The costs and benefits of symbiotic interactions are often highly contextual and can vary with host ontogeny. Effects of symbionts on different times in host development do not necessarily contribute equally to the fitness of the host. Therefore, a population dynamics approach that integrates over the host life cycle is necessary for documenting the net costs or benefits and, thus, the mutualistic or parasitic nature of the symbiosis. Vertically transmitted endophytic fungi are typically strongly beneficial for invasive, agronomic grasses, but inconsistent results, including both costs and benefits, have been documented for endophytes in native grasses. Using a native grass Agrostis hyemalis, we asked how an endophyte (Epichloe amarillans) affected the population dynamics of its host, how variation in poorly studied early demographic transitions may mediate endophyte effects, and what ecological costs and/or benefits might be driving those effects. We used a continuously size-structured integral projection model parameterized with three years of data from entirely endophyte-symbiotic (E+) and entirely endophyte-free (E-) experimental field populations. We used a megamatrix approach to link E- and E+ populations via imperfect vertical transmission and found that this parameter strongly influenced the dynamics of the total host population.
Results/Conclusions
Integrating over the host life cycle reveals substantial effects of symbiosis on host population dynamics, as seen here with an endophyte-grass symbiosis. We found size-dependent benefits of endophyte-symbiosis to A. hyemalis in individual growth and seed production that outweighed slight costs in survival. This net benefit translated to a 21% higher population growth rate, a difference found to be significant by a permutation test that resampled endophyte treatment over all plants. Both E+ and E- populations were in decline, but symbiosis significantly slowed extinction. When the E+ and E- populations were linked using a megamatrix, we were able to model how increases in both the vertical transmission rate and the probability that seedlings establish increase the population growth rate in complex, interacting ways. In contrast, the equilibrium frequency of symbiosis tends to increase with the vertical transmission rate and decrease with the probability of establishment. These results suggest that particular attention should be paid to imperfect vertical transmission and to demographic transitions in early life history stages of the host that are highly variable. Overall, our novel population dynamics approach improves the ability to characterize the outcome of the symbiotic interaction.