- The study examined the impacts of insect herbivory on population dynamics of tree cholla cactus across an elevational gradient in New Mexico. - Insect herbivore abundance and damage to cacti decreased with increasing elevation, inversely correlated with cactus density. - Experimental exclusion of insects showed their negative effects on cactus growth and seed production were strongest at low elevation and decreased at higher elevations. - Demographic modeling predicted insect herbivory depressed population growth rates the most at low elevation, less so at mid elevation, and hardly at high elevation.