Plant neighborhood effects (NE) occur when the fitness of a plant species or genotype is influenced by the density or frequency of neighboring plants. Plant competition or facilitation can cause NE at the scale of immediate neighbors, while plant interactions with mobile consumers such as herbivores or pollinators can mediate NE at larger scales. The consequences of neighborhood effects through plant competition are relatively well understood, but while it is speculated that density or frequency dependence created by insect-mediated NE should influence plant population growth and competition, few studies quantify the frequency or density dependence of insect-mediate NE. Critically, there is also little information about the scale at which these neighborhood effects occur. We considered how the strength of neighborhood effects (density and frequency dependence) changed with increasing neighborhood radius and distances among plants. We planted replicate populations of two old field plants (Solanum carolinense and Solidago altissima) in a two-species Nelder fan design, keeping track of the genotype of each plant as a covariate. This design creates both variation in distances between individuals of the two species and a gradient in local frequencies.
Results/Conclusions
We quantified the influence of local density and frequencies of plants on herbivore damage to focal plants as a function of neighborhood radius. In general, focal plant size had no significant effects, whereas focal plant genotype did affect leaf damage levels. We also found significant neighborhood effects on damage to Solanum and Solidago, but the factors influencing damage differed for the two species. The presence or identity of immediate neighbors (up to 15cm away) had no significant effects on leaf damage for either species. However the density of Solidago in a larger neighborhood influenced percent leaf damage on Solanum carolinense and this was strongest at spatial scales of approximately 1 meter. For leaf damage to Solidago, the frequency of Solanum vs Solidago in small neighborhoods (< 1m) significantly affected leaf damage (more damage when Solidago was rare), whereas in larger neighborhoods (> 1m) the number of Solidago genotypes significantly affected leaf damage (lower damage in more genetically diverse neighborhoods). Because herbivory negatively affected plant fitness, estimated as flowering stalk biomass, plant neighborhoods also had the potential to affect patterns of selection on resistance traits.