Journal of Field Ornithology J. Field Ornithol. 82(1):60–67, 2011
DOI: 10.1111/j.1557-9263.2010.00308.x
Comparison of two methods for sampling invertebrates: vacuum and sweep-net sampling Elizabeth D. Doxon,1 Craig A. Davis, and Samuel D. Fuhlendorf Department of Natural Resource Ecology and Management, Oklahoma State University, 008C Agricultural Hall, Stillwater, Oklahoma 74078, USA Received 5 April 2010; accepted 1 September 2010 ABSTRACT. With numerous invertebrate sampling techniques available, deciding which technique to use under certain circumstances may be difficult. Many researchers interested in invertebrate abundance and availability relative to the foraging ecology of birds may use a technique (e.g., vacuum sampling or sweep-netting) without understanding the impacts their choice may have on the samples collected and the ability of the method to meet research objectives. We compared the characteristics, including overall biomass, morphospecies richness, average size, diversity, and body length categories, of invertebrates collected using a sweep-net and a Dietrick vacuum sampler along paired transects in Woodward County, Oklahoma, from May to July 2007 and 2008. These sampling techniques differed in the taxa collected, with the orders Diptera, Homoptera, and Hymenoptera dominating vacuum samples and the orders Homoptera, Orthoptera, and Araneae dominating sweep-net samples. Although morphospecies richness was similar for the two techniques, the mean size of invertebrates collected and overall invertebrate biomass were greater for sweep-netting than vacuum sampling. Vacuum sampling was more effective at collecting small (e.g., 5 cm) Orthopteran and Lepidopteran larvae at higher rates. Thus, our results indicate that neither sampling method effectively sampled all invertebrate families and investigators should be aware of the potential biases of different sampling techniques and be certain that the technique selected will allow study objectives to be met. ´ de dos m´etodos para muestrear invertebrados: aspiradoras y RESUMEN. Comparacion redes de barrido Con la disponibilidad de un gran n´umero de t´ecnicas de muestreo para invertebrados, puede ser dificil decidir que t´ecnica utilizar bajo circunstancias particulares. Muchos investigadores interesados en determinar la abundancia y disponibilidad de invertebrados relacionados a la ecolog´ıa de forrajeo de aves pueden utilizar una t´ecnica particular (ej. muestreo con aspiradoras o redes de barrido) sin entender el impacto que pueda tener dicha selecci´on en las muestras obtenidas y la capacidad del m´etodo para cumplir con los objetivos de la investigaci´on. Comparamos las caracter´ısticas, incluyendo la biomasa en general, la riqueza de morfoespecies, tama˜no promedio, diversidad y categor´ıas del largo del cuerpo, de invertebrados colectados utilizando una red de barrido o un aspirador de muestras Dietrick. El estudio se llev a cabo en el condado de Woodward, Oklahoma de mayo a julio de 2007 y 2008. Estas t´ecnicas de muestras se diferenciaron en los taxones coleccionados, con los d´ıpteros, hom´opteros e himen´opteros dominando en las muestras tomadas con aspiradoras, y los ordenes hom´optera, ort´optero y aranae dominando en las muestras de barrido. Aunque la riqueza de morfoespecies result similar utilizando ambas t´ecnicas, el tama˜no promedio de los invertebrados coleccionados y en general la biomasa de estos result mayor con la t´ecnica de barrido. El muestreo con la aspiradora resulto m´as efectivo en la colecci´on de invertebrados peque˜nos (ej. 5 cm) y en mayor n´umero. Por lo tanto, nuestros resultados indican que ninguno de los dos m´etodos muestran efectivamente todas las familias de invertebrados y que los investigadores deben estar concientes del sesgo potencial de diferentes t´ecnicas de muestreo. A tales efectos deben cerciorarse que la t´ecnica seleccionada les permita cumplir con los objetivos propuestos. Key words: Dietrick vacuum, grassland, invertebrate, Oklahoma, sampling, body length
Invertebrates are important prey for many birds, and investigators often assess invertebrate abundance and availability relative to habitat use and the foraging ecology of avian species (O’Leske et al. 1997, Davis and Smith 2001, McIntyre and Thompson 2003, Taylor et al. 1
2006, Doxon and Carroll 2007). Several methods are available for sampling invertebrates (Cooper and Whitmore 1990, New 1998), and two frequently used techniques are sweepnetting and vacuum sampling. Sweep-netting is commonly used because the equipment is lightweight and simple to use (Buffington and Redak 1998, Southwood and Henderson 2000), but might be biased toward foliar insects near the tips
Corresponding author. Email:
[email protected]
C 2011
C 2011 Association of Field Ornithologists The Authors. Journal of Field Ornithology
60
Methods of Sampling Invertebrates
Vol. 82, No. 1
of the vegetation because sweep-nets typically cannot penetrate the vegetation without damaging plants or the sweep-net (Buffington and Redak 1998). Sweep-netting may also be biased toward heavier, more active insects (Cooper and Whitmore 1990) because the air pressure and velocity caused by the sweeping action can disperse small insects such as aphids (Homoptera: Aphididae) before they are collected in the net (Buffington and Redak 1998). Sweep-netting can also produce highly variable results depending on sampling intensity and weather conditions, such as temperature and wind velocity (Romney 1945, Hughes 1955, Callahan et al. 1966). In addition, invertebrates may be damaged by sweep-nets during collection, potentially hindering species-level identification (Callahan et al. 1966). Vacuum sampling involves using a Dietrick vacuum sampler (D-vac, model 24, RinconVitova Insectaries, Ventura, CA; Dietrick et al. 1960) that is more difficult to operate than a sweep-net (Wilson et al. 1993, Stewart and Wright 1995). However, vacuum sampling may be more effective than sweep-netting in collecting invertebrates near the ground and in low vegetation where many birds forage (Cooper and Whitmore 1990, Harper and Guynn 1998). Vacuum sampling may also be less effective at collecting large insects, particularly grasshoppers (Orthoptera: Acrididae), than sweep-netting (Cooper and Whitmore 1990, Mommertz et al. 1996). Because prey size is an important cue in avian prey selection (Davis and Smith 2001, VanEmden and Rothschild 2004), comparing the size of invertebrates collected using these two techniques would allow investigators to determine which technique is more appropriate given the objectives of their study. Moreover, a comparison of invertebrates collected using each technique is needed to determine their possible biases. Thus, our objective was to evaluate the use of sweep-netting and vacuum sampling for collecting invertebrates. Specifically, we compared the invertebrate biomass, abundance, diversity, and size classes collected using the two techniques. METHODS
Our study was part of a larger project that evaluated the response of shrubland birds and invertebrates to fire and grazing at the
61
Hal and Fern Cooper Wildlife Management Area (hereafter, Cooper WMA) in northwestern Oklahoma (36◦ 34 N, 99◦ 34 W). Cooper WMA is 6507 ha in size, with approximately 30% of vegetation cover consisting of sand sagebrush (Artemisia filifolium) and other shrubs such as sand plum (Prunus angustifolia). The rest of the areas consists of grasses associated with the mixed grass prairie, including little bluestem (Schizachyrium scoparium), blue grama (Bouteloua gracilis), side-oats grama (Bouteloua curtipendula), sand bluestem (Andropogon hallii), and sand lovegrass (Eragrostis trichodes; Vermeire et al. 2004). Cattle were stocked on Cooper WMA at a rate of 24.7 animal unit d/ha (approximately 6.8 ha/steer). As part of the larger study, one-third of each pasture was burned each year so sampled habitats included unburned areas as well as areas that had been burned 0–4 years previously. We established 21 and 38 points in 2007 and 2008, respectively, at Cooper WMA for collecting invertebrates by vacuum sampling (Dietrick vacuum, or D-vac; Dietrick 1961) and sweep-netting. We sampled invertebrates from May to July 2007 and 2008. Using each method, we sampled along four paired 25-m line transects for a total of eight samples per point. Pairs of transects were parallel and 2 m apart to minimize any possible disturbance of sampling while maximizing the likelihood that the invertebrate communities would be similar. We randomly determined the sampling method used first to eliminate potential biases of one sampling method on the other. We sampled paired transects within 15 min of each other. For vacuum sampling, we collected invertebrates by holding the intake cone of the vacuum sampler 15 cm above the ground and walking at a constant pace along transects, with invertebrates collected in a bag attached to the vacuum (Jackson et al. 1987, Burger et al. 1993). At the end of each transect, we removed the collection net while the vacuum was still running, cinched the opening closed with one hand, and placed the net into a 3.8-l (1-gallon) freezer bag. For sweep-netting, we used a standard 38-cm diameter canvas “American-type” sweep-net. We walked at a constant pace (87% of the invertebrates collected. These taxa have also been documented in the diets of numerous bird species (Maher 1979, Johnson and Boyce 1990, Doxon and Carroll 2010). Invertebrates such as Lepidoptera larvae and those in the orders Araneae and Diptera were analyzed at the level of order because previous studies have not differentiated among families for these taxa (e.g., Johnson and Boyce 1990). We included year and time since burn in the model to partition variance attributed to these factors. We used MANOVA because our response variables were not independent and, therefore, were correlated. For significant MANOVAs, we used analysis of variance (ANOVA) to determine the effect of sampling method on each response variable separately. We further employed general linear models to examine differences in morphospecies richness, Shannon-Weiner diversity, total invertebrate dried biomass, and mean size of invertebrates between sampling methods. When differences were significant, we conducted a means separation test using Tukey’s HSD. Because time since burn and year were not part of the study objectives, we do not report their results. We conducted all analyses using the Statistical Analysis System (SAS Institute, 2005). Values are reported as means ± SE. RESULTS
We collected 83,041 invertebrates representing 63 families in 19 orders by vacuum sampling, and 55,029 invertebrates representing 68 families in 18 orders by sweep-netting. Two orders, Dermaptera (earwigs) and Opiliones (harvestmen), were only collected by vacuum sampling, whereas one order, Phasmatodea (walkingsticks), was only collected by sweep-netting. One family, Phalangiidae (harvestmen), was only collected by vacuum sampling and six families, Coenagrionidae (damselwings), Heteronemiidae (brown lacewings), Ixodidae (ticks), Melyridae (softwinged flower beetles), Meloidae (blister beetles), and Rhynchitidae (thief weevils), were only collected by sweep-netting. Relative abundance by count in vacuum samples was dominated by the orders Homoptera (24.7 ± 0.6%), Diptera (24.0 ± 0.9%), and Hymenoptera (19.4 ± 0.8%). Conversely, sweep-net samples were dominated by the orders Homoptera (23.5 ± 0.5%), Orthoptera
Methods of Sampling Invertebrates
Vol. 82, No. 1
63
Table 1. Comparison of the mean relative biomass (%) collected for 12 orders of invertebrates by vacuum sampling and sweep-netting at the Cooper Wildlife Management Area during 2007–2008. Vacuum-sampling Invertebrate taxon x¯ Araneae 1.02 Coleoptera 0.87 Diptera 10.38 Hemiptera 2.33 Homoptera 76.05 Hymenoptera 2.26 0.41 Lepidopterab Mantodea 0.76 Odonata 4.39 Orthoptera 1.77 Phasmatodea 0.00 Thysanoptera 1.93 a Degrees of freedom = 1, 58. b larvae.
SD 0.81 0.95 6.92 1.59 11.65 1.48 0.66 2.80 6.73 5.93 0.00 3.04
(20.6 ± 0.8%), and Araneae (11.4 ± 0.4%). By biomass, relative abundance of vacuum samples was dominated by the orders Homoptera (76.1 ± 1.6%) and Diptera (10.4 ± 1.0%), whereas sweep-net samples were dominated by the orders Orthoptera (50.2 ± 2.8%) and Homoptera (42.8 ± 2.6%). Sweep-netting resulted in the collection of more total invertebrate biomass than vacuum sampling (Wilks’ = 0.4, F 17,2124 = 212.2, P < 0.0001). In addition, biomass of 10 of 18 orders differed significantly for the two sampling methods. Biomass of two orders, Orthoptera and Lepidoptera larvae, was significantly higher in sweep-net samples, whereas biomass of eight orders, including Acari, Araneae, Diptera, Hemiptera, Homoptera, Hymenoptera, Neuroptera, and Thysanoptera, was higher in vacuum samples. We collected 11.4 times more Orthopteran biomass using sweep-nets than vacuum sampling, and >31 times more Dipteran and Thysanopteran biomass in vacuum samples than sweep-nets (Table 1). We also collected 3.5 times more Lepidoptera larvae biomass by sweep-netting than vacuum sampling (Table 1). For the orders Acari, Araneae, Hemiptera, Homoptera, Hymenoptera, and Neuroptera, we collected 1.5 to 18.1 times more biomass by vacuum sampling than sweep-netting (Table 1). Overall, the proportion of invertebrate taxa in each size category differed significantly (Wilks’ = 0.6, F 154,615 = 25.9, P < 0.0001). In the
