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Plant species richness has been shown to affect pollinator richness and the frequency of pollinator visits, including hoverflies, as a result of enhanced spatial and temporal availability of pollen and nectar resources (Ebeling et al., 2008), and flower density to affect hoverfly density and diversity (Haenke et al., 2009). Where grazing impacts influence plant communities this may be expected to also have implications for the associated hoverfly assemblage. Extensively rather than intensively grazed grassland had higher species richness of four insect taxa including parasitoids, the best predictor of which was vegetation height (Kruess, 2002). A study of Irish plantation forests found almost 80

% of the hoverfly species to be associated with open space rather than closed canopy forest (Gittings et al., 2006). Flying Diptera are affected by microclimatic conditions such as wind speed and solar radiation (Peng et al., 1992) and where there is structural variation in vegetation characteristics microclimatic conditions may be altered and cause habitat preferences to be observed in Diptera and other flying insects.

Alteration of plant species composition in the herb layer is driven by selective grazing on palatable species, avoidance of unpalatable species, and stimulation of shade intolerant species through the opening or removal of higher layers (Stewart, 2001). Many host specific insect herbivores are obligatley dependent on common herb layer plants such as violets Viola spp, enchanters nightshade Circaea lutetiana, and dogs mercury Murcurialis perennis, while others, including hoverflies and other pollinator species rely on various physiological conditions of herb layer plants such as flowering status, stature and microclimate (Stewart, 2001).

Plant architectural diversity is predicted to increase species richness of herbivorous invertebrate hosts of parasitoids (Fraser et al., 2007, Strong et al., 1984), leading these authors to hypothesise that habitats with greater plant species richness and vegetation architecture will support a richer parasitoid fauna, with some empirical evidence provided

by Fraser et al., (2008b). Increased hoverfly diversity with increased structural complexity was reported by Haenke et al. (2009).

Bramble, Rubus fructicosus, is an important element of the herb layer that is preferentially browsed by deer early in the growing season. Its flowers provide important pollen and nectar resources for a wide range of insects such as hoverflies, including Criorhina spp., whose larvae are saproxylic. The limited dispersal capabilities of these species render them dependent on dead wood sources and open flower rich rides or clearings, of the type maintained by grazing (Stewart, 2001). Other important floral resources eaten by deer include hawthorn Crataegus monogyna, blackthorn Prunus spinosa, elder Sambucus nigra, and species of the Umbelliferae and Compositae (Stewart, 2001). Heavy deer grazing pressure in the herb layer is likely to result in the removal of important host plant species, their replacement with unpalatable gramminoids and bracken, leading to overall reductions in invertebrate richness (Stewart, 2001).

The basic habitat needs of parasitoids include a host population, host food plants, adult food sources, and diapause, roosting, shelter and mating sites (Shaw, 2006), all of which may feasibly be altered by the action or absence of grazing on the vegetative habitat.

Species richness and architecture of vegetation were predicted to increase niche availability for herbivores and their parasitoids (Hawkins, 1988) potentially as a result of alteration of the cues that parasitoids use for host location (Saaksjarvi et al., 2006), while there is empirical evidence that herbivore mediated production of plant volatiles may play a role in attracting different parasitoid species (Godfray, 1994). The increasingly complex architecture of herbivore host plants along the gradient of grasses, herbs, shrubs and trees, significantly affected insect (Lawton and Schroder, 1977) and parasitoid species richness (Hawkins and Lawton, 1987; Hawkins, 1988) on host plants, and tree species richness and density was correlated with parasitoid diversity (Sperber et al., 2004). Evidence of a correlation between plant species richness and parasitoid species richness was observed by (Saaksjarvi et al., 2006) although the weak nature of the relationship led the authors to speculate that the observed pattern may be more likely a result of increased plant functional group richness and architectural complexity. Searching behaviour of parasitoids is affected by the architecture of prey host plants; plant size, height, leaf number, leaf surface area, and branch number were negatively correlated with the attack rate of a parasitoid of the citrus mealybug (Cloyd and Sadof, 2000). In a rare example of an anthropogenic disturbance affecting a tertiary trophic level, indirect and direct effects on host-parasitoid interactions in cattle grazed verses cattle un-grazed woodland were

observed in a study by Vanbergen et al., (2006). Grazing dependent habitat variation resulting in increased species richness and cover in the herb layer vegetation, led to a doubling of the parasitism rate in the grazed habitat for a herbivore-parasitoid interaction.

Humphrey et al., (1999) found vertical stand structure was correlated with higher field layer cover, which supported greater syrphid diversity, but lower carabid diversity.

4.2 Rationale for this study

There are relatively few studies on the impact of wild ungulate grazing on invertebrates in the epigeal and herb layers of the UK’s native woodlands (Baines et al., 1994, Rambo and Faeth, 1999, Putman et al., 1989, Suominen, 2003, Melis et al., 2006), but grazing is known to alter the abundance, richness and distribution of invertebrates in the epigeal layer (Dennis, 2002; Suominen, 2003). Use of one group of invertebrates as an indicator of general trends across invertebrate groups is desirable but not consistent, as a measure of the impacts of habitat modification (Lawton, 1998). Carabidae and Staphylinidae have been effectively used to quantify the impacts of grazing regimes in Nardus stricta grasslands (Dennis et al., 1997) and heather moorland (Gardner et al., 1997), and woodlands are important habitat for many relict epigeal invertebrate species (Sroka and Finch, 2006). Managers of woodland grazed by wild herbivores must seek to implement effective policies for the maintenance of habitats in favourable condition. This study is intended to investigate the impacts of red deer grazing management in Atlantic oak woodland, for which there is a particular paucity of empirical research, in order to help create an evidence base on which managers can found sound policy decisions.

Table 4. 1 Average percentage ground cover including flora in grazed and un-grazed treatment plots

Betula pendula (Betula sp.) 1.52 1.98

Blechnum spicant 7.90 9.45

Pteridium aquilinum 44.52 43.58

Quercus spp. 1.84 2.32

Ranunculus acris 2.27 2.27

Ranunculus repens 0.62 0.62

Rosa cania 0.51 0.51

Rubus fruticosus agg. 4.45 4.42

Senecio jacobea 0.60 0.60

Sorbus acuparia 2.74 3.02

Spagnum spp. 47.97 47.92

Succiusa pratensis 7.45 7.45

Taraxacum agg. 0.91 0.94

Teucrium scorodonia 1.40 1.40

Urtica dioica 0 0

Vaccinium myrtillus 15.61 15.61

Vaccinium vitis-idaea 0.00 11.28

Viola riviniana 6.81 6.95

Leaf litter 27.11 26.55

Lichen 0.33 0.33

Rock 1.22 0.78

Soil 10.41 10.41

Note: Species indicative of grazed woodland indicated in bold, un-grazed underlined and bold, taken from (McEvoy et al., 2006)

4.3 Aims

The hypothesis that the ecosystem engineering action of grazing by deer, or its absence through exclusion, may affect habitat conditions such as vegetation architecture and botanical species diversity, affecting the availability of resources to invertebrates was tested. The aim was to experimentally examine these effects on species inhabiting both the epigeal and herb layers, which will provide a useful tool with which to examine the impact of anthropogenic management of grazing disturbance on biodiversity. There is a need for information on the effects of woodland management on a range of taxonomic groups, particularly the impacts of red deer grazing in Scottish woodlands for which there are very few empirical studies. The aims of this chapter are:

• To assess the role of fencing to exclude grazing as a red deer management practise, in impacting biodiversity in the Letterewe oak woodland.

• To determine the factors affecting the variation in invertebrate biodiversity in the Letterewe oak woodland.

• To make recommendations for planning and management of the woodland to maintain and enhance biodiversity.

The hypotheses that invertebrate community assemblages (1) do not vary between grazed and un-grazed woodland plots, and (2) show no association with habitat variables were tested. By identifying the primary components that determine invertebrate communities in the Letterewe oak woodland, it will be possible to assess the consequences of management practises and to develop grazing policy for biodiversity maintenance in red deer habitats.