Test of biotic and abiotic correlates of latitudinal variation in defences in the perennial herb Ruellia nudiflora

1. Geographic variation in abiotic factors and species interactions is widespread and is hypothesized to generate concomitant patterns of species trait variation. For example, higher rates of herbivory at lower latitudes are thought to select for increased plant defences, although latitudinal variation in defences may also be influenced directly by abiotic factors and indirectly by predators and parasitoids reducing herbivore pressure. 2. We measured defences of the herb Ruellia nudiflora among 30 populations spanning a latitudinal gradient from northern Yucatan to southern Belize that vary substantially in leaf herbivory (four-fold), seed herbivory (25-fold), and seed herbivore parasitism (14-fold). These surveyed populations span one-third of the species’ latitudinal distribution (5° of latitude), the entire precipitation gradient of its distribution, and one-third of the temperature gradient of its distribution. Our prior work showed that leaf herbivory decreased with latitude and th...