There is an increasing demand to predict how global changes will alter infectious disease dynamics. Of particular concern are generalist insect-vectored viral pathogens, a common life history for emerging plant and animal pathogens. Global change such as invasion and nitrogen deposition may cause these and other enemies to build up on highly susceptible hosts and spillover onto other, less susceptible hosts. The predicted outcome of such enemy-mediated apparent competition, however, depends on the assumed tradeoffs, namely competition-resistance tradeoffs or competition-tolerance tradeoffs. Competition-resistance tradeoffs are expected to facilitate local coexistence between competitively superior, poorly defended plants and competitive inferior, resistant plants. In contrast, competition-tolerance tradeoffs are predicted to allow competitively inferior, tolerant plants to locally exclude competitively superior, intolerant plants.

We predict that competition-tolerance tradeoffs are more common in plants than competition-resistance tradeoffs. We base our prediction on empirical evidence for a global leaf economics spectrum. At one end of this spectrum are slow return (SR) species that are relatively slow growing with high survival under low resource conditions (i.e., superior competitors).  SRs have high C:N and leaf toughness (i.e., traits conferring high resistance), but low photosynthetic capacity, specific leaf area, and leaf turnover (i.e., traits conferring low tolerance). In contrast, quick return (QR) species are relatively fast growing species that temporarily preempt resources (i.e., inferior competitors). QRs have lower C:N and leaf toughness (i.e., traits conferring low resistance), but higher photosynthetic capacity, specific leaf area, and leaf turnover rates (i.e., traits conferring high tolerance).

Research suggests an aphid-vectored pathogen, barley yellow dwarf virus (BYDV), may contribute to invasion of Mediterranean perennial grasslands by exotic annuals.  The presence of exotic annuals, coupled with nitrogen deposition, may increase aphid abundance and therefore BYDV prevalence in less susceptible natives. We tested for competition-resistance vs. competition-tolerance tradeoffs in phylogenetically controlled greenhouse experiments using three native perennials and three invasive annuals from the Mediterranean grasslands of western North America. We measured leaf traits and, to estimate infection probability, we conducted experimental BYDV inoculations using aphid vectors.

Results/Conclusions

For all significant differences in trait values, except photosynthetic capacity in the family Triticeae-Hordeinae, results were consistent with a QR strategy for annuals and a SR strategy for perennials. For example, in two out of three families, perennials had lower photosynthetic capacity and infection probabilities. These results suggest competitively inferior QR annuals are more likely to be infected and there is no evidence for a competition-resistance tradeoff.