Tackling a spiny threat in the Falkland Islands
For the past five years the Royal Botanic Gardens, Kew has been working with Falklands Conservation (FC) and the Falkland Islands Government (FIG) to research the impacts of invasive alien plants and take action against some of the worst offenders. FC's Richard Lewis updates us on the control of thistles (Cirsium spp. & Carduus spp.), some of the most troublesome species.
Thistles are vigorous, spiny plants which thrive in a range of habitats. In the Falklands they typically spread from core populations in moist, neutral grassland (locally called ‘greens’) or disturbed areas into coastal and heathland habitats.
These invasive plants come from the UK, accidentally introduced as weed seeds on people or goods arriving in the islands. They are taller than most native and pasture plants and aren’t eaten by livestock or local insects, so they out-compete some of the islands’ rarest plants. They are also bad news for local sheep farmers, displacing nutritious pasture grasses and helping spread disease amongst sheep. In addition any spines that get caught in fleeces injure shearers and lower the market value of wool.
Working together to stop the thistles spreading
Co-operation has been vital to the success of this work. Not only are we working with Falklands Conservation (FC) and Falkland Islands Government (FIG), but many local farmers and landowners, local conservation workers and volunteers, the Falklands Department of Agriculture (DoA), Royal Society for the Protection of Birds, Ministry of Defence (MoD), Interserve Defence Ltd and the Mount Pleasant Airbase (MPA) conservation group have all contributed significantly to research, practical action and funding. Additional funds have come from Department for Environment, Food and Rural Affairs (Defra), Joint Nature Conservation Committee (JNCC), Overseas Territories Environment Programme (OTEP), European Union and Kew’s Bentham-Moxon Trust.
Thanks to all their work over several years, old plant records have been followed up, areas surveyed, baseline data collected and an Invasive Species Action Plan has been drawn up and control measures including weedkillers and ‘chisel hoes’ have been successfully trialled. The examples below illustrate the different biology and histories of the three known species in the Falklands. However, there are several more thistle species in the UK which could be accidentally imported and the biosecurity work of the DoA will be of increasing importance to stop even more of these spiny weeds arriving.
The main culprits
Slender thistles (Carduus tenuiflorus) were first seen at a farm on West Falkland in the early 1980s. This biennial species was successfully controlled by the local farmer for several years and never spread far or increased in numbers. It is now believed extirpated from the islands. This highlights the value in taking action as early as possible, before a species becomes well established.
Creeping thistle (Cirsium arvense) is more widely established in several locations across the Falklands. In early 2012 a single, large patch was reported from Philimore Island, which is managed as a nature reserve. The MPA conservation group arranged a helicopter trip and volunteers to undertake initial control in December 2012, with follow-up control planned for future years. This species spreads by creeping rhizomes to form large patches, but has male and female flowers on separate plants. With just one patch on Philimore Island, arising from one individual plant, no seeds have been produced, and it is likely that this island can be declared thistle-free after a few years control and follow-up monitoring.
Unfortunately, spear thistles (Cirsium vulgare) are much harder to control. Although individual plants only live for two years, each plant sets hundreds or thousands of seeds, which can survive in the soil for decades. Fortunately, they are currently limited to only a handful of locations.
Saunders Island has one of the largest populations of this species, a particular concern owing to the status of the Island as an Important Plant Area (IPA). Home to several rare and threatened species, including the endemic, globally threatened hairy daisy (Erigeron incertus) and Antarctic cudweed (Gamochaeta antarctica), control of spreading invasives is urgent at this site. Several nearby islands, including further IPAs, are also at risk from wind-blown seeds. Control here is a high priority, but is logistically difficult and expensive as the plants are spread over several square kilometres in a remote part of the island.
To prevent seeding, control is needed at least twice a year, potentially for several decades. Previous control has been effective in the densest parts of the population, close to the coast, but has been less effective higher up the hills, where the population appears to be expanding. Ongoing monitoring will allow reassessment of the thistle population. Although it will be a long time before success can be declared, we are in a strong position to continue this control programme and safeguard the plants and habitats not only of this IPA, but other islands nearby.
- Richard -