Herbivory in Cacti: Fitness Effects of Two Herbivores, One Tending Ant on Myrtillocactus geometrizans (Cactaceae)

Plants in the cacti family are almost entirely restricted to the American continent; one third of the species are endemic to Mexico and many of them are threatened, with herbivory being one of the factors affecting their species populations. A range of herbivores has been reported for cacti, ranging from those that consume tissues from the stems, flowers, pollen, fruits and seeds to those that feed on fluids such as sap and nectar. Although the occurrence of this interaction is well documented, relatively few studies have assessed its effect on the plants’ demography and fitness; even fewer studies have examined how the interaction of herbivores with other species (competitors, mutualistic species, predators, etc.) affects the plants’ growth and net performance. This chapter briefly reviews the occurrence and effects of herbivory on cacti and how this is affected by the presence of other species and interactions. In addition, we use a biological system associated with the garambullo cactus Myrtillocactus geometrizans-two phytophagous species and other insects-to show how, depending on the phytophagous species and its interactions with other species also associated with the plant, the final effects on the growth, reproductive success and progeny’s quality of M. geometrizans vary. We found that both phytophagous species affect the plant negatively, but with different intensity. The soft scale Toumeyella martinezae often has a greater negative effect on M. geometrizans, affecting its growth, reproductive effort and progeny’s quality and performance. We also showed that the two scale insects compete asymmetrically: Under natural conditions, the soft scale affects Opuntiaspis philococcus more heavily, but the competitive advantage of T. martinezae is facilitated by the presence of the mutualistic ant Liometopum apiculatum. Thus, by eliciting a high abundance of the soft scale, the species that most affects the plant, the ant has a negative net effect on the plant. The parasitoids found in this system would compete with the ants for the phytophagous resource and, although they have a “moderate” effect (25% incidence) on the herbivore populations, they would favour the plant by reducing the population of phytophagous insects. The complex multitrophic system hereby studied clearly shows that analysing interspecific interactions by examining only species pairs in isolation is insufficient to understand and predict the overall impact on plants.