Enemy release from the effects of generalist granivores can facilitate Bromus tectorum invasion in the Great Basin Desert

Abstract

The enemy release hypothesis (ERH) of plant invasion asserts that natural enemies limit populations of invasive plants more strongly in native ranges than in non‐native ranges. Despite considerable empirical attention, few studies have directly tested this idea, especially with respect to generalist herbivores. This knowledge gap is important because escaping the effects of generalists is a critical aspect of the ERH that may help explain successful plant invasions. Here, we used consumer exclosures and seed addition experiments to contrast the effects of granivorous rodents (an important guild of generalists) on the establishment of cheatgrass ( Bromus tectorum ) in western Asia, where cheatgrass is native, versus the Great Basin Desert, USA, where cheatgrass is exotic and highly invasive. Consistent with the ERH, rodent foraging reduced cheatgrass establishment by nearly 60% in western Asia but had no effect in the Great Basin. This main result corresponded with a region‐specific foraging pattern: rodents in the Great Basin but not western Asia generally avoided seeds from cheatgrass relative to seeds from native competitors. Our results suggest that enemy release from the effects of an important guild of generalists may contribute to the explosive success of cheatgrass in the Great Basin. These findings corroborate classic theory on enemy release and expand our understanding of how generalists can influence the trajectory of exotic plant invasions.

An exotic herbivore reinforces competition between exotic and native plants

“…Therefore, it is important to consider the local herbivore community of the system to predict the impact of exotic plants on native plants. Although our study focused on herbivores that are shared between closely related plants, generalist herbivores such as mammals with a broader feeding range (Lucero et al, 2019; Orrock et al., 2008), may influence apparent competition across a broad range of plant taxa. Herbivore‐mediated indirect effects among plant species may be common and play a critical role in determining plant invasiveness.…”

Escape from natural enemies depends on the enemies, the invader, and competition

“…A handful of studies have experimentally shown that natural enemies inhibit invasive plant species more in their native ranges than in their non‐native ranges (e.g., DeWalt et al., 2004; Lucero et al., 2019; Williams et al., 2010), but we examined the effects of multiple enemy guilds on more than one invasive species in both ranges. We found that enemy release from insect herbivores, but not antagonistic soil biota, and antagonistic soil biota, but not insect herbivores, may enhance the performance of tansy and goldenrod, respectively, in their non‐native ranges relative to their native ranges.…”

“…The ERH has attracted considerable empirical attention (see reviews by Jeschke et al., 2012; Liu & Stiling, 2006), but relatively few studies have employed the experimental, biogeographically explicit approach outlined above. Also, most studies have examined the ERH in the context of specialist herbivores, but enemy release from several kinds of enemies has been demonstrated, including antagonistic soil biota (Maron, Klironomos, Waller, & Callaway, 2014; Reinhart, Packer, van der Putten, & Clay, 2003), aboveground fungal pathogens (DeWalt, Denslow, & Ickes, 2004), interspecific competitors (Callaway et al., 2011), generalist herbivores (Vermeij, Smith, Dailer, & Smith, 2009), and even postdispersal seed predators (Lucero et al., 2019).…”

Ecological and environmental context shape the differential effects of a facilitator in its native and invaded ranges

“…Novel weapons such as allelopathic chemicals may increase the negative effects of invading species on the recipient community (Callaway et al 2012). Invaders, released from their natural parasites and consumers, may become competitive dominants in their invaded range (Keogh et al 2017;Lucero et al 2019). Despite the growing list of invasion impact case studies conducted in invaded ranges, explicit comparisons of the effects of non-native species between their native and introduced ranges are rare (Hierro et al 2005;Gribben & Byers 2020).…”