Consequences to Threatened Plants and Insects of Fragmentation of Southland Floodplain Forests

Consequences to threatened plants and insects of fragmentation of Southland floodplain forests S. Walker, G.M. Rogers, W.G. Lee, B. Rance, D. Ward, C. Rufaut, A. Conn, N. Simpson, G. Hall, and M-C. Larivière SCIENCE FOR CONSERVATION 265 Published by Science & Technical Publishing Department of Conservation PO Box 10–420 Wellington, New Zealand Cover: The Dean Burn: the largest tracts of floodplain forest ecosystem remaining on private land in Southland, New Zealand. (See Appendix 1 for more details.) Photo: Geoff Rogers, RD&I, DOC. Science for Conservation is a scientific monograph series presenting research funded by New Zealand Department of Conservation (DOC). Manuscripts are internally and externally peer-reviewed; resulting publications are considered part of the formal international scientific literature. Individual copies are printed, and are also available from the departmental website in pdf form. Titles are listed in our catalogue on the website, refer www.doc.govt.nz under Publications, then Science and Research. © Copyright April 2006, New Zealand Department of Conservation ISSN 1173–2946 ISBN 0–478–14070–3 This report was prepared for publication by Science & Technical Publishing; editing by Geoff Gregory and layout by Ian Mackenzie. Publication was approved by the Chief Scientist (Research, Development & Improvement Division), Department of Conservation, Wellington, New Zealand. In the interest of forest conservation, we support paperless electronic publishing. When printing, recycled paper is used wherever possible. CONTENTS Abstract 5 1. Introduction 6 1.1 Habitat fragmentation 6 1.1.1 Edge effects 6 1.1.2 Habitat isolation 7 1.1.3 Effects on biodiversity 8 1.1.4 Fragmentation studies in New Zealand 8 1.2 Southland Plains study area 9 1.2.1 Geologic and geomorphic setting 9 1.2.2 Loss of forest on the Southland Plains since the arrival of humans 10 1.2.3 Current levels of protection of remaining forest 12 1.3 Southland’s floodplain ecosystem 12 1.3.1 Floodplain ecosystem 12 1.3.2 Abiotic characteristics 12 1.3.3 Threatened plants 13 1.3.4 Habitat classification 15 1.4 Fragmentation in floodplain forests 17 1.4.1 Processes and symptoms of fragmentation 17 1.4.2 Modification of the hydrological regime 18 1.5 Scope of this report 18 2. Flora of Southland’s floodplain ecosystem 20 2.1 Introduction 20 2.2 Methods 20 2.2.1 Sampling design 20 2.2.2 Main study sites 20 2.2.3 Vegetation and flora 22 2.2.4 Habitat–environment parameters 23 2.2.5 Indicators of fragmentation 23 2.2.6 Data analyses 24 2.3 Results 25 2.3.1 Floodplain forest communities 25 2.3.2 Floodplain forest vegetation gradients 28 2.3.3 Effects of fragmentation 33 2.4 Discussion 37 2.4.1 Regional vegetation patterns 37 2.4.2 Habitat specificity 38 2.4.3 Rare plant strategies for recruitment and persistence 39 2.4.4 Rare plant responses to fragmentation 40 2.5 Conclusions from the vegetation study 41 3. Effects on floodplain forest invertebrates 42 3.1 Introduction 42 3.1.1 Invertebrates in plant communities 42 3.1.2 Invertebrates as potential environmental indicators 43 3.1.3 Problems and approaches in invertebrate community studies 43 3.1.4 Invertebrate studies in the Southland floodplain ecosystem 44 3.2 Study 1—Beetle assemblages of floodplain habitats 47 3.3 Study 2—Invertebrates on Coprosma species 55 3.4 Study 3—Lepidoptera on Sophora microphylla 64 3.5 Discussion of study findings 68 3.5.1 Effects of fragmentation on invertebrate biodiversity 68 3.5.2 Relict shrubs and trees as reservoirs of indigenous biodiversity 69 3.5.3 Spatial turnover and biogeographic differences in invertebrate communities 70 3.5.4 Determinants of invertebrate patterns 70 3.5.5 Host specificity 71 4. General discussion 72 4.1 Defining characteristics of the floodplain ecosystem 72 4.2 Fragmentation of the floodplain ecosystem 73 4.3 Lowland biodiversity loss and fragment viability 73 4.4 Conservation goals for remnants of the ecosystem 74 4.4.1 Components of ecological integrity 74 4.4.2 Larger tracts with intact hydrological regimes 75 4.4.3 Smaller fragments with intact or restorable hydrological regimes 75 4.4.4 Small fragments with modified hydrological regimes 76 4.5 Future work 77 4.5.1 Hydrological alteration and reversal 77 4.5.2 Restoration strategies and techniques 77 4.5.3 Autecological work 77 4.5.4 Advocacy and inter-agency co-operation 77 5. Acknowledgements 78 6. References 78 Appendix 1 Landscape and vegetation of the Southland Plains study sites 83 Appendix 2 Characteristics of Southland’s floodplain forest communities 86 Consequences to threatened plants and insects of fragmentation of Southland floodplain forests S. Walker1, G.M. Rogers, W.G. Lee, B. Rance, D. Ward, C. Rufaut, A. Conn, N. Simpson, G. Hall, and M-C. Larivière 1 Corresponding author: Landcare Research, Private Bag 1930, Dunedin, New Zealand ABSTRACT The processes of ecosystem fragmentation and their effects on persistence of key plant and invertebrate groups were investigated within remnants of alluvial podocarp forests in Southland, New Zealand. Vegetation was sampled at 135 plots in floodplain vegetation covering a range of habitat units, plant communities, and fragmentation states. Special note was taken of six rare plant species. In addition, beetle assemblages were sampled intensively across local fragmentation and habitat gradients within a single catchment, and individual shrubs of Coprosma species and kowhai trees were sampled for key invertebrate groups across a landscape-scale fragmentation gradient. The distinctive floodplain forest communities and their rare plant species appeared to be maintained by conditions of reduced competition. Natural disturbance events (primarily periodic severe frost and waterlogging floods) impose periods of extreme stress within the floodplain ecosystem. The consequences of forest fragmentation for the plant community are tier simplification, loss of epiphytes, decreasing richness first in the ground layer and then in taller tiers, and invasion of ground and then lower forest tiers. Fragmentation also leads to reduced and patchy recruitment of the target rare plant species. Beetle assemblages showed significant compositional changes reflected in altered functional group proportions, par- ticularly a marked increase in predators in the ground beetle fauna. Isolated Coprosma shrubs had significantly greater native invertebrate loadings than those in core fragment areas. However, Lepidoptera faunas were not significantly different between isolated and core kowhai trees. The ecological integrity of the floodplain ecosystem depends largely on the maintenance of natural hydrological regimes, and their alteration by drainage and flood control works is likely to have profound effects on community composition of remaining floodplain remnants, and adverse consequences for the persistence of its rare plant components. Remaining floodplain ecosystem remnants with intact hydrological regimes should be given very high priority for protection. Keywords: ecosystem, fragmentation, podocarp forests, floodplain, hydrology, rare plants, beetle assemblages, moth assemblages, Southland, New Zealand © Copyright April 2006, Department of Conservation. This paper may be cited as: Walker, S.; Rogers, G.M.; Lee, W.G.; Rance, B.; Ward, D.; Rufaut, C.; Conn, A.; Simpson, N.; Hall, G.; Larivière, M-C. 2006: Consequences to threatened plants and insects of fragmentation of Southland floodplain forests. Science for Conservation 265. 86 p. Science for Conservation 265 5 1. Introduction 1.1 HABITAT FRAGMENTATION Habitat fragmentation occurs when the continuity of original vegetation is disrupted and reduced into a number of smaller patches (Collinge & Forman 1998). It is most often a consequence of human activities (e.g. road construction, clearing for agriculture, urbanisation), which have in the past and continue to dramatically alter the pattern, composition, and extent of global vegetation (Houghton 1994; Turner & Corlett 1998; Laurance et al. 2002). Fragmentation has also been described as the disruption of structural and spatial continuity (Lord & Norton 1990; Didham et al. 1996; Thomas et al. 1998; Laurance et al. 2002). Under this definition, the concept can be relevant to any ecosystem where continuity is important to ecosystem functions, regardless of scale. Fragmentation affects ecosystems by altering the conditions within patches and the flow of resources (organisms, propagules, nutrients) among patches. Disruption and modification to environmental drivers by fragmentation include altered irradiance, increased wind exposure (windthrow, reduced understorey humidity), increased permeability of edges to alien plants and animals, disruptions to hydrological regimes, and wind- and water-borne nutrient inputs. Biological effects of fragmentation are also induced by the physical distances between habitat fragments and intact areas, reduced habitat area, and changes in fragment composition and structure, including changes in the surrounding matrix (Ricketts 2001). There are many references in the literature documenting the abiotic (environmental) and biotic (biological) consequences of fragmentation (Saunders et al. 1991; Forman 1995; Olff & Ritchie 2002; and for review see Harrison & Bruna 1999). A brief summary is given below. 1.1.1 Edge effects Environmental conditions change along a gradient away from the boundary of a patch towards the interior. To account for this relationship, a patch is typically divided into ‘core’ and ‘edge’ areas (Morrison et al. 1992; Forman 1995). Core areas lie at least a certain distance from the edge and tend to have abiotic conditions similar to those found in the interior of larger patches. Edge areas receive the most influence from neighbouring patches and have a higher degree of alteration. Long and narrow patches may effectively have no core area despite being quite large. By reducing habitat area, fragmentation increases the amount of edge habitat relative to core habitat (Hanski et al. 1995; Redpath 1995). New, more extensive edges created by fragmentation of a forest are often adjacent to patches with a more open physical structure such as pasture or urban areas.

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