Does delaying the first mowing date increase biodiversity in European farmland meadows? (systematic review)
Meadows are regularly mown in order to provide fodder or litter for livestock and to prevent vegetation succession. However, the time of year at which meadows should be first mown in order to maximize biological diversity remains controversial and may vary with respect to context and focal taxa. We carried out a systematic review and meta-analysis on the effects of delaying the first mowing date upon plants and invertebrates in European meadowlands.
Following a CEE protocol, ISI Web of Science, Science Direct, JSTOR, Google and Google Scholar were searched. We recorded all studies that compared the species richness of plants, or the species richness or abundance of invertebrates, between grassland plots mown at a postponed date (treatment) vs plots mown earlier (control). In order to be included in the meta-analysis, compared plots had to be similar in all management respects, except the date of the first cut that was (mostly experimentally) manipulated. They were also to be located in the same meadow type. Meta-analyses applying Hedges’d statistic were performed.
Plant species richness responded differently to the date to which mowing was postponed. Delaying mowing from spring to summer had a positive effect, while delaying either from spring to fall, or from early summer to later in the season had a negative effect. Invertebrates were expected to show a strong response to delayed mowing due to their dependence on sward structure, but only species richness showed a clearly significant positive response. Invertebrate abundance was positively influenced in only a few studies.
The present meta-analysis shows that in general delaying the first mowing date in European meadowlands has either positive or neutral effects on plant and invertebrate biodiversity (except for plant species richness when delaying from spring to fall or from early summer to later). Overall, there was also strong between-study heterogeneity, pointing to other major confounding factors, the elucidation of which requires further field experiments with both larger sample sizes and a distinction between taxon-specific and meadow-type-specific responses.
Cutting, Grassland, Restoration, Systematic review, Meta-analysis
Concern about the decline of farmland fauna emerged in the late 1960’s (Carson, 1962) and has amplified until today (Chamberlain et al., 2000; Krebs et al., 1999; Robinson & Sutherland, 2002; Scherr & McNelly, 2007). As a response, most countries have implemented agri-environmental schemes (AES), in which farmers are financially supported to modify their farming practice to provide environmental benefits. Agri-environmental schemes mostly aim at protecting and restoring farmland biodiversity (Kleijn & Sutherland, 2003). They are voluntary programmes in which farmers usually receive direct payments for providing services that go beyond current good agricultural practices, such as management of semi-natural habitats. Currently, 25% of European farmland is under some sort of agri- environmental contract (Kleijn et al., 2001).
Hay and litter meadow are among the most common type of surfaces integrated in European AES. The most important management action on these surfaces is mowing. Mowing vegetation at least once a year has a positive effect on vascular plant species richness, especially when cuttings are removed (Parr & Way, 1988). Parr and Way (1988) also demonstrated that early-summer mowing has a detrimental effect on species richness of flowering plants. However, it has been suggested that later mowing dates (e.g. July 21st) might be more favourable for vascular plant biodiversity (Smith et al., 2000; Smith et al., 2002).
Anual mowing has a contrasting effect on invertebrates (Morris & Rispin, 1987; Volkl et al., 1993). Although detrimental to many insects in the short term (Erhardt, 1985; Feber, Smith & Macdonald, 1996; New, 1997; Valtonen & Saarinen, 2005), mowing has a largely positive long term effect because it prevents the growth of bushes and trees and thus maintains semi-natural grasslands beneficial to a large number of heliophilous and thermophilous species. It has also been suggested that delaying dates of first mowing could be positive for multiple invertebrates, including butterflies, spiders, and ground beetles, depending on various vegetation structures (Knop et al., 2006; Koller et al., 2000; Morris, 2000; Pozzi, 2004).
For vertebrates, the situation is different: mowing renders food resources suddenly available (e.g. insects and rodents) that were so far hidden in the sward. Foragers may massively congregate towards these rich, although ephemeral food supplies (Arlettaz, 1996). On the other hand, ground-breeding birds are likely to be heavily impacted by early mowing.
While most AES have the manifest objective of restoring biodiversity and ecosystem services (Tscharntke et al., 2005), they often bind farmers to threshold dates for agricultural operations. The date of the first mowing of meadows is usually defined as a trade-off between expected agricultural yield and impacts on wildlife. Given that this first mowing date is the most easily changed management practices (Chamberlain et al., 2000; Valtonen, Saarinen & Jantunen, 2006), it is the most likely to provide environmental benefits at little agronomic loss.
The objective of this review is to evaluate the evidence available to estimate the biodiversity benefits of a delayed mowing date on European farmland meadows.