This blog post reports on the discovery of a small population of Acaena inermis near Danby Beacon in the North York Moors National Park. Unlike other piri piri bur species such as A. novae-zelandiae (e.g. Invasive Species Compendium, CABI, 2021, https://www.cabi.org/isc/datasheet/111979), A. inermis is not known as an undesirable alien, however population dynamic is relatively unknown in Britain.
The population described here is nonetheless of biological interest based on the distance to known records and the site being located within a designated area for nature conservation. It is also an opportunity to highlight the species and some of its characteristics to other field botanists and conservationists, hopefully to improve recording coverage.
Distribution in Britain and Ireland and description of the Danby Beacon location
A. inermis is a relatively rare alien plant in Britain and Ireland, with scattered records from lowland location (from Southern England to Edinburg) and a few well known clusters of records from upland natural habitats: Isle of Skye, the Grampians, the Southern Uplands of Scotland, and in England in the Cheviot Hills (Coquet dale). These small clusters of upland populations appear well established, some since the 1970s and the Skye population may be increasing with new sites having been reported in recent years (e.g. Bungard, 2019, https://skyeraasayplants.wordpress.com/2019/05/31/aliens-day/; BSBI 2021 map).
The new location reported here is isolated by about 100 km from three other English presences hitherto reported (South Yorkshire, Skipton area, Coquet Valley). Although there may be unknown sites in between that could have acted as stepping stones, the population presented here is more likely to be an independent introduction from cultivation into the wild.
At Danby Beacon, the population is located on a lay-by popular with visitor (NZ726085, at or adjacent to NZ726308656), with a lovely view to the SW, popular with visitors for picnics and for quiet time in a camping chair by the car. Due to this relative popularity, it can be speculated that propagules of the plant reached the site on car tyres. The propagules could have been directly brought from a garden or a drive way to to this lay-by by visitors of the national park. The habitat is a species rich acidic grasssland, with species such as Antennaria dioica, Euphrasia sp., Veronica officinalis, Galium saxatile, Nardus stricta, Festuca rubra, Cynosurus cristatus, Calluna vulgaris, Trifolium repens, Polygala serpyllifolia, Carex pilulifera, Pilosella officinals, Leontodon saxatile, and other species.
Invasives and conservation
Reporting the first introduction of alien plant in an area can be important to identify the pace and vectors of colonisation, especially in genera with well known undesirable species. The North York Moors national park authority, has already taken action against A. novae-zelandiae in one location and may want to keep on eye on this new comer in the area, until we know more about the population dynamic of A. inermis in the area. Acidic grassland where the population is located is a highly threatened habitats in Britain (see e.g. Carly Stevens 2011 paper).
Improving our understanding of A. inermis distribution
Acaena inermis is a species from New Zealand, first recorded in the wild in the Britain or Ireland in the 1970s. Identification of the Danby Beacon population was based on the CRIB account for Acaena (Yeo, 1988, https://bsbi.org/wp-content/uploads/dlm_uploads/Acaena_Crib.pdf). There is also abundant photography on the internet as the species is a popular rockery plant. The species, as well as one cultivated variety (Acaena inermis cv ‘Purpurea’) is widely available from nurseries for outdoor gardens. It is not clear to me whether the population described here belongs to this the ‘Purpurea’ cv or not, and even not clear from internet searches whether all specimens of this species can be found under the name ‘Purpurea’ due to the type colouration of the foliage. I would be keen to hear from experienced nursery-people on this point.
At the Danby Beacon site the plant is extremely difficult to spot due to its size, prostrate stem and colour, which mimics topsoil litter. In addition, the bright red spines that can develop on the fruits were not observed by the end of July 2021.