Diffuse knapweed (Centaurea diffusa) : reproductive thresholds, population ecology and responses to the introduction of the buprestid beetle Sphenoptera jugoslavica
Creators: name: Powell, Robert David
Publisher: name: University of British Columbia
Year: 2010

description: Diffuse knapweed (Centaurea diffusa), like other semelparous perennials, was found to have a minimum size requirement for flowering. This critical size conditions the plant's phenotypic response to variation in growth conditions. Experimentally crowded rosettes grew slowly, failed to reach the critical size and did not flower in the following season, whereas uncrowded rosettes grew rapidly, reached the critical size in a single year and flowered in the next. The proportion of plants that flowered and subsequently died in both field and experimental populations decreased with individual crowding. The effect of interference-related growth reduction was therefore to accentuate gaps in the population's spatial pattern. Interference-related mortality of seedlings and rosettes acted in the opposite direction. The three interference-related processes of recruitment, pre-reproductive and post-reproductive mortality determine the fine scale spatial pattern of the population, and its density. There is no theoretical basis for interpreting a shift toward regularity as evidence for interference. A model, proposed to explain the apparent ubiquity of critical size-dependent switching to reproductive development in semelparous perennials, shows that this adaptation maximizes the intrinsic rate of increase of semelparous species subject to high juvenile mortality followed by low rosette mortality, seasonality, and extensive variability in rosette growth rates. The model suggests that biological control organisms might be chosen strategically to disrupt selection for the critical size. Age-structured field populations exhibit considerable spatial variation in density. To determine whether that variation could account for a significant proportion of the variability in rosette growth rates under field conditions, the growth rates of individual plants were observed and regressed against a series of 'crowding indices'. Two methods of delimiting a sample of neighbours for this purpose were compared. 'Circular sampling' defines neighbours as plants within a specified radius of a focal plant; 'polygonal sampling' defines neighbours as plants that share a boundary when the ground is partitioned into Dirichlet polygons. A simulation study of the sampling characteristics of the two methods in relation to the degree of aggregation in the population showed that polygon samples are statistically preferable in aggregated populations, but the two methods performed about equally, accounting for approximately 25% of the variance in the growth rates of rosettes in the field. The buprestid beetle Sphenoptera jugoslavica is the third insect established in Canada as a potential biological control for diffuse knapweed. The beetle population at the release site in British Columbia was monitored, and experiments were conducted to determine the beetle's effects on its host. S. jugoslavica reduces the survivorship of seedlings and rosettes, delays reproduction, and finally reduces seed output. Under favorable conditions the beetle can contribute to a significant reduction in knapweed population growth. Its effectiveness at the release site is limited by a phenological requirement for arrested plant growth during the oviposition period which leads to large fluctuations in the size of the beetle population, and only intermittent damage to the knapweed population.