Peter Dennis

Distribution and abundance of small insects and arachnids in relation to structural heterogeneity of grazed, indigenous grasslands

1. The species composition and spatial distribution of small insects (Hemiptera, Coleoptera, Lepidoptera) and arachnids (Araneae, Opiliones, and Pseudoscorpiones) were investigated in three indigenous, upland grasslands identified as the National Vegetation Classification Festuca–Agrostis–Galium typical subcommunity (code U4a), Festuca–Agrostis–Galium, Vaccinium–Deschampsia subcommunity (code U4e), and Nardus stricta species‐poor sub‐community (code U5a), on which grazing management was manipulated experimentally.

Field margins: can they enhance natural enemy population densities and general arthropod diversity on farmland?

Abstract The interactions of predatory arthropods between natural habitats and arable land were explored. Three questions were asked: (1) Does habitat quality in field margins determine the density of natural enemies overwintering in field margins? (2) Are good sites for natural enemies also habitats which support greater general arthropod diversity? (3) Does the density of predators overwintering in field margins determine their distribution and abundance in adjacent cereal fields in spring? The value of field margins to agriculture was assessed as the density of arthropods overwintering in these habitats. The arthropod groups used for the assessment were known aphid predators which moved out into cereal crops in the spring. This assessment was compared with general arthropod diversity at each site to test whether field margins supporting high densities of predators were also of value to nature conservation in supporting greater arthropod biodiversity. In spring, the spatial dynamics of predatory arthropod species were investigated in cereal crops adjacent to field-margin overwintering sites. The influence of field-margin habitats on natural enemy populations within adjacent crops was evaluated. The enhancement of natural enemies and general arthropod conservation are discussed as components of overall biological diversity at the farm scale.

The effects of varied grazing management on epigeal spiders, harvestmen and pseudoscorpions of Nardus stricta grassland in upland Scotland

Dennis, P., Young, M. R., Bentley, C. (2001). The effects of varied grazing management on epigeal spiders, harvestmen and pseudoscorpions of Nardus stricta grassland in upland Scotland. Agriculture, Ecosystems and Environment, 86 (1), 39-57.

Relationships between agricultural management and ecological groups of ground beetles (Coleoptera: Carabidae) on Scottish farmland

The primary aim of this research was to classify ground beetle species, collected from farmland habitats in Scotland, into distinct groups based on their ecological traits. The objective classification of 68 species on this basis, using multivariate analyses, identified seven distinct ecological groups that were primarily determined by size, diel activity and diet (e.g. large Carabus spp., nocturnal plant feeders and species feeding on Collembola). The influence of agricultural land use and management intensity on these ecological groups was then investigated. The percentage of both Carabus spp. and individuals was greater on heather moorland and semi-natural grassland sites than in intensively managed arable or grassland sites. The percentage of Carabus spp. was also adversely affected by intensive agricultural management. A higher percentage of carabids feed specifically on Collembola in intensive grassland sites than in arable or semi-natural grassland sites suggesting that Collembola were more available in intensive grassland. This study introduces a non-taxonomic method of classifying carabids on the basis of their ecology. Such classification methods not only enable influences of agriculture to be detected across broad ecological groups rather than being reliant on the presence of a few key indicator species, but may also help in predicting how such changes to the community structure may influence ecosystem functioning.

Low intensity, mixed livestock grazing improves the breeding abundance of a common insectivorous passerine

Abstract Livestock grazing is a major driver of ecosystem change and has been associated with significant declines in various bird species in Britain and worldwide. However, there is little experimental evidence to show how grazing affects bird populations. We manipulated livestock densities in a replicated field experiment and found that mixed sheep and cattle grazing, at low intensity, improved the breeding abundance of a common upland passerine, the meadow pipit Anthus pratensis, after two years. Plots stocked with sheep alone (at high or low density) or not stocked at all held fewer pipit territories. Despite a year-on-year decline in pairs of meadow pipits in intensively grazed plots, we found no effect of sheep number on breeding abundance. Our results support the hypothesis that mixed species of herbivores generate greater heterogeneity in vegetation structure, which modifies prey availability, resulting in a greater abundance of birds. The results of our study should inform the management of grassland areas and enhance the abundance of some bird species, particularly in areas that have seen significant shifts from mixed livestock grazing to grazing dominated by single species of animals.

Comparing the effects of farming practices on ground beetle (Coleoptera: Carabidae) and spider (Araneae) assemblages of Scottish farmland

Multivariate techniques were used to compare and contrast the effects of land cover and farming practice on ground beetle and spider assemblages of Scottish farmland. For both ground beetles and spiders, the ordination and fuzzy clustering of sites were related to land cover rather than geographical location or year of sampling. The same four types of land cover group were identified: that is, heather moorland, semi-natural grassland, intensive grassland and arable land. The robustness of these land cover groups was tested using previously unsampled sites and it was found that 79 and 86% of sites, for ground beetle and spider assemblages respectively, were allocated to the land cover group predicted from their actual land cover. Furthermore, procrustes rotational analysis found a strong relationship between ground beetle and spider assemblages in intensively managed sites, suggesting that the assemblage structure of one group could be used to predict that of the other. The observed relationship between spider and ground beetle assemblages does not necessarily indicate that both groups were responding to agricultural practices in the same way. Indeed, the highest number of beetle species occurred in intensively managed grassland and arable sites while the highest number of spider species occurred in semi-natural grassland and heather sites. When conducting ecological assessments, one might wish to collect information on a wide range of ecologically different taxa; however, financial constraints make this unfeasible. From the results it could be concluded that spiders should be chosen in preference to ground beetles when seeking to make predictions on how farming practices influence invertebrates. However, such a conclusion would be premature since not only were spiders more numerous in the traps, but they were also more time consuming to process. In addition, the strong relationship found between the spider and ground beetle assemblages further justifies carabids as a target group when monitoring the influence of farming practices on biodiversity.

Gains to species diversity in organically farmed fields are not propagated at the farm level

Organic farming is promoted to reduce environmental impacts of agriculture, but surprisingly little is known about its effects at the farm level, the primary unit of decision making. Here we report the effects of organic farming on species diversity at the field, farm and regional levels by sampling plants, earthworms, spiders and bees in 1470 fields of 205 randomly selected organic and nonorganic farms in twelve European and African regions. Species richness is, on average, 10.5% higher in organic than nonorganic production fields, with highest gains in intensive arable fields (around +45%). Gains to species richness are partly caused by higher organism abundance and are common in plants and bees but intermittent in earthworms and spiders. Average gains are marginal +4.6% at the farm and +3.1% at the regional level, even in intensive arable regions. Additional, targeted measures are therefore needed to fulfil the commitment of organic farming to benefit farmland biodiversity.

Livestock grazing affects the egg size of an insectivorous passerine.

Livestock grazing is a major driver of ecosystem change, and has been associated with significant declines in various bird species worldwide. In Britain, there is particular concern that severe grazing pressure is deleteriously affecting vegetation and birds in upland regions. However, the mechanism by which grazing affects birds is unclear. Here, we report for the first time, to our knowledge, that sheep grazing pressure affects the egg size of a common upland passerine: the meadow pipit Anthus pratensis. We manipulated sheep stocking densities in a replicated field experiment, and found that plots with the highest stocking density contained nests with the smallest eggs, and that plots with low stocking density contained nests with the largest eggs. However, eggs laid in ungrazed plots were also small, suggesting that either too many sheep or their removal from upland areas might have a detrimental effect on pipit egg size. We found no significant effect on fledging success but the reduced post-fledging survival of young from smaller eggs, as seen in other studies, could partly explain declines in upland birds.

Spatial distribution of upland beetles in relation to landform, vegetation and grazing management

Dennis, P., Aspinall, R. J., Gordon, I. J. (2002). Spatial distribution of upland beetles in relation to landform vegetation and grazing management. Basic and Applied Ecology, 3 (2), 183–193. Sponsorship: SEERAD RAE2008

The Impact of Field Boundary Habitats on the Diversity and Abundance of Natural Enemies in Cereals

Semi-natural biotopes of agricultural landscapes take the form of linear and insular structures which remain in predominantly cultivated areas (Lubbe, 1988). The linear structures are typically grass boundaries or hedges between fields, along farm tracks, roadsides, drainage ditches, water courses and forest edges (Greaves and Marshall, 1987). These semi-natural, remnant biotopes have been included in recent ecological studies of agricultural ecosystems because they provide habitat for farmland gamebird species (Potts, 1980; Sotherton, 1991), wildlife, in particular, song birds (Parish et al., 1994) and butterflies (Dover, 1991). They were also considered to influence the species composition and population size of natural enemies in arable fields which were either the stenophagous predators or parasitoids of crop pests (van Emden, 1965) or polyphagous, generalist predators, which predate on crop pests only as part of a general diet which is typically composed of arthropods, mycoflora and herbage (Sotherton, 1985; Coombes and Sotherton, 1986). It was suggested that the densities of these predators of insect pests in crop fields may be increased by these adjacent habitats because they could provide shelter, breeding sites and sources of alternative food (Hagen et al., 1976).