Granivory of invasive, naturalized, and native plants in communities differentially susceptible to invasion

Abstract

Seed predation is an important biotic filter that can influence abundance and spatial distributions of native species through differential effects on recruitment. This filter may also influence the relative abundance of nonnative plants within habitats and the communities' susceptibility to invasion via differences in granivore identity, abundance, and food preference. We evaluated the effect of postdispersal seed predators on the establishment of invasive, naturalized, and native species within and between adjacent forest and steppe communities of eastern Washington, USA that differ in severity of plant invasion. Seed removal from trays placed within guild-specific exclosures revealed that small mammals were the dominant seed predators in both forest and steppe. Seeds of invasive species (Bromus tectorum, Cirsium arvense) were removed significantly less than the seeds of native (Pseudoroegneria spicata, Balsamorhiza sagittata) and naturalized (Secale cereale, Centaurea cyanus) species. Seed predation limited seedling emergence and establishment in both communities in the absence of competition in a pattern reflecting natural plant abundance: S. cereale was most suppressed, B. tectorum was least suppressed, and P. spicata was suppressed at an intermediate level. Furthermore, seed predation reduced the residual seed bank for all species. Seed mass correlated with seed removal rates in the forest and their subsequent effects on plant recruitment; larger seeds were removed at higher rates than smaller seeds. Our vegetation surveys indicate higher densities and canopy cover of nonnative species occur in the steppe compared with the forest understory, suggesting the steppe may be more susceptible to invasion. Seed predation alone, however, did not result in significant differences in establishment for any species between these communities, presumably due to similar total small-mammal abundance between communities. Consequently, preferential seed predation by small mammals predicts plant establishment for our test species within these communities but not between them. Accumulating evidence suggests that seed predation can be an important biotic filter affecting plant establishment via differences in consumer preferences and abundance with important ramifications for plant invasions and in situ community assembly.

Biotic constraints on the establishment and performance of native, naturalized, and invasive plants in Pacific Northwest (USA) steppe and forest

“…We substantiate the immigrant class (naturalized vs. invasive) of each non-native test species based on 1) a preliminary vegetation analysis conducted at all 8 study sites (Connolly et al 2014) and 2) state and regional published accounts habitats (e.g., Gaines andSawn 1972, Roche andTalbot 1986) of the relative abundance of these species. Importantly, some work has evaluated the mechanisms driving competition dynamics between these specific native perennials and introduced annuals (e.g., Madsen et al 2012), but outcomes remain unclear and suggest evaluation of their respective establishment potential and relative performance across environmental and disturbance gradients may help identify the drivers of introduced plant colonization and persistence in natural sites.…”

“…Secale cereale, a naturalized annual, is a Washington Class C noxious weed that appears as a volunteer in many cultivated crops (Gaines and Swan 1972, Washington Noxious Weed Control Board [WNWCB]: http://www. nwcb.wa.gov) and establishes, albeit rarely, in meadow steppe and P. ponderosa forest (Connolly et al 2014, USDA PLANTS database: http://plants.usda.gov). Centaurea cyanus, a naturalized annual, is also registered on the WNWCB monitor list and is widely established at low density throughout the meadow steppe and P. ponderosa forest in eastern Washington (Roche and Talbot 1986, USDA PLANTS database: http:// plants.usda.gov).…”

“…Limitation of naturalized species in their abundance and geographic range may result from demographic restrictions, dispersal limitations, abiotic constraints, or trophic interactions in the novel range (Davis 2009, Richardson and Pyšek 2012, Connolly et al 2014. Furthermore, naturalized species often establish more readily and have higher fitness in disturbed habitats (MacDonald and Kotanen 2010, Maron et al 2013, suggesting that competition for microsites may be a major determinant of plant naturalization (Going et al 2009, Kempel et al 2013.…”

“…When the understory is removed, seedling establishment of some non-native species is not however otherwise limited by differences between these communities (Connolly 2013). Additionally, preferential granivory partially explains differences in the abundance of naturalized and invasive species within the steppe (Connolly et al 2014) but fails to account for the low abundance of non-native species in these forests. To an un-quantified degree, the realized distribution of native, naturalized, and invasive species within the steppe and forest communities may be a function of seed limitation and the ability for species to persist in undisturbed habitat (Pierson and Mack 1990).…”

“…We examined the effect of seed addition and local disturbance (i.e., removal of all plant material <1.5 m above the ground) on the establishment and performance of native, naturalized, and invasive species in meadow-steppe and forest habitats in eastern Washington (USA) as part of a multi-pronged investigation of the forces that restrict/ enhance naturalization (Connolly 2013, Connolly et al 2014. Our objectives were to 1) quantify the severity of seed limitation for a set of representative native, naturalized, and invasive species, 2) evaluate how disturbance of the understory canopy cover influenced recruitment and performance of each class of immigrant, and 3) evaluate the effect of these factors within invaded steppe and uninvaded forests.…”