A groundbreaking study published in the open-access journal PLOS One reveals that the long-assumed replacement fertility rate of 2.1 children per woman may be significantly underestimated when considering the complex realities of population dynamics. The research, led by Takuya Okabe of Shizuoka University and his colleagues, demonstrates that human populations require an average fertility rate of at least 2.7 children per woman to reliably avoid extinction over the long term. This finding challenges conventional demographic assumptions and sheds new light on the nuanced interplay between fertility, mortality, and population sustainability.
Traditional demographic models have long held that a fertility rate of approximately 2.1 children per woman ensures population replacement, as it accounts for average mortality rates excluding migration. However, Okabe and his team point out a crucial oversight in these calculations: the role of stochasticity, or random variation, in births and deaths within populations. Especially in small populations, random fluctuations can drastically alter survival outcomes. These variations include differences in individual reproductive success, mortality rates, sex ratios at birth, and the proportion of adults who never reproduce. Such demographic heterogeneity can increase extinction risk in ways not captured by deterministic models.
By employing sophisticated computational simulations grounded in mathematical modeling, the researchers explored how these random demographic factors influence population persistence across generations. Their probabilistic framework incorporated numerous variables such as the variability in family sizes, skewed sex ratios, and the statistical chances that certain lineages might terminate abruptly. Their results indicate that when these random processes are taken into account, the population’s fertility threshold to ensure survival rises notably. The study’s simulations consistently showed that a fertility rate below 2.7 children per woman carries a significant risk of eventual extinction, particularly under conditions of population stress or size constraints.
One intriguing aspect of the research is its examination of sex ratios and their impact on population sustainability. The models suggest that populations with a female-biased birth ratio—where more females are born relative to males—face lower extinction risks. This counterintuitive result aligns with empirical observations from ecology and anthropology, where stressful environmental or social conditions often correlate with increased female births. The study posits that such adjustments in sex ratio may serve as an evolutionary response to bolster the survival chances of populations facing critical pressures, effectively offsetting some risks introduced by fertility variability.
The implications of these findings extend far beyond theoretical population biology. Practically, the research compels us to reconsider existing fertility targets used in policy-making, conservation biology, and demographic forecasting. In human societies, sustaining cultural diversity and lineage continuity may require higher fertility rates than current replacement thresholds suggest. This is particularly salient in small or isolated populations, which are more vulnerable to random demographic fluctuations that can erode genetic diversity and cultural transmission over time.
Moreover, conservation strategies for endangered species can benefit from incorporating such stochastic demographic effects into their population viability analyses. Setting fertility or reproduction goals that ignore random birth and death fluctuations may unintentionally underestimate extinction risks, leading to insufficient conservation efforts. The study’s modeling approach offers a more nuanced, statistically rigorous tool to assess these risks and better inform management practices.
The research also highlights the significance of demographic stochasticity in shaping evolutionary outcomes. Over many generations, the random failure of family lineages inherently limits the continuity of genetic and cultural traits, even within large populations. This insight challenges deterministic perspectives that often treat populations as homogeneous units, instead emphasizing the importance of considering individual-level variability and its cumulative impact on population trajectories.
Central to these conclusions is the emphasis on “true” population sustainability, defined not just by maintaining overall population numbers but by preserving the diversity and continuity of lineages that underpin cultural, genetic, and ecological stability. The authors underline that sustainable populations are those that balance fertility, mortality, and sex ratio dynamics sufficiently to offset random extinctions of family lines, thus preserving rich biodiversity and human cultural heritage alike.
Diane Carmeliza N. Cuaresma, one of the study’s co-authors, summarizes the essence of the findings, stating, “Considering stochasticity in fertility and mortality rates, and sex ratios, a fertility rate higher than the standard replacement level is necessary to ensure sustainability of our population.” This statement encapsulates the study’s challenge to traditional demographic dogma and underscores the necessity of integrating stochastic models into future population research and regulation.
The study was funded by the Japan Society for the Promotion of Science (JSPS) through multiple KAKENHI grants and involved collaboration among expert researchers across Japan and the Philippines. Their independent work declares no competing interests, emphasizing the scientific integrity and transparency of their findings.
Published on April 30, 2025, this research represents a crucial advancement in our understanding of population biology and demographic science. It calls for a reevaluation of fertility goals globally, especially in the context of small or vulnerable populations, as well as endangered species conservation. In a world facing increasing environmental challenges, recognizing the elevated fertility thresholds required to maintain population viability may become integral to safeguarding biodiversity and cultural legacies for future generations.
For scientists, policymakers, and conservationists alike, this study provides a potent reminder that the realities of population survival are governed not by averages alone but by the unpredictable complexities of life’s intrinsic variability. Ensuring that this complexity is adequately modeled and incorporated into strategies will be essential for addressing the demographic challenges of the 21st century and beyond.
Subject of Research: Not applicable
Article Title: Threshold fertility for the avoidance of extinction under critical conditions
News Publication Date: April 30, 2025
Web References: https://doi.org/10.1371/journal.pone.0322174
References: Cuaresma DCN, Ito H, Arima H, Yoshimura J, Morita S, Okabe T (2025) Threshold fertility for the avoidance of extinction under critical conditions. PLoS ONE 20(4): e0322174.
Image Credits: Rafael AS Martins, Unsplash, CC0
Keywords: Extinction, Female fertility, Birth rates, Mortality rates, Sustainability, Human population, Sex ratios, Cultural diversity, Adults, Population studies, Social research, Mathematical modeling, Mass extinctions, Evolution
