Drier climatic conditions may lead to increased herbivorous insect pressure on a native tree, but not on an invasive competitor

In this study we assessed how an invasive tree (Acacia mearnsii) and an ecologically equivalent native species (Virgilia divaricata) interact with their insect pests and fungal pathogens in sympatric populations along forest edges in the Cape Floristic region of South Africa. We determined how insect herbivore abundance and fungal disease development differ between the two species across a moisture gradient and whether observed differences can be explained by moisture availability and/or plant nutrient levels. The two host plants had similar foliar nutrient content, but measurements of δ12C / δ13C isotope ratios in leaves indicated that only the native plant experienced drought stress at drier sites. The degree of disease development after tree wounding was similar for both species and was not correlated with soil moisture content in either species. As predicted by the biotic release hypothesis, herbivore numbers were significantly higher on the native plant. Herbivore numbers on A. mearnsii were unaffected by moisture availability, but herbivore numbers on V. divaricata increased at drier sites. Consequently, under conditions of increased drought, V. divaricata may experience higher levels of drought stress than the invasive A. mearnsii and may suffer from increased insect herbivory, rendering it a weaker competitor. Herbivore abundance and disease development were significantly influenced by plant nutrient content for A. mearnsii, but not for V. divaricata. Relatively nutrient-poor A. mearnsii trees experienced higher herbivore loads but slower disease development than nutrient-rich trees. Therefore, the susceptibility of A. mearnsii seems to be determined by plant nutrient levels, a factor that varies independently from water availability.