Grazing Impacts on Auchenorrhyncha Diversity and Abundance on a Scottish Upland Estate
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Insect Conservation and Diversity (2012) 5, 67–74 doi: 10.1111/j.1752-4598.2011.00135.x Grazing impacts on Auchenorrhyncha diversity and abundance on a Scottish upland estate NICK A. LITTLEWOOD, ROBIN J. PAKEMAN and GABOR POZSGAI Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, UK Abstract. 1. Livestock grazing impacts on insect populations in a variety of ways. For phytophagous insects the impact is primarily a result of altering the structure and species assemblage of vegetation. However not all species react in similar ways and even within an order there may be winners and losers from different grazing regimes. 2. A long-term, replicated, controlled experiment, comprising four grazing treat- ments, was established within an upland acid grassland area in Scotland. Auc- henorrhyncha were sampled by suction sampling and sweep-netting in the fifth year following the start of the treatments. 3. A significant treatment effect was apparent in the suction samples with Auc- henorrhyncha abundance being three to four times higher in the ungrazed plots com- pared to the other treatments. Abundance was also highest from the ungrazed plots in the sweep net samples, but this effect was not statistically significant. Redundancy analysis (RDA) showed that a suite of species which are typical of shaded positions responded with increased abundance in the ungrazed plots. 4. The findings demonstrate that the assemblages found in ungrazed areas can be vastly different to those found in even lightly grazed areas and therefore, underline the benefits of varied grazing regimes in maximising diversity. Furthermore, the work underlines the benefit of employing multiple sampling methods. Key words. Cattle grazing, grassland, grazing experiment, Hemiptera, insect, leafhopper, plant bug, Scotland, sheep grazing. Introduction Kruess and Tscharntke (2002) showed plant species richness to be higher on formerly cattle-grazed pastures where grazing had MuchoftheopengroundinuplandScotlandcomprisesa ceased than on pastures that were currently cattle-grazed whilst, mosaic of dwarf-shrub-dominated plant communities and semi- in unimproved mesotrophic pasture, Stewart and Pullin (2008) natural acid grasslands. The grasslands are extensive (semi-natu- suggested that grazing at low levels maximised forb species rich- ral acid grassland covers 12.3% of the area of Scotland – Carey ness. Tallowin et al. (2005), on the other hand, found no increase et al., 2008) and host distinct plant and animal assemblages (e.g. in plant diversity on lowland neutral grassland with a reduction Littlewood et al., 2006b). However, they are sometimes viewed in livestock grazing over a 5 year period. by conservation biologists as degraded habitats and thus are Although responses of individual species will vary, in general often understudied (e.g. Thompson & Brown, 1992). insect abundance, species richness or diversity has been shown Grazing by livestock has an important effect on vegetation to be higher in less-grazed sites with tall vegetation than in more structure and plant species composition in grasslands (e.g. Olff heavily-grazed sites with shorter vegetation (Morris, 2000). This & Ritchie, 1998) although responses can vary. For example, is particulary true of primarily phytophagous taxa (Woodcock et al., 2009) with, for example, Lepidoptera (Po¨yry et al., 2006; Littlewood, 2008) and Auchenorrhyncha (Gibson et al., 1992; Correspondence: Nick A Littlewood, Macaulay Land Use Fisher Barham & Stewart, 2005) showing clear responses. Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK. Assemblages of these and other groups may also be closely E-mail: [email protected] linked with plant species composition (e.g. Sanderson et al., Ó 2011 The Authors Insect Conservation and Diversity Ó 2011 The Royal Entomological Society 67 68 Nick A. Littlewood, Robin J. Pakeman and Gabor Pozsgai 1995; Littlewood et al., 2006a; Schaffers et al.,2008).However, There were four treatments and these were arranged in six there may be discrepancies between the rate of change of plant replicate blocks. Altitude and aspect were similar within each and insect assemblages (Nickel & Achtziger, 2005; Littlewood replicate block but varied between blocks with an altitudinal et al., 2009) and changes in insect diversity may lag behind range across all plots of 220–500 m. Grazing treatment I was changes in plant diversity (Southwood et al., 1979). Assem- high-intensity sheep grazing at commercial stocking density of 9 blages of grassland Auchenorrhyncha are also sensitive to habi- sheep per plot (=2.7 sheep ha)1). Treatment II was low-inten- tat quality (Hollier et al., 2005). They generally are closely sity sheep grazing of 3 sheep per plot (=0.9 sheep ha)1); this rep- related to vegetation with abundance being positively related to resented a continuation of the pre-experimental management on sward height and species richness being related to plant species the site. Treatment III was low-intensity mixed grazing compris- and structural diversity (Hartley et al., 2003; Morris et al., ing 2 sheep per plot (=0.6 sheep ha)1) and, for 4 weeks in 2005). autumn, two cows each suckling a single calf (=0.6 cows and Grasslands are rarely managed specifically for insects and calves ha)1), giving an equivalent aggregate quantity of livestock indeed detailed ecological knowledge of the full suite of species units ⁄offtake to treatment II. Treatment IV was ungrazed. Ani- present would be impossible to obtain. However, understanding mals were removed from the plots during severe winter weather of insect responses to management is a helpful contribution and for usual farming practices such as dipping against external when trying to make informed decisions based on established parasites. The grazing treatments were applied from January aims. Hemiptera (of which Auchenorrhyncha form a part) form 2003. a significant part of the diet of adult passerine birds (Buchanan et al., 2006). Such predators have been shown to structure their foraging based on both abundance and accessibility of prey Sample collection (Evans et al., 2006; Douglas et al., 2008; Vandenberghe et al., 2009). Information on how insect populations are structured in Sampling was carried out between 1 June and 9 July 2007. grasslands is, though, incompleteandthereisaneedforawide Two sampling methods were used: D-vac and sweep-netting. evidence body on which to base generalised management advice. The D-vac (D-vac co., Ventura, CA, USA) takes standard Here we look at the impact of different grazing levels on the suction samples through a funnel with diameter of 34.3 cm. Auchenorrhyncha assemblage within a long-term experiment at Samples consisted of five pooled sub-samples of duration an upland estate in central Scotland. We hypothesise that abun- 45 s each. Sweep-netting was carried out along a 20 · 0.5 m dance and species richness will be inversely related to grazing transect running perpendicular to the slope from the sample intensity. We further examine which elements of the Auc- point. The sample consisted of approximately 80 sweeps with henorrhyncha assemblage are most impacted by grazing levels a net of diameter 45 cm (number of sweeps varied slightly as in order to be able to provide an ecological interpretation for the transect was standardised by length rather than sweep observed effects. number). Sampling was carried out in fine dry weather once each at up to five randomly selected locations within each grazing plot. However, poor weather during the sampling Methods period prevented this number from being taken from all plots and, in total, 110 D-vac and 79 sweep net samples were col- Field site lected. Auchenorrhyncha were identified using Biedermann and Glen Finglas, Perthshire, Scotland (56°16¢N, 4°24¢W), is a Niedringhaus (2004) and Holzinger et al. (2003). Nomenclature 4085 ha estate grazed by sheep and cattle. The dominant vegeta- follows that used by Biedermann and Niedringhaus (2004). All tion in the study areas was acid grassland and mire. The males were identified to species. Females were identified to spe- most represented National Vegetation Classification (NVC) cies except for those of Delphacidae, which were aggregated as communities (Rodwell, 1991, 1992) were M23 (Juncus effu- one group, as were the very small numbers of female Aphrodi- sus ⁄acutiflorus–Galium palustre rush-pasture), M25 (Molinia nea and of the genus Cixius. caerulea–Potentilla erecta mire), U4 (Festuca ovina–Agrostis cap- illaris–Galium saxatile grassland) and U5 (Nardus stricta–Galium saxatile grassland). Some areas were covered by bracken (Pteri- Statistical analysis dium aquilinum,NVCU20).Asmallnumberofisolatedtrees grew in lower plots, comprising downy birch (Betula pubescens Analysis was carried out in Genstat 11.1 (Lawes Agricultural Ehrh.); eared willow (Salix aurita L.) and rowan (Sorbus aucupa- Trust, Hertfordshire, UK). Generalized Linear Mixed Models ria L.), while one block had substantial patches of the shrub bog were used to test for the significance of grazing treatment on myrtle (Myrica gale L.). abundance and on species richness within both sweep net and D-vac samples. Treatment was present as a fixed effect and graz- ing plots were nested within blocks which formed the random Grazing treatments model. This random model allowed the variation within plots from the different samples to contribute to the analysis whilst A replicated, randomised block experiment was established in keeping the appropriate degrees of freedom for testing the main 2002 which consisted of 24 enclosures, each measuring 3.3 ha. effects. A Poisson distribution and log-link function was used Ó 2011 The Authors Insect Conservation and Diversity Ó 2011 The Royal Entomological Society, Insect Conservation and Diversity, 5, 67–74 Auchenorrhyncha responses to grazing 69 for the abundance data and a normal distribution was used for 35 species richness data. Significance values were generated by a (28) (25) (29) (28) Wald test. The proportion of variation in the species data that could be 30 explained by grazing treatment was assessed for each sampling method by carrying out a Redundancy analysis (RDA) within CANOCO 4.5 (ter Braak & Sˇmilauer, 2002).