Public Release: 

It's mostly luck, not pluck, that determines lifetime reproductive success

Cornell University

ITHACA, N.Y. - A tree will drop hundreds of seedlings over the course of its lifetime, but only a small fraction will ever get to grow into the forest canopy like their progenitor. Studies have also shown that dominant female birds can produce up to 95 percent of the offspring within a group.

Why are those things true? Can one seedling, or one female bird, be so superior to the rest that it will inevitably become the "lucky" one to grow to the sky, or help perpetuate the species? The short answer: No.

"It's mostly a matter of not dying," Stephen Ellner, professor of ecology and evolutionary biology at Cornell University, said with a wry smile.

Ellner and Robin Snyder, associate professor of biology at Case Western Reserve University, are co-authors of "Pluck or Luck: Does Trait Variation or Chance Drive Variation in Lifetime Reproductive Success?" It was published online in American Naturalist.

"Not dying" sounds pretty simplistic, but it gets to the heart of the pair's research. In short, lifetime reproductive success (LRS) is in great measure a product of luck and not superior traits.

"The motivation for all of it is the fact that reproductive skew is very common," Ellner said. Reproductive skew is the unequal sharing of reproduction within a group.

"We talk about the one percent of people with most of the money," he said, "but it's the same thing with the birds and the bees and the trees and the grasses and all of that. Often, a small fraction of individuals have most of the offspring."

Ellner and Snyder used theoretical modeling, along with two published case studies - one involving a bird (Rissa tridactyla, the black-legged kittiwake), the other a shrub (Artemisia tripartita) and a perennial grass (Pseudoroegneria spicata) - to make the case that while trait variation can influence the fate of a population, the fates of individuals are often determined by "dumb luck."

"How much of the variability in outcomes is because of differences in quality between individuals, and how much is sheer luck? It turns out that a lot of it is sheer luck," Ellner said, noting that ecologists are likely to take issue with their contentions.

"It's a bit contrarian at this point to a lot of the things that ecologists have been focusing on for the last couple of decades," Ellner said. "It lines up very much with what economists like [Henrietta Johnson Louis Professor of Management] Robert Frank have been saying about people: Our intrinsic differences in talent are much smaller than the variability in outcomes."

And while Ellner's work has not touched on variability in human success, "for birds and shrubs, and in the theoretical models we've looked at, the answer is, it's best to be above average and be lucky."

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The pair's research was supported by grants to both researchers from the National Science Foundation.

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