Research Profile

Darwin's barberry flowers

What contributes to invasion success in the environmental weed Darwin’s barberry (Berberis darwinii)?

Darwin’s barberry (Berberis darwinii) is an invasive tree species that is considered a serious threat to native species and ecosystems in many parts of New Zealand. Originally introduced as an attractive garden plant, it has since ‘escaped’ and invaded a range of vegetation types throughout the country, including remnant forest stands, scrub, and along roadsides. There is a very large population of Darwin’s barberry on Wright’s Hill, adjacent to the Karori Wildlife Sanctuary, and this creates a constant rain of seeds into the KWS during the extensive summer fruiting period.

In this study, Kate McAlpine measured patterns and mechanisms of seed dispersal, germination, seedling growth, and seedling establishment in Darwin’s barberry to find out why this species is such a successful invader. In most of the experiments she also compared the performance of Darwin’s barberry to four common native species: Coprosma grandifolia, Pseudopanax arboreus, Melicytus ramiflorus, and Schefflera digitata.

Darwin's barberry fruit

Darwin’s barberry produces copious quantities of fruit, earlier in the summer than many other species (both native and exotic). Birds consume the fruits and disperse large numbers of seeds many hundreds of metres from the parent plants. Seeds germinate in higher numbers, and in a wider range of light environments than all four native species studied, and seedling establishment is also much more successful in Darwin’s barberry. Interestingly, seedling establishment of all study species was restricted to sunny conditions. This was unexpected for Darwin’s barberry, because it is generally considered a highly shade-tolerant species. Darwin’s barberry seedlings were also ten times bigger in the sun compared to the shade, but proportional patterns of biomass allocation to roots, stem, and leaves did not change much between light environments. Results suggest this superior performance in the sun may be due to Darwin’s barberry’s photosynthetic capacity, which was double that of the natives. The good news is that Darwin’s barberry seeds in the soil generally don’t live for more than one year.

This research contributes to the growing body of international literature on invasion ecology and, in particular, ecological characteristics that are disproportionately represented in invasive species. It also provides useful information for the management of this weed in New Zealand; results suggest that the first priority should be to remove the seed source (adult plants). Follow-up seedling control need only continue for 1-2 years, since the seed bank is short-lived. When the seed source cannot be completely removed and re-invasion is likely, seedling control can be concentrated on high light environments, because first-year seedlings occurring in the shade will largely die out naturally. Plants growing in high light can produce fruit within two years, so new seedlings should be removed within this time frame wherever possible.

Profile Kate McAlpine Kate did her BSc (Hons) fieldwork at the Karori Wildlife Sanctuary, investigating seed germination of six species (including Darwin's barberry) in treefall gaps. She has been employed as a weed ecologist by the Department of Conservation since 2001, and completed her PhD at Victoria University in 2005. Photo by Clayson Howell

Papers & presentations

• Technical Summary.
• McAlpine, K. G. (2005). Factors contributing to invasion success in the environmental weed Berberis darwinii: seed dispersal, germination, and seedling establishment. Joint conference of the New Zealand Freshwater Sciences Society and the New Zealand Ecological Society, 28 August-1 September, Nelson, New Zealand.
• McAlpine, K. G. (2002). Biomass allocation in a woody weed: does it contribute to invasion success? Joint conference of the Ecological Society of Australia and the New Zealand Ecological Society, 1-6 December, Cairns, Australia.
• McAlpine, K. G. & Drake, D. R. (2003). The effect of small-scale environmental heterogeneity on seed germination in experimental treefall gaps. Plant Ecology, 165, 207-215. (Abstract available)