Biotic interactions are key components driving community level processes.  One type of biotic interaction that affects plant performance at the population scale is herbivore damage.  Herbivores have direct negative effects by feeding on photosynthetic tissues and destroying conducting tissue, which can deter performance and decrease survival within a competitive environment. Trophic cascades can limit the negative effect that herbivores have on the plants. Trophic cascade theory predicts that top predators suppress the density of their prey, thereby releasing the next trophic level (plants) from predation. Historically, trophic cascades have been documented to be more prevalent within aquatic systems. Their prevalence and importance within terrestrial systems currently are difficult to predict. The objective of the experiment reported here was to test for the occurrence of a trophic cascade and its effect on the performance of the native Cirsium altissimum (tall thistle) within its tallgrass prairie ecosystem.  The predator for this experiment was a tangle web spider (Family Theridiidae) and the herbivore was a grasshopper (Phoetaliotes nebrascensis) that frequently feeds on tall thistle foliage.  In summer 2007, circular aluminum enclosures were placed around each individual tall thistle rosette (juvenile stage) and the treatments were as followed: (i) spider + grasshopper + plant and (ii) grasshopper + plant.  Plant herbivore damage was analyzed by measuring the percent of leaf area removed per plant by the herbivores.

Results/Conclusions

Tall thistle plants with spiders had significantly (p < 0.01) less damage than plants without spiders. In addition, 20% of the leaves on plants without spiders were heavily damaged (>60%) compared to only 5% of plants with spiders. The opposite pattern was seen (90% vs. 70%) when analyzing low level damage (<30%) on plants with or without spiders. This study provides new evidence for the existence of a terrestrial trophic cascade and its effect on native plant performance.