Low structural complexity of nonnative grassland habitat exposes prey to higher predation.

The structural complexity of vegetation can have profound effects on the hunting efficiency of predators, thereby affecting their intake rate of prey. While studies have shown that vegetation complexity can play an important role in managing unwanted impacts of predators, it is less clear how structural complexity of invasive vegetation affects the vulnerability of terrestrial prey. Short nonnative pasture species bred for agricultural production, for example, are highly invasive and pervade grassland ecosystems worldwide. They generally have low structural complexity compared with taller native vegetation they often displace. We conducted controlled experiments to test whether nonnative pastures expose fauna to greater predation risk. Survival of invertebrates (tethered locusts) subject to predation by invasive mammalian insectivores (European hedgehogs) in nonnative pasture (0.10 per 24 h; 95% CI, 0.08-0.13) was less than one-half that in structurally complex native perennial tussock (bunch) grass (0.24; 95% binomial CI, 0.18-0.31). A significant positive relationship was apparent between structural complexity (grass dry stem density) surrounding each locust and their survival. In a second experiment, survival of locusts placed solely in tussock increased with decreasing locust density in tussock, presumably reflecting fewer resource-rich patches on which predators could focus. These results demonstrate that invasion by structurally simple nonnative vegetation exposes prey to greater risk of predation. This is concerning from a global nature conservation perspective given that conversion of nearly one-half of the world's temperate grasslands to agriculture includes a range of invasive, structurally simple, nonnative, plant species. Minimizing invasion and maintaining and restoring complex habitat structure may be a useful conservation option for reducing unwanted predation.