Research interests

My PhD research explores the eco-evolutionary feedback of competition on the persistence of genotypes, populations, and species. The role of competition is well documented and highlighted from community assembly to ecological speciation; however, competition is rarely investigated in ways that can quantify evolutionary dynamics and predict evolutionary outcomes. To make progress, I apply novel experimental designs by combining classical and modern competition theory used to predict ecological persistence to answer evolutionary questions.

Fig. 1. Testing the evolution of coexistence across allopatric and genetically diverging lineages. (A) The role of tempo and mode of ecological differentiation on the speciation process. (B) We reconstructed evolutionary relationships and estimated genetic distance among pairwise populations using genome-wide SNP data. The numbers at the tips correspond to different populations. (C) Each green point corresponds to a unique S. polyrhiza population, with black lines denoting intraspecific populations that were competed to test coexistence. To examine how coexistence evolves beyond the speciation boundary, we also sourced one outgroup population of S. polyrhiza’s closest relative S. intermedia from Uruguay (orange star) that was competed against all 126 S. polyrhiza populations (orange lines). (D) We parameterized coexistence and its mechanisms by conducting ‘invasibility tests’ in a common garden in the greenhouse using experimental arenas containing artificial pond media. For each population, we quantified the low-density, invasion growth rate of every population (n=6 starting individuals of each population) growing either in the absence of competitors (i.e., ‘alone’) or in the presence of a competing population held at their carrying capacity (i.e., ‘together’; see supplimentary materials for equilibrium conditions of competing populations). (E) We quantified per capita invasion growth rates by tracking the number of individuals over eight days, which we then used to calculate niche differences (ND), fitness differences (FD), and the outcome of coexistence. See supplementary materials for experimental details.

Coexistence accumulates prior to speciation in duckweed

Overview: I investigate the origin of ecological differences and uncover the rapid accumulation of coexistence mechanisms at evolutionary scales critical to speciation. I quantify the component mechanisms of coexistence both before and after the speciation boundary, and show coexistence mechanisms rapidly evolve prior to speciation.

(CSEE 2024 talk on youtube here), (GRC Speciation 2023 Poster here)

Fig. 2. Key finding from my first research chapter. Panel A: Showcasing differences in evolutionary history among populations of V. microstatchys in response to invasive species B. hordeaceus. Panel B: Key finding that intraspecific competition evolves in response to an invader.

The evolution of competitive ability

Sakarchi, J., and R. M. Germain. 2023. The evolution of competitive ability. The American Naturalist. (PDF here)

Overview: I explore how all the component parameters of competitive ability can evolve in response to a dominant invasive competitor. I find that: (i) life history trade-offs do not constrain the evolution of competitive ability, (ii) intraspecific competition can evolve in response to a dominant competitor, and (iii) using a novel experimental design, competition parameters can be further decoupled into their component parts first theoretically described over 30 years ago.

(Evolution 2021 talk on youtube here)

Fig 3. Key finding from my second research chapter. Panel A: Arrow width denotes interaction strengths of V. microstachys, and seed count indicates per germinant low density growth rate. Panel B showcases posterior distributions of foreign genotypes relative to local genotype I on the coexistence biplot along two axes: frequency dependent niche differences (ND) and density dependent competitive asymmetries (CA) . Region in blue represents demographic coexistence (ND >0) and priority effects (ND <0).

How density dependence can affect evolutionary outcomes

Sakarchi, J., and R. M. Germain. (IN PREP)

Overview: I apply modern coexistence theory to experimentally quantify eco-evolutionary outcomes of competing genotypes. I showcase the importance of ecological parameterization and find that ecological differences among genotypes can be highly asymmetrical (A), play an important role in predicting demographic outcomes (B), and possibly counteract selective advantages expected with density independent approximations of fitness (C).

Fig. 4. Clarifying confusion about differences between attack rates, consumption rates, and utilization (panels A-D). Panels (A, C) depict the attack rate functions of two consumers for any given level of resource abundance. Panel (E) shows an underpacked community where some resources are underutilized, leaving the community susceptible to invasion, whereas panel (F) presents a community of species that fully utilize available resource production and thus are fully “packed”.

See manuscript for more detail.

MacArthur's consumer resource model: A 'Rosetta Stone' for competitive interactions

Sakarchi, J., and R. M. Germain. (Accepted at AmNat). (PREPRINT here)

Overview: Written for a special issue on ‘demystifying theory’, I decipher MacArthur’s consumer resource model and showcase its centrality to competitive interactions. Ultimately this article aims to showcase the modern utility for MacArthur’s consumer resource model through building a deeper intuition of how competition operates, exploring assumptions and nuances of resource competition (including original perspectives), and where the model can be used today.