Beetle Assemblages of Kahikatea Forest Fragments in a Pasture-Dominated Landscape
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HARRIS, BURNS: BEETLES IN KAHIKATEA FRAGMENTS 57 Beetle assemblages of kahikatea forest fragments in a pasture-dominated landscape Richard J. Harris1 and Bruce R. Burns2 1Landcare Research, Private Bag 6, Nelson, New Zealand (E-mail: [email protected]) 2Landcare Research, Private Bag 3127, Hamilton, New Zealand __________________________________________________________________________________________________________________________________ Abstract: Fragments of kahikatea (Dacrycarpus dacrydioides) forest provide a major opportunity for conservation of indigenous biodiversity in the heavily deforested landscape of the Waikato Basin, New Zealand. However, there is little documented information on what indigenous fauna survives in these fragments. Using Malaise traps set 20 m and 50 m into fragments and 20 m and 50 m into the adjacent pasture, we analysed the beetle (Coleoptera) assemblages associated with two such kahikatea forest fragments in the south Waikato Basin in order to determine the scope of biotic invasion by adventive species and use of the surrounding pasture by indigenous species. A total of 3706 beetles were caught, encompassing 37 families and 206 recognisable taxonomic units. The forest fragments had a sharply defined edge, and were dominated by indigenous beetle species, with only a few adventive species present. Beetle assemblages sampled in the surrounding pasture were numerically dominated by adventive species. Despite no indigenous plant species being present in the pastures, 55 indigenous beetles species (61% of total species sampled in the pasture) were recorded in this habitat. Traps in the pasture of greatest floral diversity caught the most indigenous beetle species. Beetles of the detritivore guild dominated the samples from forest, but in samples from pasture, detritovores and predatores were co-dominant. Indigenous herbivore species were poorly represented in samples from pasture compared to other guilds. The kahikatea fragments have a rich indigenous beetle fauna and represent important refuges in the pastoral landscape of the Waikato. Many indigenous species are utilising the pasture to some degree, although their abundance and species richness declines with distance from the forest edge. This may have implications for successful dispersal to new patches. Species that could potentially be used as indicators of kahikatea forest fragment community structure and its resistance to invasion are suggested. __________________________________________________________________________________________________________________________________ Keywords: Coleoptera; forest fragments; edge effects; pasture; community structure; biological diversity; indicator species. Introduction and Lawton, 1999). This has flow on effects for the biotic community, for example, causing tree dieback Fragmentation of indigenous ecosystems is of global and reduced seedling abundance (Laurance et al., 1997; concern for the conservation of indigenous biodiversity Benitezmalvido, 1998), allowing weed invasion (Burke and ecosystem processes. Forest fragmentation has and Nol, 1998), altering decomposition rates (Didham, been linked to the loss of species from forest systems 1998), and altering species composition and guild and alteration of ecosystem function due to loss of sturcture of the insect community (Golden and Crist, microhabitats, changes in the physical and chemical 1999; Didham et al., 1998). As a result, communities at environment, invasive alien species, and changes in the fragment edge may be intermediate between the metapopulation dynamics (de Souza and Brown, 1994; fragment and the surrounding habitat. Small fragments Didham, 1998; Didham et al., 1998). The effects of can become all “edge” without any of the original forest decline on invertebrates are not always obvious miroclimates remaining resulting in greater species and not all species are affected equally (Darveau et al., loss than for larger fragments (Didham et al., 1998). 1997; Didham et al., 1998). One result of fragmentation Although edge effects are widely accepted as is that remnant forest becomes surrounded by new impacting on the health and composition of forest ecosystems. At the boundary of the remnant forest and fragments (Murcia, 1995), the scale of the impact on the new ecosystem the physical conditions are altered the fauna will be largely influenced by the structural and the changed conditions can extend into the forest integrity of the vegetation, the composition of the (Candenassoet al., 1997; Burke and Nol, 1998; Didham surrounding vegetation, and the resistance of the biotic New Zealand Journal of Ecology (2000) 24(1): 57-67 ©New Zealand Ecological Society 58 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 24, NO. 1, 2000 community to disturbance (Webb et al., 1984; Camargo the use of the surrounding pasture by indigenous species. and Kapos, 1995; Didham and Lawton, 1999). Species that could be used as indicators of the current The Waikato basin of the North Island of New community composition of other kahikatea fragments, Zealand has largely been in pastoral production since or to monitor habitat degradation or restoration over the indigenous cover of forest and scrub was cleared time are suggested. and wetlands drained in the latter half of the 19th and early part of the 20th century (Ministry for the Environment, 1997). Before deforestation, Dacrycarpus Methods dacrydioides (A. Rich.) Laubenf. (kahikatea) forest occurred on swamp-land (reflecting kahikatea’s Study sites tolerance of waterlogged soils) and alluvial plains subject to flooding (reflecting a regeneration niche Yarndley’s Bush is 14.5 ha and occupies the bottom of requiring high light and humidity) (Wardle, 1974; a shallow catchment, 4 km north of Te Awamutu (grid Duncan, 1991; 1993). In the Waikato basin, remnant reference NZMS260 S15 135552). The forest is bisected forest fragments are almost all kahikatea-dominated. by a drain, and comprises an eastern lobe of about 140 These fragments are of relatively even age (75-125 m by 250 m and a larger western lobe of about 250 m years old) and appear to have established around a few by 500 m (from which samples were taken). The forest surviving older trees (200-450 years old) soon after the had previously been browsed by cattle, but was fenced period of land clearance (Burns et al., 2000). As a result in the summer of 1980-81 (Edmonds, 1982). Kahikatea of the formation or expansion of the kahikatea- formed an almost mono-specific canopy up to 30 m dominated fragment after clearing for farming, plant over a dense understorey dominated by Laurelia novae- species associated with the fragment are often species zelandiae Cunn. and Melicytus ramiflorus Forester et of moderately well-drained soils (e.g., Beilschmiedia Forester f. Soils are poorly drained Puniu soils, derived tawa (Cunn.) Kirk, Alectryon excelsus Gaertner, from rhyolitic alluvium (M. McLeod, Landcare Knightia excelsa R. Br.; Burns et al., in press), and not Research, Hamilton, N.Z., pers. comm.). necessarily wetland species, a feature exacerbated by Pirirakau (listed as “Garrett M.A. & P.M. Open drainage of the surrounding land (Champion, 1988). Space Covenant” in Wassilieff and Timmins, 1984) is Although common across the landscape a 6.8 ha forest 10 km east of Yarndley’s (grid reference (15 fragments per 100 km2), the kahikatea forest NZMS260 S15 237537). A farm road bisects the forest. fragments of the Waikato basin are small (mean 1.1 ha) About half the forest was fenced in 1968 to exclude (Burns et al., 2000) and so represent only about 0.2% of cattle, with further sections fenced in 1981 and 1993. the land area. Furthermore, the understorey vegetation The canopy vegetation is up to 30 m and dominated by of many fragments has been modified by browsing kahikatea, but with some L. novae-zelandiae. The from domestic cattle. Despite their small size and often understorey mostly comprises a dense shrub layer of modified vegetation structure, the fragments provide L. novae-zelandiae, M. ramiflorus, and Schefflera the major opportunity for conservation of indigenous digitata Forester et Forester f., although a strip 20-30 m biodiversity in this pastoral landscape. Yet, with the wide at the forest edge, fenced for only 3 years, had exception of the flora (e.g., de Lange 1987; 1989; a dense fern understorey of Hypolepis ambigua Whaley et al., 1997), there is little documented (A. Rich.) Brownsey et Chinnock, Histiopteris incisa information on what indigenous biota survives in these (Thumb.) J. Smith, and Diplazium australe (R. Br.) forests. Wakef. The forest occurs on Puniu silt loam (Wassilieff To determine the value of forest fragments as and Timmins, 1984). stores of indigenous biodiversity and identify the threats Both forests were surrounded by grassland fields to their conservation and persistence, we need to used that were grazed once a month by dairy cows. understand their biotic composition better. In this study, Trifolium repens L. and Lolium perenne L. were the beetles (Insecta: Coleoptera) were chosen for study as dominant forage species amongst a range of grasses they represent a large component of the measurable and herbs. The herb Polygonum hydropiper L. was also biodiversity (Southwood, 1978) accounting for about abundant. A small ditch with open water passed near 50% of New Zealand’s insect species (Watt, 1982; the trap located 50 m into the pasture adjacent to Kuschel, 1990), have representatives from all trophic Yarndley’s Bush. groups, and have been shown