Herbivore impacts on upland habitats. Update on the, 2007 field season

Summary

Field work in 2007 focussed on measuring the effect of grazing by deer and sheep and its consequences for plant species diversity at sixteen sites across Scotland. We are currently analysing data on herbivore dung counts, grazed heather shoots, sward heights and plant species composition from 140 plots within these sites. We will now determine the extent to which heather utilisation is related to the relative density of deer and sheep. In addition we will take into account local site factors such as rainfall and soil fertility when investigating how plant species diversity is affected by deer and sheep grazing. We are very grateful for the access granted to our field teams in 2007. On-going analysis will shape our future work in this area and we will keep you informed about any new developments.

Backgound

In the Scottish uplands, grazing by deer and sheep is a key factor affecting the condition of many habitats. Of particular interest is heather moorland which is recognised as important on an international scale because of its biodiversity and landscape characteristics. In the last decades the overall area of heather moorland has declined; in part due to grazing. Woody species such as heather are less resistant to grazing pressure than herbaceous plants and heavy grazing can lead to the replacement of heather with grassland communities (Ratcliffe 1959).

Over the last few years there have been a number changes relating to herbivores that have occurred simultaneously. For example, recent changes in the Common Agricultural Policy (CAP) reforms have led to a steady reduction in sheep numbers in the uplands. Coincidentally, deer numbers are reported to have increased substantially possibly due to climate and habitat changes. However, whilst these findings may be true at the large geographic scale, local situations can be quite different.

Reduction in sheep grazing is predicted to have a beneficial effect on upland heathlands by reducing grazing pressure, however many people working with deer suggest that in some areas deer may move in and replace the sheep and there is some evidence for this in the scientific literature (Hope et al. 1996). As yet there isn’t a good understanding of what the consequences of this shift may be on the habitats they graze.

Methods

In 2007 Macaulay staff carried out fieldwork to collect data on grazing impacts on heather moorland in areas with and without sheep to investigate the consequences of sheep removal. Initially sixteen sites were selected along an east west gradient in northern Scotland. Eight of these sites had reduced sheep numbers and eight nearby or adjacent sites had sheep at higher stocking densities, where possible sites were paired high sheep/ low sheep to minimise variation. Measurements at each site included heather utilisation, vegetation height; plant species composition and dung counts (red, roe, sheep, grouse, hare, rabbit and others).

Approach

Following our first year of field work, we are working on analysing the data to inform three questions which we would like to investigate further:

1). Is there any evidence for increased use by deer in sites where sheep have been removed?

To answer this we will analyse dung count data from all the sites to determine if sites with reduced sheep numbers have higher deer dung counts than sites that are grazed by both deer and sheep. Interpretation of the results needs to take into account any differences in how many deer or sheep the dung counts represent. This result may to depend on the density of deer before sheep removal so we may be in touch to ask for further information on this.

2). Does the degree of grazing impact on heather differ between sheep and deer?

To address this we are analysing heather utilisation data from across all the sites. This is a measure of the extent of grazing on heather shoots. We will test whether the degree to which heather utilisation is associated with the density of sheep, deer or other herbivores. Unsurprisingly, initial findings suggest that both deer and sheep have an impact on heather. However, more work needs to done to convert dung counts to animal density estimates before any firm conclusions can be drawn about the impact of deer compared to that of sheep.

3). Does the diversity of plant species in heather moorland depend on grazing pressure?

Although the grazing impact on heather is likely to vary in relation to sheep and deer grazing pressure, what is more important is the grazing impact on the biodiversity of these habitats. This is likely to vary from one location to another with some areas able to support more grazing animals than other areas and still maintain their biodiversity. Soil fertility and local precipitation may well be partially responsible for this variation. Our analysis will initially investigate whether plant species diversity changes in relation the dung counts of deer, sheep, grouse and other herbivores and we will go on to explore how site soil fertility and precipitation affect these interactions.

Next steps

Further analysis of the 2007 data is needed. We intend to investigate how grazing impacts and plant species diversity has responded to the length of time that some of the sites have been sheep free. We are currently reviewing the research topics and the existing data to refine field work for 2008.

Other related work at Macaulay

Complementary work on sheep and deer interactions and grazing effects is also taking place at the Macaulay Institute’s research farm. Here we have red deer hinds and hill sheep grazing on areas of heather moorland throughout the year. The animals are fitted with GPS collars and we will investigate deer and sheep grazing patterns and impacts in relation to habitat types and weather. Over the next 3 years we will also be testing how deer grazing pattern responds to the removal of sheep. This work provides more detail on key aspects of deer and sheep grazing and will augment the findings of the data collected on your site.

Publishing the results.

We would like to thank you for allowing us access to the sites we used and we will endeavour to keep you informed as results emerge. However, the process of publishing scientific research takes some time because the work needs to be reviewed by independent and anonymous referees, which is central to the quality control of scientific outputs. Any manuscript that is published will have had to address the issues raised by the referees to the satisfaction of the editor before it is allowed to be published.

Contact: Dr. Justin Irvine