Effects of seedling conspecific density and heterospecific frequency on insect herbivory in a tropical dry forest

1. Conspecific plant density and heterospecific frequency are key drivers of herbivore damage. However, most studies have investigated their effects separately and for single (rather than multiple) focal plant species. 2. We conducted an experiment involving three tree species, namely: Cordia dodecandra (Boraginaceae), Manilkara zapota (Zapotaceae), and Piscidia piscipula (Fabaceae). We manipulated understory densities of M. zapota and C. dodecandra (focal species) and their frequency relative to P. piscipula. Three months after planting, we surveyed insect leaf chewer and sucking damage on the former two. Because these species are attacked by different herbivores, we predicted a negative effect of heterospecific frequency on herbivory. 3. Density and frequency varied in the direction and function of their effects on herbivory depending on the plant species and attacking herbivore. As expected, Piscidia piscipula frequency had a negative linear effect on M. zapota leaf-chewer damage, whereas conspecific density did not affect chewer damage on this species. In contrast, density and frequency had non-linear effects on C. dodecandra chewer damage, namely positive (hump-shaped) and negative (U-shaped) relationships, respectively. In addition, density and frequency had positive linear effects on C. dondecandra damage by leafhoppers. 4. These findings call for more work jointly assessing plant inter-specific variation in density- and frequency-dependent variation in herbivory and its underlying drivers.