The distribution of Acaena novae-zelandiae T. Kirk. (the Piri-Piri Burr) on lindisfarne national nature reserve

Abstract

SUMMARY 1) The aims of this study were to map the present distribution of Acaena on the Snook and Shiel of Lindisfarne and to relate that distribution with the major factors determining where Acaena grows. Past distribution was compared to investigate invasion dynamics and predict future spread. 2) Mapping was done by pacing out a grid of 25m x 25m and sampling, using a lm x lm quadrat, at each intersection on the grid. For each quadrat percentage cover of Acaena, Ammophila and bare ground were recorded as well as the presence of paths and warrens. Aspect was also recorded and a subjective record of habitat type was made. 3) The greatest abundances of Acaena were found on the Shiel and in patches around the edges of the Snook, this correlates with the presence of yellow dunes. The Shiel is the most probable site of introduction to the island so it is not surprising that Acaena is most abundant here. 4) Preferred habitat of Acaena is open, yellow , and to a certain extent fixed, dune habitat, with ten to 50 percent of Ammophila and one to 30 percent bare ground; no Acaena was found in the slacks. There is, however, no association with aspect, but Acaena is associated with paths. 5) This preference for open ground and low vegetation cover is probably due to an inability of seeds to establish in densely vegetated areas, and because Acaena is a poor competitor in the vegetative phase. 6) Rate of spread of Acaena appears to have been constant since its introduction. It has certainly not shown any signs of decreasing in the last 29 years, despite a decline in the number of colonisation events. This indicates that vegetative spread is more important than colonisation. 7) Acaena threatens the rare flora of Lindisfarne, but so far control is ineffective and there is little that can be done about the problem. Acaena is not, however, affecting the rare species in the slack (nor is it likely to in the future); the lower areas of the transitional stages are similarly uninvaded offering refuge to a number of orchid species.