A native annual forb locally excludes a closely related introduced species that co-occurs in oak-savanna habitat remnants

Abstract

Despite the ubiquity of introduced species, their long-term impacts on native plant abundance and diversity remain poorly understood. Coexistence theory offers a tool for advancing this understanding by providing a framework to link short-term individual measurements with long-term population dynamics by directly quantifying the niche and average fitness differences between species. We observed that a pair of closely related and functionally similar annual plants with different origins-native Plectritis congesta and introduced Valerianella locusta-co-occur at the community scale but rarely at the local scale of direct interaction. To test whether niche and/or fitness differences preclude local-scale long-term coexistence, we parameterized models of competitor dynamics with results from a controlled outdoor pot experiment, where we manipulated densities of each species. To evaluate the hypothesis that niche and fitness differences exhibit environmental dependency, leading to community-scale coexistence despite local competitive exclusion, we replicated this experiment with a water availability treatment to determine if this key limiting resource alters the long-term prediction. Water availability impacted population vital rates and intensities of intraspecific versus interspecific competition between P. congesta and V. locusta. Despite environmental influence on competition our model predicts that native P. congesta competitively excludes introduced V. locusta in direct competition across water availability conditions because of an absence of stabilizing niche differences combined with a difference in average fitness, although this advantage weakens in drier conditions. Further, field data demonstrated that P. congesta densities have a negative effect on V. locusta seed prediction. We conclude that native P. congesta limits abundances of introduced V. locusta at the direct-interaction scale, and we posit that V. locusta may rely on spatially dependent coexistence mechanisms to maintain coexistence at the site scale. In quantifying this competitive outcome our study demonstrates mechanistically how a native species may limit the abundance of an introduced invader.

Keywords: Coexistence; determinants of plant community diversity and structure; environmental heterogeneity; invasion ecology; plant competition; plant population and community dynamics.

Figure 1.

Relationships between P. congesta and V. locusta abundances in habitat remnants where they co-occur in (A) 0.1-m² and (B) 1-m² plots across two field sites in the Garry oak savanna (southern Vancouver Island, BC), with 95 % CIs.

Figure 2.

Density-dependent seed production of P. congesta in the dry treatment (A) and in the wet treatment (B) and seed production of V. locusta in the dry treatment (C) and in the wet treatment (D) are plotted against number of neighbour plants per growing pot. Seed production values against conspecific neighbours are denoted with filled circles and solid trend lines, while seed production values against heterospecific neighbours are denoted with open circles and dashed trend lines. The same set of individuals grown with no neighbours (neighbour density = 0) were used to fit relationships for both intraspecific and interspecific competition for each focal species by water treatment. Points were jittered for each neighbour density to improve legibility.

Figure 3.

Coexistence predictions for P. congesta and V. locusta from a controlled experiment with dry and wet soil moisture conditions. (A) Coexistence is possible when niche overlap (ρ) is low, i.e. when ρ approaches 0 (or when niche difference (1 − ρ) approaches 1), and the fitness ratio (κ) is relatively even between species, i.e. when κ approaches 1, specifically when ρ < κ < 1/ρ. Here, fitness ratios >1 represent a V. locusta competitive advantage, while fitness ratios <1 represent a P. congesta competitive advantage. Error bars indicate standard errors. (B) Coexistence outcome can also be described using the low-density population growth rates of competing species. Species are expected to coexist when both species can invade and increase in abundance in a patch dominated by the opposing species, i.e. when both species display low-density invasion growth rates >1 (dashed line). Error bars indicate standard errors. SeeEquations 2–8, Supporting Information-Supplemental Methods for details on parameter estimation.

Figure 4.

Relationships between predicted seed production and neighbour density and variation in environmental composition in natural plant communities. Effect of intraspecific (filled circles, solid trend lines) and interspecific (open circles, dashed trend lines) competition on predicted fecundity of (A) P. congesta and (B) V. locusta with 95 % CI for significant trend. Relationship between predicted seed production and (C) soil moisture at 7.6 cm depth, and (D) soil moisture at 12 cm depth for P. congesta (filled circles, solid trend lines) and V. locusta (open circles, dashed trend lines). (E) Mean and SE for P. congesta (black) and V. locusta (grey) growing in soils shallower or deeper than 12 cm.