The invasive shrub Buddleia davidii performs better in its introduced range
It is commonly assumed that invasive plants grow more vigorously in their introduced range than in their native range. This is attributed to two major hypotheses: the Enemy Release Hypothesis (ERH) which postulates that a lack of natural enemies in the new area results in increased abundance of the invader, and the Evolution of Increased Competitive Ability Hypothesis (EICA) which states that this lack of natural enemies should select for increased competitive ability at the expense of defence mechanisms. However, few studies have tested these assumptions by comparing the performance of invasive species in their native versus introduced ranges.
Buddleia davidii (Buddleiaceae, EPPO List of Invasive Alien Plants) is a shrub native to China and was introduced to Europe and other continents for ornamental purposes around 1900. It naturalized and became invasive in Europe, North America, Africa, Australia and New Zealand. In New Zealand, the species invaded riversides and forest plantations where it out-competes native vegetation and causes significant economic problems. In both its native and invaded range, B. davidii prefers naturally or anthropogenically disturbed sites along roads, river banks, and railways. It also survives in walls and on rock faces and can tolerate a wide range of soil conditions. B. davidii populations in Europe are restricted to oceanic and sub oceanic climates in the temperate and sub Mediterranean zones (the eastern range margin currently crosses Germany). It is thought that its further spread is limited by a lack of frost tolerance. There is still poor knowledge of the species’ potential to spread and to become a problematic weed on a larger scale.
The abundance, growth, reproduction as well as leaf herbivory have been compared between 10 native populations of B. davidii in China and 10 invasive populations in Germany.
Large variations were found among B. davidii populations within each range: population size varied from 12 to 15,000 individuals in the native range in China, and from 9 to 860 individuals in the invasive range in Germany. Population density ranged from 150 to 4200;ha-1 in China, and from 38 to 12;700 ha-1 in Germany. However, these differences between ranges were not statistically significant.
There was highly significant variation among populations within ranges for all measured traits. Strong evidence was found for increased plant growth and reproduction in invasive populations: mean length of stems was 79% greater, and mean diameter of thickest stem was 73% greater in invasive populations than in native ones. European plants had on average twice as many stems as Chinese plants, although this difference was marginally significant. Furthermore, inflorescences were 23% longer on average in European populations than in Chinese populations. Since the number of stems predicts the number of inflorescences, and inflorescence length predicts the number of seed capsule, the data indicate increased seed production in invasive populations. Moreover, on average seeds in Europe were 25% heavier than in Asia, but did not germinate significantly better.
Leaf herbivory was quantified visually by estimating the percentage area destroyed of 20 randomly chosen leaves from each plant. Leaf herbivory was more pronounced in B. davidii populations in the native range. Here, about 15% of leaf area had been lost, as opposed to plants in the invasive range that showed almost no signs of herbivory. The data suggest that low levels of herbivory may contribute to the increased size and fecundity of invasive B. davidii populations (according to the ERH hypothesis). Possible explanations for this decreased herbivory in invasive populations is the lack of related native plant species in Europe, and the fact that no herbivores from the native range are present in Europe. The reduced herbivory in the new range, in particular the lack of specialist herbivores, may have led to the evolution of increased plant size and fecundity in B. davidii, as proposed by the EICA hypothesis, but this cannot be explained by the field experiments and deserves further attention.
In addition to phenotypic plasticity, genetic differentiation might be a second mechanism underlying the increased vigour of invasive B. davidii populations. The ability of exotic species to undergo evolutionary adjustments to novel environments is a key feature of successful adaptations. However, genetic differentiations may also be due to artificial selection, since B. davidii was cultivated as an ornamental plant. Selection of desirable cultivars may lead to changes in traits related to growth and reproduction, thus enhancing the invasive potential. Hence, plant breeding has to be taken into account when assessing the evolutionary ecology of plant invaders.
Sources
Ebeling SK, Hensen I, Auge H (2007) The invasive shrub Buddleia davidii performs better in its introduced range. Diversity and distribution 14(2), 225-233. (OnlineEarly Articles: http://www.blackwell-synergy.com/doi/abs/10.1111/j.1472-4642.2007.00422.x)