Biology subjects

Hammarlund, S. P.

Publications and source records attributed to Hammarlund, S. P..

3 recordsLinked to original sources

bioRxiv2021

In an "ecological-belonging" intervention to reduce inequities in STEM, context matters

Doubts about belonging in the classroom are often shouldered disproportionately by students from historically marginalized groups, which can lead to underperformance. Ecological-belonging interventions use a classroom-based activity to instill norms that adversity is normal, temporary and surmountable. Building on prior studies, we sought to identify the conditions under which such interventions are effective. In a chemistry course (Study 1), students from underrepresented ethnic backgrounds underperformed relative to their peers in the absence of the intervention. This performance gap was eliminated by the intervention. In an introductory biology course (Study 2), there were no large performance gaps in the absence of the intervention, and the intervention had no effect. Study 2 also explored the role of the instructor that delivers the intervention. The intervention boosted scores in classrooms of instructors with a fixed (versus growth-oriented) intelligence mindset. Our results suggest that ecological-belonging interventions are more effective in more threatening classroom contexts.

scientific communication and education↗

bioRxiv2020

Limitation by a shared mutualist promotes coexistence of multiple competing partners

Although mutualisms are often studied as simple pairwise interactions, they typically involve complex networks of interacting species. How multiple mutualistic partners that provide the same service and compete for resources are maintained in mutualistic networks is an open question. We use a model bacterial community in which multiple partner strains of Escherichia coli compete for a carbon source and exchange resources with a shared mutualist strain of Salmonella enterica. In laboratory experiments, competing E. coli strains readily coexist in the presence of S. enterica, despite differences in their competitive abilities. We use ecological modeling to demonstrate that a shared mutualist can create temporary resource niche differentiation by limiting growth rates, even if yield is set by a resource external to a mutualism. This mechanism can extend to maintain multiple competing partner species. Our results improve our understanding of complex mutualistic communities and aid efforts to design stable microbial communities.

ecology↗

bioRxiv2018

A shared limiting resource leads to competitive exclusion in a cross-feeding system

Species interactions and coexistence are often highly dependent upon environmental conditions. This is especially true for cross-feeding bacteria that rely on one another for essential nutrients. The addition of a cross-fed nutrient to the environment can release one species from its dependence on another, thereby altering the species interaction and potentially affecting coexistence. Using invasion-from-rare experiments with cross-feeding bacteria, genome-scale metabolic modeling, and classical ecological models, we explored the potential for coexistence when one cross-feeding mutualist becomes independent. We show that whether nutrient addition shifts an interaction from mutualism to commensalism or parasitism depends on whether the limiting nutrient can be metabolized by only one species or by both species. Furthermore, we show that coexistence is only lost when the interaction becomes parasitism, and the obligate species has a slower maximum growth rate. Surprisingly, models suggest that rates of cross-fed nutrient production have a negligible effect. These results contribute to an understanding of how resource changes, whether intentional or not, will manipulate interactions and coexistence in microbial communities.

ecology↗