Print PageGeographic variation in herbivore pressure can result in population-level variation in ecologically important plant traits. In dioecious plants, males and females differ not only in their gametes and floral structures, but more broadly in the allocation of resources that affect secondary traits, which can in turn influence ecological interactions. We investigated the mechanisms by which individual and population traits of the montane perennial Valeriana edulis vary and influence plant-herbivore interactions across a 2000 m elevation gradient in western Colorado, USA. We collected data on individual, sex-specific traits such as size, herbivore load, and recruitment of natural enemies as well as population-level traits including sex ratio, mortality, and herbivore distribution.
Results/Conclusions
There were significant trait differences within and among populations over the elevation gradient, which had consequences for herbivores and their associated arthropods. Within populations, female plants were larger than males and supported higher loads of sap-feeding (aphid) herbivores. In addition, aphids feeding on female plants attracted more ants and aphid natural enemies even when controlling for aphid density. Among populations, a skew in sex ratio towards females increased with elevation. Our analysis of data on the survival of mature plants from these populations suggest that the survival of female plants relative to that of males increases with elevation. Concurrently, aphid abundance and thus herbivore pressure decrease with elevation. This clinal variation in the magnitude of survival differentials is thus consistent with aphid host preference (preferring females) and elevational distribution (more common lower where sex ratios and survival rates are less skewed). Thus, we suggest herbivore-mediated mortality as a plausible mechanism to explain clinal variation in plant secondary sex ratios.
