1 2 TOPPING CHRIS J. TOPPING and LÖVEI: and SPIDERS GÁBORAND L. LÖVEI DISTURBANCE IN AGROECOSYSTEMS 121 1 Horticulture Research International, Littlehampton, West Sussex BN17 5HL, U.K. Present address: Department of Landscape Ecology, National Environmental Research Institute, Kalø, Grenåvej 14, DK-8410 Rønde, Denmark. E-mail:cjt@dmu.dk 2 AgResearch, Flock House Agricultural Centre, Bulls and Horticulture and Food Research Institute, Private Bag 11030, Palmerston North, New Zealand. E-mail: loveig@hort.cri.nz 2 Author for correspondence. SPIDER DENSITY AND DIVERSITY IN RELATION TO DISTURBANCE IN AGROECOSYSTEMS IN NEW ZEALAND, WITH A COMPARISON TO ENGLAND __________________________________________________________________________________________________________________________________ Summary: Spider assemblages were sampled by quantitative sampling in pasture and arable habitats under different management regimes in the lower North Island of New Zealand. Density and species diversity increased with decreasing frequency and/or intensity of disturbance from two species and 1.8 individuals m-2 in wheat to 16 species and 130 indiv. m-2 in an abandoned, ungrazed pasture. The spider fauna was dominated by introduced species of money spiders (Linyphiidae). The most abundant species, Lepthyphantes tenuis, is also the most abundant one in British cultivated habitats. Additional pitfall trap samples from the same location and the Waikato, central North Island, indicated a similar species range containing mainly European species. A sample from a native tussock habitat had a completely different fauna, with only one species shared with the most undisturbed cultivated area. Comparative samples showed that similarly structured, but about twice as species-rich assemblages live in similar cultivated habitats in England. __________________________________________________________________________________________________________________________________ Keywords: spiders, Lyniphiidae, density, diversity, agroecosystems, disturbance, New Zealand, England. Introduction Many spiders are preadapted to habitats with large spatial and temporal variability (Wise, 1993) and constitute an abundant and widespread group of polyphagous predators in ephemeral and disturbed habitats, including cultivated land. In the Northern Hemisphere, spiders are regarded as significant natural enemies of arthropod pests (Riechert and Lockley, 1984; Nyffeler and Benz, 1988; Sunderland et al., 1986). Agricultural habitats in New Zealand are evolutionally recent and bear close resemblance to similar habitats in the Northern Hemisphere, especially Europe, both in terms of flora and fauna (Lövei, 1991). It is likely that spiders play an important role in these habitats. However, information on spiders in agricultural fields in New Zealand is scarce. Martin (1983) listed 47 species collected in pasture during three years near Nelson, South Island. Most of the spiders were not identified to species and general notes on abundance are given for a few species only. European species of linyphiids, Eperigone fradeorum (Berland)*, *Misidentified as Erigone tridentata (A. MacLachlan, pers. comm., Lincoln University, Lincoln, N.Z.) E. wiltoni Locket, Leptyphanthes tenuis (Blackwall) dominated in pitfall trap catches. In this paper, we provide the first quantitative data on the density and diversity of spiders in New Zealand cultivated land under different management regimes, and compare the species assemblages with those found in similar habitats in England. Methods Quantitative sampling, New Zealand Our main study site was on the AgResearch farm at Flock House near Bulls (40o10’S, 175o23’E), in southern North Island, New Zealand. Spiders were collected from nine different agricultural habitats during the southern spring (late November) of 1992. In each habitat, five sampling units of 1 m2 each were selected and isolated by a 150 mm tall steel ring. Spiders were collected by a modified D-vac suction sampler (30 sec suction) with subsequent hand-searching (Topping and Sunderland, 1994). This method provides a good estimate of ‘true’ density data for spiders; D-vac samples that are not supplemented by hand-searching underestimate spider density (Topping and Sunderland, 1994). New Zealand Journal of Ecology (1997) 21(2): 121-128 ©New Zealand Ecological Society 122 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 21, NO. 2, 1997 We selected nine habitats forming a series of management operations (disturbance) of different intensity and frequency: 1. Conventionally managed spring wheat paddock {CW}, in its second consecutive year in wheat. 2. Organically managed spring wheat {OW}, first year in wheat. This plot had an ‘organic’ certification, conforming to the international Bio-Gro standards. No artificial fertiliser or pesticide was applied. Weeds were controlled mechanically, twice during early spring, when the wheat plants were not taller than 15 cm. The previous crop was a ryegrass (Lolium perenne L.) - white clover (Trifolium repens L.) pasture. 3-5. Ryegrass - white clover pasture blocks subjected to rotational sheep grazing at high (HG; 28 ewes ha-1), medium (MG; 15 ewes ha-1) and low stocking rates (LG; 5 ewes ha-1). An area within the paddock was grazed for about one week every 15 weeks. 6. a ryegrass - white clover pasture ungrazed for 6 months {NG}, previously under rotational grazing at medium stocking rates by sheep. 7. a ryegrass - white clover pasture abandoned for one year {AP}, previously under rotational grazing at medium stocking rates by sheep. Habitats marked 1-7 were 2 - 2.3 ha in size. 8. a roadside grassy verge, subject to mowing once a year, during the summer {RV}. The dominant plant species were perennial ryegrass, Poa annua L., Phleum pratense L., and sparse flowering weeds (Matricaria sp., Lamium sp., A. Maclean and J.M. Hickman, pers. comm.). 9. An experimental plot of species-rich pasture (0.5 ha), planted with a grass/herb mixture containing 16 species {HP}. This plot was planted two years before the sampling, and was not treated, cut or grazed. Pitfall trapping, New Zealand Additionally, spiders were collected by pitfall trapping from an organically managed, ungrazed plot of serradella (Ornithopus sativus Brotero) at Flock House, over seven weeks between 20 December 1990 and 7 February 1991. The serradella was in its first year, and the plot was free of weeds, except sparse mayweed (Matricaria sp.) plants around the edges. Ten pitfall traps were deployed along a straight line, starting from the edge of the paddock and running towards the centre. Individual traps were 10 m from each other. A pitfall trap consisted of a 0.5 L plastic pot filled with 300 mL of 70% ethylene glycol, and covered by a metal square ‘roof’ to protect the catch from birds, small mammals and rain. Traps were controlled weekly and the catch was collected, sieved, and stored in 70% ethanol until identification. The total trapping effort was 490 trap-days. Two further series of pitfall trap sampling were used for comparisons, collected from two pastures near Hamilton (37o47’S, 175o17’E), in the Waikato region of the North Island, New Zealand, during October-December 1990. Site A at Ruakura was sown in 1980 with perennial ryegrass and white clover (Trifolium repens L.). At the time of sampling, it had substantial ingress of P. annua and was grazed by dairy cows. Trapping effort here was 350 trap-days. Site B, Rukuhia, was sown in 1978 with perennial ryegrass cv. ‘Grasslands Nui’ and white clover cv. ‘Grasslands Pitau’. By 1990, it also had ingress of P. annua. This pasture was grazed by sheep. The trapping effort was 560 trap-days. At both sites, 10 pitfall traps, with ethylene glycol as preservative, were deployed. Quantitative sampling from non-cultivated habitat, New Zealand For comparison with the spider assemblage of cultivated habitats, ten D-Vac suction samples of 1m2 (using the metal ring and hand searching as for the other quantitative samples) were also taken from a scarce tussockland habitat, dominated by Chinochloa rubra on the central Volcanic Plateau of the North Island near the top of the Desert Road (850 m a.s.l.; 39o13’S, 175o45’E), in November 1992. Quantitative sampling, England Intercontinental comparisons of the assemblage structure were made with spider assemblages collected from two fields of winter wheat in Sussex, southern England (Grid Refs. TQ0403, 50o49’N, 00o31’W, and TQ1807, 50o51’N, 00o19’W, Both fields were sampled using 15 density samples of 0.5 m2 each in an identical manner to that used for the New Zealand sampling (Topping and Sunderland, 1994). These were conventionally managed winter wheat fields receiving fungicide and fertiliser but no insecticide applications during the sample seasons (i.e. none were necessary). Both fields were in rotation with grass and oil-seed rape. TQ 1807 was in grass the year before, the other in wheat. The fields were at 20 m and 70 m a.s.l., respectively. TOPPING and LÖVEI: SPIDERS AND DISTURBANCE IN AGROECOSYSTEMS 123 Table 1: Total spider catch resulting from density sampling of nine agricultural habitats, arranged in decreasing degree of disturbance, at Flock House, Bulls, New Zealand. CW - conventional wheat; OW - organic wheat; HG, MG, LG, NG grazed paddocks, with high, medium, low and no grazing; RV - roadside verge; AP - abandoned pasture; HP - herb/ pasture. For more details, see Methods. __________________________________________________________________________________________________________________________________ Species CW OW HG MG LG NG RV AP HP Total 0 0 4 0 0 3 2 0 0 0 3 4 0 0 0 5 11 0 0 0 76 0 50 0 0 2 11 0 0 0 24 0 41 0 0 37 69 0 0 2 15 0 1 0 0 5 49 0 3 15 1 0 0 0 1 40 403 0 35 98 2 0 0 2 0 37 184 1 4 136 0 0 1 2 0 21 268 0 32 213 129 4 118 4 1 153 998 1 74 464 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 5 1 0 0 0 0 0 0 0 4 3 0 0 1 1 2 8 1 1 30 1 1 0 0 5 3 1 15 34 1 1 1 1 7 11 0 0 0 0 0 0 0 2 2 0 0 0 0 0 0 0 0 0 0 1 0 1 2 1 1 3 3 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 0 0 2 0 0 0 1 2 3 0 0 4 85 11 16 17 107 0 0 0 0 0 0 0 0 0 0 0 0 2 3 0 1 2 4 0 0 0 0 0 0 0 0 2 1 0 0 0 0 0 0 2 1 0 0 0 0 0 0 0 0 0 0 0 2 1 3 0 0 1 5 0 0 1 0 0 0 0 0 1 0 0 0 0 0 2 2 42 46 25 12 13 4 5.0 142 81 61 4 28.4 174 62 112 5 34.8 99 30 69 7 19.8 __________________________________________________________________________________________________________________________________ LINYPHIIDAE Erigone wiltoni 8 Eperigone fradeorum 0 Erigonine Immatures 21 Diplocephalus cristatus 0 Microtenonyx subitaneus (O.-P. Cambridge) 0 Lepthypantes tenuis 3 L. tenuis immatures 1 Diplocenta spp. 0 Mynoglenes diloris 0 M. diloris immatures 0 THERIDIIDAE Th1 0 Th2 0 Th2 immature 0 Th3 0 Th4 0 Th5 0 Th5 immature 0 Th7 0 Th7 immature 0 TETRAGNATHIDAE Tetragnatha spp. 0 ARANEIDAE Ar1 immatures 0 Ar2 immatures 0 SALTICIDAE Sa1 immature 0 Sa2 0 LYCOSIDAE Ly1 0 Ly11 Immatures 0 OXYOPIDAE Ox1 0 Immatures 0 GNAPHOSIDAE Anzacia gemmea Dalmas 0 Anzacia spp. immature 0 CLUBIONIDAE Clubiona clima Forster 0 Immatures 0 CYATHOLIPIDAE Tekelloides spp. 0 Unknown Family Un1 immatures 0 Total Adults Immatures Number of species Density, individuals m-2 33 11 22 2 6.6 9 3 6 2 1.8 599 491 82 60 507 431 8 16 120.0 98.2 651 2213 77 418 574 1795 14 23 130.0 __________________________________________________________________________________________________________________________________ 124 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 21, NO. 2, 1997 Pitfall trapping, England Pitfall trap samples from two perennial ryegrass pastures, obtained from the northeast of England (Grid Ref. NZ 1367, 55o00’N, 01o48’W, 100 m a.s.l.) were compared to pitfall trap catches from New Zealand. Pitfall traps consisted of 110 mm deep x 85 mm diameter polypropylene cups, with undiluted ethylene glycol as preservative. Lids or covers were not used on the traps. Traps were placed in a transect across the sampled area consisting of 12 traps 1 m apart. The first nine undamaged traps recovered were used in the analysis. Both pasture sites were 1 y old perennial ryegrass swards. Two cuts of silage were taken during the sample year and the sites were grazed by cattle afterwards. The samples were taken in late May which corresponded in season to those taken in New Zealand. The total trapping effort was 360 trap-days Evaluation and identification To describe the diversity of spider assemblages, we used species richness (S), which is the simplest but legitimate measure of diversity, and is often as informative as the more complicated diversity indices (Southwood, 1978). All spiders were identified using the available identification keys (Forster, 1967, 1970, Forster and Wilton, 1968, 1973; Forster and Blest, 1979; Roberts, 1987, Forster et al., 1988). The taxonomic status of many New Zealand spider species is still in doubt, hence a number of morpho-species encountered could not be identified and were allotted code numbers. Nomenclature of the European species follows Roberts (1987). Results Spider assemblages in relation to disturbance in New Zealand A total of 23 species of spiders were identified in the quantitative samples from nine habitats sampled at Flock House. Among habitats, spider species richness ranged between two and 16, and the densities varied between 1.8 and 130 individuals m-2 (Table 1). There was a general trend towards higher numbers of individuals and species as disturbance (defined as the frequency/intensity of agricultural management and/or grazing) decreased. Very high disturbance levels of the two cultivated sites resulted in low spider density and a species-poor assemblage (two species). The most disturbed habitats were the pasture and cultivated ones. These had a simple spider assemblage, comprising the introduced species L. tenuis, E. wiltoni, Mynoglenes diloris (Urquhart) and a few incidentals such as Eperigone fradeorum and Diplocephalus cristatus (Blackwall) (Tables 1- 3). Only three native species were regularly recorded in the agricultural samples Table 2: Spider assemblages on an experimental pasture plot, Bulls, North Island, New Zealand. Each sample consited of 10 pitfall traps. __________________________________________________________________________________________________________________________________ Species 27 Dec 3 Jan 10 Jan Date 17 Jan 24 Jan 31 Jan 7 Feb Total 5 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 5 2 2 1 10 13 0 15 5 0 3 10 0 9 6 0 19 8 0 20 6 0 40 4 1 116 52 2 2 2 2 0 9 12 29 1 0 0 1 0 0 0 2 1 0 0 0 0 1 0 2 33 23 15 18 27 36 59 211 __________________________________________________________________________________________________________________________________ LINYPHIIDAE Erigone wiltoni Eperigone fradeorum Diplocephalus cristatus Ostearius melanopygius (O-P. Cambridge) Lepthyphantes tenuis Mynoglenes diloris LYCOSIDAE Ly1 DOLOMEDIDAE Do1 THERIDIIDAE Th2 __________________________________________________________________________________________________________________________________ Total __________________________________________________________________________________________________________________________________ TOPPING and LÖVEI: SPIDERS AND DISTURBANCE IN AGROECOSYSTEMS 125 Table 3: Spider assemblages on pastures in the Waikato region, North Island, New Zealand, according to results of pitfall trap catches. __________________________________________________________________________________________________________________________________ Location, species 25 Oct 15 Nov Date 22 Nov 29 Nov Total 0 2 2 1 34 1 2 1 0 0 5 2 19 6 4 3 53 9 13 7 2 7 5 43 4 11 4 36 15 97 4 Oct 11 Oct 25 Oct 15 Nov 6 Dec Total 38 11 0 1 5 3 19 8 0 0 10 3 4 4 0 0 1 0 33 10 0 0 12 6 96 26 1 1 15 11 190 59 1 2 43 23 1 59 4 44 5 14 8 69 6 156 24 342 __________________________________________________________________________________________________________________________________ Ruakura LINYPHIIDAE Erigone wiltoni Eperigone fradeorum Lepthyphantes tenuis Mynoglenes diloris LYCOSIDAE Ly1 Total Rukuhia LINYPHIIDAE Erigone wiltoni Eperigone fradeorum Latesia bellissima Diplopecta spp. Lepthyphantes tenuis Mynoglenes diloris LYCOSIDAE Ly1 Total __________________________________________________________________________________________________________________________________ (M. diloris, Th2 and Ly1). Almost all species recorded from these habitats were money spiders (Linyphiidae). The organically managed wheat field did not support more spiders than the conventionally managed one: both had only two species, and the density ratio was 3.5:1 in favour of the conventional field (Table 1). For the grazed pastures, the species richness steadily increased with the decreasing intensity of grazing (from high intensity grazing to abandoned pasture). In contrast, the density was highest in the least grazed habitat, and the mediumgrazed habitat had higher spider population density than the abandoned one (Table 1). Density but not species richness was three times greater in the roadside verge than the least grazed pasture. Spider density in the other two types of ‘abandoned’ habitats (AP and HP) was similar to that recorded in the roadside verge habitat. Species richness, however, was higher (Table 1). In habitats with reduced disturbance, E. wiltoni no longer formed a significant part of the assemblage. In contrast, L. tenuis numbers were higher. M. diloris did not seem to tolerate grazing: of all the grazed habitats, only two immatures (and no adults) were caught in the low-intensity grazed habitat. The prevalence of this species was consistently higher in less disturbed habitats. The trends in abundance estimates of the adult and immature stages roughly coincided, although habitats with the highest densities had disproportionately high numbers of immatures (Table 1). Spider fauna sampled by pitfall traps Flock House, Manawatu A total of 211 individuals of nine species were collected (Table 2). Weekly catches ranged between two and seven species, and 15- 59 individuals. L. tenuis was the most numerous species in pitfall trap samples followed by the native M. diloris. These two species made up 80% of the total catch. The third-ranking species was an unidentified native lycosid spider, which was caught, although in small numbers, on all but one sampling occasion. Ruakura, Waikato Five species and 97 individuals were caught over the five weeks of pitfall trapping. Four of these were introduced linyphiid spiders. The only native species was the same lycosid caught at Flock House, also 126 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 21, NO. 2, 1997 constantly present in the captures in small numbers. On two occasions, all the species on the total list were present in the weekly catch. The number of individuals per week ranged between 7 and 56. The catch was dominated by E. wiltoni and L. tenuis (Table 3). Rukuhia, Waikato Seven species and 312 individuals were collected over eight weeks. Five of these were introduced linyphiids, with two natives: M. diloris and the unknown lycosid (Table 3). All the species were caught in several weekly or fortnightly catches, and the number of individuals per week varied between 7 and 78. The most numerous species in the catch was E. wiltoni, followed by E. fradeorum, L. tenuis, and M. diloris. Table 4: Spiders collected by suction sampling in tussockland habitat on the Volcanic Plateau, North Island. Altitude: 850 m a.s.l. All species thought to be native. ______________________________________________________________ Species Number of individuals ______________________________________________________________ LINYPHIIDAE Diploplecta communis Millidge Laetesia trispathulata (Urquhart) Laetesia amoena Millidge SYMPHYTOGNATHIDAE Sy1 THERIDIIDAE Th2 Th8 Th9 Th10 Th11 Th12 LYCOSIDAE Ly2 Total Adults Immatures Number of species Density, individuals m-2 12 4 1 2 14 1 1 1 1 1 1 305 39 266 11 30.5 ______________________________________________________________ Spider density in native tussockland Suction sampling from native New Zealand tussock grassland yielded 305 individuals comprising ten species (Table 4), of which only three were linyphiids. Only the most abundant species (theridiid Th2; see Table 2), was found in samples taken from the least disturbed agricultural habitats (AG, RV, AP and HP). Intercontinental comparisons Only partial comparisons were feasible, because of the lack of comparative material. We compared spider density estimates in English and New Zealand wheat fields, and pitfall trap catches in ryegrass pastures in New Zealand and England. Samples taken using identical methods in Britain showed that wheat fields in England contained more spider species than New Zealand fields (14 and 15 species vs. two species in New Zealand), and densities were five times higher (Table 5). Both faunas were dominated by linyphiid spiders, and L. tenuis was abundant in both. The New Zealand assemblage can be considered a subset of the European fauna. In ryegrass pastures, the same trends were observed as for the density sampling: there were more species and higher numbers in Britain than in New Zealand (Table 6). Linyphiidae again dominated in both areas. Discussion Although the structure of the spider assemblages in our study area of agricultural land in New Zealand was similar to that found in England, both species richness and density were lower in New Zealand. The New Zealand samples had approximately half the number of species expected for comparable habitats in England. The majority of the spiders in agricultural habitats in both countries were money spiders (Linyphiidae). Money spiders are highly dispersive (Duffey, 1956; Sunderland, 1991). In contrast, New Zealand native species generally have an allopatric distribution. M. diloris appears to be an exception to this and has been observed ballooning in large numbers (R.R. Forster, pers. comm.). Thus, its dispersal ability, atypical of New Zealand native spiders, may explain the high abundance of M. diloris in agricultural habitats. M. diloris seemed to prefer a complex vegetational architecture, as the highest densities were found in the two habitats which are floristically and structurally the most complex ones (G.L. Lövei and V.K. Brown, unpublished). Possibly the most interesting aspect of New Zealand agricultural spider assemblages is that the species appear to occupy very similar habitats to those found in Europe. L. tenuis is the most abundant spider, with E. wiltoni, also common, favouring the shorter, more heavily managed swards. This is almost identical to the situation in Britain where L. tenuis is often the most abundant and ubiquitous spider while short swards or arable land TOPPING and LÖVEI: SPIDERS AND DISTURBANCE IN AGROECOSYSTEMS Table 5: Spider assemblages in two fields of winter wheat in Sussex, England, during spring (May 1990). Samples obtained by D-Vac suction and subsequent hand-searching on 7.5m2. ______________________________________________________________ Species Field A, 17 May Field B, 21 May 0 1 1 1 4 2 10 1 4 2 5 41 3 2 1 6 4 0 1 0 3 0 5 4 9 1 0 1 1 0 2 3 0 1 1 0 0 1 283 204 79 15 37.7 285 243 42 14 38.0 ______________________________________________________________ LINYPHIIDAE Oedothorax apicatus (Blackwall) O. fuscus (Blackwall) O. retusus (Westring) Tiso vagens (Blackwall) Dismodicus bifrons (Blackwall) Pananamops sulcifrons (Wider) Milleriana inerrans (O.P.-Cambridge) Erigone atra E. dentipalpis E. promiscua Meioneta rurestris (C.L. Koch) Lepthyphantes tenuis Bathyphantes gracilis (Blackwall) THERIDIIDAE Theridion pallens Blackwall TETRAGNATHIDAE Pachygnata degeeri Sundevall LYCOSIDAE Pardosa palustris (L.) THOMISIDAE Oxyptila sanctuaria (O.P.-Cambridge) CLUBIONIDAE Clubiona reclusa O.P.-Cambridge C. brevipes Blackwall Total Immatures Adults Number of species Density, individuals m-2 ______________________________________________________________ are favoured by the Erigone species E. atra (Blackwall), E. dentipalpis (Wider) and E. promiscua (O.P.-Cambridge) (Topping, 1991; Topping and Sunderland, 1992). This suggests that the ecological niches occupied by these species are broadly similar between continents. If this is the case, then the lower spider density and diversity in New Zealand agroecosystems could be due to differences in habitat structure and a lower availability of niches, a lack of available colonists or lower prey resource. New Zealand pastures are relatively recent, and were established with European grasses and forage legumes, on land formerly in forest. New Zealand pastures are in appearance similar to those in England. Poor dispersal power may mitigate against successful invasion of this new habitat by endemic species, whilst non-native species may not have had the opportunity to invade. For similar reasons, suitable prey may be low in abundance and thus limit the 127 occupation of the agricultural habitats to only the most efficient competitors. Comparative trophic studies in agroecosystems in Europe and New Zealand would be necessary to give support to either of these hypotheses. Since most of lowland and mid-altitude areas in New Zealand were covered with forests until the arrival of man (Wardle, 1991), tussockland was chosen as an example of a native habitat most likely to contain spiders pre-adapted to conditions in cultivated habitats. Although our sample was very limited, the structure of the spider community found in tussock was remarkably different from the agricultural samples. All species collected from the tussock grassland site were thought to be endemic to New Zealand and only one (theridiid Th2) was shared with the agricultural sites sampled. Thus, the tussock grassland habitat, although apparently structurally close to some agricultural habitats, seems to be sufficiently different to almost totally exclude species overlap. This pattern is typical of many other groups of animals and plants in New Zealand (Kuschel, 1990; Lövei, 1991) leading to the peculiar situation whereby the ‘imported’ agricultural habitats with virtually no evolutionary history in New Zealand are Table 6: Adult spider assemblages on two ryegrass pastures from the Tyne Valley, England, obtained by pitfall trapping. ______________________________________________________________ Species Field 1 Field 2 ______________________________________________________________ LINYPHIIDAE Oedothorax fuscus 32 Oe. retusus 11 Monocephalus fuscipes (Simon) 1 Gongylideillum vivum (O.P.-Cambridge) 0 Savignya frontata (Blackwall) 0 Areoncus humilis (Blackwall) 0 Milleriana inerrans 8 Erigone dentipalpis 327 E. atra 206 Meioneta rurestris 3 Centromerita bicolor (Blackwall) 0 Bathyphantes gracilis 0 Lepthyphantes tenuis 6 TETRAGNATHIDAE Pachygnatha degeeri 1 P. clerki Sundevall 2 LYCOSIDAE Pardosa amentata (Clerck) 1 P. palustris 6 P. pullata (Clerck) 1 CLUBIONIDAE Clubiona reclusa 0 Total 605 Number of species 13 6 1 6 1 4 3 3 98 143 9 1 7 17 1 0 3 0 4 0 307 16 ______________________________________________________________ 128 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 21, NO. 2, 1997 found side-by-side with the native habitats. Since agroecosystems are largely species-poor and populated by introduced species, there is great potential for studying the ecological aspects of community organisation in a simplified natural environment. With fewer species, niches may be more easily defined and inter-species interactions may be less complex and more easily investigated. Acknowledgements We would like to thank G. Barker for the use of his pitfall trap samples, D.J. Hodgson and K.D. Sunderland for their help with density sampling in New Zealand, and Britain, respectively, R.R. Forster, J.M. Hickman, A. Maclachlan, A. Macleod, W. Stiefel, C. Vink for information, and two anonymous reviewers for comments on the manuscript. CJT was supported by a fellowship under the OECD Project on Biological Resource Management and a travel grant from the British Council (Higher Education Link 00984). GLL would like to thank N.D. Barlow for taking over the editorial responsibilities for this manuscript. References Duffey, E. 1956. 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Limitations to the use of pitfall traps in ecological studies exemplified by a study of spiders in a field of winter wheat. Journal of Applied Ecology 29: 485-491. Topping, C.J.; Sunderland, K.D. 1994. Methods for quantifying spider density and migration in cereal crops. Bulletin of the British Arachnological Society. 9: 209-213. Wardle, P. 1991. Vegetation of New Zealand. Cambridge University Press, Cambridge, U.K. 672 pp. Wise, D.H. 1993. Spiders in ecological webs. Cambridge University Press, Cambridge, U.K. 328 pp.