The Distribution of Soil Insects Across Three Spatial Scales in Agricultural Grassland
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ORIGINAL RESEARCH published: 21 April 2016 doi: 10.3389/fevo.2016.00041 The Distribution of Soil Insects across Three Spatial Scales in Agricultural Grassland Carly M. Benefer 1*, Karzan S. D’Ahmed 2, Rod P. Blackshaw 3, Hadewij M. Sint 4 and Phil J. Murray 4 1 School of Biological Sciences, Plymouth University, Plymouth, UK, 2 Department of Plant Protection, Salahaddin University–Erbil, Erbil, Iraq, 3 Blackshaw Research and Consultancy, Chudleigh, UK, 4 Rothamsted Research, Okehampton, UK The effects of specific environmental factors on abundance and distribution of some individual soil insect taxa is known, but how scale influences spatial distribution is less well evaluated, particularly at the community level. However, given that many soil insects are pests or beneficial natural enemies, and that collectively they play a role in soil processes, this information is of potential value for predictive modeling and in furthering our understanding of soil ecology and management. The objectives of this study were to characterize the spatial distribution, relative population sizes, effect of sampling scale and taxa co-occurrence on a range of soil insects at the family level over 2 years. Soil cores were taken from agricultural grassland soils across three different sampling scales (farm, Edited by: field, and core) using a systematic sampling approach. Spatial distribution was assessed Ivan Hiltpold, using the variance-to-mean (VMR) ratio and taxa distribution plots and the contribution Western Sydney University, Australia of scale, spatial (geographical location), and biotic (presence-absence of other species) Reviewed by: factors determined using deviance partitioning. Tipulid larvae (leatherjackets) were the Aidan M. Keith, Centre for Ecology and Hydrology, UK most abundant taxa in both years, but the composition of other Dipteran and Coleopteran Alan Kergunteuil, taxa varied between years. The VMRs revealed differences in spatial distribution between University of Neuchâtel, Switzerland taxa across scales and years, showing a range of underlying distributional patterns. Scale *Correspondence: Carly Benefer was the most important factor influencing species distributions, but a large proportion of [email protected] deviance remained unexplained and there was much variation between taxa, suggesting biological and scale-specific factors are driving distributions, in agreement with a previous Specialty section: study. This article was submitted to Agroecology and Land Use Systems, Keywords: deviance partitioning, variance-to-mean ratio, Tipulidae, pest, belowground, soil communities a section of the journal Frontiers in Ecology and Evolution Received: 04 February 2016 INTRODUCTION Accepted: 04 April 2016 Published: 21 April 2016 Whilst there is increasing research interest and focus on the roles of soil organisms in above Citation: and belowground processes and their interactions (e.g., Johnson et al., 2013), there remains a Benefer CM, D’Ahmed KS, considerable lack of knowledge on the abundance and distribution of soil insects at an individual Blackshaw RP, Sint HM and and a community level. Grassland soils support a diverse range of insects, many of which are direct Murray PJ (2016) The Distribution of Soil Insects across Three Spatial pests of grass (e.g., Tipulidae) or nearby arable crops (e.g., wireworms) when present in sufficient Scales in Agricultural Grassland. numbers. Conversely, the stable habitat that grassland provides is also of value to beneficial natural Front. Ecol. Evol. 4:41. enemies such as Carabids, adults, and larvae of which can help to suppress pest populations to non- doi: 10.3389/fevo.2016.00041 damaging levels (Kromp, 1999). This is likely to be important in mixed farming systems that form Frontiers in Ecology and Evolution | www.frontiersin.org 1 April 2016 | Volume 4 | Article 41 Benefer et al. The Distribution of Soil Insects a mosaic of both cropped and grass fields, since grasslands annum. The soil types common on the NWFP are typical can provide a reservoir of generalist natural enemies (Gravesen of soils under grassland management in England. The soil is and Toft, 1987). Therefore, an understanding of the basic predominantly of 2 similar soil types, Harrod and Hogan (2008), biology and ecology of these organisms is important for which are of a slightly stony clay loam topsoil (∼36% clay) implementing sustainable management strategies. In order to overlying mottled stoney clay (∼60% clay), derived from the do this, knowledge of the spatial structuring of populations, carboniferous Culm measures. Below 30 cm the soil is highly and the underlying mechanisms leading to this, is needed. It impermeable to water and is seasonally waterlogged. The mean is known that there is natural stochasticity in insect abundance annual rainfall (1982–2011) at the North Wyke site was 1042 ◦ over short timescales, linked to their high reproductive rate and mm with a mean temperature ranging from 6.6-13.4 C. North environmental factors (Schowalter, 2011), such as temperature, Wyke has relatively high and consistent summer rainfall which moisture, food availability and soil texture (King, 1939) and that is characteristic of the major agricultural grassland areas in the abundance and distributions can vary even between even closely UK. It has been calculated (Wilkins, 1982) that the environmental related species (Benefer et al., 2012). While factors affecting the conditions are sufficient to support 280 days of grass growth, but biology of some notable pest taxa (e.g., wireworms) have been the grazing season is restricted to ∼180 days due to soil wetness. particularly well-studied in the laboratory and in the field at Soil samples were taken from 19 permanent grassland fields small scales (e.g., Campbell, 1937; Lees, 1943; Furlan, 1996), this (>5 years old) split over 3 farms, ranging from 1.3 to 7.9 ha, with has rarely been replicated at larger scales. Benefer et al. (2010) a total study area of 67 ha. Sampling points were plotted on a investigated the effects of spatial, biotic and scale variables on 25 m sampling grid using ESRI ArcMap (V.10) GIS software and observed soil insect distributions and associations in a grassland determined in the field using a GPS system (a Trimble R8 base soil, focusing on the soil core, site (within field), and field scale. station with an R6 rover and TSC3 controller). Samples were It was found that the observed distributions and associations of taken between April and May 2012 and repeated between April most taxa changed according to scale, with this being the most 2013 and May 2013. Soil cores were taken using a standard soil important variable influencing distributions. corer with a 6.5 cm plastic pipe which was inserted to a depth of In this study, we used the North Wyke Farm Platform, which 10 cm. A total of 2260 soil samples were collected (1130 in each is a highly instrumented and monitored grassland farm to: year, representing a total sampled area of ∼9 m2). The individual ◦ samples were placed in labeled plastic bags and held at 4 C until 1. Characterize the spatial distribution of subterranean grassland processed, within 14 days of sampling. insects over three spatial and time scales: soil core, field and farm, and 2 years. Extraction and Identification of Soil Insects 2. To assess the relative population sizes and associations of soil A modified heat extraction method based on that of Blasdale insects at the family level over these spatial scales. (1974) was used to recover soil insects from the cores over 3. To determine the relative importance of scale, spatial, and a 24 h period. The modified apparatus consists of two layers, biotic factors on the observed distribution and abundance of each layer holding 25 soil samples of 6.5 cm diameter. The soil insects. insects collected were stored in 70% ethanol before being With limitations on the chemical controls that can now be individually identified to family level using morphological implemented to manage grassland pests, understanding pest keys (Freeman, 1983; Stubbs and Drake, 2001; Luff, 2007). (and natural enemy) distribution and using this information in Some specimens which could not be identified to Family with predictive modeling has increasing importance for sustainable confidence were sent to the Natural History Museum, London, management. Assessing these distributions and associations including those in the families Scatopsidae, Anthomyiidae and in the same location over time aids in strengthening our two types of Dolichopodidae larvae: Dolichopodidae “type A” understanding of the variation in ecological processes occurring was subsequently further identified as belonging to the Genus in the soil; this is again important for making predictions based Campsicnemus, while Dolichopodidae “type B” was classified as on observational data. Use of the same data analysis approach as an undescribed larva. In addition, there were larval individuals Benefer et al. (2010) has allowed a direct comparison of the use of an unknown beetle family which were labeled as “unknown of these techniques for assessing soil insect distributions across Coleoptera.” sites, and tests whether the conclusions of the earlier study are robust or site-specific. Data Analysis To visualize the distribution of each taxa across the sampling METHODS area in each year, the British National Grid coordinates (Easting and Northing) of each soil core sample were used to create a Study Site and Soil Sampling spatial distribution map for each taxa using ArcGIS 10.2. The The