To answer research question one, “Do high quality field margins deliver ecosystem services more effectively than low quality field margins?”, margin plots were selected according to their value as flower-visitor foraging habitat. A large body of empirical evidence suggests that early successional biotopes with an abundant and diverse flower community provide high quality foraging habitat for flower-visiting insects (Haaland et al., 2011, Scheper et al., 2013). Consequently, forb-rich margins were designated as ‘high quality’ and forb-poor margins were
17 designated as ‘low quality’. Quality was defined in this way for three reasons. Firstly, it has been hypothesised that creating forb-rich biotopes on farmland to enhance flower-visiting
insect populations will provide additional biodiversity and ecosystem service benefits (Blaauw
and Isaacs, 2014a, Pywell et al., 2015, Balzan et al., 2016b). Despite such examples, few studies have examined whether forb-rich field margins can provide multiple ecological benefits simultaneously (i.e. multifunctionality) (but see Olson and Wackers, 2007, Sutter et al., 2017a). Secondly, most studies focus on the benefits of forb-rich field margins and often ignore the ecosystem dis-services they might support. For example, we know very little about the potential of forb-rich field margins to support populations of crop pests (but see Frank, 1998, Winkler et al., 2010, Balzan et al., 2016a, Balzan et al., 2016b, Grass et al., 2016). Thirdly, additional information on the biodiversity and ecosystem services provided by forb-rich field margins will help to highlight the ecological role they play, which may increase their uptake by farmers and will also facilitate a greater understanding of when and where their implementation will be most appropriate. (Wratten et al., 2012). For example, not only do forb-rich field margins support greater numbers of flower-visiting natural enemies than either margins that are forb-poor or crop edges (Haenke et al., 2009, Haaland et al., 2011, Campbell et al., 2017a), but they also increase pest suppression in the adjacent crop (Blaauw and Isaacs, 2015, Woodcock et al., 2016a). Furthermore, compared to forb-poor biotopes, forb-rich biotopes deliver better foraging resources for granivorous and insectivorous birds (Wood et al., 2013, Westbury et al., 2017), increased carbon and nitrogen storage (De Deyn et al., 2011), and greater levels of pollination (Orford et al., 2016).
Initially, margin plots were selected by conducting a rapid visual assessment of forb richness and cover. After selection, the percent cover of individual species was recorded
within three 2 x 2 mquadrats placed at 25, 50 and 75 m along the centre of each 100 m plot
(Figure 2.2). Only individuals rooted within the quadrat were recorded. In total, two vegetation surveys were carried out: one in August 2014 and one in July 2016. Percent cover values were averaged across the three quadrats during each survey, whereas plant richness and diversity values were calculated by summing the total number of species found within each margin plot during each survey year. Data from both surveys was averaged and then a quality Index was constructed for each field margin plot based on forb richness and forb cover (%). First, both variables were normalised using the following formula taken from Herzog et al. (2006):
18 Where Yi is the observed value, Ymin is the minimum observed value and Ymax is the maximum observed value. After normalising forb richness and forb cover values for each margin, these variables were themselves averaged to give a quality index score. Using this index, a margin was classified as high quality if it had a score >30 (Figure 2.3). Quality index scores for individual margin plots used during this study are listed in Table A2 in the Appendix. Low quality margins had index scores ranging from 0.96 to 25.11, whereas index scores for high quality margins ranged between 34.49 and 81.01. Mean index scores were also significantly greater within high quality margin plots (χ² = 167.45, d.f. = 1, P <0.001; Table 2.2). In general, high quality margins were characterised by having: significantly greater vascular vegetation richness, forb richness, Shannon diversity and Simpson’s diversity; significantly greater cover of forbs, Fabaceae and Asteraceae; and, significantly less cover of grasses (Table 2.2). In addition, quality was not always determined by the original seed mix used to create each field margin plot (Figure 2.4). For example, whilst 73.3% of margins designated as high quality were originally sown with a seed mixture containing forbs, 11.1% were originally sown with a grass only seed mixture and 15.6% were created by natural regeneration (Figure 2.4). In contrast, 50.9% of margins designated as low quality were originally left to naturally regenerate, 37.7% were originally sown with a grass only seed mixture and 11.3% were originally sown with a seed mixture containing forb species (Figure 2.4).
Figure 2.2. Schematic diagram showing vegetation quadrat locations within each field
19 Figure 2.3. Quality index scores for individual high and low quality field margin
plots used during this project.
Table 2.2. Mean (± SEM) difference in individual vegetation (a) richness, (b) diversity, (c) abundance
and (d) margin quality metrics between high and low quality field margin plots. Chi-square test statistics and P-values are from linear/generalised linear mixed-effect model likelihood ratio tests with quality as a fixed effect and site as a random effect (d.f. = 1). Significant results (P <0.05) are presented in bold.
Vegetation metric High quality Low quality χ² p
a) Richness metrics
Vegetation species richness 26.76 ± 1.14 16.53 ± 0.49 66.98 <0.001
Forb species richness *1 19.16 ± 0.76 8.91 ± 0.40 110.45 <0.001
Grass species richness 6.29 ± 0.41 6.42 ± 0.28 0.01 0.933 b) Diversity metrics
Species evenness 0.17 ± 0.00 0.16 ± 0.01 3.47 0.062 Shannon-Weiner index 1.99 ± 0.06 1.47 ± 0.05 39.95 <0.001
Simpson's index 0.70 ± 0.02 0.58 ± 0.02 14.70 <0.001
c) Abundance metrics
Total forb cover (%) *2 60.20 ± 3.86 8.29 ± 0.73 140.34 <0.001
Fabaceae cover (%)1 27.75 ± 4.31 1.02 ± 0.24 111.94 <0.001
Apiaceae cover (%)1 1.26 ± 0.32 1.31 ± 0.34 0.24 0.625 Asteraceae cover (%)1 17.02 ± 2.32 2.66 ± 0.38 59.56 <0.001
Other forb cover (%)1 14.17 ± 2.38 3.30 ± 0.52 42.84 <0.001
Grass cover (%) 39.35 ± 3.82 86.18 ± 1.72 98.41 <0.001
d) Margin quality index 55.42 ± 2.04 13.08 ± 0.81 167.45 <0.001
*Variables used to construct margin quality index scores 1
Negative binomial (log-link) Generalised linear mixed-effect model 2
20 Figure 2.4. The number of high and low quality margin plots used during this study
that were originally created by natural regeneration, sowing of a grass only seed mixture or the sowing of a seed mixture that included forb species.