Student Feature - Samuel Fleischer


Sam Fleischer

My introduction to dynamical systems came from CSU Northridge mathematician Dr. Jing Li, who started the first lecture of an Ordinary Differential Equations class this way: "A differential is a measure of change, and an equation is a relationship between two quantities.  Therefore, the study of differential equations is the study of changing relationships."  This deeply affected my relationship to mathematics, and as I learned more about dynamical systems I was struck by the simplicity and beauty of the flowing phase plane and the steady bifurcation diagram.

Prior to attending CSU Northridge and majoring in mathematics, I was a student at the Conservatory of Theatre Arts at Purchase College in New York.  While I enjoyed my time acting, I was not inspired by old theatrical texts or by practicing getting into character.  My friends in my cohort spent their free time analyzing movies and going to plays in the city, while I spent many hours enjoying what I now call "popular math," obsessing over Vi Hart YouTube videos and exploring the patterns of Punnett Squares.

One of my acting professors said, "If you can envision yourself doing anything other than acting, you should quit this program now."  It seemed harsh at the time, but she was only trying to protect us from a cold and unforgiving career in the entertainment industry.  After months of reflection, I figured out that I was not fully committed to pursuing an acting career and made the decision to drop out of school and move home.  It took me six months to decide to enroll myself in community college.

Having been inspired to study math from Vi Hart and other YouTubers, I began to feel a sense of agency about academia that I had never felt before.  Community college was the perfect place for me at that time in my life.  I had a general sense of excitement about school, but very little direction.  Community college in America is significantly more affordable than public universities, which made it possible for me to explore intro courses in a variety of fields and get as much help from professors as I needed.

I started to envision myself as a high school math teacher, and my plan was to major in math and get my teaching credential when I transferred to CSU Northridge.  That all changed in Dr. Li's class.  Her statement about differential equations as the "the study of changing relationships" was so intriguing to me, especially since I had studied changing relationships in my script analysis classes in acting school.  When I expressed to her my interest in the subject, Dr. Li was kind enough to introduce me to the world of academic research.  We worked with CSUN biologist Dr. Casey terHorst on what is now an eco-evolutionary theory paper regarding the effects of different evolutionary tradeoffs on predator-prey coevolutionary dynamics [1].  That project was heavily influenced by a theory paper from UC Davis mathematical biologist Dr. Sebastian Schreiber et al [2].  I eventually enrolled in graduate school at UC Davis to be part of Dr. Schreiber’s lab, and am now a fourth-year PhD Candidate in the Graduate Group in Applied Mathematics.

My research at UC Davis is a natural extension of my work with Dr. Li and Dr. terHorst.  I am interested in the ecological and evolutionary dynamics of trophically transmitted parasites (parasites which move from host to host via predation) and the communities in which they reside.  One such parasite is the tapeworm Schistocephalus Solidus, which begins its parasitic life when an egg hatches in a lake.  For the tapeworm to reproduce, it must first be consumed by a plankton, which must then be eaten by a particular fish (the threespine stickleback), which itself must then be eaten by a bird.  The tapeworm lays eggs in the bird's gut, and those eggs are reintroduced to a lake when the bird defecates, and the cycle begins again [3].

In addition to stealing their host's energy, many parasites manipulate their host's behavior.  A stickleback infected with S. solidus is significantly less likely to react to predation threats from birds and larger predatory fish [4,5].  The flatworm Leucochloridium infects snails and causes their antenna to pulsate, which attracts predation by birds (their definitive hosts) [6].  Parasitic fungi infect a variety of insects and influence them to climb to the tops of plants in what seems like an optimal position for the dispersal of fungal spores by wind [7,8].  My research is centered on the question "how does the evolution of parasitic manipulation affect the ecological and evolutionary dynamics of the hosts?"  I am also interested in the converse, "how do community dynamics affect the evolution of parasitic manipulation?"  I am currently working with Dr. Schreiber and University of Connecticut biologist Dr. Dan Bolnick to begin to answer these questions.

Another major influence in my professional life has been my time as an intern at NASA Jet Propulsion Laboratory.  In my first semester at CSU Northridge, I was lucky enough to make friends with a JPL intern who eventually recommended me to his boss.  She gave me very challenging but feasible tasks, and my programming skills improved dramatically in just a few months.  As an undergraduate intern, I improved the ground-based software system testing process for the Cassini spacecraft by creating an organizational tool to run nightly software tests.  In Summer 2018, I transferred part of a model of the Europa Clipper spacecraft to a new discrete event simulator currently in development at JPL.  In Summer 2019, I will work on cost and risk assessment for the Europa Clipper spacecraft team to meet a directive from the Office of Planetary Protection.  In particular, I will be working on proving the probability that the spacecraft contaminates Europa with terrestrial organisms is less than 1/10000 [9].

I think all of this work is fun and exciting, and I am so lucky to have been given these opportunities.  I went from being an acting school drop-out to a NASA intern with graduate school prospects in less than two years.  While I worked hard to get to where I am, I often reflect on my immense amount of privilege I have in society.  I wonder if I would have been taken as seriously if I hadn't been born a white, heterosexual, cisgender, upper-middle-class, able-bodied man in a community with well-funded schools and extracurriculars.  I had fine grades and a positive attitude, but in retrospect my resume was sparse compared to many of my current undergraduate students.

I want everyone to have the opportunities I've been given, so I use my position of privilege to join in the dismantling of structural white supremacy, patriarchy, and other forms of wrongful discrimination in my communities.  This includes taking action in my classrooms, my department, and my school.  As a TA and instructor, I have made a number of low-cost efforts to make my classroom a more inclusive space.  My friends and I cofounded the UC Davis chapter of the Association for Women in Mathematics, and we use our resources for outreach to K-12 schools in local underprivileged communities, workshops to educate members of our own department regarding social justice issues within academia, and a campaign to push for gender-neutral bathrooms in the math department building.  Our next major project is a fundraising campaign to reimburse the hefty graduate school application fee for low-income and underrepresented applicants to the Math and Applied Math PhD programs.

Wherever my future takes me, I hope to stay passionate about life, engaged with my work, and humbled by those less fortunate than me.


[1] S. Fleischer, C. terHorst, and J. Li. Pick your trade-offs wisely: Predator-prey eco-evo dynamics are qualitatively different under different trade-offs. Journal of Theoretical Biology 456 (2018), pp. 201-212.

[2] S. J. Schreiber, R. Bürger, and D. I. Bolnick. The community effects of phenotypic and genetic variation within a predator population. Ecology 92 (2011), pp. 1582-1593.

[3] I Barber and J. P. Scharsack. The three-spined stickleback-Schistocephalus solidus system: an experimental model for investigating host-parasite interactions in fish. Parasitology 137 (2010), pp. 411-424.

[4] C. Wedeking and M. Milinski. Do three-spined sticklebacks avoid consuming copepods, the first intermediate host of Schistocephalus solidus? - an experimental analysis of behavioural resistance. Parasitology 112 (1996), pp. 371-383.

[5] J. H. Ness and S. A. Foster. Parasite-Associated Phenotype Modifications in Threespine Stickleback. Oikos 85 (1999), pp. 127-134.

[6] C. Wesenberg-Lund. Contributions to the Development of the Trematoda Digenea: The Biology of Leucochloridium paradoxum. Levin & Munksgaard, 1931.

[7] D. P. Maitland. A parasitic fungus infecting yellow dungflies manipulates host perching behaviour. Proc. R. Soc. Lond. B 258 (1994), pp. 187-193.

[8] R. Poulin. Evolutionary ecology of parasites. Princeton university press, 2011.

[9] Board, Space Studies. Preventing the forward contamination of Europa. National Academies Press, 2000.

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