Tag Archives: evolutionary biology

When less is more: Death in moderation boosts population density in nature (Trends in Ecology and Evolution)

A study by Princeton researchers and European colleagues found that the positive effect that mortality can have on populations depends on the size and developmental stage of the creatures that die. The finding could aid the management of wildlife and fish such as the Atlantic cod (Image source: NOAA).

A study by Princeton researchers and European colleagues found that the positive effect that mortality can have on populations depends on the size and developmental stage of the creatures that die. The finding could aid the management of wildlife and fish such as Atlantic cod (Image source: NOAA).

By Morgan Kelly, Office of Communications

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).

Read the abstract.

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

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).

From dark hearts comes the kindness of mankind (Evolution)

By Morgan Kelly, Office of Communications

The kindness of mankind most likely developed from our more sinister and self-serving tendencies, according to Princeton University and University of Arizona research that suggests society’s rules against selfishness are rooted in the very exploitation they condemn.

The report in the journal Evolution proposes that altruism — society’s protection of resources and the collective good by punishing “cheaters” — did not develop as a reaction to avarice. Instead, communal disavowal of greed originated when competing selfish individuals sought to control and cancel out one another. Over time, the direct efforts of the dominant fat cats to contain a few competitors evolved into a community-wide desire to guard its own well-being.

The study authors propose that a system of greed dominating greed was simply easier for our human ancestors to manage. In this way, the work challenges dominant theories that selfish and altruistic social arrangements formed independently — instead the two structures stand as evolutionary phases of group interaction, the researchers write.

Second author Andrew Gallup, a former Princeton postdoctoral researcher in ecology and evolutionary biology now a visiting assistant professor of psychology at Bard College, worked with first author Omar Eldakar, a former Arizona postdoctoral fellow now a visiting assistant professor of biology at Oberlin College, and William Driscoll, an ecology and evolutionary biology doctoral student at Arizona.

To test their hypothesis, the researchers constructed a simulation model that gauged how a community withstands a system built on altruistic punishment, or selfish-on-selfish punishment. The authors found that altruism demands a lot of initial expenditure for the group — in terms of communal time, resources and risk of reprisal from the punished — as well as advanced levels of cognition and cooperation.

On the other hand, a construct in which a few profligate players keep like-minded individuals in check involves only those members of the community — everyone else can passively enjoy the benefits of fewer people taking more than their share. At the same time, the reigning individuals enjoy uncontested spoils and, in some cases, reverence.

Social orders maintained by those who bend the rules play out in nature and human history, the authors note: Tree wasps that police hives to make sure that no member other than the queen lays eggs will often lay illicit eggs themselves. Cancer cells will prevent other tumors from forming. Medieval knights would pillage the same civilians they readily defended from invaders, while neighborhoods ruled by the Italian Mafia traditionally had the lowest levels of crime.

What comes from these arrangements, the researchers conclude, is a sense of order and equality that the group eventually takes upon itself to enforce, thus giving rise to altruism.

Read the abstract.

Eldakar, O. T., Gallup, A. C. and Driscoll, W. W. (2013), When Hawks Give Rise To Doves: The Evolution and Transition of Enforcement Strategies. Evolution. doi: 10.1111/evo.12031

This work was supported by the National Institutes of Health.