Effects of mutualists on the responses of demographic rates to inter-annual variability

Background/Question/Methods

Interactions between endosymbionts and their hosts are highly variable and often context dependent.  Endosymbionts can significantly alter host demographic rates and thereby influence population dynamics.  Additionally, costs or benefits owing to the presence of an endosymbiont could be magnified or dampened by inter-annual variation in the environment (e.g., climate).  Effects of symbionts on host responses to inter-annual variability should have consequences at the population level, since variance in demography generally reduces the long-term population growth rate (in a geometric series, a bad year is more costly than an equally good year is beneficial).  We collected demographic data from experimental plots of the cool-season grass host Poa autumnalis. We compared demographic rates between hosts that were naturally symbiotic with the fungal endophyte and hosts from which the endophyte was experimentally eliminated.  We asked whether fungal endophytes alters their host demography, and specifically if they exacerbate or ameliorate responses to inter-annual variation.  

Results/Conclusions

We found that endophytes do alter host demographic rates.  However, effects of endophytes on their hosts were not consistent across demographic rates.  We found that there was support for models including endophyte effects on survival and reproduction, but not growth.  Interestingly we observed conflicting patterns for survival and reproduction.  Endophytes are beneficial to their host’s survival at small sizes, yet costly at large sizes.  In contrast, endophytes provide a reproductive advantage to large host plants but the effect is unnoticeable at smaller sizes.  We also detected weak effects of endophyte symbiosis on variance in host vital rates. An integral projection model showed that endophytes affect the population dynamics of their grass hosts via effects on both mean vital rates as well as variance in vital rates. Our study demonstrates the important effects that symbionts can have on host population dynamics in temporally fluctuating environments.