Invertebrate richness and abundances
Our results show that the richness of species groups increased with increasing age of the field margins and that this trend was consistent during the first 11 years. This represents an important finding, indicating the conservation value of long-lasting semi-natural elements in agricultural areas. To our knowledge, this is the first time that such a pattern has been described for field margins for a broad range of invertebrates and over a considerable period of time. It is not surprising that there is a slow but steady increase in richness, because the small margins have to be colonised by small invertebrates moving through a hostile environment (Steffan-Dewenter and Tscharntke
1999; Öckinger and Smith
2007; Kohler et al.
2008), and similar patterns of increasing diversity have been described for other habitats (Mook
1971; Judd and Mason
1995; Desender et al.
2006; Cameron and Bayne
2009). Increasing functional diversity in species communities will lead to a greater variety of ecosystem processes (Naeem et al.
1994; Tilman et al.
1996; Heemsbergen et al.
2004) and with time, therefore, margins left on their own may develop towards more natural ecosystems.
Predators form an important aspect of our study, as some of these invertebrates are beneficial to farmers because of their potential as pest control (Carter and Rypstra
1995; Obrycki and Kring
1998; Collins et al.
2002). Predator abundance decreased with progressing age of the margins (in contrast to Denys and Tscharntke
2002, but in line with Woodcock et al.
2008), due probably to the vegetation developing from a recently sown, open situation to higher standing biomass and a denser sward, although in our analyses this development was only expressed by a significant effect of age (Noordijk et al.
2010). Ground-dwelling predatory invertebrates often depend on open, sun-lit places where they can easily move to find prey (Harvey et al.
2008). Those species potentially invading the arable fields have a particular preference for the open vegetation in the margins, as this is quite similar to conditions in the fields themselves (Samu and Szinetar
2002). Consequently, young margins appear to provide the best conditions for providing pest-control services. On the other hand, it has been shown that high vegetation cover in winter provides most opportunities for predators to hide during this period (e.g., Dennis et al.
1994; Collins et al.
2003).
We found herbivore abundance to be favoured by the width of the margin, but most significantly by the age of field margin and vegetation cover in summer (see also Meek et al.
2002; Harvey et al.
2008). This latter relationship can be explained by more plant biomass being available to provide food for more individuals (e.g., McFarlin et al.
2008) and more shelter against predators, which appear to be less abundant in these situations anyway (this study; Harvey et al.
2008). More than half the herbivores counted were Gastropoda, but Cicadellidae and Aphidoidea were also caught in high numbers. All these groups include polyphagous species, which may be damaging to crops and thus result in economic loss to farmers (Glen and Moens
2002; Nickel
2003; Van Emden and Harrington
2007).
The abundance of detritivores increased with the age of the margins. This is not surprising, given the build-up of a substantial surface litter layer (especially because no cuttings were removed from the margins after mowing, Noordijk et al.
2010), on which these animals depend for food (Smith et al.
2008a). A well-developed detritivore assemblage is essential for decomposition and enhancement of soil structure (Ekschmitt and Griffiths
1998), thus promoting healthier soils. In addition, this invertebrate group in particular represents species unable to persist in arable fields, as a litter layer is generally absent there (Smith et al.
2008b). Old field margins with high standing biomass will therefore represent true refuge habitats for these invertebrates.
One should bear in mind that vegetation structure and/or density at ground level might affect the activity-density of invertebrates and therefore pitfall trap catches (Greenslade
1964; Thomas et al.
2006), implying certain limitations on interpretation of our results. Moreover, different species groups may have very different activity patterns that could be affected differently by vegetation, for example, Gastropods versus Carabids. And our pitfalls were only open during 1 week each year, making the catches potentially vulnerable to uncommon weather conditions. However, we think that this will have hardly any effect on our richness analyses, as these are based only on the presence of a particular group, and not on its abundance. If it did have any effect, the already significant trend would likely be stronger, since there may especially be undersampling in the older margins with denser vegetation. For predator abundances, though, caution may be in order. On the other hand, the increasing abundance of herbivores with increasing vegetation cover might have been underestimated, so our recommendations concerning management of these margins for agricultural benefits (see below) therefore remain sound and grounded in empirical findings.
Pitfalls do not catch all invertebrates (Thomas and Marshall
1999). Flying insects, for example, are missed and of these many are also predators or parasitoids that may be beneficial to farmers. Therefore, our results cannot be generalised to all predators, herbivores or detritivores that occur in field margins.
Management recommendations
We recommend creating field margins for ‘as long as possible’ at the same location, as this will increase the number of taxonomic groups present in these structures, thereby promoting a variety of functions, leading to healthy ecological systems (Brussaard et al.
2007). In contrast, most agri-environmental schemes last only for a limited number of years (Kleijn et al.
2006), a situation that needs to be changed if better conservation results are to be achieved. However, old margins where no plant biomass is removed provide habitat for many herbivores and may also lead to less suitable situations for predators. To benefit farmers, then, these margins need to be managed differently. Since scarification, in particular, can be detrimental to many soil and ground-dwelling organisms (Smith et al.
2008b), re-establishing margins will not be the best option. An alternative is to introduce a hay-making management regime, with the vegetation being cut once a year, for example (Hovd and Skogen
2005; De Cauwer et al.
2005; Manhoudt et al.
2007). Margins can then still be established to last for a long time, but with plant biomass now being removed and vegetation succession set-back, thus providing less suitable conditions for high herbivore abundances while probably promoting predators. In addition, margins managed for hay-making will have fewer noxious weeds (De Cauwer et al.
2008), but greater plant diversity (Schaffers
2002; Musters et al.
2009; Blomqvist et al.
2009), which might in turn permit higher invertebrate diversity (Thomas and Marshall
1999; Asteraki et al.
2004) and more flower-visiting insects (Noordijk et al.
2009). The actual effect of hay-making on invertebrate species richness in arable field margins needs further study. As the possibilities for overwintering invertebrates increases with vegetation cover in winter, in the case of a hay-making management regime we recommend mowing the margins not too late in autumn (and preferably in late summer), permitting a certain amount of subsequent re-growth and thus providing sufficient overwintering opportunities.