A model to quantify species interactions is proposed in the new Early View paper “Costs, benefits, and loss of vertically transmitted symbionts affect host population dynamics” by Kelsey M. Yule, Tom E.X. Miller and Jennifer A. Rudgers. Below is Kelsey’s background story to the study:
How do we quantify the relationship between two species? When individuals of the two species interact at many points throughout their lifetime, the answer is not as simple as we sometimes assume. The effect of the interaction at different points in the species’ life history may not affect fitness in the same way. Take, for example, an insect pollinator that greatly increases the reproductive success of its plant partner. We might consider this a classic example of a mutualism, as the presence of the pollinator allows the plant to produce more seeds and the presence of the plant allows the pollinator to produce more eggs. However, that same pollinator may lay those eggs directly onto the plant, which later suffers significant herbivory damage from the larvae. So, which plant produces more offspring: the one that interacts with that particular pollinator or the one that doesn’t? Without following the plants’ growth, reproduction and survival throughout their lifetimes, it’s difficult to say. Even humans harbor many symbionts of which the varying positive and negative effects are only recently beginning to be understood and debated. This tension between cooperation and conflict is not uncommon in many of the systems we traditionally call mutualisms or parasitisms. In our paper, we argue that a snapshot at one point in the life history is not sufficient for understanding the population-level effects or evolutionary significance of any interspecific interaction.
Fungal endophytes that produce herbivore-deterring alkaloids are generally considered clear mutualistic partners of their grass hosts in agronomic systems. Yet, confusion and debate over their role in native systems has arisen due to some documented costs, notably reduction in host survival. Theory suggests that when these endophytes are vertically transmitted, harming their hosts should doom them to extinction, as the endophytes can only increase their own reproduction by increasing their hosts’ reproduction. Yet, vertically transmitted endophytes are often present at high frequencies in native systems, despite transmission rates that can be well under 100%. Therefore, we wondered whether an approach that could integrate the effect of endophytes across the entire life history of the host could shed some light on this problem.
To do this, we developed a new modeling approach in which we could structure a native grass host’s population both continuously by size and discretely by the presence or absence of endophyte symbionts. With our integral projection model (IPM) megamatrix, which we parameterized with experimental field data, we were able to show that endophyte symbiosis provided a net benefit to its host by increasing population growth. Indeed, we saw costs to host survival that were outweighed by boosts to growth and reproduction.
More surprisingly, we saw some patterns that we did not expect given previous theory. Early life history stages, like germination and seedling establishment, were critically important for determining the locations of transmission rate thresholds below which these beneficial symbionts go extinct. For example, if seedlings are able to survive to adulthood 2% of the time, endophytes will not be able to persist if endophyte symbiotic adults produce endophyte symbiotic seeds, as opposed to endophtye-free seeds, less than 50% of the time. However, if seedlings establish 3% of the time, endophyte persistence requires a transmission rate of more than about 75%. This complex interaction between early life history and vertical transmission arises because higher seedling establishment leads to a greater proportion of the population being made up with seedlings, which will be dominated by endophyte-free plants at low transmission rates. This result also highlights the need for a greater understanding of the mechanisms behind variation in endophyte transmission.
We believe that the modeling approach we developed will be broadly applicable to understanding how species interactions, especially those involving vertically transmitted symbionts, influence populations. For myself, this research, which I began as an undergraduate, is influencing my thoughts on species interactions as I start my graduate career at the University of Arizona. In the future, I hope to continue integrating models with empirical data. I believe that doing so is a particularly powerful way to improve our understanding of relationships in nature that so often slide on the continuum between mutualism and parasitism.