There is a critical need to assure future food security, and increasing emphasis will be placed on greater crop productivity while reducing environmental impact and the reliance on chemical use in modern agriculture (OECD-FAO 2008; Royal Society 2009). One way of achieving this aim will be to make farmland biodiversity ‘work harder’ by identifying ecological processes that may be managed to deliver robust ecosystem services. Recent studies have put emphasis on ecosystem services provided in agro-ecosystems (Moonen & Barberi 2008; Macfadyen et al. 2009). After several decades of intensive use of chemicals in agriculture and landscape simplification, a key question that has emerged is whether the extent of biodiversity loss that has occurred in agro-ecosystems (Benton, Vickery & Wilson 2003) still allows ecosystem services to be delivered in intensive agricultural landscapes (Loreau, Mouquet & Gonzalez 2003; Tscharntke et al. 2005).
The regulation and control of pests that results from the activity of naturally present predators (natural enemies) is frequently cited as an important ecosystem service in arable agriculture (Losey & Vaughan 2006). To date, however, few natural enemy functions have been demonstrated to elicit regulation or apply with robustness and generality in real agro-ecosystems. Although mass release of natural enemies has been shown to work in closed systems, such as greenhouses, management of agro-ecosystems to enhance natural enemies rarely matches expectations (Gurr, Wratten & Altieri 2004).
Policy-driven changes in herbicide use may lead to increases in weed plant densities in arable fields, and reductions in crop productivity (Kim et al. 2002) and more generally the economic performance of agriculture. In the UK, in 2008 alone, 3 229 254 ha of cereal crops were treated with some 5 717 110 kg of herbicides (Garthwaite et al. 2010). The move away from chemical weed control will only be possible if ecological services are available and function well enough to substitute for these chemical inputs. For farmers to adopt these alternatives, it will be necessary to show that ecological processes could be employed to replace herbicides with little or no additional risk.
Carabid beetles have been studied as potential natural enemies of weeds, through predation of weed seeds by omnivorous and granivorous species (Tooley & Brust 2002; Westerman et al. 2003; Honek et al. 2007; Baraibar et al. 2009). It has been suggested that an annual seed loss of 25–50% may be enough to slow down weed population growth substantially (Firbank & Watkinson 1985), and predation rates observed in the field can exceed this level. Weed seed predation studies have shown that certain carabid species can aggregate to weed patches in the field (Holland, Perry & Winder 1999; Hough-Goldstein, VanGessel & Wilson 2004) and readily eat weed seeds under laboratory conditions (Honek, Martinkova & Jarosik 2003).
Although one might expect a positive relationship between seed predation rate and activity density of granivorous ground beetles in the field (Kromp 1999; Tooley & Brust 2002), field data are relatively scarce and the results are equivocal; some data show a relationship (Honek, Martinkova & Jarosik 2003; Honek, Martinkova & Saska 2005; Menalled et al. 2007), while other data sets indicate a lack of spatio-temporal correspondence (Mauchline et al. 2005; Saska et al. 2008). Ongoing, unpublished analyses of large-scale data sets suggest that the abundance of many granivorous and omnivorous carabid species are positively associated with weed seed abundance, while predominantly carnivorous species are not (D. R. Brooks, pers. comm.). It is not clear, however, whether these associations indicate that granivorous and omnivorous carabids can regulate weeds and represent an ecosystem service.
For seed predation by carabids to be considered an important ecosystem service, it would be necessary to show that the beetles are capable of regulating the long-term store of seed in the weed seedbank. It might be expected (Expectation 1) that regulation would be apparent as a negative relationship between the change in the weed seedbank over 1 year and the abundance of carabids in that year, all other factors being equal. In our simple model, the seedbank changes as weed seed are shed as seed rain from plants and return to the soil. Some of this seed rain may be found (intercepted), at the soil surface, and eaten by seed predator carabids reducing the amount returned to the seedbank. If the interception rate is high and enough seeds are eaten, there will be a net decline in the seedbank over the year. This simple model suggests two subsidiary expectations that would be required for systematic changes in the weed seedbank to be attributable to regulation by carabid beetles; Expectation 2 that ‘seedbank population density is positively related to seed rain abundance’; and Expectation 3 that, with successful interception, ‘carabid abundance is positively related to seed rain’. Here, we test these three expectations for seedbank regulation using data on carabid, seedbank and seed rain counts collected in 257 fields of four crops located across regions of Great Britain (GB).