

Researchers have argued over which of two nitrogen-removal mechanisms, denitrification and anammox, is most important in the oceans. The question is not just a scientific curiosity, but has real world applications because one mechanism contributes more greenhouse gases to the atmosphere than the other.

Bess Ward

Bess Ward, Princeton’s William J. Sinclair Professor of Geosciences (Photo courtesy of Georgette Chalker)

“Nitrogen controls much of the productivity of the ocean,” said Andrew Babbin, first author of the study and a graduate student who works with Bess Ward, Princeton’s William J. Sinclair Professor of Geosciences. “Understanding nitrogen cycling is crucial to understanding the productivity of the oceans as well as the global climate,” he said.

In the new study, the researchers found that both of these nitrogen “exit strategies” are at work in the oceans, with denitrification mopping up about 70 percent of the nitrogen and anammox disposing of the rest.

The researchers also found that this 70-30 ratio could shift in response to changes in the quantity and quality of the nitrogen in need of removal. The study was published online this week in the journal Science.

The two other members of the research team were Richard Keil and Allan Devol, both professors at University of Washington’s School of Oceanography.

Research vessel

The researchers collected the samples in 2012 in the ocean off Baja California. Click on the image to read blog posts from a similar expedition that took place the following year.

Essential for the Earth’s life and climate, nitrogen is an element that cycles between soils and the atmosphere and between the atmosphere and the ocean. Bacteria near the surface help shuttle nitrogen into the ocean food chain by converting or “fixing” atmospheric nitrogen into forms that phytoplankton can use.

Without this fixed nitrogen, phytoplankton could not absorb carbon dioxide from the air, a feat which is helping to check today’s rising carbon dioxide levels in the atmosphere. When these tiny marine algae die or are consumed by predators, their biomass sinks to the ocean interior where it becomes food for other types of bacteria.

Test tubes

Researchers added specific amounts and types of nitrogen and organic compounds to test tubes containing seawater, and then noted whether denitrification or anammox occurred.

Until about 20 years ago, most scientists thought that denitrification, carried out by some of these bacteria, was the primary way that fixed nitrogen was recycled back to nitrogen gas. The second process, known as anaerobic ammonia oxidation, or anammox, was discovered by Dutch researchers studying how nitrogen is removed in sewage treatment plants.

Both processes occur in regions of the ocean that are naturally low in oxygen, or anoxic, due to local lack of water circulation and intense phytoplankton productivity overlying these regions. Within the world’s ocean, such regions occur only in the Arabian Sea, and off the coasts of Peru and Mexico.

In these anoxic environments, anaerobic bacteria feast on the decaying phytoplankton, and in the process cause the denitrification of nitrate into nitrogen gas, which cannot be used as a nutrient by most phytoplankton. During this process, ammonium is also produced, although marine geochemists had never been able to detect the ammonium that they knew must be there.

That riddle was solved in the early 2000s by the discovery of the anammox reaction in the marine environment, in which anaerobic bacteria feed on ammonium and convert it to nitrogen gas.

Glove bag

Graduate student Andrew Babbin filling incubation vials under an anoxic atmosphere in a transparent container called a “glove bag.” (Photo courtesy of Andrew Babbin)

But another riddle soon appeared: the anammox rates that Dutch and German teams of researchers measured in the oceans appeared to account for the entire nitrogen loss, leaving no role for denitrification.

Then in 2009, Ward’s team published a study in the journal Nature showing that denitrification was still a major actor in returning nitrogen to the air, at least in the Arabian Sea. The paper further fueled the controversy.

Back at Princeton, Ward suspected that both processes were necessary, with denitrification churning out the ammonium that anammox then converted to nitrogen gas.

To settle the issue, Ward and Babbin decided to look at exactly what was going on in anoxic ocean water when bacteria were given nitrogen and other nutrients to chew on.

They collected water samples from an anoxic region in the ocean south of Baja California and brought test tubes of the water into an on-ship laboratory. Working inside a sturdy, flexible “glove bag” to keep air from contaminating the low-oxygen water, Babbin added specific amounts and types of nitrogen and organic compounds to each test tube, and then noted whether denitrification or anammox occurred.

“We conducted a suite of experiments in which we added different types of organic matter, with variable ammonium content, to see if the ratio between denitrification and anammox would change,” said Babbin. “We found that not only did increased ammonia favor anammox as predicted, but that the precise proportions of nitrogen loss matched exactly as predicted based on the ammonium content.”

The explanation of why, in past experiments, some researchers found mostly denitrification while others found only anammox comes down to a sort-of “bloom and bust” cycle of phytoplankton life, explained Ward.

“If you have a big plankton bloom, then when those organisms die, a large amount of organic matter will sink and be degraded,” she said, “but we scientists are not always there to measure this. In other words, if you aren’t there on the day lunch is delivered, you won’t measure these processes.”

The researchers also linked the rates of nitrogen loss with the supply of organic material that drives the rates: more organic material equates to more nitrogen loss, so the quantity of the material matters too, Babbin said.

The two pathways have distinct metabolisms that turn out to be important in global climate change, he said.  “Denitrification produces carbon dioxide and both produces and consumes nitrous oxide, which is another major greenhouse gas and an ozone depletion agent,” he said. “Anammox, however, consumes carbon dioxide and has no known nitrous oxide byproduct. The balance between the two therefore has a significant impact on the production and consumption of greenhouse gases in the ocean.”

The research was funded by National Science Foundation grant OCE-1029951.

Read the abstract.

Andrew R. Babbin, Richard G. Keil, Allan H. Devol, and Bess B. Ward. Organic Matter Stoichiometry, Flux, and Oxygen Control Nitrogen Loss in the Ocean. Science. Published Online April 10 2014. DOI: 10.1126/science.1248364