Ben Thornes

Impact of Sika grazing at Arne Saltmarsh

Tuesday 15th July 2014

The impact of sika deer grazing on the vegetation and infauna of Arne saltmarsh.

Saltmarshes are extremely important coastal habitats which have long been impacted upon by human activities. Such activities as: land claim for port and marina facilities; waste disposal; agriculture; erosion and/or coastal squeeze due to coastal protection works and changes in estuarine morphology on the seaward edge and fixed flood defence walls and other construction on the landward side.

Saltmarshes are a particularly important habitat for many breeding and over-wintering bird species, and they are of particular importance for the Redshank (Tringa totanus), with roughly 50% of breeding pairs nesting in such a habitat. Britain’s coastal saltmarshes are of national importance with regard to the conservation of Redshank. This species has been declining in Britain in recent years, reflecting a decline throughout Europe. These declines are believed to be due to the loss and degradation of important breeding sites combined with an intensification of agriculture. However, in Britain this decline has been attributed to a change in grazing practices on saltmarshes rather than loss of habitat.

Redshank build their nests in grass tussocks or patches of relatively tall vegetation, however they are visual feeders so require swards of shorter vegetation with patches of surface water. Therefore redshanks need a habitat of varied vegetation structure, such a habitat is characteristic of a saltmarsh moderately grazed by cattle.

Poole Harbour is recognised as an important area for wildlife, with many designations including Special Site of Scientific Interest (SSSI), Special Protection Area (SPA), and a RAMSAR site. The Arne RSPB nature reserve is also located within the harbour. Concerns have been raised by the RSPB regarding the impact of sika deer (Cervus nippon) grazing on the saltmarsh habitat and, consequently, its impact on the redshank population of the harbour.

Sika deer are an alien species in Britain, originally imported between 1860 and 1920 from eastern Asia and Japan, and are generalists that can quickly change their behaviour to adapt to changes in environmental variables such as food variability.

The aim of this study was to assess the impact of sika deer on the saltmarsh by investigating the effect of deer grazing on the vegetation diversity and structure and also on the invertebrate fauna of the harbour.

A survey of Arne saltmarsh was undertaken to assess the general extent of deer grazing at Arne. From this survey, three sites were identified for more focused analysis: Grip Heath; Crichton’s Heath north and Crichton’s Heath south. Fifty Plots measuring 2m x 2m were established across these sites, allowing data to be collected for 20 grazed, 20 ungrazed and 10 fenced plots.

The impact of deer grazing was investigated using a standard vegetation survey within each plot to determine species composition and abundance. Above ground vegetation volume was assessed by visually recording the percentage occupancy of slices of the plot cube at 10 cm height intervals. To investigate the possible impact of deer grazing below ground, root biomass was also investigated and the relationship between above ground vegetation cover and below ground root biomass was assessed using a Spearman’s Rank Correlation.

The findings of this study indicated that deer grazing at Arne had a significant impact on the structural and species diversity of the saltmarsh vegetation. Spartina anglica was found to dominate in ungrazed areas, whilst Salicornia ramosissima, and, to a lesser extent, Puccinellia maritima dominated the grazed sites. In the grazed areas the vegetation cover was significantly lower, as was vegetation height and volume. Additionally, significant changes were observed in the root biomass, which was lower in grazed areas. Infaunal diversity was generally low throughout the survey area, with only three invertebrate species recorded, but significant variations were observed. Invertebrates were found to be more abundant in grazed plots than in ungrazed plots, and least abundant in the fenced plots.

The study indicated that in its current condition, localised areas of Arne saltmarsh do not provide adequate habitat requirements needed for the Redshank, Tringa totanus. Redshank forage for a wide variety of invertebrates, in particular, Corophium spp. a species not observed in this study at Arne. Additionally, Redshank require a range of foraging habitats as juveniles tend to feed on saltmarshes whilst adults feed on more open habitats such as mudflats and mussel beds. The saltmarsh provided a better food source, but a higher risk of predation, particularly from sparrowhawks.

Read the full article here.

Find out more about Dr Anita Diaz here.

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