Stronger effect of gastropods than rodents on seedling establishment, irrespective of exotic or native plant species origin

Abstract

Experimental evidence about how generalist consumers aff ect exotic plant invasions is equivocal, but most tests have been limited to few plant species, single herbivore guilds, and single locations. Using a seed-addition experiment, we studied eff ects of gastropods and rodents on recruitment success of 37 exotic and 37 native plant species affi liated to three diff erent functional groups (i.e. grasses, legumes and non-legume herbs). We replicated our seed addition x herbivore exclusion experiment at multiple grassland sites, located within a few km of each other in two regions, coastal central California (USA) and southern Saxony -Anhalt (Germany). Th e two study regions diff ered in climate, land-use, invasion history and species pools which allowed us to disentangle general from context-specifi c eff ects. In both regions, herbivory by gastropods had a stronger impact on the proportion of recruited seedlings and the proportion of recruited species than rodent herbivory, but this eff ect was much more pronounced in California than in Germany. Especially, seedling recruitment of non-legume herbs and legumes suff ered from gastropod herbivory. Contrastingly, the eff ect of rodents was negative at the German sites and positive at the Californian sites, likely driven by context-specifi c diff erences in the rodent assemblages. Across both study regions, exotics had higher seedling recruitment than natives, indicating that higher recruitment success constitutes an inherent feature of exotic species. After two years, more exotic than native species established at grassland sites in California while the opposite was true for the German grassland sites. Consistently across regions, native and exotic species did, however, not diff er in their response to herbivory, suggesting that generalist consumers suppress recruitment and colonization of plant species irrespective of their origin. Our results demonstrate the importance of a multi-species, multi-site approach to separate general responses of exotic and native plants to generalist herbivory from local, regional or species-specifi c peculiarities.

Snail herbivory affects seedling establishment in a temperate forest in the Ozarks

“…Instead, this finding is in line with the biotic resistance hypothesis which proposes that resident herbivores (and other antagonists such as competitors) can decrease invader success (Levine, Adler, & Yelenik, ; Maron & Vilá, ). Our finding that native snails reduce establishment of these invasive plant species is in contrast with previous studies that suggest that snails do not differentially prefer native or exotic grassland and woody plant species (Buschmann, Edwards, & Dietz, ; Korell et al, ; Motheral & Orrock, ; Shiels et al, ). In addition to snail herbivory, the low abundance of both invasive species might also be explained by the lack of major disturbances within the 12‐ha forest plot.…”

“…can decrease invader success (Levine, Adler, & Yelenik, 2004;Maron & Vilá, 2001). Our finding that native snails reduce establishment of these invasive plant species is in contrast with previous studies that suggest that snails do not differentially prefer native or exotic grassland and woody plant species (Buschmann, Edwards, & Dietz, 2006;Korell et al, 2016;Motheral & Orrock, 2010;Shiels et al, 2014).…”

“…However, the exclusion of white-tailed deer did not affect biomass or survival of young seedlings as did the exclusion of snails in our experiment. This is consistent with previous studies in grasslands that showed invertebrate herbivory to be more influential to seedling establishment than vertebrate herbivory (Allan & Crawley, 2011, Korell et al, 2016, but see Hulme, 1994).…”

“…Even though snails and slugs are known as essential components of forest ecosystems, our current knowledge is largely limited to their importance to forest litter decomposition and nutrient cycling (Mason, 1970;Richter, 1979), or as food sources for many other forest fauna (Digweed, 1993;Graveland, Vanderwal, Vanbalen, & Vannoordwijk, 1994;Nyffeler & Symondson, 2001). This lack of studies is surprising, as many studies performed in grasslands do find strong impacts of gastropod herbivory on plant species composition (Allan & Crawley, 2011;Buschmann, Keller, Porret, Dietz, & Edwards, 2005;Cleland, Peters, Mooney, & Field, 2006;Hanley, Fenner, & Edwards, 1995) that can even be stronger than the effect of herbivory by vertebrates (Korell et al, 2016). Generally, it is expected that the negative effects of herbivores on seedling establishment lead to greater changes in plant community composition than herbivory on adult plants (Fenner, Hanley, & Lawrence, 1999).…”

Interactions count: plant origin, herbivory and disturbance jointly explain seedling recruitment and community structure

“…In contrast, the biotic resistance hypothesis 12 states that resident generalist herbivores may constrain exotic plant invasions by suppressing those exotic species that have not evolved effective defense mechanisms against them. However, so far studies on the effects of generalist herbivores on early seedling recruitment and establishment success have provided inconsistent results, as they found stronger 13 , 14 , equal 2 , 15 , 16 or weaker 17 , 18 effects on exotic compared to native species. This apparent inconsistency may at least be partly explained by the different feeding preferences of the key herbivores in the particular study system.…”