Fungal Symbionts Buffer Stochastic Population Dynamics

July 27, 2020 | 3 Minute Read

Increased climatic variability is a critical expectation of climate change. Mutualisms often provide context-dependent benefits to hosts that are beneficial under abiotic stress while being neutral or even slightly costly in non-stressful conditions. This context-dependence has the potential to reduce the extremely good and the extremely bad years that the host experiences. Symbiotic mutualists may benefit their hosts by buffering demographic variability, even if their effects are neutral on average across years because environmental variability is generally bad for long term population growth.

Fungal endophytes are widespread symbionts of grasses that have been shown to provide a variety of context dependent benefits under environmental stress such as drought or salinity tolerance. These fungal endophytes are vertically transmitted through the seeds of their plant hosts. Context-dependence may make interactions seem unpredictable, but it provides a distinct mechanism by which symbionts may act as mutualists beyond influencing mean population growth rates. In collaboration with Dr. Jenn Rudgers and Dr. Ken Whitney we are quantifying the mean and variance effects of symbionts using long-term demographic data from experimental plots in Indiana at Lilly-Dickey Woods.

The experiment, started in 2007, comprises 10-18 plots for each of 7 species of grass hosts. Half the plots were planted with 20 endophyte-infected plants, and half with 20 endophyte-free plants. Endophyte free plants were generated by heat treating seeds, which kills the fungus, but still permits seeds to germinate. Each year, plots are censused for the survival, growth (measured as the number of tillers), flowering, and seed production of each plant, including new recruits. Endophytes tend to have small effects on mean vitals and do reduce variance across years. These effects are variable across vital rates and across species.

From these vital rate estimates, we build stochastic matrix population models to make population projections which allow us to assess the contributions of endophyte partnership on both the mean and variance components of host fitness.

While increasing mean temperature and precipitation are common predictions of climate change, increases in environmental variability are also expected. I'm particularly interested in extending this project by forecasting the consequences that expected increased variability in the future may have for variance buffering, as well as investigating the causes behind the differences that we observe between species. Certain host life histories may benefit more from variance buffering than others, and there may be tradeoffs between a strong mean endophyte effect and variance buffering.

You can see a poster presented at ESA 2020 for this project here, and see select references for this project below:

1. Lewontin, R C, and D Cohen. “ON POPULATION GROWTH IN A RANDOMLY VARYING ENVIRONMENT,” 1969. https://www.pnas.org/content/pnas/62/4/1056.full.pdf.
2. Boyce, Mark S., Chirakkal V. Haridas, Charlotte T. Lee, and the NCEAS Stochastic Demography Working Group. “Demography in an Increasingly Variable World.” Trends in Ecology & Evolution 21, no. 3 (March 2006): 141–148. https://doi.org/10.1016/J.TREE.2005.11.018.
3. Morris, William F., and Daniel F. Doak. “Buffering of Life Histories against Environmental Stochasticity: Accounting for a Spurious Correlation between the Variabilities of Vital Rates and Their Contributions to Fitness.” The American Naturalist 163, no. 4 (April 1, 2004): 579–90. https://doi.org/10.1086/382550.
4. Caswell, H. (2010). Life table response experiment analysis of the stochastic growth rate. Journal of Ecology, 98(2), 324-333.
5. Faeth, Stanley H. “Are Endophytic Fungi Defensive Plant Mutualists?” Oikos 98, no. 1 (July 2002): 25–36. https://doi.org/10.1034/j.1600-0706.2002.980103.x.
6. Saikkonen, K, S H Feath, M Helander, T J Sullivan, and S H Faeth. “FUNGAL ENDOPHYTES: A Continuum of Interactions with Host Plants.” Annual Review of Ecology and Systematics 29, no. 1998 (1998): 319–343. https://doi.org/10.1146/annurev.ecolsys.29.1.319.