Conservation Scientist/Geneticist Chicago Botanic Garden/Northwestern University, United States
Abstract: Theory states that for populations that are small the risk of extinction is large. The small population paradigm was theorized to describe how demographic dynamics and population genetics contribute to increasing small populations’ susceptibility to stochastic events. However, numerous species have always persisted at low numbers with minimal impact, suggesting that this idea might be an over generalization. Population extinction from stochastic events is inherently unpredictable, and uncoupling the demographic drivers from the impacts of limited genetic diversity and inbreeding on small population size is a major challenge. This study aims to examine the predictions of the small population paradigm and importantly attempt to untangle the demographic effects from the genetic ones to improve our ability to predict extinction risk and better manage small, rare plant populations. This study aims to 1) assess the demographic consequences of population size, 2) assess the genetic differences by populations size, and 3) finally determine if demographic factors, genetic parameters, or the interaction of both are best at predicting population fitness. To address these objectives, censuses of a rare midwestern endemic plant were conducted to identify a gradient of small to large populations. Plant size and reproductive fitness data were collected from each population, and genetics metrics were calculated from sequence data. To determine further fitness differences, we conducted a drought experiment on plants grown from seed collected in small and large populations and that had high and low genetic diversity, and different combination of both. This experiment aimed to assess each population’s ability to cope with an ecologically relevant event. We found a positive correlation between population size and reproductive fitness and genetic diversity, but not plant size. There were population level differences between other fitness metrics, including response in plant growth and survival under experimental drought conditions. These results hint at the genetic and fitness increased stress placed on small populations. While some small populations may persist through stochastic events, if inbreeding increases and genetic diversity is lost, fitness reductions may hinder the populations’ ability to adapt. It may be prudent to consider pathways for preventing the loss of genetic diversity and minimize inbreeding when managing small populations. Future work should address how isolation, gene flow, and potential genetic rescue can aid in decreasing the susceptibility of certain small populations to stochastic events such as drought.
