Density and reproduction in native and invasive Linaria vulgaris populations at multiple spatial scales

Comparing invasive plants in their native and invasive ranges can answer questions about invasion mechanisms and inform management options. However, few studies have considered how density varies with spatial scale or how individual fitness is affected by conspecific density at different spatial scales. A census was carried out of 15 native UK populations of the perennial herb Linaria vulgaris Miller (Plantaginaceae), and of seven invasive populations from a climatically matched area of Alaska. There was no difference in density between native and invasive populations when compared at spatial scales of 0.0625, 0.5, 1 or 4 m2, or when density was measured as a mean field of each population. However, invasive populations covered a larger area, so density was greater at broad spatial scales. The effect of conspecific density on the height and reproduction of ramets varied with the spatial scale and between ranges. Invasive ramets were shorter than native ramets, and therefore produced fewer mature fruit. However, this was more than compensated for by the greater number of viable black seed in invasive fruit than native fruit. One of the reasons for this was the presence of the seed feeding weevil Rhinusa antirrhini in over half of the native, but none of the invasive, fruit. The majority of seed was estimated to fall within 1 m of maternal plants when surrounded with vegetation, but seed travelled further in an unvegetated area. Germination rates were very low in both the field and laboratory. The thesis ends with a description of the biology of L. vulgaris. This work demonstrates that invasion and escape from natural enemies can occur at a broad spatial scale, without increased density and vigour at a fine spatial scale.