The basic approach was a large-scale comparison during the period 2000–2003 of organic and non-organic farms paired on the basis of proximity, crop type and cropping season. Organic farms of at least 30
ha with contiguous organic fields containing arable land were identified from the databases of the Soil Association and Organic Farmers and Growers. Data were collected from 89 pairs of farms; 80% of pair members were within 10
km of one another (median distance=6.4
km) but pair members were non-contiguous. Virtually all suitable organic farms in England growing relevant crop types (winter wheat and spring cereals) at the time of the study were examined for a minimum of three taxa (plants, spiders, carabid beetles). One organic and one non-organic cereal field were randomly selected (‘target fields’) in each of the 89 farm pairs. Plants and invertebrates were sampled on these 89 pairs of fields. All fields sampled in 2000 were spring cereals; all fields thereafter were winter cereals. The organic target fields covered a range of ages since conversion. Birds and bats were sampled at a larger spatial scale extending over several fields.
Habitat data were collected at farm and field levels once during the project period. Ground-based surveys of habitat extent were undertaken for a sample of the bird survey areas. Locations of crops and ‘habitat patches’ (e.g. hedges, ponds) were mapped at 1
2500. Hedge height and width were measured at 10 evenly spaced points around the boundary of each target field; tree/shrub composition, numbers of trees and gaps were recorded within 5
m of these points. Farmers were asked 40 questions concerning management of the target field and the whole farm.
Plants within target fields were sampled in 3 years (2000, 2002, 2003) with each field sampled in one of the years. Three plot types were used; the first two followed the procedure of Smart et al. (2003)
with one plot per field. (i) Crop margin plots recorded species presence in plots extending 1
m from the ploughed edge and 100
m along the field edge. (ii) Field boundary plots recorded presence and abundance (% cover) of species in plots extending 1
m from the centre of the uncultivated field boundary and 10
m parallel to the boundary. (iii) Percent cover of within-crop plants was recorded in 0.5×0.5
m quadrats placed at distances of 2, 4, 8, 16 and 32
m from the ploughed margin on 12 transects per field.
For invertebrates, years of sampling and fields used were as for plants. A grid of 18 pitfall traps was set in each target field, comprising nine within the crop and nine within the uncropped boundary. Traps were set for one week before emptying. Paired target fields were always sampled at the same time. Because of seasonal variation in animal activity and trapping efficiency, separate samples were collected before and after harvest. Spiders and carabids were identified to species level.
Sampling of birds was carried out on 61 farm pairs in winters 2000/2001 and 2002/2003 (29 in both winters and 32 in one winter). Surveys took place on the target field and up to five adjacent fields once per month at each site between October and February. During each visit, the observer walked the perimeter of each field and once across the centre of each field. Birds were mapped on large-scale maps and individual records were subsequently allocated to habitat categories. Abundance values for individual farms were based on mean counts across visits.
Bat surveys were completed pre-harvest on 65 farm pairs between June and August in 2002 and 2003. Using transects of approximately 3
km starting in the target field, activity of Nyctalus leisleri
, Nyctalus noctula
and Eptesicus serotinus
was identified using heterodyne bat detectors tuned to 25
kHz. Transects were as close as possible to triangular with an apex pointing north. Bat passes and feeding calls were counted for each 125
m transect section, at the end of which the detector was retuned to 50
kHz and the number of Pipistrellus
passes and feeding buzzes counted for 1
min. An abundance index for all bats was based on total passes per 3
km. Bat activity along transects was also recorded onto minidisc and sonograms were analysed using Batsound
software. The data were adjusted for recording duration and used to derive indices of bat activity, species density and dominance.
Comparisons of habitat and management attributes are based on Wilcoxon matched-pair tests. Analyses in follow the format of Perry et al. (2003)
Effects of farming system on number of species (species density), diversity (dominance) and abundance.