Effects of soil nutrient enrichment on trophic interactions between herbivorous insects and foliage spiders in Patagonian forests

Abstract

The addition of nutrients to soil can induce changes through “bottom-up” effects in soil–plant–animal interactions, impact- ing higher levels in food chains. Changes in foliar nutrient content may influence diets of herbivorous insects, subsequently altering their feeding rates and abundance. Spiders established in foliage rely on these insects for sustenance; therefore, fluctuations in the abundance of herbivorous insect communities ultimately determine the population dynamics of their spider predators. We established a long-term field experiment to investigate the effects of nutrient addition of N (nitrogen), P (phosphorus), K (potassium) and their combinations on plant–arthropod interactions. We carried out sampling in 32 plots under different nutrient addition treatments, where we captured herbivorous insects and spiders on Nothofagus antarctica plants, the most abundant plant species in the Patagonian scrubland. Our results showed that foliar nutrient concentrations had cascading effects throughout the forest food chain. Herbivorous insects (Diptera, Coleoptera, Hemiptera and Lepidoptera orders) exhibited a positive response to the increased foliar phosphorus content, resulting in higher population densities. In addition, we found a strong positive relationship between herbivorous insect abundance and spider abundance. Our find- ings indicate that variations in the availability of soil nutrients play a crucial role in defining the structure of predator–prey interactions in forest ecosystems.