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Responses in plant and carabid communities to farming practises in boreal landscapes
Agriculture, Ecosystems and Environment 135 (2010) 288–293
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Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee
Responses in plant and carabid communities to farming practises in boreal landscapes Johan Ekroos a,*, Terho Hyvo¨nen b, Juha Tiainen c, Mikko Tiira c,1 a
Department of Biological and Environmental Sciences, University of Helsinki, P.O. Box 65, Viikinkaari 9, FI-00014, Helsinki, Finland MTT Agrifood Research Finland, Plant Production Research, FI-31600, Jokioinen, Finland c Finnish Game and Fisheries Research Institute, P.O. Box 2, FI-00791, Helsinki, Finland b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 February 2009 Received in revised form 14 October 2009 Accepted 15 October 2009 Available online 22 November 2009
The effect of organic farming compared to conventional mixed and cereal farming on arable weeds and carabid beetles in boreal landscapes was studied by comparing the distribution of ecological traits, diversity partitions, species richness and abundance. Organic farming increased both insectpollinated as well as overall weed species richness, whereas the proportion of insect-pollinated weed species of the total species richness was unaffected by farming practises. Carabid species richness was mainly unaffected by farming practises although a higher alpha diversity of large and intermediate carabid species in organic and conventional mixed farms was marginally significant. Activitydensities of carabids were highest on conventional mixed farms. Landscape variables did not affect weed diversity but carabid beta diversity increased with increasing landscape heterogeneity. Local richness of large and intermediate carabid species showed a marginally significant decrease with increasing field cover. It is concluded that arable weed diversity is affected by organic farming to a higher extent than carabids. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Additive partitioning Beta diversity Conventional cereal farming Conventional mixed farming Landscape complexity Organic farming Species richness
1. Introduction Intensification of agriculture has been detrimental for farmland biodiversity. Agricultural intensification has led to a regional specialization with farms producing either cereal crops or livestock (Robinson and Sutherland, 2002). The recent expansion of organic farming in Europe may benefit farmland biodiversity at least in areas with intensive agriculture (Bengtsson et al., 2005; Hole et al., 2005). Organic farming generally enhances species diversity more in homogeneous than complex landscapes, where any biodiversity benefits attributable to farming practises tend to be masked by effects of landscape structure (Roschewitz et al., 2005; Rundlo¨f and Smith, 2006). Particularly arable weeds (Hyvo¨nen et al., 2003; Gabriel et al., 2006; Gabriel and Tscharntke, 2007) and flower-visiting insects (see e.g. Rundlo¨f and Smith, 2006; Holzschuh et al., 2007) benefit from organic farming. The effects of organic farming on carabid beetles have been less consistent in different studies (reviewed by Hole et al., 2005), although generally organic farming enhances
* Corresponding author. Tel.: +358 9 191 57810; fax: +358 9 191 57694. E-mail addresses: johan.ekroos@helsinki.fi (J. Ekroos), terho.hyvonen@mtt.fi (T. Hyvo¨nen), juha.tiainen@rktl.fi (J. Tiainen), [email protected] (M. Tiira). 1 Current address: Pietila¨nkatu 15 FI-11130, Riihima¨ki, Finland. 0167-8809/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2009.10.007
carabid diversity and abundance (Bengtsson et al., 2005). Species richness is enhanced by mixed farming as it combines cereal production and animal husbandry (Kromp, 1999; Robinson and Sutherland, 2002). The majority of studies reporting beneficial effects of organic farming are conducted in landscapes with high proportions of cultivated fields, at least 50–60% of overall land use (Purtauf et al., 2005; Roschewitz et al., 2005; Rundlo¨f and Smith, 2006; Holzschuh et al., 2007). Little is known on the impact of organic farming in boreal landscapes, where arable land is concentrated to the most productive areas and consists of farmland patches interspersed in a landscape dominated by forests (Luoto, 2000). An ecological trait classification combined with diversity partitioning may reveal meaningful ecological interpretations to diversity responses mediated by farming practises or landscape structure at different spatial scales (Gabriel et al., 2006; Clough et al., 2007a,b). Carabid body size, foraging strategy and wing length allow relevant ecological groupings, as these relate to dispersal ability, tolerance against land-use intensity and diet preference (Ribera et al., 1999, 2001). Concerning arable weeds, insectpollinated species provide food resources for pollinating insects. The relationship between insect-pollinated weeds and farming practises can indicate whether organic farming has resulted in changes in the community structure of arable weeds. Insectpollinated arable weeds have been found to benefit more of
J. Ekroos et al. / Agriculture, Ecosystems and Environment 135 (2010) 288–293
organic farming than non-insect-pollinated species (Gabriel and Tscharntke, 2007), which raises the question whether this relationship can be found also in boreal agricultural landscapes. In this paper, the impact of farm type and landscape structure on arable weeds and carabid beetles in cereal fields was studied in six sites consisting of mosaic landscapes with farmland and forest in different proportions. Ecological trait classifications were combined with the additive partitioning approach of overall species richness. Regional-scale alpha and beta diversity of ecological groups were compared between study sites, each containing three farm types (organic, conventional cereal and mixed farms) and differing in landscape structure. At a local scale, species richness and abundance of species belonging to ecological trait groups was compared between study fields within study sites. The impact of farm type and landscape structure (measured at two spatial scales) was expected to differ between ecological groups and diversity partitions of weeds and carabids. As carabid beetles are mobile insects, landscape factors were expected to be more important than farm type, whereas for arable weeds the a priori expectation was the opposite. 2. Materials and methods The study area was situated in the southern boreal vegetation zone in Southern Finland (618050 –608400 N, 258000 –258420 E). Coarse and clay soils prevailed in the study area, and spring-sown cereals dominated over autumn-sown cereals (Table 1), a condition typical of boreal regions. Six patches of farmland (units of agricultural landscape consisting mainly of fields surrounded by forests) were selected as study sites, situated 10–30 km apart. The selected study sites included landscapes dominated by forests with low field cover (e.g. the regions Soukkio and Lammi, see Table 2) and those dominated by more continuous farmland areas with some patches of forests between fields (e.g. region Kantele). The farm types compared in this study were conventional cereal, conventional mixed and organic farms. The organic farms were both cereal and mixed farms but were treated collectively as organic farms, since both cereal and mixed farms under organic management were not available in the same study sites. Table 1 Characteristics of the study fields in organic, conventional mixed and conventional cereal farms.
Soil type Coarsea Claya pHb Fertilizationb,c N P K Herbicide usea Treated Not treated Cropa Barley Oats Rye Wheat Pre-cropa Spring cereal Winter cereal Ley a
Organic
C. mixed
C. cereal
6 9 5.8 (4.7, 6.1)
9 7 5.6 (4.9, 6.6)
5 12 5.8 (5.2, 6.6)
0 (0, 27) 0 (0, 27) 0 (0, 72)
77 (40, 116) 16 (0, 44) 40 (4, 130)
90 (24, 124) 15 (4, 24) 28 (4, 43)
– 15
11 5
17 –
– 8 3 4
4 12 – –
7 6 – 4
8 1 6
10 – 6
17 – –
Number of fields. Median values; minimum and maximum values are given in parentheses. c Mineral fertilizer and manure in N, P, K (kg ha1) (for C. mixed: mineral fertilizers: N 59.8 (40, 98.8), P 6 (0, 14.3), K 21 (2.1, 42.8); manure: N 43.6 (5.5, 132), P 18 (4, 43.6), K 58 (22, 270)). b
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Table 2 Land-cover percentages around the central coordinate point for each of the six study sites. Forest %
Pasture %
Semi-natural %
Radius
Field % 1 km
10 km
10 km
10 km
10 km
Lammi Ha¨meenkoski Soukkio Ka¨rko¨la¨ Savijoki Kantele
29 37 40 50 72 72
25 26 26 28 40 45
58 63 64 61 30 47
0.3 0.03 0.3 0.03 0.5 0.5
0.3 0.3 0.3 0.4 0.5 0.6
In each of the six study sites, two to three cereal fields from each farm type were selected. The total number of fields studied was 48, with 15 (ten cereals and five mixed) in organic farms, 16 in conventional mixed farms and 17 in conventional cereal farms. Fields varied somewhat in size (organic farms [average S.D.] 6.3 4.69 ha, conventional mixed 4.4 2.83 ha, conventional cereal 3.0 1.65 ha), but did not differ significantly between farm types. The fields included in the study were cultivated with spring-sown cereals, with the exception of three autumn-sown rye fields under organic management in two regions (Savijoki and Soukkio). Although species richness (average S.D.) of arable weeds was higher in rye fields (32.7 2.31) than in other organic fields (21.8 6.18), they were included in the analysis as they represented the only available cereal fields in organic production in the particular study region. Landscape structure and related habitat descriptive statistics are shown in Table 2 (for more details see Lammi study area in Hyvo¨nen et al., 2003). Information on cropping practises and field properties was obtained by interviewing the farmers. The studied cropping practises represent typical agricultural conventions of agriculture in Southern Finland. The studied farm types differed in terms of herbicide and fertilizer use and crop rotation practises. In organically cultivated fields, herbicides and mineral fertilizers had not been applied within 5 years prior to sampling (with one exception: 2 years), the fields being fertilized with manure and legumes (n = 5) or only legumes (n = 10). Fields in organic farms treated with manure were mixed farms with both cereal production and animal husbandry. Crop rotation in organic farms included winter and spring cereals and leys (1–3 years sequence; mixed clover Trifolium and grasses). Oats was the most common spring cereal in organically cultivated fields (Table 1). In the study year, conventional mixed farms had used manure as a fertilizer along with mineral fertilizers (n = 6) or only mineral fertilizers (n = 9) or manure (n = 1). Crop rotation included spring cereals and perennial (2–4 years) ley (Table 1). Conventional cereal farms applied mineral fertilizers, and crop rotation was based solely on cereal cropping (Table 1). Both types of conventional farms used herbicides for weed control, but five study fields in conventional mixed farms were not treated with herbicides in the study year (four of which were left unsprayed due to undersown grass-clover mixture). None of the farmers, including those at organic farms, had applied mechanical weed control (cf. e.g. Gabriel et al., 2006). Insecticides had been applied in three conventional cereal fields within 5 years prior to sampling but not in the study year. Weed surveys were conducted in July–August 1998 during a two-week period, roughly a month after herbicide treatments of conventional fields. Each field was divided into a 10 10 grid (with grid cells varying in size with varying field size) before sampling; thus, each field had 100 possible sampling plots. Ten cells, two of which situated in the edge of the grid, were selected randomly and sampling plots were established by placing a sample quadrangle (25 40 cm) in each of the selected cells. In the cells situated in the edge of the grid, the quadrangle was placed less than 5 m from the
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edge of the field. All shoots of arable weed species were identified and counted individually within the sample plots. Observations on the ten sample plots of each field were summed before analyses. Carabid beetles were collected using pitfall traps (mouth diameter 70 mm, volume 170 ml, half-filled with 70% ethylene glycol and a few drops of detergent) in three two-week sampling periods during the first half of June, July and August, respectively. In each of the 48 studied fields, a nine-trap transect (traps 5 m apart) was placed starting 1 m from the field margin towards the centre of the field. Carabids were identified to species level using identification keys by Lindroth (1985, 1986). For statistical analysis the carabid data from the three sampling periods were pooled. Arable weeds were classified into insect-pollinated and noninsect-pollinated species (including wind- and self-pollination) according to their species-specific traits (Klotz et al., 2002). For carabids three traits were chosen, each with two classes. The classification was based on body size (small species [<0.55 cm] vs. intermediate and large species [0.55 cm]), feeding guild (herbivores and omnivores vs. predators) and wing development (short-winged and dimorphic species vs. long-winged species) (Thiele, 1977). Overall carabid and arable weed diversity as well as diversity within the trait groups was partitioned into alpha and beta components between the six different study sites for each farm type separately (six regions and three farming regimes, n = 18) by using the additive partitioning approach a + b = g (Allan, 1975; Wagner et al., 2000). Gamma diversity was defined as the total species richness observed in the study fields in each study region and farm type separately. Alpha diversity corresponded to the average species richness observed per study field within each study region and farm type, and beta diversity was obtained by subtracting alpha diversity from gamma diversity. For local-scale analyses (n = 48), landscape structure was quantified by computing field cover (including all types of cultivated land, set-asides and fallows, but excluding meadows and pastures) for a 300 m radius. Landscape diversity was calculated by using the Shannon–Wiener index for 37 CORINE land-use categories with 25 m resolution (Ha¨rma¨ et al., 2004) within a 1 km radius surrounding the studied fields using GRASS GIS software (GRASS Development Team, 2006). Land-use categories were divided between five main groups: built-up areas (12 categories), agricultural areas (five categories), forests (including open heaths and rocky outcrops; nine categories), wetlands and open bogs (six categories) and water bodies (five categories). Field-specific landscape metrics was averaged to obtain field cover and landscape diversity variables for each farm type in each study site (n = 18). Arable weed and carabid beetle alpha and beta diversities were analyzed with general linear models using normal error structure, where each farm type was represented in each study site (n = 18). At the local scale (n = 48), field-specific arable weed density (i.e. number of shoots per study field) and carabid beetle activity-density (number of individuals caught per field) were analyzed using generalized linear mixed models with a log link function and negative binomial error distribution, accounting for overdispersed count data (Littell et al., 2006). Study site (n = 6, see Table 2) was included as a random term. Species richness was otherwise analyzed similarly, but with Poisson error distributions. Field cover, landscape diversity and farm type (three-level factor) were included in all models. Field size did not correlate with species richness or activitydensity of carabid beetles (r < 0.12, P < 0.41) whereas arable weed species richness (r = 0.53, P < 0.001) and density did (r = 0.39, P = 0.01). This was due to the fact that fields in organic management were larger on average and therefore field size was not considered important in the analysis. Variables describing proportional species richness and field cover were arcsine square-root transformed prior to analysis. Upon convergence problems maximally two extreme
values were deleted one at a time, after which non-converged models were discarded. All models were run in SAS 9.1 for Windows (SAS Institute Inc., 2004). GLM assumptions were verified by a visual inspection of model residuals. Furthermore, all models were tested for collinearity of independent variables. As even weak correlations may bias inference (Graham, 2003), the pair-wise collinearity between field cover and landscape diversity was removed by regressing the smaller-scale variable (field cover) against the larger-scale variable (landscape diversity) and residuals were used instead of raw values to describe landscape heterogeneity. In this context (see Graham, 2003), field cover can be viewed as the determining variable since it directly influences the structure of any larger-scale landscape variable. This procedure made it possible to include two independent landscape variables measuring landscape structure at different scales. 3. Results A total of 60 carabid beetle species (10 888 individuals) and 63 arable weed species (13 536 individuals) were recorded. Depending on farm type, 72–78% of all arable weed species were insect-pollinated. Carabid species richness differed between organic fields in cereal and mixed farming systems (see Table 3), but due to insufficient replication this variation could not be accounted for in this study. Arable weed species richness did not differ between organic cereal and mixed farms (Table 3). Alpha and beta diversity of arable weeds and carabid beetles in conventional cereal, conventional mixed and organic farms (including both cereal and mixed systems) are shown in Table 4. Table 3 Species richness and density or activity-density (mean S.D.) of arable weeds and carabid beetles in conventional and organic fields under cereal and mixed production systems. Organic cereal and organic mixed farms were treated as one farm type in the analyses due to low sample size of organic mixed farms. Conventional Cereal
Organic Mixed
Cereal
Mixed
Weeds Species richness 13.5 3.1 12.2 2.8 18.9 5.9 22.8 1.9 Density 209.1 147.0 180.2 168.1 451.6 303.8 558.0 213.3 Carabid beetles Species richness 16.6 3.4 17.9 3.4 15.7 2.2 Activity-density 163.4 122.8 290.9 154.3 188.7 95.0
22.8 1.9 307.0 225.1
Table 4 Regional-scale (n = 18) alpha and beta diversity (mean S.D.) of arable weeds and carabid beetles in organic, conventional mixed and conventional cereal farms.
Arable weeds a, overall richness b, overall richness a, insect-pollinated b, insect-pollinated Carabid beetles a, predatory species b, predatory species a, omnivores b, omnivores a, long-winged species b, long-winged species a, short-w. and dimorphic b, short-w. and dimorphic a, small species b, small species a, large and intermediate b, large and intermediate
Organic
C. mixed
C. cereal
20.4 4.3 14.8 3.8 8.6 1.9 6.0 2.0
12.3 2.0 9.4 1.5 7.1 1.4 5.1 1.7
13.5 1.7 10.0 1.4 7.8 2.7 5.0 1.8
11.1 1.8 2.6 0.4 6.6 0.9 3.3 1.8 8.4 1.6 4.5 2.1 7.3 1.3 2.0 1.0 5.4 1.1 1.5 0.4 12.3 1.9 4.4 1.8
11.4 1.4 3.6 1.7 6.5 1.2 3.5 1.5 8.9 1.4 4.8 1.7 7.6 1.2 2.7 1.8 5.6 0.4 2.1 0.9 12.3 2.0 5.0 2.3
10.9 1.4 3.1 2.8 5.6 0.9 3.8 0.9 8.2 1.3 4.6 1.8 7.2 1.5 3.1 1.5 6.3 0.9 1.9 1.8 10.2 1.8 4.9 1.9
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At the local scale, arable weed species richness benefited of organic farming (F2,38 = 17.67, P < 0.0001). This was also true for insect-pollinated species (F2,38 = 11.31, P < 0.0001). Organic farms supported highest density of overall and insect-pollinated species, whereas the two conventional systems did not differ from each other (overall: F2,38 = 11.39, P = 0.0001; and insect-pollinated: F2,38 = 11.79, P = 0.0001, respectively). Landscape variables did not affect arable weed species richness or density. The proportional species richness of insect-pollinated species showed no significant differences between farm types (F2,38 = 1.31, P = 0.28). At a regional scale organic farming enhanced arable weed alpha diversity
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(overall: F2,13 = 11.8, P = 0.0012, insect-pollinated: F2,13 = 7.1, P = 0.008), whereas no explanatory variable explained variation in beta diversity. Carabid beta diversity increased with increasing landscape diversity (all species F1,13 = 6.8, P = 0.022; predators F1,13 = 7.51, P = 0.017; herbivores and omnivores F1,13 = 4.1, P = 0.065; longwinged species F1,13 = 4.65, P = 0.05; wing-dimorphic and shortwinged species F1,13 = 4.94, P = 0.045; large and intermediate species F1,13 = 15.3, P = 0.0018). Diversity of large and intermediate carabid species increased with decreasing field cover (alpha diversity F1,13 = 6.54, P = 0.024; beta diversity F1,13 = 5.59,
Fig. 1. Activity-density (mean S.D.) of carabid beetles in cereal fields in organic, conventional mixed and conventional cereal farms for: (a) all species, (b) small carabids, (c) large and intermediate carabids, (d) predatory carabids, (e) herbivorous and omnivorous carabids and (f) short-winged and dimorphic carabids. Each panel includes F-statistics for farmtype effects (*P < 0.05).
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P = 0.034). Farm type was not a significant predictor for carabids although a higher alpha diversity of large and intermediate carabid species on organic and conventional mixed farms as compared to conventional cereal farms was marginally significant (F2,13 = 3.1, P = 0.082). Local-scale carabid species richness did not differ between farm types (F2,38 = 0.44, P = 0.64) and neither was it affected by landscape variables (field cover: F1,38 = 1.65, P = 0.21, landscape diversity: F1,38 = 0.02, P = 0.88). Species richness of ecological trait groups was not related to farm type or landscape variables, although species richness of larger species tended to decrease with increasing field cover (F1,38 = 3.59, t = 1.89, P = 0.066). Carabid beetle activity-density was, when farm type was a significant predictor, lowest on conventional cereal and highest on conventional mixed farms (Fig. 1). Organic farms were either intermediate, overlapping with both conventional types (Fig. 1a, b and d), or differed from one conventional farm type but not from the other (highest activity-densities on mixed farms, Fig. 1c and e, or lowest on cereal farms, Fig. 1f). Landscape diversity had a negative impact on large (F1,37 = 15.34, P = 0.0004, one extreme observation deleted) and herbivorous/omnivorous carabid activity-density (F1,37 = 5.74, P = 0.022, one extreme observation deleted). The model for long-winged species activity-density did not converge. 4. Discussion Analysis of six separate regions with different proportions of field cover revealed that farm type generally had much less effect on carabid beetles as compared to arable weeds. However, arable weeds were unaffected by landscape factors but carabid beta diversity increased with increasing landscape heterogeneity within a 1 km radius. The measure of landscape heterogeneity decreased with increasing field cover and increased with increasing forest cover within the same landscape. It therefore depicted a gradient ranging from forest-dominated landscapes and small farmland patches (high landscape diversity) to rather large patches of arable land with islets of forests (low landscape diversity). Large and intermediate carabid species richness also increased with decreasing field cover within a 300 m radius, showing that landscape structure both within and between farmscale determines carabid species assemblages in agricultural landscapes. Recently, Vesely and Sarapatka (2008) found organic farming to be beneficial for large carabid species. The present study could not confirm this result as regional alpha diversity of large and intermediate species was only marginally significant and equally high in organic and conventional mixed farms. Furthermore, local species richness was unaffected by farm type. However, five study fields under organic management were situated in mixed farms with both cereal production and animal husbandry, and these had higher carabid species richness than organic fields in farms with only cereal production. These figures suggest that organic mixed farming may be beneficial for carabid species richness. The use of manure is likely to enhance food availability for carabids, but mixed farming may also provide profitable over-wintering habitats as it increases habitat heterogeneity at the farm scale (Benton et al., 2003). Neither of the two landscape variables affected carabid species richness at the local scale. These results differ from earlier work (e.g. Purtauf et al., 2005) where landscape structure has been found to be an important predictor. Boreal landscapes with comparatively low field cover (study region average varying between 25–45% within a 10 km radius and 29–72% within a 1 km radius) does apparently not constitute a gradient wide enough to affect species richness in local carabid communities. Purtauf et al.
(2005) found strong landscape effects, quantified as the percentage of grasslands around study sites in landscapes with contrasting field cover ranging from >50% to >80% arable land in sectors within a 1.5 km radius. These results suggest that farmland carabids may not be affected by landscape composition if field cover remains low, at least <60%, although landscape configuration (as well as farming intensity, see below) may additionally affect carabid assemblages. Arable weed diversity remained unaffected by landscape factors both at regional and local scales. The most likely explanation is either that conventional farming practises are not as intensive as in Central Europe (see Hyvo¨nen et al., 2003), where landscape effects have been found (Roschewitz et al., 2005; Holzschuh et al., 2007), or the fact that the present study did not include landscapes with >80% arable field cover or due to a combination of these two factors (compare Table 1 with Table 2 in Holzschuh et al., 2007). Arable weed alpha diversity was enhanced only by organic farming, but beta diversity was not affected by any explanatory variables. As also insect-pollinated arable weeds were enhanced by organic farming, it is likely that fields in organic farms provide more resources for insect pollinators than fields in conventional farms as suggested by Gabriel and Tscharntke (2007). However, the proportional density of insect-pollinated species did not differ between farm types, suggesting that no shift in plant–pollinator interactions has taken place between farm types (cf. Gabriel and Tscharntke, 2007). Compared to Central Europe, boreal agricultural practises are not as intensive even in conventional farms, which may explain the contrasting results on the proportions of insect-pollinated weeds (compare Table 1 with Table 2 in Holzschuh et al., 2007). Earlier studies have either found higher carabid activitydensities at organic farms (Ma¨der et al., 2002) or no differences between organic and conventional farms (Purtauf et al., 2005). In this study, activity-densities were highest at conventional mixed farms while organic farms were mostly intermediate. Although field sizes varied somewhat between farm types this could not explain the observed variation in carabid activity-density, since there was no correlation between carabid activity-densities and field size. However, activity-densities varied between farming systems within organic regimes, which suggests that mixed farming irrespective of farming regime may enhance carabid populations by increasing the amount of resources and/or suitable over-wintering habitats in the surroundings, which results in larger population sizes (Purtauf et al., 2005). This study suggests that organic farming will benefit arable weeds to a higher extent than carabid beetles both at local and regional scales in boreal agricultural landscapes. However, mixed farming may benefit carabid diversity irrespective of whether the farm is under conventional or organic management, but this issue needs to be clarified by further studies on mixed farming in conventional and organic regimes. Conventional mixed farming nevertheless increases activity-density of carabid beetles, suggesting that carabid assemblages may benefit from animal husbandry even though it is based on conventional farming practises. Conventional mixed farming does not benefit arable weeds since their diversity is limited by herbicide applications. Acknowledgements Critical comments by Matti Koivula, Doreen Gabriel, Markus Piha and Erja Huusela-Veistola made substantial improvements to earlier drafts of the manuscript. Matti Koivula provided help on carabid ecology and Jyrki Holopainen on GIS-related matters. The Finnish Cultural Foundation (JE) and the Finnish Ministry of Agriculture and Forestry are greatly acknowledged for financial support.
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Kromp, B., 1999. Carabid beetles in sustainable agriculture: a review on pest control efficacy, cultivation impacts and enhancement. Agric. Ecosyst. Environ. 74, 187–228. Lindroth, C.H., 1985. The Carabidae Coleoptera of Fennoscandia and Denmark. Fauna Entomol. Scand. 15, 1–232. Lindroth, C.H., 1986. The Carabidae Coleoptera of Fennoscandia and Denmark. Fauna Entomol. Scand. 15, 233–497. Littell, R.C., Milliken, G.A., Stroup, W.W., Wolfinger, R.D., Schabenberger, O., 2006. SAS For Mixed Models, second ed. SAS Institute Inc., Cary, NC. Luoto, M., 2000. Spatial analysis of landscape ecological characteristics of five agricultural areas in Finland by GIS. Fennia 178, 15–54. Ma¨der, P., Fliessbach, A., Dubois, D., Gunst, L., Fried, P., Niggli, U., 2002. Soil fertility and biodiversity in organic farming. Science 296, 1694–1697. Purtauf, T., Roschewitz, I., Dauber, J., Thies, C., Tscharntke, T., Wolters, V., 2005. Landscape context of organic and conventional farms: influences on carabid beetle diversity. Agric. Ecosyst. Environ. 108, 165–174. Ribera, I., Dole´dec, S., Downie, I.S., Foster, G.N., 2001. Effect of land disturbance and stress on species traits of ground beetle assemblages. Ecology 82, 1112– 1129. Ribera, I., Foster, G.N., Downie, I.S., McCracken, D.I., Abernethy, V.J., 1999. A comparative study of the morphology and life traits of Scottish ground beetles (Coleoptera, Carabidae). Ann. Zool. Fenn. 36, 21–37. Robinson, R.A., Sutherland, W.J., 2002. Post-war changes in arable farming and biodiversity in Great Britain. J. Appl. Ecol. 39, 157–176. Roschewitz, I., Gabriel, D., Tscharntke, T., Thies, C., 2005. The effects of landscape complexity on arable weed species diversity in organic and conventional farming. J. Appl. Ecol. 42, 873–882. Rundlo¨f, M., Smith, H.G., 2006. The effect of organic farming on butterfly diversity depends on landscape context. J. Appl. Ecol. 43, 1121–1127. SAS Institute Inc., 2004. What’s New in SAS 9.0, 9.1, 9.1.2 and 9.1.3. SAS Institute Inc., Cary, NC. Thiele, H.U., 1977. Carabid Beetles in Their Environments. Springer Verlag. Vesely, M., Sarapatka, B., 2008. Effects of conversion to organic farming on carabid beetles (Carabidae) in experimental fields in the Czech Republic. Biol. Agric. Hortic. 25, 289–309. Wagner, H.H., Wildi, O., Ewald, K.C., 2000. Additive partitioning of plant species diversity in an agricultural mosaic landscape. Landscape Ecol. 15, 219– 227.
Contents lists available at ScienceDirect
Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee
Responses in plant and carabid communities to farming practises in boreal landscapes Johan Ekroos a,*, Terho Hyvo¨nen b, Juha Tiainen c, Mikko Tiira c,1 a
Department of Biological and Environmental Sciences, University of Helsinki, P.O. Box 65, Viikinkaari 9, FI-00014, Helsinki, Finland MTT Agrifood Research Finland, Plant Production Research, FI-31600, Jokioinen, Finland c Finnish Game and Fisheries Research Institute, P.O. Box 2, FI-00791, Helsinki, Finland b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 February 2009 Received in revised form 14 October 2009 Accepted 15 October 2009 Available online 22 November 2009
The effect of organic farming compared to conventional mixed and cereal farming on arable weeds and carabid beetles in boreal landscapes was studied by comparing the distribution of ecological traits, diversity partitions, species richness and abundance. Organic farming increased both insectpollinated as well as overall weed species richness, whereas the proportion of insect-pollinated weed species of the total species richness was unaffected by farming practises. Carabid species richness was mainly unaffected by farming practises although a higher alpha diversity of large and intermediate carabid species in organic and conventional mixed farms was marginally significant. Activitydensities of carabids were highest on conventional mixed farms. Landscape variables did not affect weed diversity but carabid beta diversity increased with increasing landscape heterogeneity. Local richness of large and intermediate carabid species showed a marginally significant decrease with increasing field cover. It is concluded that arable weed diversity is affected by organic farming to a higher extent than carabids. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Additive partitioning Beta diversity Conventional cereal farming Conventional mixed farming Landscape complexity Organic farming Species richness
1. Introduction Intensification of agriculture has been detrimental for farmland biodiversity. Agricultural intensification has led to a regional specialization with farms producing either cereal crops or livestock (Robinson and Sutherland, 2002). The recent expansion of organic farming in Europe may benefit farmland biodiversity at least in areas with intensive agriculture (Bengtsson et al., 2005; Hole et al., 2005). Organic farming generally enhances species diversity more in homogeneous than complex landscapes, where any biodiversity benefits attributable to farming practises tend to be masked by effects of landscape structure (Roschewitz et al., 2005; Rundlo¨f and Smith, 2006). Particularly arable weeds (Hyvo¨nen et al., 2003; Gabriel et al., 2006; Gabriel and Tscharntke, 2007) and flower-visiting insects (see e.g. Rundlo¨f and Smith, 2006; Holzschuh et al., 2007) benefit from organic farming. The effects of organic farming on carabid beetles have been less consistent in different studies (reviewed by Hole et al., 2005), although generally organic farming enhances
* Corresponding author. Tel.: +358 9 191 57810; fax: +358 9 191 57694. E-mail addresses: johan.ekroos@helsinki.fi (J. Ekroos), terho.hyvonen@mtt.fi (T. Hyvo¨nen), juha.tiainen@rktl.fi (J. Tiainen), [email protected] (M. Tiira). 1 Current address: Pietila¨nkatu 15 FI-11130, Riihima¨ki, Finland. 0167-8809/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2009.10.007
carabid diversity and abundance (Bengtsson et al., 2005). Species richness is enhanced by mixed farming as it combines cereal production and animal husbandry (Kromp, 1999; Robinson and Sutherland, 2002). The majority of studies reporting beneficial effects of organic farming are conducted in landscapes with high proportions of cultivated fields, at least 50–60% of overall land use (Purtauf et al., 2005; Roschewitz et al., 2005; Rundlo¨f and Smith, 2006; Holzschuh et al., 2007). Little is known on the impact of organic farming in boreal landscapes, where arable land is concentrated to the most productive areas and consists of farmland patches interspersed in a landscape dominated by forests (Luoto, 2000). An ecological trait classification combined with diversity partitioning may reveal meaningful ecological interpretations to diversity responses mediated by farming practises or landscape structure at different spatial scales (Gabriel et al., 2006; Clough et al., 2007a,b). Carabid body size, foraging strategy and wing length allow relevant ecological groupings, as these relate to dispersal ability, tolerance against land-use intensity and diet preference (Ribera et al., 1999, 2001). Concerning arable weeds, insectpollinated species provide food resources for pollinating insects. The relationship between insect-pollinated weeds and farming practises can indicate whether organic farming has resulted in changes in the community structure of arable weeds. Insectpollinated arable weeds have been found to benefit more of
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organic farming than non-insect-pollinated species (Gabriel and Tscharntke, 2007), which raises the question whether this relationship can be found also in boreal agricultural landscapes. In this paper, the impact of farm type and landscape structure on arable weeds and carabid beetles in cereal fields was studied in six sites consisting of mosaic landscapes with farmland and forest in different proportions. Ecological trait classifications were combined with the additive partitioning approach of overall species richness. Regional-scale alpha and beta diversity of ecological groups were compared between study sites, each containing three farm types (organic, conventional cereal and mixed farms) and differing in landscape structure. At a local scale, species richness and abundance of species belonging to ecological trait groups was compared between study fields within study sites. The impact of farm type and landscape structure (measured at two spatial scales) was expected to differ between ecological groups and diversity partitions of weeds and carabids. As carabid beetles are mobile insects, landscape factors were expected to be more important than farm type, whereas for arable weeds the a priori expectation was the opposite. 2. Materials and methods The study area was situated in the southern boreal vegetation zone in Southern Finland (618050 –608400 N, 258000 –258420 E). Coarse and clay soils prevailed in the study area, and spring-sown cereals dominated over autumn-sown cereals (Table 1), a condition typical of boreal regions. Six patches of farmland (units of agricultural landscape consisting mainly of fields surrounded by forests) were selected as study sites, situated 10–30 km apart. The selected study sites included landscapes dominated by forests with low field cover (e.g. the regions Soukkio and Lammi, see Table 2) and those dominated by more continuous farmland areas with some patches of forests between fields (e.g. region Kantele). The farm types compared in this study were conventional cereal, conventional mixed and organic farms. The organic farms were both cereal and mixed farms but were treated collectively as organic farms, since both cereal and mixed farms under organic management were not available in the same study sites. Table 1 Characteristics of the study fields in organic, conventional mixed and conventional cereal farms.
Soil type Coarsea Claya pHb Fertilizationb,c N P K Herbicide usea Treated Not treated Cropa Barley Oats Rye Wheat Pre-cropa Spring cereal Winter cereal Ley a
Organic
C. mixed
C. cereal
6 9 5.8 (4.7, 6.1)
9 7 5.6 (4.9, 6.6)
5 12 5.8 (5.2, 6.6)
0 (0, 27) 0 (0, 27) 0 (0, 72)
77 (40, 116) 16 (0, 44) 40 (4, 130)
90 (24, 124) 15 (4, 24) 28 (4, 43)
– 15
11 5
17 –
– 8 3 4
4 12 – –
7 6 – 4
8 1 6
10 – 6
17 – –
Number of fields. Median values; minimum and maximum values are given in parentheses. c Mineral fertilizer and manure in N, P, K (kg ha1) (for C. mixed: mineral fertilizers: N 59.8 (40, 98.8), P 6 (0, 14.3), K 21 (2.1, 42.8); manure: N 43.6 (5.5, 132), P 18 (4, 43.6), K 58 (22, 270)). b
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Table 2 Land-cover percentages around the central coordinate point for each of the six study sites. Forest %
Pasture %
Semi-natural %
Radius
Field % 1 km
10 km
10 km
10 km
10 km
Lammi Ha¨meenkoski Soukkio Ka¨rko¨la¨ Savijoki Kantele
29 37 40 50 72 72
25 26 26 28 40 45
58 63 64 61 30 47
0.3 0.03 0.3 0.03 0.5 0.5
0.3 0.3 0.3 0.4 0.5 0.6
In each of the six study sites, two to three cereal fields from each farm type were selected. The total number of fields studied was 48, with 15 (ten cereals and five mixed) in organic farms, 16 in conventional mixed farms and 17 in conventional cereal farms. Fields varied somewhat in size (organic farms [average S.D.] 6.3 4.69 ha, conventional mixed 4.4 2.83 ha, conventional cereal 3.0 1.65 ha), but did not differ significantly between farm types. The fields included in the study were cultivated with spring-sown cereals, with the exception of three autumn-sown rye fields under organic management in two regions (Savijoki and Soukkio). Although species richness (average S.D.) of arable weeds was higher in rye fields (32.7 2.31) than in other organic fields (21.8 6.18), they were included in the analysis as they represented the only available cereal fields in organic production in the particular study region. Landscape structure and related habitat descriptive statistics are shown in Table 2 (for more details see Lammi study area in Hyvo¨nen et al., 2003). Information on cropping practises and field properties was obtained by interviewing the farmers. The studied cropping practises represent typical agricultural conventions of agriculture in Southern Finland. The studied farm types differed in terms of herbicide and fertilizer use and crop rotation practises. In organically cultivated fields, herbicides and mineral fertilizers had not been applied within 5 years prior to sampling (with one exception: 2 years), the fields being fertilized with manure and legumes (n = 5) or only legumes (n = 10). Fields in organic farms treated with manure were mixed farms with both cereal production and animal husbandry. Crop rotation in organic farms included winter and spring cereals and leys (1–3 years sequence; mixed clover Trifolium and grasses). Oats was the most common spring cereal in organically cultivated fields (Table 1). In the study year, conventional mixed farms had used manure as a fertilizer along with mineral fertilizers (n = 6) or only mineral fertilizers (n = 9) or manure (n = 1). Crop rotation included spring cereals and perennial (2–4 years) ley (Table 1). Conventional cereal farms applied mineral fertilizers, and crop rotation was based solely on cereal cropping (Table 1). Both types of conventional farms used herbicides for weed control, but five study fields in conventional mixed farms were not treated with herbicides in the study year (four of which were left unsprayed due to undersown grass-clover mixture). None of the farmers, including those at organic farms, had applied mechanical weed control (cf. e.g. Gabriel et al., 2006). Insecticides had been applied in three conventional cereal fields within 5 years prior to sampling but not in the study year. Weed surveys were conducted in July–August 1998 during a two-week period, roughly a month after herbicide treatments of conventional fields. Each field was divided into a 10 10 grid (with grid cells varying in size with varying field size) before sampling; thus, each field had 100 possible sampling plots. Ten cells, two of which situated in the edge of the grid, were selected randomly and sampling plots were established by placing a sample quadrangle (25 40 cm) in each of the selected cells. In the cells situated in the edge of the grid, the quadrangle was placed less than 5 m from the
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edge of the field. All shoots of arable weed species were identified and counted individually within the sample plots. Observations on the ten sample plots of each field were summed before analyses. Carabid beetles were collected using pitfall traps (mouth diameter 70 mm, volume 170 ml, half-filled with 70% ethylene glycol and a few drops of detergent) in three two-week sampling periods during the first half of June, July and August, respectively. In each of the 48 studied fields, a nine-trap transect (traps 5 m apart) was placed starting 1 m from the field margin towards the centre of the field. Carabids were identified to species level using identification keys by Lindroth (1985, 1986). For statistical analysis the carabid data from the three sampling periods were pooled. Arable weeds were classified into insect-pollinated and noninsect-pollinated species (including wind- and self-pollination) according to their species-specific traits (Klotz et al., 2002). For carabids three traits were chosen, each with two classes. The classification was based on body size (small species [<0.55 cm] vs. intermediate and large species [0.55 cm]), feeding guild (herbivores and omnivores vs. predators) and wing development (short-winged and dimorphic species vs. long-winged species) (Thiele, 1977). Overall carabid and arable weed diversity as well as diversity within the trait groups was partitioned into alpha and beta components between the six different study sites for each farm type separately (six regions and three farming regimes, n = 18) by using the additive partitioning approach a + b = g (Allan, 1975; Wagner et al., 2000). Gamma diversity was defined as the total species richness observed in the study fields in each study region and farm type separately. Alpha diversity corresponded to the average species richness observed per study field within each study region and farm type, and beta diversity was obtained by subtracting alpha diversity from gamma diversity. For local-scale analyses (n = 48), landscape structure was quantified by computing field cover (including all types of cultivated land, set-asides and fallows, but excluding meadows and pastures) for a 300 m radius. Landscape diversity was calculated by using the Shannon–Wiener index for 37 CORINE land-use categories with 25 m resolution (Ha¨rma¨ et al., 2004) within a 1 km radius surrounding the studied fields using GRASS GIS software (GRASS Development Team, 2006). Land-use categories were divided between five main groups: built-up areas (12 categories), agricultural areas (five categories), forests (including open heaths and rocky outcrops; nine categories), wetlands and open bogs (six categories) and water bodies (five categories). Field-specific landscape metrics was averaged to obtain field cover and landscape diversity variables for each farm type in each study site (n = 18). Arable weed and carabid beetle alpha and beta diversities were analyzed with general linear models using normal error structure, where each farm type was represented in each study site (n = 18). At the local scale (n = 48), field-specific arable weed density (i.e. number of shoots per study field) and carabid beetle activity-density (number of individuals caught per field) were analyzed using generalized linear mixed models with a log link function and negative binomial error distribution, accounting for overdispersed count data (Littell et al., 2006). Study site (n = 6, see Table 2) was included as a random term. Species richness was otherwise analyzed similarly, but with Poisson error distributions. Field cover, landscape diversity and farm type (three-level factor) were included in all models. Field size did not correlate with species richness or activitydensity of carabid beetles (r < 0.12, P < 0.41) whereas arable weed species richness (r = 0.53, P < 0.001) and density did (r = 0.39, P = 0.01). This was due to the fact that fields in organic management were larger on average and therefore field size was not considered important in the analysis. Variables describing proportional species richness and field cover were arcsine square-root transformed prior to analysis. Upon convergence problems maximally two extreme
values were deleted one at a time, after which non-converged models were discarded. All models were run in SAS 9.1 for Windows (SAS Institute Inc., 2004). GLM assumptions were verified by a visual inspection of model residuals. Furthermore, all models were tested for collinearity of independent variables. As even weak correlations may bias inference (Graham, 2003), the pair-wise collinearity between field cover and landscape diversity was removed by regressing the smaller-scale variable (field cover) against the larger-scale variable (landscape diversity) and residuals were used instead of raw values to describe landscape heterogeneity. In this context (see Graham, 2003), field cover can be viewed as the determining variable since it directly influences the structure of any larger-scale landscape variable. This procedure made it possible to include two independent landscape variables measuring landscape structure at different scales. 3. Results A total of 60 carabid beetle species (10 888 individuals) and 63 arable weed species (13 536 individuals) were recorded. Depending on farm type, 72–78% of all arable weed species were insect-pollinated. Carabid species richness differed between organic fields in cereal and mixed farming systems (see Table 3), but due to insufficient replication this variation could not be accounted for in this study. Arable weed species richness did not differ between organic cereal and mixed farms (Table 3). Alpha and beta diversity of arable weeds and carabid beetles in conventional cereal, conventional mixed and organic farms (including both cereal and mixed systems) are shown in Table 4. Table 3 Species richness and density or activity-density (mean S.D.) of arable weeds and carabid beetles in conventional and organic fields under cereal and mixed production systems. Organic cereal and organic mixed farms were treated as one farm type in the analyses due to low sample size of organic mixed farms. Conventional Cereal
Organic Mixed
Cereal
Mixed
Weeds Species richness 13.5 3.1 12.2 2.8 18.9 5.9 22.8 1.9 Density 209.1 147.0 180.2 168.1 451.6 303.8 558.0 213.3 Carabid beetles Species richness 16.6 3.4 17.9 3.4 15.7 2.2 Activity-density 163.4 122.8 290.9 154.3 188.7 95.0
22.8 1.9 307.0 225.1
Table 4 Regional-scale (n = 18) alpha and beta diversity (mean S.D.) of arable weeds and carabid beetles in organic, conventional mixed and conventional cereal farms.
Arable weeds a, overall richness b, overall richness a, insect-pollinated b, insect-pollinated Carabid beetles a, predatory species b, predatory species a, omnivores b, omnivores a, long-winged species b, long-winged species a, short-w. and dimorphic b, short-w. and dimorphic a, small species b, small species a, large and intermediate b, large and intermediate
Organic
C. mixed
C. cereal
20.4 4.3 14.8 3.8 8.6 1.9 6.0 2.0
12.3 2.0 9.4 1.5 7.1 1.4 5.1 1.7
13.5 1.7 10.0 1.4 7.8 2.7 5.0 1.8
11.1 1.8 2.6 0.4 6.6 0.9 3.3 1.8 8.4 1.6 4.5 2.1 7.3 1.3 2.0 1.0 5.4 1.1 1.5 0.4 12.3 1.9 4.4 1.8
11.4 1.4 3.6 1.7 6.5 1.2 3.5 1.5 8.9 1.4 4.8 1.7 7.6 1.2 2.7 1.8 5.6 0.4 2.1 0.9 12.3 2.0 5.0 2.3
10.9 1.4 3.1 2.8 5.6 0.9 3.8 0.9 8.2 1.3 4.6 1.8 7.2 1.5 3.1 1.5 6.3 0.9 1.9 1.8 10.2 1.8 4.9 1.9
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At the local scale, arable weed species richness benefited of organic farming (F2,38 = 17.67, P < 0.0001). This was also true for insect-pollinated species (F2,38 = 11.31, P < 0.0001). Organic farms supported highest density of overall and insect-pollinated species, whereas the two conventional systems did not differ from each other (overall: F2,38 = 11.39, P = 0.0001; and insect-pollinated: F2,38 = 11.79, P = 0.0001, respectively). Landscape variables did not affect arable weed species richness or density. The proportional species richness of insect-pollinated species showed no significant differences between farm types (F2,38 = 1.31, P = 0.28). At a regional scale organic farming enhanced arable weed alpha diversity
291
(overall: F2,13 = 11.8, P = 0.0012, insect-pollinated: F2,13 = 7.1, P = 0.008), whereas no explanatory variable explained variation in beta diversity. Carabid beta diversity increased with increasing landscape diversity (all species F1,13 = 6.8, P = 0.022; predators F1,13 = 7.51, P = 0.017; herbivores and omnivores F1,13 = 4.1, P = 0.065; longwinged species F1,13 = 4.65, P = 0.05; wing-dimorphic and shortwinged species F1,13 = 4.94, P = 0.045; large and intermediate species F1,13 = 15.3, P = 0.0018). Diversity of large and intermediate carabid species increased with decreasing field cover (alpha diversity F1,13 = 6.54, P = 0.024; beta diversity F1,13 = 5.59,
Fig. 1. Activity-density (mean S.D.) of carabid beetles in cereal fields in organic, conventional mixed and conventional cereal farms for: (a) all species, (b) small carabids, (c) large and intermediate carabids, (d) predatory carabids, (e) herbivorous and omnivorous carabids and (f) short-winged and dimorphic carabids. Each panel includes F-statistics for farmtype effects (*P < 0.05).
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P = 0.034). Farm type was not a significant predictor for carabids although a higher alpha diversity of large and intermediate carabid species on organic and conventional mixed farms as compared to conventional cereal farms was marginally significant (F2,13 = 3.1, P = 0.082). Local-scale carabid species richness did not differ between farm types (F2,38 = 0.44, P = 0.64) and neither was it affected by landscape variables (field cover: F1,38 = 1.65, P = 0.21, landscape diversity: F1,38 = 0.02, P = 0.88). Species richness of ecological trait groups was not related to farm type or landscape variables, although species richness of larger species tended to decrease with increasing field cover (F1,38 = 3.59, t = 1.89, P = 0.066). Carabid beetle activity-density was, when farm type was a significant predictor, lowest on conventional cereal and highest on conventional mixed farms (Fig. 1). Organic farms were either intermediate, overlapping with both conventional types (Fig. 1a, b and d), or differed from one conventional farm type but not from the other (highest activity-densities on mixed farms, Fig. 1c and e, or lowest on cereal farms, Fig. 1f). Landscape diversity had a negative impact on large (F1,37 = 15.34, P = 0.0004, one extreme observation deleted) and herbivorous/omnivorous carabid activity-density (F1,37 = 5.74, P = 0.022, one extreme observation deleted). The model for long-winged species activity-density did not converge. 4. Discussion Analysis of six separate regions with different proportions of field cover revealed that farm type generally had much less effect on carabid beetles as compared to arable weeds. However, arable weeds were unaffected by landscape factors but carabid beta diversity increased with increasing landscape heterogeneity within a 1 km radius. The measure of landscape heterogeneity decreased with increasing field cover and increased with increasing forest cover within the same landscape. It therefore depicted a gradient ranging from forest-dominated landscapes and small farmland patches (high landscape diversity) to rather large patches of arable land with islets of forests (low landscape diversity). Large and intermediate carabid species richness also increased with decreasing field cover within a 300 m radius, showing that landscape structure both within and between farmscale determines carabid species assemblages in agricultural landscapes. Recently, Vesely and Sarapatka (2008) found organic farming to be beneficial for large carabid species. The present study could not confirm this result as regional alpha diversity of large and intermediate species was only marginally significant and equally high in organic and conventional mixed farms. Furthermore, local species richness was unaffected by farm type. However, five study fields under organic management were situated in mixed farms with both cereal production and animal husbandry, and these had higher carabid species richness than organic fields in farms with only cereal production. These figures suggest that organic mixed farming may be beneficial for carabid species richness. The use of manure is likely to enhance food availability for carabids, but mixed farming may also provide profitable over-wintering habitats as it increases habitat heterogeneity at the farm scale (Benton et al., 2003). Neither of the two landscape variables affected carabid species richness at the local scale. These results differ from earlier work (e.g. Purtauf et al., 2005) where landscape structure has been found to be an important predictor. Boreal landscapes with comparatively low field cover (study region average varying between 25–45% within a 10 km radius and 29–72% within a 1 km radius) does apparently not constitute a gradient wide enough to affect species richness in local carabid communities. Purtauf et al.
(2005) found strong landscape effects, quantified as the percentage of grasslands around study sites in landscapes with contrasting field cover ranging from >50% to >80% arable land in sectors within a 1.5 km radius. These results suggest that farmland carabids may not be affected by landscape composition if field cover remains low, at least <60%, although landscape configuration (as well as farming intensity, see below) may additionally affect carabid assemblages. Arable weed diversity remained unaffected by landscape factors both at regional and local scales. The most likely explanation is either that conventional farming practises are not as intensive as in Central Europe (see Hyvo¨nen et al., 2003), where landscape effects have been found (Roschewitz et al., 2005; Holzschuh et al., 2007), or the fact that the present study did not include landscapes with >80% arable field cover or due to a combination of these two factors (compare Table 1 with Table 2 in Holzschuh et al., 2007). Arable weed alpha diversity was enhanced only by organic farming, but beta diversity was not affected by any explanatory variables. As also insect-pollinated arable weeds were enhanced by organic farming, it is likely that fields in organic farms provide more resources for insect pollinators than fields in conventional farms as suggested by Gabriel and Tscharntke (2007). However, the proportional density of insect-pollinated species did not differ between farm types, suggesting that no shift in plant–pollinator interactions has taken place between farm types (cf. Gabriel and Tscharntke, 2007). Compared to Central Europe, boreal agricultural practises are not as intensive even in conventional farms, which may explain the contrasting results on the proportions of insect-pollinated weeds (compare Table 1 with Table 2 in Holzschuh et al., 2007). Earlier studies have either found higher carabid activitydensities at organic farms (Ma¨der et al., 2002) or no differences between organic and conventional farms (Purtauf et al., 2005). In this study, activity-densities were highest at conventional mixed farms while organic farms were mostly intermediate. Although field sizes varied somewhat between farm types this could not explain the observed variation in carabid activity-density, since there was no correlation between carabid activity-densities and field size. However, activity-densities varied between farming systems within organic regimes, which suggests that mixed farming irrespective of farming regime may enhance carabid populations by increasing the amount of resources and/or suitable over-wintering habitats in the surroundings, which results in larger population sizes (Purtauf et al., 2005). This study suggests that organic farming will benefit arable weeds to a higher extent than carabid beetles both at local and regional scales in boreal agricultural landscapes. However, mixed farming may benefit carabid diversity irrespective of whether the farm is under conventional or organic management, but this issue needs to be clarified by further studies on mixed farming in conventional and organic regimes. Conventional mixed farming nevertheless increases activity-density of carabid beetles, suggesting that carabid assemblages may benefit from animal husbandry even though it is based on conventional farming practises. Conventional mixed farming does not benefit arable weeds since their diversity is limited by herbicide applications. Acknowledgements Critical comments by Matti Koivula, Doreen Gabriel, Markus Piha and Erja Huusela-Veistola made substantial improvements to earlier drafts of the manuscript. Matti Koivula provided help on carabid ecology and Jyrki Holopainen on GIS-related matters. The Finnish Cultural Foundation (JE) and the Finnish Ministry of Agriculture and Forestry are greatly acknowledged for financial support.
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