Abstract
Invasive species are a threat to every ecosystem. There is a strong incentive to predict which species will become invasive before they become too widespread and unmanageable. Different approaches have been advocated to assess invasive species potential. These include examining plant functional traits, quantifying competitive ability and phylogenetic comparison. In this study, we conducted experiments based on the above approaches in a multi-year, temporally replicated, set of experiments to compare these assessment methods to determine the invasive potential of Japanese chaff flower (Achyranthes japonica). We compared plant traitsand competitive ability of Japanese chaff flower with two agricultural invasive species, Palmer amaranth (Amaranthus palmeri) and tall waterhemp (Amaranthus tuberculatus), and one endangered plant species, bloodleaf (Iresine rhizomatosa), in the Amaranthaceae. Additionally, we assessed the invasive potential based on each of these approaches and determined the degree of agreement between them. A relatively conservative assessment integrating all three approaches would be that the competitive ability of closely related individuals with similar functional traits would share invasive potential. In a greenhouse experiment, each of the study species and soya beans were grown as monocultures and were evaluated to assess the drawdown of an aboveground (light) and a belowground (nitrogen) resource. In a field experiment, each study species was grown at varying densities per 15-cm-diameter pot with or without one or two soya bean plants, to simulate relative densities for soya beans grown in 38- and 76-cm-wide row spacing, respectively. In addition, Japanese chaff flower seedlings were planted either as un-manipulated seedlings or as a seedling cut back to the soil surface at the four-node stage (cut Japanese chaff flower) at which point seedlings have reached a perennial growth stage. The greenhouse experiment showed that each species drew down light differently, but not nitrogen. Shading decreased the aboveground biomass of the species in comparison with unshaded controls. Nitrogen, however, increased the aboveground biomass of Palmer amaranth and Japanese chaff flower. In the field experiment, a competitive effect ranking was determined to be: tall waterhemp ≥ Palmer amaranth = cut Japanese chaff flower ≥ uncut Japanese chaff flower ≥ bloodleaf, with the competitive response ranking being the inverse. These results suggest that under specific conditions, these closely related species do exhibit similar competitive abilities. Furthermore, the invasiveness and not the life history or habitat of these closely related species appeared to be the driving factor of competitiveness.
Highlights
Invasive species have large ecological impacts on native species, communities and ecosystems (Elton 1958; Simberloff et al 1997; Blackburn et al 2014)
Palmer amaranth and soya bean drew down the greatest amount of light (26.8 + 3.5% and 30.2 + 2.8 %, respectively), and tall waterhemp and Japanese chaff flower drew down an intermediate level of light in comparison with the other species (41.4 + 2.9 and 45.6 + 2.2 %, respectively)
There was no significant difference for aboveground biomass between N treatment levels and species, except for Japanese chaff flower and Palmer amaranth (Table 1, Fig. 2A)
Summary
Invasive species have large ecological impacts on native species, communities and ecosystems (Elton 1958; Simberloff et al 1997; Blackburn et al 2014). 50 000 invasive species and the number is steadily increasing (Sakai et al 2001). About 42 % of the species on the threatened and endangered species list are at risk primarily because of invasive species (Sakai et al 2001; Pimentel et al 2005). In Illinois, the location of this study, 1156 invasive plant species had escaped cultivation and became naturalized by 2014, equivalent to 32.1 % of the state’s total flora (Mohlenbrock 2014). Predicting whether or not an introduced species will become invasive can be difficult there is a strong incentive to determining which plants are likely to become invasive before they become too widespread and unmanageable (Westbrooks 2004)
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