Herbivores cause a rapid increase in hereditary symbiosis and alter plant community composition

Abstract

Microbial symbioses are ubiquitous in nature. Hereditary symbionts warrant particular attention because of their direct effects on the evolutionary potential of their hosts. In plants, hereditary fungal endophytes can increase the competitive ability, drought tolerance, and herbivore resistance of their host, although it is unclear whether or how these ecological benefits may alter the dynamics of the endophyte symbiosis over time. Here, we demonstrate that herbivores alter the dynamics of a hereditary symbiont under field conditions. Also, we show that changes in symbiont frequency were accompanied by shifts in the overall structure of the plant community. Replicated 25-m2 plots were enriched with seed of the introduced grass, Lolium arundinaceum at an initial frequency of 50% infection by the systemic, seed-transmitted endophyte Neotyphodium coenophialum. Over 54 months, there was a significantly greater increase in endophyte-infection frequency in the presence of herbivores (30% increase) than where mammalian and insect herbivory were experimentally reduced by fencing and insecticide application (12% increase). Under ambient mammalian herbivory, the above-ground biomass of nonhost plant species was reduced compared with the mammal-exclusion treatment, and plant composition shifted toward greater relative biomass of infected, tall fescue grass. These results demonstrate that herbivores can drive plant-microbe dynamics and, in doing so, modify plant community structure directly and indirectly.

Fig. 1.

Tissue-print immunoblot analyses. Red circles indicate N. coenophialum. Hyphal cross sections are clustered around vascular bundles of L. arundinaceum. Pink circles indicate uninfected tillers.

Fig. 2.

Efficacy of experimental treatments. (A) Distribution of the initial proportion of infected tillers per plot, which did not deviate significantly from 0.5 (mean, 0.496; 95% confidence interval, 0.452–0.540), by using the bias-corrected accelerated bootstrap with 10,000 resamples. (B) The number of voles per trap for fenced vs. unfenced plots across 10 census dates. (Fence, F_1,55 = 13.1, P = 0.0007; fence × time, F_9,47 = 0.6, P = 0.8.) (C) The number of herbivorous insects collected per plot for plots sprayed with the insecticide (Malathion) vs. plots sprayed with water for three census dates. Within a bar, data are divided by insect orders. (Insecticide, F_1,56 = 6.7, P = 0.012; insecticide × time, F_2,55 = 1.3, P = 0.3.) Error bars indicate means ± SE. The insecticide did not affect voles (F_1,55 = 0.9, P = 0.3; fence × insecticide, F_1,55 = 0.04, P = 0.8), nor did fencing affect herbivorous arthropods (F_1,56 = 1.6, P = 0.2; fence × insecticide, F_1,56 = 0.7, P = 0.4) or predaceous arthropods (F_1,56 = 1.7, P = 0.2; fence × insecticide, F_1,56 = 0.61, P = 0.4).

Fig. 3.

The change in endophyte frequency among treatments. The change in frequency was determined by subtracting the initial proportion of tillers infected in that plot from the proportion of tillers infected on each date. The change in proportion is bounded by –0.5 and 0.5 (0% and 100% infected, respectively). Over time, infection increased in all plots (time, F_8,47 = 29.6, P < 0.0001). Treatments diverged over time (fence × insecticide × time interaction, F_8,47 = 2.8, P = 0.01). No main effects or two-way interactions were significant (fence, F_1,54 = 3.1, P = 0.08; fence × time, F_8,47 = 1.4, P = 0.2; insecticide, F_1,54 = 1.9, P = 0.2; insecticide × time, F_8,47 = 0.8, P = 0.6; fence × insecticide, F_1,54 = 0.43, P = 0.5). Symbols show means ± SE and are slightly offset to show error bars clearly. P values indicate a significant difference between the dual herbivore-exclusion treatment (fenced plus insecticide) and the control (unfenced plus water) for each date.

Fig. 4.

Plant composition among treatments. Fencing significantly affected tall fescue biomass (fence, F_1,56 = 4.4, P = 0.04) (A) and forb biomass (fence, F_1,56 = 4.4, P = 0.04) (B) but not total biomass (fence, F_1,56 = 3.0, P = 0.1) (C). Total biomass was calculated as the sum of tall fescue, forb, nonfescue grasses, and thatch biomass. Error bars show means ± SE. Different letters indicate significant differences among treatments. The insecticide treatment did not significantly affect the biomass of tall fescue (F_1,56 = 2.1, P = 0.2; insecticide × fence, F_1,56 = 1.8, P = 0.2), forbs (F_1,56 = 0.6, P = 0.4; insecticide × fence, F_1,56 = 0.1, P = 0.8), nonfescue grasses (F_1,56 = 0.7, P = 0.4; insecticide × fence, F_1,56 = 0.0, P = 0.9), or total biomass (F_1,56 = 0.0, P = 0.9; insecticide × fence, F_1,56 = 1.6, P = 0.2).