Spatial variability of insect communities in a homogenous system: Measuring biodiversity using Malaise trapped beetles in a Pinus radiata plantation in New Zealand

Forest Ecology and Management 118 (1999) 93±105

Spatial variability of insect communities in a homogenous system: Measuring biodiversity using Malaise trapped beetles in a Pinus radiata plantation in New Zealand J. Hutchesona,*, D. Jonesb a

Forest Research Associates (FRASS), PO Box 1031, Rotorua, New Zealand b School of Forestry, Canterbury University, Christchurch, New Zealand Received 19 May 1997; accepted 5 October 1998

Abstract Insect communities of second rotation Pinus radiata stands in Kaingaroa forest were characterised using Malaise trapped beetles. Samples were collected from the part of the adult beetle activity period previously shown to deliver best discrimination of samples from New Zealand habitats. Eight trap-sites within a 14-year-old Pinus radiata stand provide indications of community variation within this relatively homogenous forest environment. Single trap-sites in adjacent younger (®ve-year-old) and older (30-year-old) stands provided initial intra-rotation comparison. Beetle assemblages from the three stand ages were unable to be discriminated using similarity or diversity indices, but were clearly distinguishable using divisive cluster analysis. Age of stands was of primary importance in distinguishing clusters, with those from the ®ve-year-old stand being most dissimilar. Clustering of catches from within the 14-year-old stand was in¯uenced more by week of capture (temporal variation) than trap-site (spatial variation). Within the 14-year-old stand, variation of abundance was associated with dominant detritivore species, and the extent and proximity of debris resources. Species richness was more constant, although considerable variation in component species was recorded. Trophic structure was also relatively consistent, with anomalous apparent variation possibly due to ignorance of species life histories. Successional processes were apparent within the insect samples over the rotation. The majority of the beetle assemblage from the 30-year-old stand were present at mid-rotation, but relative abundance of component species had changed. Beetle assemblages from all three age classes of stands were dominated by endemic detritivore species, re¯ecting the constant addition of woody debris within this rapid growing exotic vegetation system. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Coleoptera; Pine forest; Diversity; Techniques; Systems

1. Introduction *Corresponding author. Tel.: +64-7-3627122; fax: +64-73627122; e-mail: [email protected]

Biodiversity refers to the variety of life, and the largest component of this is provided by insects,

0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0378-1127(98)00495-2

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whether measurement is made at the genetic, character or species level (Berry, 1982; Gaston, 1996; Williams and Humphries, 1996). At the species level insects provide well over 60% of all organisms on earth (Heywood and Watson, 1995). Biodiversity per se includes both endemic organisms (i.e. those which evolved and are unique to a speci®ed area), and introduced organisms, many of which society relies on for economic sustainability. A nation's contribution to global biodiversity comes from endemic species, of which New Zealand has a relatively high proportion. As with most countries these resources are very poorly understood. Of an estimated 20±30 000 species of insect in New Zealand only about half have been described (Watt, 1982; Emberson, 1994) and the life histories of most can only be assumed from their taxonomic relationships. How this major area of biodiversity relates to the vegetation systems we use to categorize the environment, and how they are affected by management is even less well understood. Like many regions, most of New Zealand's land area now lies within an economically productive matrix within which indigenous ecosystems have been radically modi®ed. Isolated alpine parks cannot represent the biodiversity of the entire country, as both fauna and ¯ora exhibit local and regional endemism. Therefore, a major step in assessing the sustainability of present management of these genetic resources is an evaluation of how various land management regimes in¯uence the insect communities they host (Hutcheson and Hosking, 1994). The relationship does not appear to be simple, e.g. systems with low plant species richness (such as NZ beech forests) may harbour relatively species-rich insect communities (Hutcheson unpublished data). As the nature of the relationship cannot be assumed and no single study can cover all the system permutations required to hypothesize models, a sound sampling approach which will allow cumulative comparison is required (Ehrlich, 1996). It is impossible to sample, identify and interpret all insect species in large systems (Disney, 1986) and pragmatism must help guide the inevitable choices of taxa, trap and timing of sampling necessary for a standardized approach. Beetles range across all trophic groups and may comprise up to

50% of insect species (e.g. Kuschel, 1990; Lawrence and Britton, 1991). Their taxonomy, biology and the dynamics of sampling are also better understood than other species-rich, multi-trophic groups such as Diptera (¯ies) and Hymenoptera (ants, bees and wasps). Beetles also occur in lower abundance and are more characteristic of sampling sites in New Zealand than swarming groups such as the Diptera (Didham, 1992). Review of a range of studies found that relative to other approaches, Malaise trapped beetles exhibited a high correlation with total invertebrate catch (Hutcheson et al., 1997). Divisive cluster analysis (TWINSPAN (Hill, 1979)) has demonstrated that weekly catches of Malaise trapped beetles are characteristic for communities of particular sites (Hutcheson, 1990). Clustering was in¯uenced by site, time of season and micro-site in descending order of importance. Smaller catches from the beginning and end of the activity season were less successfully discriminated (Hutcheson, 1990). Subsequent work has shown that in New Zealand bioclimes, four consecutive weekly catches of beetles taken from within the early adult activity peak in December provide the most robust sample discrimination in accordance with habitat type (Hutcheson and Kimberley, in press). Clusters were strongly related to habitat attributes despite separation of component catches in time (4 years) and space (100 km). Catches appear to be mainly in¯uenced by habitat attributes within 25±50 m radius of the trap, a scale of operation which circumvents some of the problems associated with microhabitat variation (Dugdale and Hutcheson, 1997). Saproxylic beetle species have shown a relationship with habitat attributes at this scale (ékland et al., 1996), and it is also compatible with system management and remotely sensed information. The point source nature of Malaise trap catches accommodates the individualistic nature of community interactions at speci®c sites, while the weekly catches are large enough to be characteristic for recognizable system types. Insects of New Zealand exotic forest habitat have not been characterised at the community level despite intensive study of individual species directly affecting trees (e.g. Kay, 1983; White, 1974; Zondag and Nuttall, 1977). It is widely assumed that the invertebrate communities of exotic forest are radically different from those of indigenous forests, and various studies

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have indicated that habitat type and management regime strongly in¯uence invertebrate communities (e.g. McColl, 1974; Watt, 1975; Winterbourne, 1983; Walsh et al., 1993; Kuschel, 1990; Dugdale and Hutcheson, 1997). However, the nature and magnitude of these differences, particularly with regard to current establishment and management practices in New Zealand exotic pine forest, e.g. non-burning prior to establishment, have yet to be documented. Improved knowledge of the natural variation of insect communities within vegetation systems will obviously be important when trying to determine the relevance of effects associated with management practices. This paper documents spatial variation of insect communities within a relatively homogenous 14-year-old Pinus radiata stand as they are depicted by Malaise trapped beetle assemblages. Samples from the 14-year-old stand were also compared with limited sampling from adjacent ®ve-year-old and 30-year-old stands. 2. Methods 2.1. Site documentation Within the 14-year-old radiata stand, eight trap-sites were arranged in four sets along a transect. Members in each pair were separated by ca. 80 m, while sets were separated by ca. 160 m. The layout of these trapsites was determined for an ongoing study where one from each pair of trap-sites was to be later subjected to treatment. A single trap was also erected in each of adjacent ®ve-year-old and 30-year-old stands for comparison. All stands were second rotation Pinus radiata. The standardised procedure of Allen and McLennan (1983) as modi®ed by Leathwick (1987) was used to document site characteristics and vegetation structure and composition at each trap-site. The technique is known as `recce plots' and has a long history in various forms for recording and comparing New Zealand vegetation (Allen, 1992). Plots are generally located within homogenous vegetation types and cover an unde®ned area of c. 30 m diameter. Physical site parameters of slope, aspect, physiography parent soil material and drainage, and cultural in¯uences etc. are documented. Coverage of vascular plant species is

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recorded within seven classes de®ned as <1%, 1±5%, 5±25%, 25±50%, 50±75%, 75±95% and 95±100%. These are recorded within six ®xed vertical tiers de®ned as <30 cm, 30 cm±2 m, 2±5 m, 5±12 m, 12 m‡ and emergent crowns. The method is very rapid and provides a semi-quantitative and immediately interpretable table of live vegetation composition and structure. In addition, the woody debris resource at each trap-site was subjectively recorded as low, moderate or high, within and beyond a 5 m radius of each trap. Trap-sites 1±8 were within the 14-year-old stand, trap-site 9 was within a ®ve-year-old stand, and trapsite 10 was within a 30-year-old stand just prior to harvest. 2.2. Insect sampling Malaise traps used conformed to Townes (1972) speci®cations, as smaller commercially available traps have been found to provide catches too small for robust classi®cation of communities (Dugdale and Hutcheson, 1997). Previous sampling within indigenous forest canopies on the volcanic plateau have provided relatively small samples which were a subset of those captured at forest ¯oor level (Hutcheson, 1996). A single trap was therefore erected on the forest ¯oor at each of the trap-sites. Collection attachment was as described by Hutcheson (1991), except collecting jar and trap were connected using hose clamps, and the pottle held 300 ml of 70% ethanol as collecting ¯uid (Cresswell, 1995). Trapping was conducted during December and traps were serviced weekly to give four, consecutive catches from each trap. Beetles from the weekly catches were selected from the bulk of the catch, curated as per Walker and Crosby (1988), and identi®ed using exterior morphology to recognizable taxonomic units (RTUs). RTUs approximate species, and provide a practical approach to the current overwhelming problems of taxonomy (Ramsay, 1986; Oliver and Beatie, 1993, 1996). They allow taxonomy to be standardised and to be revised as knowledge increases. Non-standardised taxonomy prevents data being accumulative, effectively isolating any study and thus limiting its usefulness. Species level taxonomy provides the greatest amount of information for

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the interpretation of the relationship of the beetle assemblages to their habitat. (Oliver and Beatie, 1993, 1996) found reasonably good correlation between RTUs as de®ned by non-taxonomists and species de®ned by trained taxonomists. The correlation varied for different taxonomic groups, with Curculionidae and Staphylinidae being the most dif®cult within the Coleoptera. Given the incomplete nature of insect taxonomy in New Zealand, it is impossible for all RTUs in insect community studies to be named to species level. To improve readability, species is used to refer to RTU in this paper. 2.3. Analysis Sample af®nities were assessed using polythetic divisive classi®cation (TWINSPAN) and Sorensen's similarity index (K) (Krebs, 1978). The former objectively clusters subsamples (weekly catches) into groups based on the species present and their levels of abundance. The latter compares species presence/ absence in combinations of pairs of traps (Kˆ2c/a‡b, where a, b, and c, respectively, represent species unique to each trap and those which are shared). Species richness (S) in average trap-catches were comparedP with Shannon's' diversity index (H 0 ˆ ÿ Sjˆ1 Pj ln pj) where pjˆproportionPof the individuals in the jth species, Evenness (J0 ˆÿ p loge P/loge S) and summing of the abundance classes (SAC) found to optimise sample discrimination (Hutcheson, 1990). Cut levels of 0, 2, 5, 10 and 20 specimens de®ne these classes. Since abundance is linked to each species, SAC allows diversity measures to be easily calculated for data subsets grouped by biological attributes, such as trophic assignation. Species were assigned into the three simple trophic groups: detritivores (including scavengers and fungivores); herbivores (including all live plant feeders); and predators. Simple groups were used both because autecological knowledge is sparse for most species, and because many functional assignations may be both arti®cial and constraining. Without speci®c knowledge it is dif®cult, e.g. to ascertain a clear delineation along gradients of resource use such as debilitated live plants to dead plants. The functional (trophic) structure of the samples was examined at the levels of individuals and species.

3. Results 3.1. Habitats formed by stands The general study area is a relatively ¯at terrace comprising repeated pumice layers with very limited humus accumulation prior to plantation establishment. Drainage is exceptionally good, with no permanent surface water apart from the Whaeo river 2.5 km away. Stumps and logging debris from the ®rst pine crop were still present in the area of trap-site 9 in the ®veyear-old pine. The 8 m-high radiata canopy had not yet reached closure, and a number of species contributed to the ground cover. These were dominated at 2 m by the indigenous toe toe (Cortaderia fuluida) and tutu (Coriaria arborea), and below 30 cm by the introduced Mycelis muralis and Crepis capillaris, and the indigenous Epilobium alsinoides. Other indigenous plants such as Dracophyllum subulatum, a former dominant plant of the volcanic plateau were occasionally present. The introduced legume Lotus pedunculatus was also present in low abundance. This species is seeded in as part of forest management and becomes a dominant ground cover in mid-succession. The vegetation structure of the 14-year-old stand was generally much less complex. It was dominated by a 20 m canopy of Pinus radiata, over 1 m indigenous bracken (Pteridium esculentum) and Lotus pedunculatus <30 cm. Other species were occasionally present, e.g. small patches of gorse (Ulex europaeus) and the indigenous ferns Dicksonia squarrosa and Blechnum novae-Zealandiae. Vegetation in the 30-year-old stand was also more complex than that in the 14-year-old stand. The 30 m radiata canopy covered an understorey dominated by the indigenous ferns Dicksonia squarrosa, Paesia scaberula and Blechnum novae-Zealandiae. Of a total of 26 plant species recorded in this trap-site, 19 were indigenous. 3.2. Beetle samples A total of 5466 individuals from 131 recognizable taxonomic units (RTUs) were captured. Of these, 50.4% were identi®ed to species, 22.9% to generic level, and 19.1% were simply coded within family. Data were evaluated from several perspectives because of their information-rich nature and the rela-

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tively recent development of this approach to biodiversity measurement. 3.3. Total abundance Beetle catches (i.e. total beetles/trap/week) varied over the four-week period in an inconsistent fashion between traps, as various component species peaked in abundance (Fig. 1). Some traps were relatively consistent over the four weeks (traps 1,2), while others peaked toward the beginning (traps 3,5,6), the middle (traps 7,10) or the end of the sampling period (traps 4,7). Trap 10 in the 30-year-old stand,

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and trap 4 in the 14-year-old stand provided highest total abundance. Number of individuals, species and families in the samples (1 sampleˆall four weekly catches from each trap) were depicted as an average catch from each trap (Fig. 2). Samples from the traps in 14-year-old radiata (traps 1±8) varied considerably at the level of individuals but were much more consistent at the levels of species and families. The sample from ®ve-year-old radiata (trap 9) was within the range of the 14-year-old stand, but toward the lower limits. The 30-year-old stand (trap 10) provided the highest abundance of individuals, but not of species or families.

Fig. 1. Variation in weekly catch abundance of Malaise trapped beetles from a New Zealand pine forest over the sampling period (weeks 49± 52) Traps 1±8 in 14-year-old, nine in five-year-old and 10 in 30-year-old stands.

Fig. 2. Average number of individuals, species and families of Malaise trapped beetles from a New Zealand pine forest by trap. Traps 1±8 in 14-year-old, nine in five-year-old and 10 in 30-year-old stands.

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Fig. 3. Comparison of averaged abundance class distributions of Malaise trapped beetles from a New Zealand pine forest by trap. Traps 1±8 in 14-year-old, nine in five-year-old and 10 in 30-year-old stands.

Average catch distributions of the abundance classes established by Hutcheson (1990) as optimising classi®cation of beetle catches by site, also showed considerable variation between traps (Fig. 3). The 30year-old stand provided the greatest number of species in high abundance. Those traps in the 14-year-old stand which returned highest abundance for this habitat (4,7) differed from the trap in the 30-year-old stand in that they had fewer species in the highest abundance classes.

only two species (Cortinicara hirtalis) known to be adventive. Provenance of the remainder are presently unknown (Table 1(a)). The number of species unique to each trap was generally very low at ca. 2±8% (Table 1(b)). This ®gure rose to ca. 8% for trap 4 where debris (windfall) was closer to the trap and more extensive and varied. This trap also had the highest number of species overall. The ®ve-year-old stand provided ca. 33% unique species. Their life histories associated them with the presence of open ground and logging debris from the previous rotation, i.e. this site was most different from the sites in mid- and late-rotation stages of forest. In contrast, it is notable that the single trap in the 30-year-old stand provided only 8% of unique species.

3.4. Dominant species A core group of species occurred in most of the traps in the 14-year-old stand. Known endemicity of dominant species (i.e. averaging 5 or more individuals from any one trap) ranged from ca. 50 to 80%, with

Table 1 Provenance of dominant species of Malaise trapped beetles from a New Zealand pine forest (i.e. those averaging 5‡/trap) Trap no.

1

2

3

4

5

6

7

8

9

10

(a) Traps 1±8 in 14-year-old, nine in five-year-old and 10 in 30-year-old stands Endemic 3 5 5 4 7 7 Adventive 1 Unknown 3 3 2 2 3 5 Known endemicity (%) 50 56 71 67 70 58

4 1 1 67

5

5

8

2 71

1 66

3 73

(b) Unique species as a percentage of the total species captured in each trap Total sp/trap 43 48 40 61 45 Unique sp/trap 3 1 2 8 3 Unique sp (%) 7.0 2.1 5.0 13.1 6.7

41 1 2.4

40 2 5.0

58 19 32.8

52 4 7.7

51 4 7.8

Totals 19 2 7 71

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Table 2 Sorensen's index of similarity for Malaise trapped beetles from a New Zealand pine forest (based on presence/absence of species) P. rad. age

Trap

1

(a) Traps 1±8 in 14-year-old, nine in five-year-old and 10 in 30-year-old stands 14 1 14 2 0.659 14 3 0.578 14 4 0.596 14 5 0.591 14 6 0.638 14 7 0.619 14 8 0.699 5 9 0.376 30 10 0.611

2

3

4

5

6

0.659 0.661 0.667 0.727 0.652 0.705 0.472 0.620

0.634 0.729 0.615 0.642 0.600 0.469 0.587

0.642 0.661 0.647 0.574 0.437 0.602

0.708 0.698 0.659 0.447 0.598

0.696 0.659 0.617 0.459 0.444 0.367 0.621 0.645 0.652 0.418

(b) Sorensen's similarity values averaged for stand age Age (no. of traps) 5 (1)

14 (8) 30 (1)

5 14 30

0.651 0.617 Ð

Ð 0.434 0.418

7

8

9

3.5. Presence/absence similarity Sorensen's similarity indices based on presence/ absence showed high similarity of samples from within the 14-year-old stand. Trap pairs were 1±2, 3±4, 5±6 and 7±8, however closest presence/absence similarity was between numbers 3 and 5 (Table 2(a)). Only one of the ®ve highest indices was provided by a trap pair, signifying that trap proximity was not a strong in¯uence on the similarity of species composition. Average indices indicated a clear de®nition between samples from the ®ve-year-old stand and those from the two older stands, but no clear difference between samples from the 14 and the 30-year-old stands (Table 2(b)). 3.6. Diversity measures Diversity is a measure that includes both species richness and their equability. The latter thus includes the genetic variety within species by various transformations of species abundance. Comparison of species richness (S) with the three diversity indices: H0 , J and SAC (Fig. 4) showed a similar relative pattern of diversities. H0 might be more correctly termed a dominance index as it responds to the presence of more abundant species. This response is particularly

Fig. 4. Comparison of relative patterns of species richness (S) with the measures of diversity: Shannon's (H0 ), Evenness (J') and summed abundance classes (SAC) of Malaise trapped beetles from a New Zealand pine forest. S and SAC have been transformed to this scale by dividing by 100. Traps 1±8 in 14-year-old, nine in five-year-old and 10 in 30-year-old stands.

apparent in the higher relative H0 values for samples from trap 10 in the 30-year-old stand. Relative differences are less extreme for the SAC values, which show greater similarity to relative values for evenness (J). The presence of a large component of abundant species in the 30-year-old stand is also indicated by comparison of SAC and S for traps 9 and 10. All diversity indices depicted the 30-year-old stand with

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Fig. 5. Dendrogram of meaningful TWINSPAN divisions of weekly Malaise trapped beetles from a New Zealand pine forest.

the highest diversity, although SAC better re¯ected a strong in¯uence from high species richness. SAC thus accords more importance to the genetic differences between species than to that which exists within species. 3.7. Cluster analysis The polythetic divisive classi®cation program TWINSPAN clearly differentiated between the samples from the three stand ages (Fig. 5). Beetle assemblages from the ®ve-year-old stand were most dissimilar to all others, and were discriminated at

the ®rst level of division with an eigenvalue (i.e. measure of the variance accounted for by the division) of 0.317. The low eigenvalue re¯ects both the variation of catches within clusters, and the relatively high similarity of assemblages from early and mid-rotation stands. However, the clean division (all four catches from the ®ve-year-old stand were separated from the other catches), indicated a consistent difference between this community and those in the older stands. At the second level of division, catches from the 30year-old stand were separated from those captured in the 14-year-old stand, with an eigenvalue of just 0.236. Separation was also not as clean as that at the ®rst

Fig. 6. Trophic proportions of individuals of Malaise trapped beetles from a New Zealand pine forest. Traps 1±8 in 14-year-old, nine in fiveyear-old and 10 in 30-year-old stands.

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Fig. 7. Trophic proportions of species (RTUs) of Malaise trapped beetles from a New Zealand pine forest. Traps 1±8 in 14-year-old, nine in five-year-old and 10 in 30-year-old stands.

division, with one catch from the 14-year-old stand being grouped with those from the 30-year-old stand. At the third level of division, catches from traps within the 14-year-old stand were clustered more by week of capture than by trap, showing that temporal variation was greater than spatial variation within this midrotation stand. 3.8. Functional (trophic) structure of the beetle assemblages No species known to have aquatic larvae were captured and trophic structure was dominated by detritivores. At the level of individuals (Fig. 6) almost total dominance by detritivores was apparent. The anomalous trophic structure for trap 8 in the 14year-old stand is suspected to be due to the trap being erected over a 25 cm, decaying log, which is thought to have in¯uenced a subsequent high catch of Elateridae. The Elateridae is a multi-trophic family for much of which autecological information is presently scarce. Most species in this family were assigned into the herbivore trophic group apart from Metablax spp. and Thoramus spp. , which are known to be predatory. Trap 8 captured abundant Protelater elongatus, Panspoeus guttatus and an undetermined `Ctenicera' sp. The latter species, which was not captured in the same abundance in other traps, may be a detritivore rather than a herbivore as assigned. Further autecological information on this species (and many others) is required to fully clarify interpretation of the results.

Trophic structure at the level of species was much more consistent, (Fig. 7). Proportions of both herbivores and predators were expanded, and the dominance of detritivores was lessened considerably. Comparison of Figs. 6 and 7 thus show that detritivores were represented by species of relatively high abundance. In contrast predatory species contributed a smaller proportion of individuals than species, indicating their generally being represented by species of low abundance. 4. Discussion Insect communities in New Zealand as depicted by Malaise trapped samples, comprise a continual ¯ux of populations and turnover of component species over the adult beetle activity period from spring to autumn (Hutcheson, 1990; Moeed and Meads, 1987). Trapping over four consecutive weeks provides a relatively homogenous `set' of catches from each trap. These provide a relative measure of the signi®cance of variation between different traps in the classi®cation procedure. Sample discrimination is meaningful in terms of spatial variation when four (or at least three) catches from each trap are grouped together. When this no longer occurs, variation is as much due to temporal variation (week of capture) as to spatial variation (trap-site). Further divisions may then be disregarded (Hutcheson and Kimberley, in press). Cluster analysis clearly discriminated between beetle assemblages of the three age classes of pine. Although considerable variation in beetle species

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composition and their abundance occurred between traps in the 14-year-old stand, this was of less in¯uence in the analysis than variation with time (i.e. among weeks). Given the abundance class distributions shown in Fig. 3 and the undoubted presence of many more unsampled species of low abundance, the Sorensen's index of 0.651 within the mid-rotation stand is a product of both site related community variation and of sampling effect. However, the results imply that within a homogenous habitat type, a single Malaise trap can provide the general characteristics of the community. For this reason, the single traps in the early and late-rotation stands are felt to provide strong indications of the communities in these habitats. Few species were captured in only one trap. Within the 14-year-old stand the highest number of unique species occurred in trap 4, which also provided the highest species richness and total abundance in this habitat. Trap-site 4 was not noticeably different from other mid-rotation trap-sites in terms of live vegetation, but the top of a windblown tree came within 3 m of the trap. Decay was well advanced, although ®ne twigs were still present. This meant the debris resource was more extensive and variable and closer to the trap than occurred in other trap-sites in the 14-year-old stand. The dominance of detritivore individuals in the community structure, and the association of high abundances of particular detritivore species with the proximity of debris, indicate that the insect community of Pinus radiata plantations is largely involved with the recycling of the abundant debris resource. Although some phytophagous species associated with particular plants (e.g. Apion ulicis, a small weevil that feeds on gorse seeds) were present in samples from trap-sites containing their host, community variation was not strongly in¯uenced by the occasional presence of particular plant species. Before exotic pines were planted, the general area was dominated by indigenous early successional heathland which may be only about a metre tall after 30±60 years (Smale, 1990). In contrast, Pinus radiata attains a 20 m high forest environment after a mere 14 years. In relation to indigenous systems exotic pine forest is an accelerated carbon accumulator, providing an extensive cellulose resource over the period of a tree-crop rotation. The importance of this debris resource to both the insect community, and by impli-

cation, to the productive capacity of the pine forest through nutrient cycling, is clear from the functional structure of the samples. Saprophytic fungi are thought to be largely responsible for much initial cellulose breakdown in ecosystems. Most detritivorous beetle species are thought to feed on the fungi/wood decay complex rather than on sound wood because fungi are far more nutritious. C : N ratios for wood, fungal mycelium and insect bodies lie in the regions of 500 : 1, 50 : 1 and 5 : 1, respectively (White, 1993). However, apart from economically important Platypodinae and Scolytinae, these fungal/insect relationships have been largely ignored in the past. Improved knowledge of fungal associations with dominant beetle species in systems may assist in the interpretation of plantation forest health. For example, penetration by parasitic/saprophytic fungi (e.g. Diplodia spp. ) into trees that are damaged or physiologically stressed may be re¯ected in particular beetle assemblages. Once such relationships are identi®ed, beetles may supply a quanti®ed measure of fungal presence that would be dif®cult to attain directly from the fungi. The predominance of detritivores (particularly in terms of abundance) across all traps indicates that within the rapid growing exotic pine system, recycling of cellulose rather than live plant species composition is the main driving force in the maintenance of beetle biodiversity. This is in accord with interpretations from sampling of other habitats (Hutcheson, 1996, unpublished data). The heterogenous spatial and temporal dynamics of exotic forest management (e.g. pruning and thinning) supplies a continual debris resource for the fungal and insect components of the system. This both enhances biodiversity, and through nutrient recycling, enhances growth of the remaining crop trees. The ®ve-year-old stand provided the greatest number of unique species in this study and most of the dominant beetle species from this stand have also been captured from remnant indigenous heathland of the area (Hutcheson and Kimberley, in press). Highest plant species richness was recorded within the 30year-old stand, but highest beetle species richness was provided by a trap-site in the 14-year-old stand with nearby tree-fall. The majority of the assemblage associated with this exotic softwood forest appears to be established by mid-rotation. The system provides a

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rapid accumulation of cellulose compared with indigenous systems, and samples indicate that pine forest plays a positive role in sustaining a signi®cant sector of endemic biological diversity within the economically productive landscape. This may be associated with the presence of native fungi that can colonise radiata debris (see Hood, 1992), but insuf®cient knowledge exists of such relationships to assess this. Comparable knowledge of insect communities can provide a much-improved understanding of the dynamic processes of vegetation systems. For example, populations of phytophagous species are often closely associated with the physiological state of the host plant in indigenous systems (White, 1974; Hosking and Hutcheson, 1986). At the community level, therefore, insects have the potential to provide a sensitive monitoring device for measuring the health and vitality of vegetation systems, as well as providing a means of evaluating the dif®cult area of insect biodiversity. Perceptions of the full functional range of biological diversity, rather than simply live vegetation or vertebrates may assist in the integration of goals of both economic production and of biodiversity conservation. Strong support for basic taxonomy and directed autecological study of dominant species is required to enable this approach to realise its potential for system comparison. 5. Conclusions Variation of catch abundance recorded within the 14-year-old Pinus radiata stand was associated with dominant detritivore species, and was apparently related to extent and proximity of the debris resource. Species richness and functional (trophic) structure of the community showed little variation although component species varied. Within the mid-rotation stand, variation over time was greater than that due to trapsite location. Use of a single Malaise trap would thus have provided the general characteristics of the beetle samples from this homogenous habitat. Beetle catches from early, mid- and late stages of second rotation Pinus radiata could be clearly discriminated using polythetic divisive cluster analysis. This was not possible using diversity indices, species similarity or functional (trophic) structure. The most

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useful information was drawn from the identity of the organisms, their relative abundance and knowledge of their life histories. Diversity indices showed similar relative trends to each other, but did not contribute information useful for either discriminating communities or understanding their association with habitat attributes. Successional processes in terms of beetle species composition were displayed over the crop rotation. The beetle assemblage at the beginning of the rotation was the most dissimilar, and while no more species rich, included a range of species not present in later stages. The majority of the beetle species present at the end of the rotation were present at mid-rotation, but relative abundances of component species (even within the same trophic guild) had changed signi®cantly. The beetle assemblages were totally dominated by very abundant detritivore species, in comparison with samples from indigenous early successional vegetation (Hutcheson and Kimberley, in press), re¯ecting the rapid accumulation of cellulose, and subsequent debris deposition, achieved by this exotic vegetation system. The most abundant species exhibited a high level of endemicity, indicating that this exotic softwood habitat can play an important role in sustaining a sector of endemic biodiversity within the economically productive landscape. Acknowledgements Thanks are due to the former Forestry Corporation for allowing the study to be conducted in Kaingaroa forest and to Dave Moore and Kevin Simons (Environment BOP) for determining an appropriate area for the study. Discussions with Dr Patrick Walsh of Forest Research Associates (FRASS) were most helpful. References Allen, R.B., 1992. Recce: an inventory method for describing New Zealand vegetation. FRI Bulletin no. 176. Ministry of Forestry, Wellington, New Zealand. Allen, R.B., McLennan, M.J., 1983. Indigenous forest survey manual: Two inventory methods. New Zealand Forest Service, FRI Bulletin No. 48. Forest Research Institute, Rotorua, New Zealand.

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