The impact of REPS on biodiversity: the contribution of hedgerow structure to the value of REPS farms for breeding birds.

Introduction

There has been a dramatic decline in farmland bird populations across Europe in recent decades. Ecological studies have shown that these declines are closely linked to agricultural intensification. Agri-environment schemes such as the REPS aim to ameliorate the adverse effects of agricultural intensification by promoting more extensive and environmentally friendly farming practices.

However, effective monitoring and assessment is crucial if benefits are to be measured. The REPS has been in place for 8 years yet, no comprehensive ecological monitoring system has been established to date.

As one of a number of Walsh Fellowship studies designed to examine the impact of REPS, this project focused on farmland birds. This paper provides an overview of some if its findings in relation to bird species diversity and abundance of breeding birds with particular reference to hedgerow structure.

REPS measures and birds

While all 11 compulsory REPS measures are designed to realise environmental benefits, several measures are of particular relevance to the biodiversity of wild flora and fauna present on farms. Measures pertaining to field boundaries and hedgerows are of particular significance. On the majority of farms, hedgerows are the only semi-natural habitat remaining.

A bird census conducted on a sample of REPS and non-REPS farms in 1999 confirmed that hedgerows held a greater number of breeding birds than any other farmland feature. Hedgerows have several important roles for birds; they provide feeding, nesting and roosting sites, movement corridors and cover from predators (Lack 1992). Hedgerow management considerations are thus of prime importance in the conservation and maintenance of farmland birds. Of the 11 REPS measures, 4 relate to field boundary elements including hedgerows and hedgerow/field margins. They are as follows (Department of Agriculture, Food and Rural Development 2000):

• Measure 4-Retain wildlife habitats (hedgerows are one of the listed habitats in this measure)
• Measure 5- Maintain farm and field boundaries
• Measure 6- Cease using herbicides, pesticides and fertilisers in and around hedgerows, streams and ponds
• Measure 9-Tillage crop production (an uncultivated strip of at least 1.5m retained at the margin of the field).

The quality of hedgerow is variable. Factors such as the structure, botanical composition and management of hedgerows, combined with adjacent land use have been shown to influence the value of these field boundaries for birds (Parish et al. 1994 & 1995, Chamberlain and Wilson 2000).

In 2000, 10 farms (5 REPS, 5 non-REPS) located in counties, Wexford, Offaly, Meath, Galway and Cork were selected for study. The REPS farms had been in the scheme for at least 4 years. A complete bird census was conducted on each farm using the well-established territory mapping method. This provided information on birds present on the entire farm and also on individual hedgerows. A comprehensive hedgerow survey was also conducted on each farm. Various hedgerow structural variables including, hedge height and width were measured. Botanical composition was examined and adjacent hedgerow margin was measured.

Data collected was examined at the whole farm and hedgerow level. The relationship between bird population variables and measured hedgerow variables was examined.

Results

Hedgerow structure

In a comparison of hedgerow density (total hedgerow length (m) divided by area of farmland surveyed (ha)) there was no significant difference between REPS (median value = 163.47) and non-REPS (median value = 141.59) farms examined (U= 10, n₁=5, n₂=5, P>0.05). This result showed that the farms were well-matched in terms of hedgerow cover and facilitated further comparison in relation to hedgerow structure.

Data from all study farms were pooled and 290 hedgerows were included in analysis (162 REPS and 128 non-REPS).

There were significant differences in hedgerow structure between REPS and non-REPS hedges.

• REPS managed hedgerows were significantly wider (P<0.001) and hedgerow area of the REPS hedges was significantly greater than non-REPS hedges (Fig. 1).

• REPS hedgerows had significantly wider field margins (P<0.01).
• REPS hedgerows had less gaps (P<0.001) than the non-REPS sample.
• In relation to hedge composition, REPS hedgerows had greater percentage cover of the dominant shrub.
• A greater number of herb groups were recorded on REPS hedges (P<0.01) and there was greater percentage cover of herb layer on REPS hedges.
• Of the surrounding habitat variables, only percentage cover of surrounding scrub area was significantly different between the management types with more scrub areas recorded adjacent to REPS hedges.

Bird species richness and abundance of breeding birds

Data collected during the bird census was examined at two levels: at the whole farm level and at the finer scale, hedgerow level.

• There was no significant difference in the average number of bird species recorded on the REPS (23.8 ± 2.5, n= 5,) and non-REPS farms (21.4 ± 3.1, n=5).
• A total of 44 bird species was recorded during the bird census in 2000. Of the 44 species, 30 were found on both REPS and non-REPS farms. Eleven species were recorded on REPS farms only and 3 species were recorded on non-REPS farms exclusively (Fig. 2). The species that were recorded exclusively on either farm type occurred at low frequencies and had specific habitat requirements (Appendix 1).

• Of the birds recorded during the census, 32 of the 44 were utilising hedgerows in some way.

Hedgerow birds

Further analysis of bird data was conducted at the hedgerow level. Data collected on all hedgerows on the study farms was combined and bird species richness and breeding bird density (total no. of birds/hedgerow area) were compared between REPS (n=162) and non-REPS (n=128) hedges.

• There was no difference between the average number of species recorded on REPS hedges (5.6±2.6) and non-REPS hedges (4.7± 2.59) (Fig 3).
• There was no significant difference in the density of birds (per unit hedgerow area; REPS= 0.031±0.02, non-REPS =0.037±0.03) between management types despite the fact that REPS hedgerows were generally larger.

However, when bird variables were examined per unit hedgerow length, a greater number of significant differences between farm types could be seen.

• REPS managed hedgerows had significantly higher species richness (0.036±0.02) than non-REPS hedgerows (0.031±0.02).
• The number of birds (per unit hedgerow length) was higher on REPS hedges (0.69 ± .04) than on non-REPS hedges (0.605±0.04) but failed to be significant at the 0.05 level.

Figures 4a & 4b. Species richness and numbers of birds per unit hedgerow length on REPS and non-REPS hedgerows.

Hedgerow structure and its influence on bird species on REPS and non-REPS managed hedgerows.

Correlation analysis between the bird population variables and the measured hedgerow variables highlighted many significant associations.

Multivariate statistical methods were employed to model the relationship between bird variables and the measured hedgerow characteristics.

Discussion

REPS farms did not differ from non-REPS farms with respect to the number of species present. However, REPS farmed surveyed, did hold a greater numbers of certain species, with 11 species recorded on REPS farms exclusively. Many of these species have specific habitat requirements and it appeared that REPS farms held more variable habitat than the non-REPS farms sampled. Results showed that REPS farms had more scrub areas associated with them than conventional farms. This is perhaps an indication of less intensive management as scrub growth is generally cleared on more intensive farming systems. This habitat is beneficial for many of the bird species recorded on REPS farms.

The project was designed by selecting farms of similar size and hedgerow density (both REPS and non-REPS) in order to examine the influence of hedgerow structure and management on breeding birds.

REPS farms had wider hedgerows, with wider associated field margins than conventional farms. REPS hedges were also shown to have less gaps. All of these features are known to affect the farmland bird community (Green et al. 1994, Parish et al. 1994), particularly species associated with woodland or woodland edge.

Differences in hedgerow structure between the management types did not translate into significant differences in the bird population variables. Rather, it was hedgerow length, which determined differences between REPS and non-REPS hedgerows with species richness per unit hedgerow length was shown to be higher on REPS hedgerows. While species richness is a useful index, the total number of breeding birds is probably a better reflection of habitat availability. When the total number of breeding birds and density of breeding birds was compared between management types, no significant difference was evident although REPS hedges did hold more birds.

Regression analysis showed that hedgerow size variables (including length) were the most significant in determining the total number of breeding birds on both REPS and non-REPS hedgerows. Species richness was heavily influenced by the presence of hedgerow trees and overall structural diversity.

The results of the hedgerow survey are encouraging, and while it is difficult to determine whether the differences are solely attributable to REPS, the scheme undoubtedly plays a significant role in maintaining hedgerow quality. However, the lack of difference in breeding birds suggests that other unmeasured factors may also be significant in determining the number of breeding species present such as cropping practices, off farm enterprises and bird food resources. Hedgerows are not completely independent structures and there may be much interchange between birds on adjacent hedgerows and between adjacent REPS and non-REPS managed hedgerows. A demographic study of the breeding birds present was beyond the scope of this project. However, REPS farms may be more productive, thus providing surplus birds for adjacent farmland, a factor that would not be picked up in a bird census. It is also the case that Ireland has a higher density of hedgerows than comparable UK studies and that hedgerow size per se may not be a limiting factor in the numbers and diversity of farmland birds in Ireland.

Hedgerow quality is important in determining avian diversity on farmland. Recommendations given by Feehan (2002) for promoting botanical and invertebrate diversity have been shown to benefit birds in other studies (Vickery et al, 2001, 2002). Increasing hedge margin width on grassland and tillage fields to buffer the effects of agricultural inputs increases the uncultivated area that can be exploited by birds and provides cover and food resources.

REPS, therefore has a role in maintaining good quality hedgerows on farms and should move towards a more restorative and enhancement role in terms of hedgerow habitat. The higher number of bird species recorded on the REPS sample reinforces this. The specific habitat requirements of these species indicate that these farms can provide greater habitat diversity for birds, a fact that could be important for the conservation of rare and threatened bird species.

However contribution of the REPS to hedgerow quality and bird diversity and abundance is difficult to evaluate at this stage. REPS measures were not tailored for specific biodiversity considerations, and the broad nature of the measures within the scheme makes it difficult to evaluate the effectiveness of those measures. In the absence of baseline data it is not possible to directly measure how breeding bird populations have changed on REPS farms since enrolling in the scheme. This is compounded by a continued lack of specific ecological targets, which could be used in an evaluation of the scheme. The statutory requirement for environmental monitoring and evaluation of agri-environment schemes has not been fulfilled by a number of member states. The most recent developments in agri-environment under the composite Rural Development Regulation 1257/99, puts even more emphasis on monitoring and evaluation than in the past (Bignal and Baldock 2002).

The urgent need for monitoring and evaluation combined with clear objectives and targets cannot be overstated. Only then can the true impact of these agri-environment schemes on the natural biodiversity of farmland be elucidated.

References

Bignal, E. and Baldock, D (2002) Agri-environment policy in a changing European context. In Frame, J. (Ed) Conservation Pays? British Grassland Society Occasional Symposium No. 36 3-14.

Chamberlain, D. E. and Wilson, J. D. (2000) The contribution of hedgerow structure to the value of organic farms to birds. In Aebischer, N. J., Evans, A. D., Grice, P. V. and Vickery, J. A. (Ed's.) Ecology and Conservation of Lowland Farmland Birds: proceedings from the 1999 Ornithologists' Union Spring Conference, 57-68. BOU, Tring.

Department of Agriculture, Food and Rural Development (2000). Rural Environment Protection Scheme. Specifications for REPS 2000. Government of Ireland Publications, Dublin.

Feehan, J. (2002) The Impact of the Rural Environment Protection Scheme on the diversity of flora and Caribidea fauna within the Republic of Ireland. PhD thesis submitted to Department of Geography, Trinity College Dublin.

Green, R. E., Osbourne, P. E. and Sears, E. J. (1994) The distribution of passerine birds in hedgerows during the breeding season in relation to characteristics of the hedgerow and adjacent farmland. Journal of Applied Ecology, 31, 677-692.

Lack P. (1992). Birds of lowland Farms. HMSO London.

Parish T., Lakhani K.H. and Sparks T.H (1994). Modelling the relationship between bird population variables and hedgerow and other field margin attributes. I. Species richness in winter, summer and breeding birds. Journal of Applied Ecology, 31, 764-75.

Parish, T., Lakhani, K. H. and Sparks, T. H. (1995) Modelling the relationship between bird population variables and hedgerow, and other field margin attributes. II. Abundance of individual species and of groups of similar species. Journal of Applied Ecology, 32, 362-371.

Vickery, J.A, Tallowin, J.R, Feber, R.E, Asteraki, EJ, Atkinson, P.W., Fuller, R.J, and Brown, V.K (2001) The management of lowland neutral grasslands in Britain: effects of agricultural practices on birds and their food resources. Journal of Applied Ecology, 38, 647-664.

Vickery, J.A., Carter, N and Fuller, R.J.(2002) The potential value of managed cereal field margins as foraging habitats for farmland birds in the UK. Agriculture, Ecosystems and the Environment, 89, 41-52.

Appendix 1.

From Figure 2. The numbers of bird species common to both REPS and non-REPS farms, and the number of species recorded exclusively on both farm types.

Acknowledgements

This project was funded by a Teagasc Walsh Fellowship and by the Department of Agriculture, Food and Rural Development.

Special thanks to Dr. Brendan Kavanagh, Biology Division, Royal College of Surgeons.