The biodiversity cost of reducing management intensity in species-rich grasslands: Mowing annually vs. every third year
Introduction
Ongoing changes in agriculture mean that it is a challenge for conservation efforts to maintain the rich biodiversity that is associated with traditionally managed semi-natural grasslands in Europe (Kull & Zobel 1991; Poschlod, Baumann, & Karlik 2009; D’Aniello, Stanislao, Bonelli, & Balletto 2011; Wilson, Peet, Dengler, & Pärtel 2012; Habel et al. 2013; Babai & Molnár 2014). Grazing and mowing are the two options for exploiting the biomass of grass-dominated vegetation, and both seem relatively similar in regard to the benefit that they provide for preserving biodiversity (Tälle, Fogelfors, Westerberg, & Milberg 2015; Tälle et al. 2016). However, the amount of available cattle for conservation-oriented grazing is decreasing (Kumm 2003), while the labour costs for annual mowing in most cases remain high (Schreiber, Brauckmann, Broll, & Krebs 2009; Török, Vida, Deák, Lengyel, & Tóthmérész 2011).
Although not the first choice, reduction of management intensity might allow more land to be managed. Managers can skip mowing every second year and thereby double the acreage that is mowed or reduce the stocking density (e.g. a limited number of animals spread over a larger area; or rotational grazing with grazing-free periods). Theoretically, there should be an optimal balance between the available resources (e.g., number of animals, cost of labour), area covered and biodiversity benefits provided. Put another way, a larger area with relaxed management (intensity or type) might be preferred over a smaller area under the best management option as long as the losses in biodiversity are acceptable. Hence, it is important to estimate the “cost”, in terms of lost biodiversity, for reducing management intensity (Marriott et al. 2004).
As grazing intensity is difficult to estimate and to maintain at a uniform level over many years, experiments that vary the stocking density are less suited to address this issue than experiments assessing the effect of different mowing intensities on biodiversity. The results from studies comparing the effect of higher and lower mowing intensities vary, with some in favour of higher mowing intensities (e.g., Köhler et al. 2005; Noordjik, Delille, Schaffers, & Sýkora 2009), and others in favour of lower mowing intensities (e.g., Everwand, Rösch, Tscharntke, & Scherber 2014; Kőrösi, Szentirmai, Batáry, Kövér, & Örvössy 2014). However, few studies have examined the effect of mowing less than once per year (but see e.g., Bakker, Elzinga, & de Vries 2002). Hence, there is still room for improvement in our understanding of the effect of different mowing intensities on biodiversity. In general, the outcome could be expected to resemble secondary succession when management ceases, that is, woody species and tall-grown species increase while the diversity indices and richness decrease.
Here, we analyse data from a series of eleven field trials in southern Sweden in which mowing every year was compared with mowing every third year as well an unmanaged control. Two of these trials have previously been reported in full elsewhere (Hansson & Fogelfors 2000; Wahlman & Milberg 2002) but then with mainly qualitative assessment of treatment and ignoring the substantial value of including replications over sites (which was the original intention of the series of trials). We intended to estimate the rate of loss of conservation values based on all eleven grasslands over 14 years. At one site, data were also available after 38 years. More specifically, when converting to more infrequent mowing, we expected:
- (i)
a decrease in the number of species and diversity indices (Shannon, Gini-Simpson, Shannon evenness),
- (ii)
a decrease of species indicative of good management (i.e. one that maintains species-richness typical of traditional grassland management),
- (iii)
an increase in species indicative of poor management,
- (iv)
an increase in woody species,
- (v)
an increase in tall-grown species.
Section snippets
Study sites
In the early 1970s, a long-term experiment was established at eleven experimental sites at nine locations in southern Sweden (Fig. 1). Two of the locations (Ekenäs, Tagel) had two experimental sites each. The mean annual temperature in southern Sweden is approximately 6 °C, mean annual precipitation is 500–1000 mm (Alexandersson, Karlström, & Larsson-McLann 1991) and growing period is 180–220 days (Sjörs 1999).
The sites were selected with the aid of local conservation authorities, to ensure
Effect on species richness and diversity
As expected, there was an overall drop in ln(RR) for the number of species when mowing every third year was compared with annual mowing, but with considerable variation between sites (Fig. 2A). The sites showing the weakest effect sizes were the moist one (Gränö, Ekenäs Moist, Andersby), while the drier sites Österplana (dry) and Ekenäs Mesic (dry/mesic) showed the strongest effect sizes (Fig. 2A). The two diversity indices also decreased, albeit the Gini-Simpson decreased only very modestly (
Discussion
By using data from eleven replicated field trials, we showed that relaxed management, in this case mowing every third year, means losing biodiversity attributes in vegetation after 11–14 years compared to annual mowing. This was most clearly seen in data on the species number (Fig. 2A) as well as for some of the species (Fig. 7) but also in one diversity index (Fig. 2B). However, it is important to note that the effect sizes were quite small (an ln(RR) of −0.1 represents e.g. comparing species
Acknowledgements
We thank those who helped initiate, maintain and monitor the long-term field trials analysed here. The Swedish Environmental Protection Agency initially funded these trials. Financial support for our study was provided by the Swedish Board of Agriculture.
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2022, Agriculture, Ecosystems and EnvironmentCitation Excerpt :Intensification of management can dramatically decrease the diversity and richness of grassland organisms (Heuss et al., 2019). On the other hand, reduced mowing intensity or abandonment can also lead to species loss (Milberg et al., 2017). Patterns of species composition of grasslands in different vegetation types and under the influence of various management regimes are relatively well known.