Invasive plants monopolize resources, altering biodiversity and community dynamics. Complex mechanisms underlie invasions, but understanding them is essential to control the spread of weeds. Brachypodium sylvaticum (slender false brome) is a perennial bunchgrass invasive to the Pacific Northwest US. As B. sylvaticum spreads, secondary bottlenecks and lag phases delay population growth at the edge of the expanding range before rapid expansion resumes. We hypothesize that lag phases result from decreased fitness of homozygous self-fertilizing populations until sufficient gene flow or purging of deleterious alleles occurs. Populations establish in disturbed areas with low resource competition, but an invasive weed must maintain the upper hand as other species are introduced. We investigate whether decreased population genetic diversity (Hs) from bottlenecks and habitat disturbance due to nitrogen deposition affect B. sylvaticum competition with three grasses, Holcus lanatus, Dactylis glomerata, and Elymus glaucus. Seeds of species found near 11 populations of B. sylvaticum were grown for 26 months in a randomized competition matrix in a greenhouse. Nitrogen was applied to half the replicates. Fitness of B sylvaticum individuals was measured monthly as plant size (n=384). Genetic diversity was determined by assaying eight microsatellite loci. Analysis was performed as repeated measures ANOVAs in SAS 9.0.
Results/Conclusions
There was a significant effect of source population on competitive ability (p<0.001), as well as a significant interaction between source population and species of competitor (p=0.025). Gene diversity levels varied considerably among populations, ranging from 0 (completely homozygous, inbred population) to 0.34 (outbreeding population). There was a borderline significant effect in the interaction between Hs and species of competitor (p=0.0699). We hypothesize that the effect of Hs was not significant because of the effects of genetic purging that occur during bottlenecks in some peripheral populations, as these populations possess high fitness levels despite high inbreeding. Nitrogen addition was not significant when box effects were taken into consideration. These results suggest that the main barrier to the expansion of B. sylvaticum is decreased competitive ability due to low gene diversity. However, if purging is occurring in peripheral populations as evidenced by highly fit homozygotes, B. sylvaticum’s spread may be highly accelerated.