Does rat control benefit forest invertebrates at Moehau, Coromandel Peninsula? S.R. Rate DOC RESEARCH & DEVELOPMENT SERIES 316 Published by Publishing Team Department of Conservation PO Box 10420, The Terrace Wellington 6143, New Zealand DOC Research & Development Series is a published record of scientific research carried out, or advice given, by Department of Conservation staff or external contractors funded by DOC. It comprises reports and short communications that are peer-reviewed. Individual contributions to the series are first released on the departmental website in pdf form. Hardcopy is printed, bound, and distributed at regular intervals. Titles are also listed in our catalogue on the website, refer www.doc.govt.nz under Publications, then Science & technical. © Copyright September 2009, New Zealand Department of Conservation ISSN 1176–8886 (hardcopy) ISSN 1177–9306 (web PDF) ISBN 978–0–478–14665–3 (hardcopy) ISBN 978–0–478–14666–0 (web PDF) This is a client report commissioned by Waikato Conservancy and funded from the Science Advice Fund. It was prepared for publication by the Publishing Team; editing and layout by Amanda Todd. Publication was approved by the General Manager, Research and Development Group, 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 2. Objectives 6 3. Methods 7 3.1 Study site 7 3.2 Invertebrate monitoring 8 3.3 Rat control 9 3.4 Rat and possum monitoring 10 3.5 Data analysis 10 3.5.1 Data preparation 10 3.5.2 Univariate analyses 10 3.5.3 Multivariate analyses 12 4. Results 13 4.1 Invertebrate abundance and diversity 13 4.2 Invertebrate body length 15 4.3 Invertebrate community composition 16 4.4 Rats and possums 17 5. Discussion 18 6. Conclusions 20 7. Acknowledgements 21 8. References 21 Appendix 1 Invertebrate taxa collected at Moehau during the study period 24 Does rat control benefit forest invertebrates at Moehau, Coromandel Peninsula? S.R. Rate Wildland Consultants Ltd, 764 Cumberland Street, Dunedin 9016, New Zealand Email: [email protected] Abstract Many of New Zealand’s indigenous invertebrates are predated on by introduced mammals, but the impacts of this on their populations remain little understood. The effect of rat (Rattus spp.) control on invertebrates was examined in two forest types near Moehau, Coromandel Peninsula, between 2002 and 2007. Rat control had no effect on the relative abundance, diversity and body length of pitfall-trapped invertebrates, except for a significant reduction in numbers of ants (Formicidae). There was strong spatial and temporal variation in several of the invertebrate indices measured, and site and sample date were the main factors separating samples based on invertebrate community composition. These results suggest that rat control has not benefited invertebrate populations at Moehau. However, it is possible that some invertebrates that are susceptible to predation by rats were not adequately sampled, that predatory pest mammals were not reduced to low enough levels to elicit a measurable invertebrate response, or that invertebrate responses would only be measurable over a longer time span. Therefore, further research into the effect of rodent control on mainland invertebrate populations is required. Several recommendations are made to improve future studies. Keywords: rodent control, rat, Rattus spp., invertebrates, Moehau, Formicidae, ants, mainland © September 2009, New Zealand Department of Conservation. This paper may be cited as: Rate, S.R. 2009: Does rat control benefit forest invertebrates at Moehau, Coromandel Peninsula? DOC Research & Development Series 316. Department of Conservation, Wellington. 25 p. DOC Research & Development Series 316 5 1. Introduction New Zealand’s indigenous invertebrates continue to suffer losses through predation by introduced mammals. Some indigenous species are particularly vulnerable because, in the absence of mammalian predators, they have evolved to become large-bodied, flightless, often ground-dwelling, and nocturnal (Gibbs 1998). In addition, the main defence mechanism of some invertebrates (such as weta: Orthoptera) against endemic predators (e.g. tuatara and birds) is to remain still when threatened. To complicate matters, some weta have a strong odour. Neither of these traits is advantageous when dealing with introduced predators that rely on both sight and smell to locate prey (McGuinness 2001). Evidence that rodents impact on invertebrate populations in New Zealand originally came from research investigating the effects of rodent eradications on islands (e.g. Newman 1994; Green 2002; Rufaut & Gibbs 2003; Sinclair et al. 2005). In more recent years, this research has been supplemented by studies investigating the effects of rat control on land snail populations (e.g. Sherley et al. 1998; Bennett et al. 2002) and an increasing number of mainland studies investigating the effect of rodent control on arthropods (e.g. Spurr 1996; Hunt et al. 1998; King 2007), although several of these were designed to detect potential adverse effects as a result of poisoning (Spurr & Berben 2004; Powlesland et al. 2005). Eradication of rats has been implicated in altered invertebrate abundance (Green 2002; Watts 2004), species richness (Sinclair et. al. 2005) and diversity (Hutcheson 1999), and changes in age class structure and behaviour (Rufaut & Gibbs 2003). However, these responses have not always been positive (Craddock 1997; Sinclair et al. 2005). Furthermore, many invertebrate groups have shown no response to rodent control or exclusion of rodents (e.g. Green 2002; van Aarde et al. 2004). To increase understanding of invertebrate responses to rodent control on the mainland, invertebrates were collected from treatment sites (where rats were controlled) and non-treatment sites in two forest types at Moehau, Coromandel Peninsula. 2. Objectives The aim of this study was to determine the effects of rat control on forest invertebrates at Moehau, Coromandel Peninsula. Specific questions to be answered were: • Does rat control result in changes in invertebrate abundance? • How do invertebrate community structure and composition change after rat populations are managed? • Do different invertebrate groups respond to rat control in different ways? 6 Rate—Does rat control benefit Moehau invertebrates? 3. Methods 3.1 StudY sitE The Moehau Ecological Area is situated at the northern end of the Coromandel Peninsula, c. 70 km north of Thames on the east coast of the North Island (Fig. 1). Characteristic landforms of the peninsula include long ridges and steep streams radiating out to the coast, steep and broken hillslopes, floodplains, harbours, and estuaries (Amoore & Denyer 2006). Volcanic rocks of the Coromandel Group overlay Jurassic siltstone, sandstone and conglomerate. On the Moehau Range there are intrusions of a quartz diorite pluton. Soils on hillslopes and steeplands are mainly clayey and infertile, with depth decreasing with increasing steepness of the terrain. The climate is mild, moist and oceanic, with annual rainfall of 1250–2500 mm and summer droughts (McEwen 1987). Moehau (892 m a.s.l.) is the highest point. The study area was bisected by Stony Bay Creek, with the treatment area to the north and the non-treatment area to the southeast of the creek (Fig. 1). Study sites were stratified by altitude/vegetation type (200 m a.s.l./mature kanuka (Kunzea ericoides) forest versus 400 m a.s.l./podocarp/hardwood forest) and treatment (rat control versus no rat control). Thus, ‘Knt’ (200 m a.s.l.) and ‘Pnt’ (400 m a.s.l.) were in kanuka and podocarp/hardwood forest, respectively, in the non-treatment area; and ‘Pt’ (400 m a.s.l.) and ‘Kt’ (200 m a.s.l.) were in podocarp/hardwood forest and kanuka, respectively, in the rat control area (Table 1). The treatment area comprised 357 ha, while the non-treatment area encompassed 489 ha. Figure 1. Location of the study sites at Moehau, Coromandel Peninsula. Knt: kanuka (Kunzea ericoides) forest, non-treatment; Pnt: podocarp/hardwood forest, non-treatment; Pt: podocarp/hardwood forest, treatment; Kt: kanuka forest, treatment. DOC Research & Development Series 316 7 TABLE 1. INVERTEBRATE SAMPLING SITE DETAILS AT MOEHAU. SITE TRAPS ALTITUDE TREATMENT FOREST TYPE COORDINATES (n) (m a.s.l.) Knt 1–20 200 Nil Kanuka (Kunzea ericoides) E2727394 N6516766 Pnt 21–40 400 Nil Podocarp/hardwood E2727034 N6516280 Kt 61–80 200 Rat control Kanuka E2726140 N6518000 Pt 41–60 400 Rat control Podocarp/hardwood E2725271 N6517923 Mature kanuka forest at Knt and Kt is typically c. 10 m tall, with occasional mahoe (Melicytus ramiflorus) and rewarewa (Knightia excelsa) in the canopy, and abundant silver fern (Cyathea dealbata) in the understorey, along with mingimingi (Leucopogon fasciculatus), hangehange (Geniostoma ligustrifolium) and kanono (Coprosma grandifolia). Climbing rata (Metrosideros spp.), five-finger (Pseudopanax arboreus), mahoe, hound’s tongue fern (Microsorum pustulatum), rangiora (Brachyglottis repanda) and mangemange (Lygodium articulatum) are also present. Hooked sedges (Uncinia spp.) and bush rice grass (Microlaena avenacea) are dominant in the ground cover vegetation. Podocarp/hardwood forest at Pnt and Pt contains kohekohe (Dysoxylum spectabile), and occasional miro (Prumnopitys ferruginea), Hall’s totara (Podocarpus hallii), northern rata (Metrosideros robusta), pukatea (Laurelia novae-zelandiae),