Rare species advantage in Antarctic Lakes

Abstract: The maintenance of diversity in complex ecological communities despite unpredictable dynamics and competitive exclusion is thought to require continual influx of new species or competitive advantages that accrue as species become rare. We examine isolated planktonic microbial communities under permanent ice cover in Antarctic lakes, recording prokaryotic abundance across 9 communities, 11 years, 30~m of depth, and thousands of species in the McMurdo LTER. We quantify rare species advantage by modeling community dynamics under frequency-dependent selection. We find persistent diversity and pervasive negative frequency dependence with limited immigration and turnover. While ecology and evolutionary sciences have long debated whether diversity is maintained selectively, we measure selection over a $10^4$-fold range of abundance in naturally coevolving communities and implicate rare species advantage.

• The paper demonstrates that strong negative frequency‐dependent selection gives rare Antarctic lake microbes a measurable fitness advantage.
• It employs extensive sequencing over 11 years across multiple lake layers to quantify selection effects on thousands of prokaryotic species.
• Findings challenge neutral theory by showing that intrinsic selective dynamics, not immigration, maintain microbial diversity.

Rare Species Advantage and Frequency-Dependent Selection in Antarctic Lakes

Introduction

"Rare species advantage in Antarctic Lakes" (2601.14213) investigates the mechanisms underlying persistent microbial diversity within Antarctic lake ecosystems, characterized by long-term physical isolation and negligible immigration. The study specifically quantifies frequency-dependent selection (FDS) in planktonic microbial communities over 9 communities, spanning 11 years, 30 m of depth, and encompassing thousands of prokaryotic species in the McMurdo Dry Valleys (MDV). The primary goal is to elucidate whether negative frequency dependence—advantage accrued by rare species—serves as a selective mechanism sufficient to sustain diversity in these hyperstable environments.

Background and Theoretical Context

Ecological theory traditionally predicts competitive exclusion, limiting coexistence in simple models. Nonetheless, real ecosystems exhibit rich diversity, driving empirical and theoretical debate between niche-based stabilizing mechanisms and neutral theories that posit diversity arises from continual influx via immigration, mutation, and speciation. Negative frequency dependence, where the fitness of a species declines with its abundance, has been hypothesized to provide rare species with an advantage, enabling their persistence against competitive exclusion. While this hypothesis has been debated and difficult to quantify in complex, naturally assembling communities, Antarctic lakes offer a unique setting: extreme isolation, minimal exogenous perturbation, and finely stratified microbiota present an opportunity to directly test the prevalence and strength of FDS in situ.

Methods

The authors sampled three lakes—Bonney (east and west lobes) and Fryxell—over 11 field seasons. Using Illumina NextSeq 2000 sequencing, over 52 million reads were obtained along extensive depth profiles. Rigorous DNA sequence data processing yielded 10,776 ribotypes, with community compositions analyzed across epilimnion (E), chemocline (C), and hypolimnion (H) strata. The FDS was inferred by fitting a piecewise-constant selection function $s(x)$ against interannual time series of species frequency data, following a general maximum likelihood approach [Newberry and Plotkin 2022]. The effective FDS represents the empirical association between relative growth rates and frequency across species.

Results

Persistent, Stratified Diversity

Distinct communities were observed to persist over time and depth. Although 17 ribotypes were ubiquitous among all samples, their representation within communities was highly variable (0.9–65%). Rarefied species richness spanned a 3.1-fold range across communities and lacked correlation with time, population size, or sampling effort, indicating a diversity near steady state.

Negative Frequency-Dependent Selection

The central finding is strong, nonlinear, negative FDS in all nine communities, indicating a measurable advantage to rare species that extends across a $10^4$-fold abundance range (down to frequencies of 0.003%). Selection is significantly positive for the rarest species and significantly negative for the most common, as supported by 95% bootstrap CIs. Numerically, common species (0.1–35% abundance) declined at rates between $-6.9$ and $-1.2$\%/year, while rare species (0.003–0.2%) increased in abundance at rates ranging between $6.7$ and $300$\%/year, contingent on specific lake and depth.

A common shape of $s(x)$ described by depth-dependent scaling explained the majority (79.8%) of variance in FDS coefficients. Depth modulates overall community dynamics, with shallower strata exhibiting faster selection and turnover rates.

Limited Immigration and Turnover

Cumulative species-time curves revealed limited long-term immigration, with upper bounds of 1.4% of novel species per known species per year entering the observable frequency window. Top-list turnover rates for the most abundant 25 species were depth-dependent, occurring 1.9-fold faster at surface layers compared to deep hypolimnion. These quantitative constraints support the assertion that persistent diversity cannot be attributed to sustained immigration or rapid turnover but is maintained by intrinsic selective dynamics.

Discussion

The study provides direct quantitative evidence that negative frequency-dependent selection is pervasive and robust in Antarctic lake microbial communities. This result contradicts neutral theory and competitive exclusion, explicitly supporting selective models in which rare species accrue fitness advantages that stabilize coexistence. The observed negative FDS is consistent with several hypothesized dynamical regimes, such as kill-the-winner, boom-bust cycles, diversity waves, storage effects, and exclusion-of-the-fittest [e.g., Thingstad 2000, Doebeli et al. 2021], all of which posit mechanisms favoring rarity.

Physical isolation and minimal immigration further reinforce that observed diversity is a product of coevolution and endogenous dynamics, not the result of constant external input. The analytical framework allows for generalization to other hyperstable or isolated systems, where similar measurements could clarify the role of coevolutionary history and community assembly in stabilizing diversity.

Implications and Future Directions

Practical implications extend to conservation biology and ecosystem management. The results demonstrate that loss of species interactions stabilizing coexistence may be as critical as species loss itself. As Antarctic lakes warm and approach seasonal ice-free states (predicted by 2050), increased influx might disrupt the historical equilibrium, potentially eroding the rare species advantage and resulting diversity structure.

Theoretically, this work suggests emergent effective negative frequency dependence may be an inherent property of complex, competitive dynamics in natural ecosystems. Future extensions should address the transferability of these findings to systems with less pronounced isolation or coevolutionary imprint, and how anthropogenic alterations affect stabilizing mechanisms.

Conclusion

This investigation establishes that rare species advantage, quantifiable as negative frequency-dependent selection, is a pervasive, robust, and necessary mechanism for maintaining microbial diversity in isolated Antarctic lake communities. These findings challenge the adequacy of neutral models, reinforce the centrality of selective interactions in ecological theory, and highlight the nuanced, emergent properties of complex coevolving systems (2601.14213). The study provides an analytical framework applicable for future empirical work in diverse ecological contexts.