Ecological factors affecting rates of spread in Cytisus scoparius, an invasive exotic shrub

Thesis (Ph. D.)--University of Washington, 1996Invasions of exotic species are among our most daunting conservation challenges, and range expansion models provide us with a framework for modelling them. I have focussed on ecological factors contributing to variation in population growth and spread in Cytisus scoparius (Scotch broom), an exotic shrub invading western Washington.The reproduction of C. scoparius is highly dependent on pollinators. Mean visitation rates (percent flowers tripped) varied from 3% to 30% among four populations over three years. Urban populations experienced greater visitation than prairie populations. Experimental pollen addition revealed significant pollinator limitation in all populations in two years. Fruit set was increased on average 280% to 2620%. No subsequent cost of reproduction was found, indicating that pollinators exert true population-level control over seed production.The dual effects of seed number and safe-site limitation were investigated with an experiment crossing five seed densities (0-1000 seeds/m$\sp{\wedge}$2) with four surface treatments (control, "scalping" of the cryptogram layer, burning before seed addition, burning after seed addition). Seedling/seed decreased with seed density. Burning did not increase germination relative to controls, but seeds scarified by the burn had significantly higher germination rates than seeds added after the burn. Undisturbed plots had the highest germination levels, providing evidence that invasions are not always linked to disturbance.The relative importance of all stages of the life cycle was evaluated using a matrix demographic approach. I followed marked individuals in six populations, three urban fields and three prairies, across three stages of invasion. Population growth was higher in prairie populations than urban populations and decreased as the invasion proceeded. Elasticity patterns revealed no "silver bullet" for population control in the form of a single dominant stage in early-invasion plots.Finally, I modelled areal spread of the populations with a cellular automata model. By combining life history transition probabilities with dispersal parameters estimated from the field, I investigated the relative importance of dispersal and demographic variation. The difference in demographic rates between urban and prairie populations made a large difference in rate of spread, enough to swamp out variation in dispersal