By Catherine Zandonella, Office of the Dean for Research
A group of prominent researchers from seven institutions including Princeton University are calling for the establishment of a worldwide program to collect and test blood and other human bodily fluids to aid in the study and prevention of emerging infectious diseases such as the mosquito-borne Zika fever, which is caused by the Zika virus and has spread throughout Latin America and the Caribbean since early 2015.
In an article published in The Lancet April 5, the authors call for the creation of a World Serology Bank that would include storage repositories located around the world that use a common set of best practices to collect and evaluate blood and other bodily fluids from people infected by various pathogens. The resulting data would help scientists better understand the susceptibility of humans to emerging diseases such as Zika fever. The information could be shared widely among scientists who track disease.
Why is it important to create a World Serology Bank?
Serology is the study of bodily fluids including serum, the part of the blood that contains antibodies, with the aim of detecting the body’s immune response to pathogens. Serology provides us with the clearest window we have onto the landscape of susceptibility to pathogens across human populations, and the consequent risk of outbreaks. A World Serology Bank would shed light on the global risk of infectious disease outbreaks, and would be of tremendous public health benefit.
Why do you feel it is important to address this issue now?
With the emergence of pathogens like Zika virus, it becomes ever more important to understand what enhances or limits the global spread of these pathogens, and what the consequences of such spread may be across our pathogen communities. A World Serology Bank would provide a powerful mechanism toward such a global perspective.
What are the challenges involved in creating the Bank?
Challenges range from developing systems for collecting fluids, which can be done on a regular schedule or during specific disease events, to methods for sera storage and sera testing. Other challenges include defining who will administer the World Serology Bank, and what global data-sharing agreements will be put in place. Finally, we will need to develop new methods to translate what we learn from the evaluation of sera, such as patterns of susceptibility to specific pathogens, or protection from those pathogens. These methods will be driven by the underlying biology, and are likely to require an array of analytical innovations.
The article, “Use of serological surveys to generate key insights into the changing global landscape of infectious disease,” by C. Jessica E Metcalf, Jeremy Farrar, Felicity T. Cutts, Nicole E. Basta, Andrea L. Graham, Justin Lessler, Neil M. Ferguson, Donald S. Burke and Bryan T. Grenfell was published online in the journal The Lancet on April 5, 2016. http://dx.doi.org/10.1016/S0140-6736(16)30164-7.
Stopping the outbreak of a disease hinges on a wealth of data such as what makes a suitable host and how a pathogen spreads. But gathering these data can be difficult for diseases in remote areas of the world, or for epidemics involving wild animals.
A new study led by Princeton University researchers and published in the Journal of the Royal Society Interface explores an approach to studying epidemics for which details are difficult to obtain. The researchers analyzed the 2013 outbreak of dolphin morbillivirus — a potentially fatal pathogen from the same family as the human measles virus — that resulted in more than 1,600 bottlenose dolphins becoming stranded along the Atlantic coast of the United States by 2015. Because scientists were able to observe dolphins only after they washed up on shore, little is known about how the disease transmits and persists in the wild.
The researchers used a Poisson process — a statistical tool used to model the random nature of disease transmission — to determine from sparse data how dolphin morbillivirus can spread. They found that individual bottlenose dolphins may be infectious for up to a month and can spread the disease over hundreds of miles, particularly during seasonal migrations. In 2013, the height of disease transmission occurred toward the end of summer around an area offshore of Virginia Beach, Virginia, where multiple migratory dolphin groups are thought to cross paths.
In the interview below, first author Sinead Morris, a graduate student in ecology and evolutionary biology, explains what the researchers learned about the dolphin morbillivirus outbreak, and how the Poisson process can help scientists understand human epidemics. Morris is in the research group of co-author Bryan Grenfell, Princeton’s Kathryn Briger and Sarah Fenton Professor of Ecology and Evolutionary Biology and Public Affairs.
Q: How does the Poisson process track indirectly observed epidemics and what specific challenges does it overcome?
A: One of the main challenges in modeling indirectly observed epidemics is a lack of data. In our case, we had information on all infected dolphins that had been found stranded on shore, but had no data on the number of individuals that became infected but did not strand. The strength of the Poisson process is that its simple framework means it can be used to extract important information about the how the disease is spreading across space and time, despite having such incomplete data. Essentially the way the process works is that it keeps track of where and when individual dolphins stranded, and then at each new point in the epidemic it uses the history of what has happened before to project what will happen in the future. For example, an infected individual is more likely to transmit the disease onwards to other individuals in close spatial proximity than to those far away. So, by keeping track of all these infections the model can identify where and when the largest risk of new infections will be.
Q: Why was this 2013-15 outbreak of dolphin morbillivirus selected for study, and what key insights does this work provide?
A: The recent outbreak of dolphin morbillivirus spread rapidly along the northwestern Atlantic coast from New York to Florida, causing substantial mortality among coastal bottlenose dolphin populations. Despite the clear detrimental impact that this disease can have, however, it is still poorly understood. Therefore, our aim in modeling the epidemic was to gain much needed information about how the virus spreads. We found that a dolphin may be infectious for up to 24 days and can travel substantial distances (up to 220 kilometers, or 137 miles) within this time. This is important because such long-range movements — for example, during periods of seasonal migration — are likely to create many transmission opportunities from infected to uninfected individuals, and may have thus facilitated the rapid spread of the virus down the Atlantic coast.
Q: Can this model be used for human epidemics?
A: The Poisson process framework was originally developed to model the occurrence of earthquakes, and has since been used in a variety of other contexts that also tend to suffer from noisy, indirectly observed data, such as urban crime distribution. To model dolphin morbillivirus, we adapted the framework to incorporate more biological information, and similar techniques have also been applied to model meningococcal disease in humans, which can cause meningitis and sepsis. Generally, the data characterizing human epidemics are more detailed than the data we had for this project and, as such, models that can incorporate greater complexity are more widely used. However, we hope that our methods will stimulate the greater use of Poisson process models in epidemiological systems that also suffer from indirectly observed data.
This research was supported by the RAPIDD program of the Science and Technology Directorate of the Department of Homeland Security; the National Institutes of Health Fogarty International Center; the Bill and Melinda Gates Foundation; and the Marine Mammal Unusual Mortality Event Contingency Fund and John H. Prescott Marine Mammal Rescue Assistance Grant Program operated by the National Oceanic and Atmospheric Administration.
Sinead E. Morris, Jonathan L. Zelner, Deborah A. Fauquier, Teresa K. Rowles, Patricia E. Rosel, Frances Gulland and Bryan T. Grenfell. “Partially observed epidemics in wildlife hosts: modeling an outbreak of dolphin morbillivirus in the northwestern Atlantic, June 2013–2014.” Journal of the Royal Society Interface, published Nov. 18 2015. DOI: 10.1098/rsif.2015.0676
Columns of workers penetrate the forest, furiously gathering as much food and supplies as they can. They are a massive army that living things know to avoid, and that few natural obstacles can waylay. So determined are these legions that should a chasm or gap disrupt the most direct path to their spoils they simply build a new path — out of themselves.
Without any orders or direction, individuals from the rank and file instinctively stretch across the opening, clinging to one another as their comrades-in-arms swarm across their bodies. But this is no force of superhumans. They are army ants of the species Eciton hamatum, which form “living” bridges across breaks and gaps in the forest floor that allow their famously large raiding swarms to travel efficiently.
Researchers from Princeton University and the New Jersey Institute of Technology (NJIT) report for the first time that these structures are more sophisticated than scientists knew. The ants exhibit a level of collective intelligence that could provide new insights into animal behavior and even help in the development of intuitive robots that can cooperate as a group, the researchers said.
Ants of E. hamatum automatically form living bridges without any oversight from a “lead” ant, the researchers report in the journal Proceedings of the National Academy of the Sciences. The action of each individual coalesces into a group unit that can adapt to the terrain and also operates by a clear cost-benefit ratio. The ants will create a path over an open space up to the point when too many workers are being diverted from collecting food and prey.
“These ants are performing a collective computation. At the level of the entire colony, they’re saying they can afford this many ants locked up in this bridge, but no more than that,” said co-first author Matthew Lutz, a graduate student in Princeton’s Department of Ecology and Evolutionary Biology.
“There’s no single ant overseeing the decision, they’re making that calculation as a colony,” Lutz said. “Thinking about this cost-benefit framework might be a new insight that can be applied to other animal structures that people haven’t thought of before.”
The research could help explain how large groups of animals balance cost and benefit, about which little is known, said co-author Iain Couzin, a Princeton visiting senior research scholar in ecology and evolutionary biology, and director of the Max Planck Institute for Ornithology and chair of biodiversity and collective behavior at the University of Konstanz in Germany.
Previous studies have shown that single creatures use “rules of thumb” to weigh cost-and-benefit, said Couzin, who also is Lutz’s graduate adviser. This new work shows that in large groups these same individual guidelines can eventually coordinate group-wide, he said — the ants acted as a unit although each ant only knew its immediate circumstances.
“They don’t know how many other ants are in the bridge, or what the overall traffic situation is. They only know about their local connections to others, and the sense of ants moving over their bodies,” Couzin said. “Yet, they have evolved simple rules that allow them to keep reconfiguring until, collectively, they have made a structure of an appropriate size for the prevailing conditions.
“Finding out how sightless ants can achieve such feats certainly could change the way we think of self-configuring structures in nature — and those made by man,” he said.
Ant-colony behavior has been the basis of algorithms related to telecommunications and vehicle routing, among other areas, explained co-first author Chris Reid, a postdoctoral research associate at the University of Sydney who conducted the work while at NJIT. Ants exemplify “swarm intelligence,” in which individual-level interactions produce coordinated group behavior. E. hamatum crossings assemble when the ants detect congestion along their raiding trail, and disassemble when normal traffic has resumed.
The video below shows how E. hamatum confronted a gap they encountered on an apparatus that Lutz and Reid built and deployed in the forests of Barro Colorado Island, Panama. Previously, scientists thought that ant bridges were static structures — their appearance over large gaps that ants clearly could not cross in midair was somewhat of a mystery, Reid said. The researchers found, however, that the ants, when confronted with an open space, start from the narrowest point of the expanse and work toward the widest point, expanding the bridge as they go to shorten the distance their compatriots must travel to get around the expanse.
“The amazing thing is that a very elegant solution to a colony-level problem arises from the individual interactions of a swarm of simple worker ants, each with only local information,” Reid said. “By extracting the rules used by individual ants about whether to initiate, join or leave a living structure, we could program swarms of simple robots to build bridges and other structures by connecting to each other.
“These robot bridges would exhibit the beneficial properties we observe in the ant bridges, such as adaptability to local conditions, real-time optimization of shape and position, and rapid construction and deconstruction without the need for external building materials,” Reid continued. “Such a swarm of robots would be especially useful in dangerous and unpredictable conditions, such as natural disaster zones.”
Radhika Nagpal, a professor of computer science at Harvard University who studies robotics and self-organizing biological systems, said that the findings reveal that there is “something much more fundamental about how complex structures are assembled and adapted in nature, and that it is not through a supervisor or planner making decisions.”
Individual ants adjusted to one another’s choices to create a successful structure, despite the fact that each ant didn’t necessarily know everything about the size of the gap or the traffic flow, said Nagpal, who is familiar with the research but was not involved in it.
“The goal wasn’t known ahead of time, but ’emerged’ as the collective continually adapted its solution to the environmental factors,” she said. “The study really opens your eyes to new ways of thinking about collective power, and has tremendous potential as a way to think about engineering systems that are more adaptive and able to solve complex cost-benefit ratios at the network level just through peer-to-peer interactions.”
She compared the ant bridges to human-made bridges that automatically widened to accommodate heavy vehicle traffic or a growing population. While self-assembling road bridges may be a ways off, the example illustrates the potential that technologies built with the same self-assembling capabilities seen in E. hamatum could have.
“There’s a deep interest in creating robots that don’t just rely on themselves, but can exploit the group to do more — and self-assembly is the ultimate in doing more,” Nagpal said. “If you could have small simple robots that were able to navigate complex spaces, but could self-assemble into larger structures — bridges, towers, pulling chains, rafts — when they face something they individually did not have the ability to do, that’s a huge increase in power in what robots would be capable of.”
The spaces E. hamatum bridges are not dramatic by human standards — small rifts in the leaf cover, or between the ends of two sticks. Bridges will be the length of 10 to 20 ants, which is only a few centimeters, Lutz said. That said, E. hamatum swarms form several bridges during the course of a day, which can see the back-and-forth of thousands of ants. Many ants pass over a living bridge even as it is assembling.
“The bridges are something that happen numerous times every day. They’re creating bridges to optimize their traffic flow and maximize their time,” Lutz said.
“When you’re moving hundreds of thousands of ants, creating a little shortcut can save a lot of energy,” he said. “This is such a unique behavior. You have other types of ants forming structures out of their bodies, but it’s not such a huge part of their lives and daily behavior.”
The research also included Scott Powell, an army-ant expert and assistant professor of biology at George Washington University; Albert Kao, a postdoctoral fellow at Harvard who received his doctorate in ecology and evolutionary biology from Princeton in 2015; and Simon Garnier, an assistant professor of biological sciences at NJIT who studies swarm intelligence and was once a postdoctoral researcher in Couzin’s lab at Princeton.
To conduct their field experiments, Lutz and Reid constructed a 1.5-foot-tall apparatus with ramps on both sides and adjustable arms in the center with which they could adjust the size of the gap. They then inserted the apparatus into active E. hamatum raiding trails that they found in the jungle in Panama. Because ants follow one another’s chemical scent, Lutz and Reid used sticks and leaves from the ants’ trail to get them to reform their column across the device.
Lutz and Reid observed how the ants formed bridges across gaps that were set at angles of 12, 20, 40 and 60 degrees. They gauged how much travel-distance the ants saved with their bridge versus the surface area (in centimeters squared) of the bridge itself. Twelve-degree angles shaved off the most distance (around 11 centimeters) while taking up the fewest workers. Sixty-degree angles had the highest cost-to-benefit ratio. Interestingly, the ants were willing to expend members for 20-degree angles, forming bridges up to 8 centimeters squared to decrease their travel time by almost 12 centimeters, indicating that the loss in manpower was worth the distance saved.
Lutz said that future research based on this work might compare these findings to the living bridges of another army ant species, E. burchellii, to determine if the same principles are in action.
The paper, “Army ants dynamically adjust living bridges in response to a cost-benefit trade-off,” was published Nov. 23 by Proceedings of the National Academy of Sciences. The work was supported by the National Science Foundation (grant nos. PHY-0848755, IOS0-1355061 and EAGER IOS-1251585); the Army Research Office (grant nos. W911NG-11-1-0385 and W911NF-14-1-0431); and the Human Frontier Science Program (grant no. RGP0065/2012).
Living with others can offer tremendous benefits for social animals, including primates, but these benefits could come at a high cost. New research from a project that originated at Princeton University reveals that intermediate-sized groups provide the most benefits to wild baboons. The study, led by Catherine Markham at Stony Brook University and published in the journal, Proceedings of the National Academy of Sciences, offers new insight into the costs and benefits of group living.
In the paper titled “Optimal group size in a highly social mammal,” the authors reveal that while wild baboon groups range in size from 20 to 100 members, groups consisting of about 50 to 70 individuals (intermediate size) exhibit optimal ranging behavior and low physiological stress levels in individual baboons, which translates to a social environment that fosters the health and well-being of individual members. The finding provides novel empirical support for an ongoing theory in the fields of evolutionary biology and anthropology that in living intermediate-sized groups has advantages for social mammals.
“Strikingly, we found evidence that intermediate-sized groups have energetically optimal space-use strategies and both large and small groups experience ranging disadvantages,” said Markham, lead author and an assistant professor in the Department of Anthropology at Stony Brook University. “It appears that large, socially dominant groups are constrained by within-group competition whereas small, socially subordinate groups are constrained by between-group competition and/or predation pressures.”
The researchers compiled their findings based on observing five social wild baboon groups in East Africa over 11 years. This population of wild baboons has been studied continuously for over 40 years by the Amboseli Baboon Research Project. They observed and examined the effects of group size and ranging patterns for all of the groups. To gauge stress levels of individuals, they measured the glucocorticoid (stress hormone) levels found in individual waste droppings.
“The combination of an 11-year data set and more intensive short-term data, together with the levels of stress hormones, led to the important finding that there really is a cost to living in too small a group,” said Jeanne Altmann, the Eugene Higgins Professor of Ecology and Evolutionary Biology, Emeritus and a senior scholar at Princeton University. Altmann is a co-director of the Amboseli Baboon Research Project and co-founded the project in 1971 with Stuart Altmann, a senior scholar in the Department of Ecology and Evolutionary Biology at Princeton.
“The cost of living in smaller groups is a concern from a conservation perspective,” Jeanne Altmann said, “Due to the fragmentation of animal habitats, many animals will be living in smaller groups. Understanding these dynamics is one of the next things to study.” The research was supported primarily by the National Science Foundation and the National Institute on Aging.
Markham, who earned her Ph.D. at Princeton University in 2012 with Jeanne Altmann as her thesis adviser, explained that regarding optimal group sizes for highly social species the key to the analysis is how are trade-offs balanced, and do these trade-offs actually result in an optimal group size for a social species.
She said that their findings provide a testable hypothesis for evaluating group-size constraints in other group-living species, in which the costs of intra- and intergroup competition vary as a function of group size. Additionally, their findings provide implications for new research and a broader understanding of both why some animals live with others and how many neighbors will be best for various species and situations.
The research was conducted in collaboration with Susan Alberts, a professor of biology at Duke University and co-director of the Amboseli Baboon Research Project, and with Laurence Gesquiere, a former postdoctoral researcher at Princeton who is now a senior research scientist working with Alberts. Altmann and Alberts are also affiliated with the Institute for Primate Research, National Museums of Kenya.
Additional support was provided by the American Society of Primatologists, the Animal Behavior Society, the International Primatological Society, and Sigma Xi.
A. Catherine Markham, Laurence R. Gesquiere, Susan C. Alberts and Jeanne Altmann. Optimal group size in a highly social mammal. Proceedings of the National Academy of Sciences. Published online before print October 26, 2015, doi: 10.1073/pnas.1517794112 PNAS October 26, 2015
In nature, the right amount of death at the right time might actually help boost a species’ population density, according to new research that could help in understanding animal populations, pest control and managing fish and wildlife stocks.
In a paper in the journal Trends in Ecology and Evolution, a Princeton University researcher and European colleagues conclude that the kind of positive population effect an overall species experiences from a loss of individuals, or mortality, depends on the size and developmental stage of the creatures that die.
If many juveniles perish, more adults are freed up to reproduce, but if more adults die, the number of juveniles that mature will increase because density dependence is relaxed, explained co-author Anieke van Leeuwen, a postdoctoral researcher in Princeton’s Department of Ecology and Evolutionary Biology. Van Leeuwen worked with first author Arne Schröder, a postdoctoral research fellow at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin, and Tom Cameron, a lecturer in aquatic community ecology at the University of Essex in the United Kingdom.
This dynamic wherein the loss of individuals in one developmental stage translates to more robust individuals in another stage can be important to managing wildlife, pests or resource stocks, van Leeuwen said. For instance, targeting the adults of an invasive insect could have a counterproductive effect of making more food available to growing larvae, she said.
“It doesn’t matter which developmental stage you target, if you impose mortality on one you will get overcompensation on the opposite end of the size range,” van Leeuwen said. “This effect can be especially advantageous in situations where we want to manage resources we want to harvest. Knowing that there are potential effects that result in an increase in that segment of the population we want to encourage is highly relevant.”
At a certain point, of course, mortality becomes too high and the species as a whole declines, the researchers report.
The researchers compared existing theoretical and experimental work on the effect of mortality on population density to resolve various inconsistencies between the two. Existing mathematical models have predicted this phenomenon, and laboratory and field studies have shown that the effect holds true for a variety of animal species.
Many ecological theories and models, however, have ignored differences in body size and development, and predicted that a modest amount of mortality would result in an increase in the total number of individuals, the researchers wrote. On the other hand, experiments have predominantly shown — along with certain models — that mortality has a positive effect within certain life stages or size classes. The researchers concluded that the overlap of experimental and theoretical data indicates that the benefit of mortality is likely divided by developmental stage. In addition, the number of species in which the phenomenon has been observed makes it commonplace in the natural world.
This work was supported by the Journal of Experimental Biology; the Swedish Research Council and the Leibniz-Institute of Freshwater Ecology and Inland Fisheries; the University of Leeds, the National Environment Research Council (grant no. NE/C510467/1) and the European Commission Intra-European Fellowship (FANTISIZE, #275873); and the National Science Foundation (grant no. 1115838).
Citation: Schröder, Arne, Anieke van Leeuwen, Thomas C. Cameron. 2014. When less is more: Positive population-level effects of mortality. Trends in Ecology and Evolution. Published in November 2014 edition: Vol. 29, issue 11, pp. 614–624. DOI: 10.1016/j.tree.2014.08.006
In the United States, natural-gas production from shale rock has increased by more than 700 percent since 2007. Yet scientists still do not fully understand the industry’s effects on nature and wildlife, according to a report in the journal Frontiers in Ecology and the Environment.
As gas extraction continues to vastly outpace scientific examination, a team of eight conservation biologists from various organizations and institutions, including Princeton University, concluded that determining the environmental impact of gas-drilling sites — such as chemical contamination from spills, well-casing failures and other accidents — must be a top research priority.
With shale-gas production projected to surge during the next 30 years, the authors call on scientists, industry representatives and policymakers to cooperate on determining — and minimizing — the damage inflicted on the natural world by gas operations such as hydraulic fracturing, or “fracking.” A major environmental concern, hydraulic fracturing releases natural gas from shale by breaking the rock up with a high-pressure blend of water, sand and other chemicals, which can include carcinogens and radioactive substances.
“The past has taught us that environmental impacts of large-scale development and resource extraction, whether coal plants, large dams or biofuel monocultures, are more than the sum of their parts,” Tingley said.
The researchers found that there are significant “knowledge gaps” when it comes to direct and quantifiable evidence of how the natural world responds to shale-gas operations. A major impediment to research has been the lack of accessible and reliable information on spills, wastewater disposal and the composition of fracturing fluids. Of the 24 American states with active shale-gas reservoirs, only five — Pennsylvania, Colorado, New Mexico, Wyoming and Texas — maintain public records of spills and accidents, the researchers report.
“The Pennsylvania Department of Environmental Protection’s website is one of the best sources of publicly available information on shale-gas spills and accidents in the nation. Even so, gas companies failed to report more than one-third of spills in the last year,” said first author Sara Souther, a postdoctoral research associate at the University of Wisconsin-Madison.
“How many more unreported spills occurred, but were not detected during well inspections?” Souther asked. “We need accurate data on the release of fracturing chemicals into the environment before we can understand impacts to plants and animals.”
One of the greatest threats to animal and plant life identified in the study is the impact of rapid and widespread shale development, which has disproportionately affected rural and natural areas. A single gas well results in the clearance of 3.7 to 7.6 acres (1.5 to 3.1 hectares) of vegetation, and each well contributes to a collective mass of air, water, noise and light pollution that has or can interfere with wild animal health, habitats and reproduction, the researchers report.
“If you look down on a heavily ‘fracked’ landscape, you see a web of well pads, access roads and pipelines that create islands out of what was, in some cases, contiguous habitat,” Souther said. “What are the combined effects of numerous wells and their supporting infrastructure on wide-ranging or sensitive species, like the pronghorn antelope or the hellbender salamander?”
The chemical makeup of fracturing fluid and wastewater is often unknown. The authors reviewed chemical-disclosure statements for 150 wells in three of the top gas-producing states and found that an average of two out of every three wells were fractured with at least one undisclosed chemical. The exact effect of fracturing fluid on natural water systems as well as drinking water supplies remains unclear even though improper wastewater disposal and pollution-prevention measures are among the top state-recorded violations at drilling sites, the researchers found.
“Some of the wells in the chemical disclosure registry were fractured with fluid containing 20 or more undisclosed chemicals,” said senior author Kimberly Terrell, a researcher at the Smithsonian Conservation Biology Institute. “This is an arbitrary and inconsistent standard of chemical disclosure.”
The paper’s co-authors also include researchers from the University of Bucharest in Romania, Colorado State University, the University of Washington, and the Society for Conservation Biology.
The work was supported by the David H. Smith Fellowship program administered by the Society for Conservation Biology and funded by the Cedar Tree Foundation; and by a Policy Fellowship from the Wilburforce Foundation to the Society for Conservation Biology.
Souther, Sara, Morgan W. Tingley, Viorel D. Popescu, David T.S. Hyman, Maureen E. Ryan, Tabitha A. Graves, Brett Hartl, Kimberly Terrell. 2014. Biotic impacts of energy development from shale: research priorities and knowledge gaps. Frontiers in Ecology and the Environment. Article published online Aug. 1, 2014. DOI: 10.1890/130324.
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.”
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).
While carbon dioxide is typically painted as the bad boy of greenhouse gases, methane is roughly 30 times more potent as a heat-trapping gas. New research in the journal Nature indicates that for each degree that the Earth’s temperature rises, the amount of methane entering the atmosphere from microorganisms dwelling in lake sediment and freshwater wetlands — the primary sources of the gas — will increase several times. As temperatures rise, the relative increase of methane emissions will outpace that of carbon dioxide from these sources, the researchers report.
The findings condense the complex and varied process by which methane — currently the third most prevalent greenhouse gas after carbon dioxide and water vapor — enters the atmosphere into a measurement scientists can use, explained co-author Cristian Gudasz, a visiting postdoctoral research associate in Princeton’s Department of Ecology and Evolutionary Biology. In freshwater systems, methane is produced as microorganisms digest organic matter, a process known as “methanogenesis.” This process hinges on a slew of temperature, chemical, physical and ecological factors that can bedevil scientists working to model how the Earth’s systems will contribute, and respond, to a hotter future.
The researchers’ findings suggest that methane emissions from freshwater systems will likely rise with the global temperature, Gudasz said. But to not know the extent of methane contribution from such a widely dispersed ecosystem that includes lakes, swamps, marshes and rice paddies leaves a glaring hole in climate projections.
“The freshwater systems we talk about in our paper are an important component to the climate system,” Gudasz said. “There is more and more evidence that they have a contribution to the methane emissions. Methane produced from natural or manmade freshwater systems will increase with temperature.”
To provide a simple and accurate way for climate modelers to account for methanogenesis, Gudasz and his co-authors analyzed nearly 1,600 measurements of temperature and methane emissions from 127 freshwater ecosystems across the globe.
The researchers found that a common effect emerged from those studies: freshwater methane generation very much thrives on high temperatures. Methane emissions at 0 degrees Celsius would rise 57 times higher when the temperature reached 30 degrees Celsius, the researchers report. For those inclined to model it, the researchers’ results translated to a temperature dependence of 0.96 electron volts (eV), an indication of the temperature-sensitivity of the methane-emitting ecosystems.
“We all want to make predictions about greenhouse gas emissions and their impact on global warming,” Gudasz said. “Looking across these scales and constraining them as we have in this paper will allow us to make better predictions.”
Yvon-Durocher, Gabriel, Andrew P. Allen, David Bastviken, Ralf Conrad, Cristian Gudasz, Annick St-Pierre, Nguyen Thanh-Duc, Paul A. del Giorgio. 2014. Methane fluxes show consistent temperature dependence across microbial to ecosystem scales. Nature. Article published online before print: March 19, 2014. DOI: 10.1038/nature13164 and in the March 27, 2014 print edition.
By Catherine Zandonella, Office of the Dean for Research
The drug-resistant bacteria known as MRSA, once confined to hospitals but now widespread in communities, will likely continue to exist in both settings as separate strains, according to a new study.
The prediction that both strains will coexist is reassuring because previous projections indicated that the more invasive and fast-growing community strains would overtake and eliminate hospital strains, possibly posing a threat to public health.
Researchers at Princeton University used mathematical models to explore what will happen to community and hospital MRSA strains, which differ genetically. Originally MRSA, which is short for methicillin-resistant Staphylococcus aureus, was confined to hospitals. However, community-associated strains emerged in the past decade and can spread widely from person to person in schools, athletic facilities and homes.
Both community and hospital strains cause diseases ranging from skin and soft-tissue infections to pneumonia and septicemia. Hospital MRSA is resistant to numerous antibiotics and is very difficult to treat, while community MRSA is resistant to fewer antibiotics.
The new study found that these differences in antibiotic resistance, combined with more aggressive antibiotic usage patterns in hospitals versus the community setting, over time will permit hospital strains to survive despite the competition from community strains. Hospital-based antibiotic usage is likely to successfully treat patients infected with community strains, preventing the newcomer strains from spreading to new patients and gaining the foothold they need to out-compete the hospital strains.
The researchers made their predictions by using mathematical models of MRSA transmission that take into account data on drug-usage, resistance profiles, person-to-person contact, and patient age.
Published February 28 in the journal PLOS Pathogens, the study was conducted by postdoctoral researcher Roger Kouyos, now a scholar at the University of Zurich, and Eili Klein, a graduate student who is now an assistant professor in the Johns Hopkins School of Medicine. They conducted the work under the advisement of Bryan Grenfell, Princeton’s Kathryn Briger and Sarah Fenton Professor of Ecology and Evolutionary Biology and Public Affairs at Princeton’s Woodrow Wilson School of International and Public Affairs.
Kouyos R., Klein E. & Grenfell B. (2013). Hospital-Community Interactions Foster Coexistence between Methicillin-Resistant Strains of Staphylococcus aureus. PLoS Pathogens, 9 (2) e1003134. PMID: 23468619
RK was supported by the Swiss National Science Foundation (Grants PA00P3_131498 and PZ00P3_142411). EK was supported by Princeton University (Harold W. Dodds Fellowship), as well as the Models of Infectious Disease Agent Study (MIDAS), under Award Number U01GM070708 from the National Institute of General Medical Sciences. BG was supported by the Bill and Melinda Gates Foundation; the Research and Policy for Infectious Disease Dynamics (RAPIDD) program of the Science and Technology Directorate, Department of Homeland Security; and the Fogarty International Center, National Institutes of Health.
By Catherine Zandonella, Office of the Dean for Research
Trees in the continental U.S. could send out new spring leaves up to 17 days earlier in the coming century than they did before global temperatures started to rise, according to a new study by Princeton University researchers. These climate-driven changes could lead to changes in the composition of northeastern forests and give a boost to their ability to take up carbon dioxide.
Trees play an important role in taking up carbon dioxide from the atmosphere, so researchers led by David Medvigy, assistant professor in Princeton’s department of geosciences, wanted to evaluate predictions of spring budburst — when deciduous trees push out new growth after months of winter dormancy — from models that predict how carbon emissions will impact global temperatures.
The date of budburst affects how much carbon dioxide is taken up each year, yet most climate models have used overly simplistic schemes for representing spring budburst, modeling for example a single species of tree to represent all the trees in a geographic region.
In 2012, the Princeton team published a new model that relied on warming temperatures and the waning number of cold days to predict spring budburst. The model, which was published in the Journal of Geophysical Research, proved accurate when compared to data on actual budburst in the northeastern United States.
In the current paper published online in Geophysical Research Letters, Medvigy and his colleagues tested the model against a broader set of observations collected by the USA National Phenology Network, a nation-wide tree ecology monitoring network consisting of federal agencies, educational institutions and citizen scientists. The team incorporated the 2012 model into predictions of future budburst based on four possible climate scenarios used in planning exercises by the Intergovernmental Panel on Climate Change.
The researchers included Su-Jong Jeong, a postdoctoral research associate in Geosciences, along with Elena Shevliakova, a senior climate modeler, and Sergey Malyshev, a professional specialist, both in the Department of Ecology and Evolutionary Biology and associated with the U.S. National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory.
The team estimated that, compared to the late 20th century, red maple budburst will occur 8 to 40 days earlier, depending on the part of the country, by the year 2100. They found that the northern parts of the United States will have more pronounced changes than the southern parts, with the largest changes occurring in Maine, New York, Michigan, and Wisconsin.
The researchers also evaluated how warming temperatures could affect the budburst date of different species of tree. They found that budburst shifted to earlier in the year in both early-budding trees such as common aspen (Populus tremuloides) and late-budding trees such as red maple (Acer rubrum), but that the effect was greater in the late-budding trees and that over time the differences in budding dates narrowed.
The researchers noted that early budburst may give deciduous trees, such as oaks and maples, a competitive advantage over evergreen trees such as pines and hemlocks. With deciduous trees growing for longer periods of the year, they may begin to outstrip growth of evergreens, leading to lasting changes in forest make-up.
The researchers further predicted that warming will trigger a speed-up of the spring “greenwave,” or budburst that moves from south to north across the continent during the spring.
The finding is also interesting from the standpoint of future changes in springtime weather, said Medvigy, because budburst causes an abrupt change in how quickly energy, water and pollutants are exchanged between the land and the atmosphere. Once the leaves come out, energy from the sun is increasingly used to evaporate water from the leaves rather than to heat up the surface. This can lead to changes in daily temperature ranges, surface humidity, streamflow, and even nutrient loss from ecosystems, according to Medvigy.
Jeong, Su-Jong, David Medvigy, Elena Shevliakova, and Sergey Malyshev. 2013. Predicting changes in temperate forest budburst using continental-scale observations and models. Geophysical Research Letters. Article first published online: Jan. 25, 2013. DOI: 10.1029/2012GL054431
This research was supported by award NA08OAR4320752 from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.
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