Opposing Effects of Native and Exotic Herbivores on Plant Invasions

Abstract

Exotic species are widely assumed to thrive because they lack natural enemies in their new ranges. However, a meta-analysis of 63 manipulative field studies including more than 100 exotic plant species revealed that native herbivores suppressed exotic plants, whereas exotic herbivores facilitated both the abundance and species richness of exotic plants. Both outcomes suggest that plants are especially susceptible to novel, generalist herbivores that they have not been selected to resist. Thus, native herbivores provide biotic resistance to plant invasions, but the widespread replacement of native with exotic herbivores eliminates this ecosystem service, facilitates plant invasions, and triggers an invasional "meltdown."

Invasive species cause large-scale loss of native California oyster habitat by disrupting trophic cascades

“…But an invasive intermediate consumer capable of depleting an important foundation species appears to be excluded from the food web, because the invader's naivete prevents it from recognizing and successfully avoiding a native top predator before being consumed. Thus, echoing the conclusions of previous work on trophic cascades (Myers et al 2007), biological invasions (Parker et al 2006), and biodiversity-ecosystem function (Jackson et al 2001;Worm et al 2006), our results suggest that marine food webs can be conserved by protecting native top predators that simultaneously regulate the foraging of native intermediate consumers (via density-and trait-mediated cascades) and exclude invasive intermediate consumers (via density-mediated cascades). Furthermore, although native species extinctions at the local and regional scale can be balanced by invasions of functionally similar species (Sax and Gaines 2003), our results suggest that naivete can prevent top predators and/or intermediate consumers in invaded food webs from re-creating historically important trophic cascades that maintain biodiversity and ecosystem function (Schmitz 2008).…”

In a long-term experimental demography study, excluding ungulates reversed invader's explosive population growth rate and restored natives

“…Because Alliaria cannot flourish in this forest without deer, our results suggest that previous hypotheses for Alliaria's invasion success (48) may be context dependent. In contrast to studies that demonstrate how native herbivores can increase biotic resistance by consuming invaders (18), our findings show that deer, a native generalist ungulate, find the invasive Alliaria completely inedible, which undoubtedly bolstered its fitness where deer had access (39,41). Thus, our data lend support to the hypothesis that a lack of palatability may be a general trait of highly successful invaders (16,53), particularly in communities with overabundant ungulates (54, 55).…”

“…According to ecological theory, the ability of the resident community to limit the success of invading exotics [biotic resistance (9,10)] will depend upon ecological context that includes the suite of local interactors (11)(12)(13)(14)(15). The abundance of herbivores and their local impacts (11,14,16) can play a prominent role in how fast plant populations grow or shrink and how much the relative abundance of plant species changes over time (5,15), including changes associated with plant invasions (11,(16)(17)(18)(19). Recently, increased browsing pressure by overabundant ungulate herbivores on native plant communities has been proposed as a fundamental cause of a shift from native to exotic plant domination in forests and rangelands worldwide (11,16,20).…”

Trade‐off between early emergence and herbivore susceptibility mediates exotic success in an experimental California plant community

“…However, as discussed above, herbivore impact on biomass was not predicted by the duration of exposure to herbivores: Late‐emerging species showed little response to exclusion treatments that differed in herbivore exposure time by 35 days or more (Figure 4). Another potential driver of the increased susceptibility associated with earlier emergence is that exotic species, which on average emerged earlier, may be less resistant to evolutionarily novel native herbivores when encountered, perhaps because of a lack of coevolved defenses (Colautti et al., 2004; Parker et al., 2006). However, we observed no significant interaction between exclusion treatment and origin, showing that origin per se had little effect on susceptibility in monoculture.…”