Invasive plant removal increases insect herbivory pressure on a native tree due to an increase in resource quality

Abstract

Insect predator densities and plant nutritional status can affect insect herbivore densities, but these effects have not yet been assessed in the context of an invasive species management. This study investigated the influence of (i) altered predatory arthropod abundance and community composition (top-down-effects); and (ii) altered leaf nitrogen and phosphorus levels (bottom-up-effects) as possible causes of fluctuations in herbivore abundance and herbivory on the native tree Brabejum stellatifolium in riparian sites that differ in invasion status (near pristine, heavily invaded by Acacia mearnsii, and restored). Species richness, abundance and composition of herbivorous and predatory arthropods were compared between categories. Herbivore and predator abundance, species richness and assemblage composition were significantly influenced by invasion status. As expected, herbivore abundance was significantly and positively correlated to levels of leaf damage. Leaf nitrogen levels were the highest at restored sites where herbivorous arthropods were also most abundant and the abundance of predators was intermediate. These results suggest that altered leaf nitrogen content is likely a key influence for the increased herbivore pressure on B. stellatifolium at sites under pressure from invasive A. mearnsii and its status. Bottom-up influences therefore may be more important than top-down effects on controlling herbivore abundances in these altered riparian environments.