Journal of ADVANCES IN APPLIED ECOLOGICAL TECHNIQUES Applied Ecology 2000, 37, The adequacy of collecting techniques for estimating 884±893 species richness of grassland invertebrates VALERIE STANDEN Department of Biological Sciences, Durham University, Durham DH1 3LE, UK Summary 1. There is still considerable debate about the most eective methods of sampling invertebrates in monitoring and assessment programmes. 2. The above-ground invertebrates of a limestone grassland in north-east England were compared between samples from pitfall traps and from a D-vac suction trap combined with a lightweight swish net (SW/DV). 3. Over 14 000 individuals were captured, with similar numbers in the pitfall and SW/DV samples. A total of 480 species of Hemiptera, Coleoptera, Diptera and Araneae was identi®ed and placed into 14 taxa for further analysis. 4. The pitfall sample produced species/specimen curves from which it was possible to estimate species richness for all the Coleoptera and Araneae taxa and the calyp- terate Schizophora. The SW/DV sample was adequate to estimate the species rich- ness of Hemiptera, most Diptera taxa, herbivorous Coleoptera and Linyphiidae. 5. The proportion of Coleoptera and Araneae taxa that were method-unique was higher in the pitfall sample than the SW/DV sample and vice versa for the Hemi- ptera and Diptera taxa. Nevertheless, a relatively high proportion of method- unique species of most taxa was found in both sample types, indicating that they can each contribute to assessing species assemblages in grasslands. 6. Both pitfall traps and SW/DV samples are needed to estimate species richness in grasslands for all taxa except Heteroptera, Homoptera and Lycosidae. Herbivorous Coleoptera and Linyphiidae were collected in numbers adequate for assessing rich- ness in both sample types, but more specimens were required in one or other sam- ple for the remaining taxa. Key-words: D-vac technique, monitoring, pitfall traps, rarefaction, sampling, swish net. Journal of Applied Ecology (2000) 37, 884±893 Introduction group. Pitfall traps catch more specimens of the lar- ger carabid species and under-represent smaller spe- A comprehensive comparison of invertebrate taxa, cies (Spence & Niemela 1994; Ulber & Wolf- collecting techniques, vegetation morphs and habi- Schwerin 1995); they also catch more spider males tats does not exist, although comparisons between than females and more species of Erigoninae and some variables have been made. For example, Lycosidae than other spider families (Dinter 1995). Kromp et al. (1995) estimated the density of arthro- Pitfall traps captured a dierent suite of Hemiptera pod groups using emergence traps, pitfall traps and species compared with sweep netting (Cherrill & a ¯ooding method, and found that the size and Sanderson 1994; Danahar 1998). mobility of taxa in¯uenced estimates of density. Although less structurally diverse than some habi- More frequent are comparisons of dierent collect- tats, grasslands present their own problems for sam- ing methods used to sample a single taxonomic pling invertebrates. Lu (1966) demonstrated signi®cant dierences between the beetle fauna inha- # 2000 British Correspondence: Valerie Standen (e-mail valerie.stan- biting tussocks and the intervening ground. Spiders Ecological Society [email protected]). tend to be either active hunters over the surface of 885 the ground or web-builders inhabiting dierent were to establish (i) which collecting method(s) V. Standen parts of plants, and so require speci®c sampling allowed an estimate of species richness of 14 methods (Cherrett 1964). Finally, Denno (1994) Coleoptera, Diptera, Hemiptera and Araneae taxa, demonstrated the in¯uence of grassland structure on and (ii) the extent of species overlap between sam- the density and diversity of leafhoppers. pling methods and consequently whether one or The herb-rich grasslands that have developed over more methods are needed to estimate species rich- outcrops of magnesian limestone in south and east ness of each taxon in the grassland habitat. County Durham, UK, have mostly been destroyed by agricultural intensi®cation and the habitat is now nationally scarce. Thrislington, the location of this Methods study, has been considered an important area for plants and invertebrates since the 19th century. By S I T E D E S C R I P T I O N the middle of the 20th century it supported the most extensive stand of primary magnesian limestone Thrislington grassland in east County Durham (NZ grassland in Britain and 10 ha were designated as a 318 228) is 110 m a.s.l. with an annual rainfall of site of special scienti®c interest in 1984. In the early approximately 500 mm. The calcareous plant com- 1970s, an extension to the adjacent dolomite quarry munity that has developed over thin soils derived threatened to destroy the site entirely but, as a result from the magnesian limestone escarpment contains of public concern, part was left undisturbed and 5Á5 species found in northern arctic±alpine grasslands ha of ancient grassland was translocated as intact and in chalk grasslands of southern England. Blue turves to an adjacent, previously quarried, area moor grass Sesleria albicans Kit. ex Schult. and (Park 1989). Translocation took place over 8 years small scabious Scabiosa columbaria L. characterize beginning in 1982, and following completion the site the site, with over 140 vascular species recorded in was designated as a National Nature Reserve. total. A large number of nationally or locally scarce An extensive programme of monitoring plants invertebrates are associated with the grasslands, and invertebrates was initiated at Thrislington in including the Northern Argus butter¯y Aricia artax- 1982. Monitoring an invertebrate community can erxes spp. salmacis Stephens 1831 and the glow reveal how it changes over time. Once the species worm Lampyris noctiluca L. composition is known, it may be possible to target rare species or those that play a key functional role in the maintenance of the community, and design a S A M P L I N G P R O G R A M M E sampling programme to monitor changes in their Two adjacent sites approximately 5 ha in area were abundance. However, in the initial stages the feature sampled, one of which was to be translocated in most often used to assess change is species richness. 1983. The ®rst sample was taken in May 1982, then It is therefore necessary to employ a sampling each following month up to and including Septem- regime which ensures that less abundant species, or ber 1982, using three collecting techniques. The sam- those not easily captured, are recorded because they pling programme was repeated at both sites in 1985 may be the species that are most aected by distur- and 1988, giving a total of 15 months of data for bance. each. Macroinvertebrates were counted and identi- Where the aim is to monitor all the invertebrates, ®ed by specialist taxonomists, except for (i) Isopoda several collecting techniques must be used as no sin- (Philoscia muscorum (Scopoli) and Armadillidium gle method will capture animals from all microhabi- vulgare (Latreille)), which were ubiquitous and tats. The methods chosen for this work were pitfall super-abundant and of little value therefore in traps, a D-vac suction apparatus and a swish net detecting change, and (ii) Mollusca and Lepidop- designed to capture the most active and easily dis- tera, as none of the methods employed was suitable turbed invertebrates. The samples dier in two for these groups. important respects. First, they represent dierent parts of the total fauna, pitfall traps collecting mostly epigeal fauna, and swish net and D-vac C O L L E C T I N G T E C H N I Q U E S mostly epiphytic fauna. Secondly, the patterns of accretion of individuals and species will vary Pitfall traps because pitfall traps depend on the animals' activity and collect continuously from an unknown area, Nine plastic cups, 8 cm in diameter, were set without whereas the suction apparatus and swish net collect covers 2 m apart in a 3 Â 3 grid in each site. Ethy- from a known area for brief periods. lene glycol of 80% was used as preservative and the # 2000 British The aim of monitoring invertebrates in this pro- traps were set continuously and emptied monthly Ecological Society gramme was to assess changes in species composi- throughout May to the end of September in each Journal of Applied Ecology, 37, tion and richness over a number of years following year of study. No attempt was made to compensate 884±893 grassland translocation. The objectives of this paper for loss, which was approximately 10%. 886 Swish samples families. However, several groups were not identi®ed Monitoring to species owing to lack of taxonomic expertise and The swish samples were designed to collect insects grassland these were excluded: Sciarinea (Nematocera), 130 ¯ying over the sward and visiting the tallest in¯ores- invertebrates specimens; Phoridae (Brachycera), 50 specimens; cences. These often escape the standard sweep-net and Anthomyiinae (acalypterate Schizophora), 400 technique, which is designed to collect amongst the specimens. The Araneae were grouped as Linyphii- grass sward and canopy. The aluminium frame was dae, Lycosidae and `other spider' families. The 0Á4 m in width on a 1Á5-m handle and was swept families included in the non-familial groupings are only over the topmost part of the sward. One sam- listed in Table 1. ple unit was 20 strokes of 0Á75 m width taken at 0Á75-m intervals so that the area swept was 6 m2 per sample within approximately 10 m2. Five sample R A R E F A C T I O N A N D C O L L E C T O R C U R V E S units were collected, giving an area of approximately Although species abundance models such as rarefac- 30 m2 sampled each month.