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The impact of alien mammal exclusion on invertebrate food resources for
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native birds in New Zealand
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Paul S. EDDOWES
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Centre for Ecology and Conservation, University of Exeter, Cornwall Campus, Penryn,
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UK, TR10 9EZ
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The impact of alien mammal exclusion on invertebrate food resources for
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native birds in New Zealand
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Abstract
Invertebrate sampling was carried out in late summer and autumn at six treatment and
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control sites on the North Island of New Zealand to assess the impact of alien mammal
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exclusion has on invertebrate abundance, diversity and biomass. The aim was to assess
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whether increased food resources or reduced predation allows recovery of native bird
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species within fenced reserves and ‘Mainland Islands’. Across all six sites invertebrate
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abundance was only significantly higher in the areas of mammal exclusion compared to
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control sites when sampling with the portable light trap. In contrast invertebrate biomass
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was significantly higher in mammal-present areas when sampling with the beating tray,
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sweep net, malaise trap and pitfall traps. If invertebrate resources throughout the year show
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comparable patterns of abundance, then recovery of populations of avian insectivores
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within fenced reserves seems likely to benefit more from reduced predation than greater
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food availability.
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Keywords
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New Zealand conservation, Invertebrate biomass, Mainland Island, Invertebrate sampling,
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Introduced mammals.
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1. Introduction
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1.1. Overview
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Like many other island archipelagos, New Zealand has an evolutionary history that
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diverged markedly from the rest of the world about 65-80 million years ago (Cooper and
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Milliner, 1993) when it separated from the southern continent of Gondwanaland. New
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Zealand totals 26 million ha over 3 main islands, plus another 700 smaller islands greater
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than 5 ha. These stretch from the subtropics to the sub Antarctic (29oS to 52oS) across two
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tectonic plates, leading to a diverse landscape (Craig et al., 2000).
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New Zealand’s biota evolved completely free of the influence of terrestrial mammals,
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excluding two bat species, and over the past 10,000 years birds were the largest animals in
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all terrestrial ecosystems. The ratites were very common, were large in size and often
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flightless (Atkinson and Millener, 1991). The reptiles that evolved on the island include
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tuatara, geckos and skinks but no snakes or crocodiles.
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New Zealand was the last major land mass to be colonised by humans. The predecessors
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of the Maoris arrived 700-1000 years ago and the Europeans around 200 years ago (Craig et
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al., 2000). Birds and reptiles are the two groups of animals that have suffered the most from
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this anthropogenic presence. The cause of this has been ecosystem loss and fragmentation,
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hunting by humans and depredation by introduced alien species. Temperate rainforests have
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been reduced from an original 78% of land area to just 23% and wetlands have been
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reduced by over 90% of their pre-European area (Ministry for the Environment, 1997).
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Native grasslands have decreased greatly through over-sowing with European pasture
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grasses and poor land management (Craig et al., 2000). Maori hunting eliminated 26 species
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(30%) of endemic land birds including many Moa species, and 4 species (18%) of sea birds.
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Also, tuatara, many lizards and many invertebrates were eliminated from the main islands
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(Craig et al., 2000). European colonisation increased ecosystem destruction due to increase
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demand for timber and pastoral agricultural land, which in turn saw the extinction of a
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further 16 land birds as well as bat, fish and invertebrate species (Ministry for the
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Environment, 1997). Nationally, bird, bat, lizard and invertebrate species are characterised
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by low population densities and severe population fragmentation (Towns and Daugherty,
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1994).
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The consequences of this make conservation in New Zealand a key issue, and there are
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many considerations to be made by the Department of Conservation (DOC). Preservation
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versus sustainable management, amount of land represented by conservation areas,
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reintroductions of native species, the maintenance of whole ecosystems and the control of
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introduced alien species are the main considerations that are of importance in New Zealand
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conservation.
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1.2. Effects of introduced alien species on native terrestrial flora and fauna
New Zealand now has 34 species of land mammal. The introduced mammals include
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predators such as rats, dogs, cats, stoats, ferrets and weasels and browsers like red deer,
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rabbits and horses (Atkinson, 2001). Nearly all the mammals were intentionally introduced
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and they affect a wide range of organisms including the kaka (Nestor meridionalis)
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(Moorhouse et al., 2003), shorebirds like oystercatchers and snipe (Dowding and Murphy,
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2001), the long-tailed bat (Chalinolobus tuberculatus) (O’Donnell, 2000) and tuatara and
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the lizards, geckos and skinks (Towns et al., 2001). Stoats and cats have caused the
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extinction of 9 endemic bird species in the last 150 years (Mooney and Hobbs, 2000) and
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introduced mammals such as possums, red deer and goats have altered the structure and
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composition of the native forests of New Zealand (Nugent et al., 2001). Little is known
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about the effects that mammals have on invertebrates though the few studies that have been
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conducted indicate that mammal presence has no direct association with invertebrate
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activity and abundance (Watts, 2004; Hunt et al., 1998).
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The reasons for these effects on the native species vary depending on the organisms
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involved. In many cases it is predation by the mammals that is having the detrimental effect.
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The pacific rat (Rattus exulans), that probably accompanied the first travellers to New
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Zealand, spread quickly from sea level to the sub-alpine zone. Native invertebrates, frogs,
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skinks, geckos, tuatara and smaller sea birds and forest birds would have been naïve to new
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introduced ground dwelling mammalian predators (Holdaway, 1989). The three mustelids,
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stoats (Mustela erminea), ferrets (M. furo) and weasels (M. nivalis) were introduced to
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control rabbits in the 1880’s (Atkinson, 2001). Stoats have been shown to have a negative
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impact on the kaka by predation (Moorhouse et al., 2003; Wilson et al., 1998). Ferrets and
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dogs (Canis familiaris) are the main predators of the adult kiwi (Apteryx spp.), stoats and
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cats of young kiwi and possums (Trichsurus vulpecula) and ferrets are the main egg
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predators (Mclennan et al., 1996). The long-tailed bat has also suffered at the hands of
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introduced mammals in the form of predation by feral cats (Felis catus) (Daniel and
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Williams, 1984), and competition from ship rats (Rattus rattus) and even introduced
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starlings (Sturnus vulgaris) (Sedgeley and O’Donnell, 1999). The situation maybe
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exacerbated when weed invasion is increased by introduced small mammals like mice (Mus
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musculus), ship rats and possums (Williams et al., 2000).
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While alien mammals are widely recognised as inimical to native species, the effects of
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invasive plant species are less well known. Almost half of all the vascular plants growing in
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New Zealand are introduced. 2,068 out of the 19,000 species introduced are now considered
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naturalised and the DOC recognises 240 species of invasive weeds (Owen, 1998). Old mans
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beard (Clematis vitalba), wild ginger (Asarum canadense) and pampus grass (Cortaderia
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selloana) are just a few of the invasive species threatening New Zealand’s native species.
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The survival of 61 native vascular plant species is threatened (Owen, 1998). Negative
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effects can be seen in the seedling species richness and abundance in podocarp/broad leaved
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forest remnants in the presence of the invasive weed Tradescantia fluminensis (Standish et
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al., 2001). Invasive weeds like T. fluminensis can also have impacts on invertebrates. It is
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known that epigaeic invertebrates suffer reduced abundance in the presence of T.
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fluminensis (Standish, 2004). However, the effects of invasive weeds may not always be
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negative, as is shown by the wide range of impacts that Senecio jacobaea on pasture
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ecosystems (Wardle et al., 1995). Nevertheless, with predictions that invasive weeds will
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threaten areas covering more than 580,000 ha over the next 10-15 years (Owen, 1998), it
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seems that failure to manage this problem could lead to the further loss of native species.
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Introduced invertebrates in New Zealand are also of concern to the survival of native
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wildlife. The common wasp (Vespula vulgaris (L.)), is a common predator of Diptera,
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Lepidoptera and Araneida (Harris, 1991). Orb-web spiders (Eriophora Pustulosa) are
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known to suffer from predation by the common wasp and it has been shown that poisoning
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of wasps can increase the survival of orb-web spiders, although wasp abundance would
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need to be reduced by up to 90% in order for the spider population to survive (Toft and
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Rees, 1998). The effect of the common wasp may not be confined to direct predation of the
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aforementioned invertebrate orders. Many native birds in New Zealand rely on such
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invertebrate resources for food. The estimated biomass intake of the common wasp on the
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South Island of New Zealand is thought to be similar to that of the entire insectivorous
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avifauna (Harris, 1991). Hence, the common wasp may be acting as a competitor with
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native insectivorous birds for invertebrate food resources as much as introduced mammals
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do.
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Introduced birds are well established in New Zealand, especially in modified landscapes,
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but are also commonly found within large tracts of native forests. Censuses have shown that
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five introduced European passerines, chaffinch (Fringilla coelebs), blackbird (Turdus
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merula), song thrush (Turdus philomelos), dunnock (Prunella modularis) and red poll
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(Acanthis flammea), represented 18% of all bird individuals, where as all the native forest
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passerines represented only 64% of all bird individuals. It has been suggested that the ability
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of the introduced birds to colonise, is increased in heavily browsed forests with
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impoverished native bird communities (Diamond and Veitch, 1981).
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1.3. Methods of pest control
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In New Zealand there have been two routes taken to conserve the native wildlife. The
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first is restoration of populations and communities of native species on offshore islands, and
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the second is restoration of sites on the mainland, often referred to as Mainland Islands.
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Approximately 150 of New Zealand’s offshore islands above 5 ha in size have been
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colonised by introduced mammals. However, since 1920, 53 of these islands have had one
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or more mammal species removed and 36 are now completely free of mammals. Out of the
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16 islands that are greater than 50km away from the mainland, 8 have been cleared of at
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least one or more mammal species with 5 now completely free (Atkinson, 2001). Successful
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campaigns include the removal of feral cats from Stephens Island (Nogales et al., 2004) and
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the removal of goats (Capra hircus), cats and brushtail possums from Kapiti (Atkinson,
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2001).
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There are currently 46 projects (Fig. 1) on the North and South islands of New Zealand
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as well as Stewart Island, that are making a serious attempt at controlling introduced
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mammals as well as other pest animal and plant species (White, 2007). Six of these are
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funded by the DOC (Saunders and Norton, 2001). These Mainland Island projects can be
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fenced or unfenced and in most cases pest control is achieved through intensive poisoning
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and trapping regimes. Several different poisons are used in New Zealand which include
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1080, brodifacoum and feracol. 1080 (Sodium Monofluroacetate) is of particular use in New
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Zealand as New Zealand has no other native large land mammals that could be put at risk
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from 1080, so aerial drops can be made without adversely affecting any native species
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(Green, 2004). Despite difficulties in maintaining such areas, significant successes have
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been recorded. Possum and rat population densities have been reduced and maintained at
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low levels for more than 12 months at several Mainland Island sites. Feral goat populations
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at Boundary Stream have been reduced by 90%, and cattle have been excluded at Hurunui
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for the first time in 125 years (Saunders and Norton, 2001). At Mapara, control of predators
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has allowed the population of North Island Kokako (Callaeas cinerea wilsoni) to be
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successfully protected (Pryde and Cocklin, 1998). At Rotoiti, stoat control has resulted in
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successful fledging of kaka nestlings at sites previously not viable (Wilson et al., 1998;
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Paton et al., 2004). Additionally, at Trounson there has been a dramatic increase in the
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numbers of native pigeon (Hemiphaga novaeseelandiae) (Saunders and Norton, 2001).
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It appears that the DOC and many private enterprises have had considerable success in
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certain areas of New Zealand in restoring native wildlife (Atkinson, 2001, Wilson et al.,
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1998; Nogales et al., 2004; Saunders and Norton, 2001; Paton et al., 2004), and it seems that
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in most of these cases removing predation by the alien species is the main benefit to the
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native species. Some work has focused on the impact that mammal removal has on
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invertebrate abundance and diversity (Watts, 2004; Hunt et al., 1998); however, a little
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researched aspect of the management strategies mentioned above, is the role of increased
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invertebrate food supplies as opposed to reduced predation on the recovery of native fauna.
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There is considerable evidence that many mammals’ diets are rich in invertebrates in New
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Zealand. In one study at Boundary Stream reserve on the North Island the larvae of the
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Tortricid moth were found in 31% of all guts sampled (Jones and Toft, 2006). Other
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invertebrates commonly detected in the guts of mice include beetles, weta, spiders and other
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Lepidoptera larvae (Fitzgerald, 1996). Other mammals like hedgehogs (Berry, 1999) also
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have very invertebrate rich diets. So it is expected that the presence of mammals has an
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impact on the local invertebrate population. This paper aims to ascertain the extent to which
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invertebrate food resources respond to mammal exclusion and what impact this has on
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native fauna. The three hypotheses that will be tested are as follows. 1. There will be no
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difference in invertebrate biomass between mammal-absent and mammal-present sites. This
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might occur if neither mammal predation nor native vertebrates affect invertebrate
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abundances. Equally, any effect of mammal removal might be fully compensated by the
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recovery of bird populations. 2. There will be a higher invertebrate biomass in the mammal-
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free sites. This situation would arise if invertebrate populations are otherwise depressed by
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mammal predation. 3. There will be a higher invertebrate biomass in the mammal present
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sites. If avian impacts on invertebrates are paramount, the suppression of bird populations
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by predation may benefit invertebrates. These hypotheses will be discussed further below.
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2. Methods
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2.1. Site locations
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The study was carried out at a total of six sites on the North Island of New Zealand (Fig.
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1) from March 16th to May 25th. There were three fenced sites - Karori Wildlife Sanctuary,
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Tawharanui Open Sanctuary and Bushy Park and three non fenced sites - Boundary Stream
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Reserve, Mapara Reserve and Mount Bruce National Wildlife Centre. At each site there was
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a treatment area which was within the reserves and received some form of intensive
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mammal control, and a control site, matched with the treatment site for vegetation type,
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which was outside the reserves and had received no intensive mammal control. The control
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areas for each of the sites were as follows; for Karori the control area was Birdwood
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Reserve, at Boundary Stream it was Bellbird Bush, at Tawharanui it was Hubbard’s Bush.
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At Mapara it was Aratoro Reserve, at Bushy Park it was the area of bush just outside the
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fence and for Mount Bruce the control area was the W.A. Miller Reserve.
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2.2. Invertebrate sampling
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Invertebrate sampling methods involved sweep netting, beating trays, portable light
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traps, malaise traps and pitfall traps. Sweep netting was carried out over three days in each
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of the treatment and control sites. Fifty sweeps a day were carried out over five patches, ten
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sweeps over each patch. Sweep net sampling was carried out only on dry days and on dry
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vegetation with stature. Invertebrates were extracted with a pooter and identified. Sampling
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patches were chosen by walking along the path into the reserve, stopping every 50m and
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walking 10m into the bush away from the path, to ensure that the effects of disturbance
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from the path was minimised.
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The beating trays were also used over three days in each of the treatment and control
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sites. Fifty bushes/trees were sampled on each day and the beating tray was held under the
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tree while it was shaken for 30 seconds. Once again this method was only used on dry days
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and on dry vegetation. There were 5 sampling patches, each with ten bushes/trees. Sampling
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patches were chosen using the same methods as the sweep netting.
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A portable light trap was deployed from dusk till dawn over two nights in each of the
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treatment and control sites. The light trap was set out at 200m and 400m intervals along the
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path into the reserve and 10m in from the path to avoid the effects of disturbance from the
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path.
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A single malaise trap was set out for 24 hours on three days in each of the treatment and
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control sites. It was moved to a new position each day at 300m, 500m and 700m intervals
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along the path and 10m in from the path.
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Ten pitfall traps set out for 24 hours on three days in each of the treatment and control
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sites. Each trap lay on a transect 5m apart, which began 10m into the bush from the edge of
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the path. On the first day the first trap was set 50m in along the path into the reserve and the
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transect line moved along another 50m on each following day. A small plastic cup with a
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diameter of 8cm served as the trap, with care taken to ensure that the soil was level with the
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rim of the cup. A plastic plate with two nails in it was propped over the trap to ensure rain
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and intruding mammals were kept out while still allowing invertebrates to be trapped.
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The portable light trap, malaise trap and pitfall traps were used regardless of weather
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conditions. The regime used in order to ensure that all sampling was carried out as
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efficiently as possible is outlined in Table 1. This regime ensured enough time to travel
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between and move equipment from the control to treatment, and gave enough time to move
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the static sampling methods (malaise trap, pitfall traps and portable light trap) to a new
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position each day. Where rain postponed sweep net and beating tray sampling, they were
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carried out on the first subsequent dry day.
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Invertebrate specimens were identified to family level and where possible to genus level
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using Crowe (1999) and Grant (1999). Invertebrate biomass (dry weight (mg)) was worked
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out using methods described in Collins (1992) for Gastropoda, Sage (1982) for Orthoptera
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and Araneida and for the remaining invertebrates Sample et. al. (1993) was used. Where
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biomass models were not available for certain invertebrates, the general insect model used
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in Sample et. al. (1993) was applied.
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2.3. Bird sampling
Five minute bird counts were carried out on two suitable mornings with settled and still
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weather in each of the treatment and control areas. There were five stops made at 200m
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intervals along the path through the study site. At each stop bird calls were identified by
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sound and recorded if within a 200m radius. No bird was knowingly identified more than
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once. The target species that were identified were the insectivorous Grey Warbler, Fantail,
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Tomtit and Robin. Presence of other native birds such as the nectivorous Silvereye and
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Bellbird and the herbivorous Tui, Kaka and Kereru were also recorded. This would enable
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comparisons to be made between the insectivorous and non-insectivorous birds which
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depending on the results of the invertebrate biomass, may indicate if it is reduced predation
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or increased invertebrate food resources that are allowing bird populations to recover.
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2.4. Statistical analyses
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There were four levels of analysis carried out on the invertebrate data. Firstly, a linear
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mixed effects model was used to analyse the effect that treatment had on the invertebrate
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biomass and invertebrate abundance categorised by sampling method. Site was added as the
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random effect allowing correction for between site variations. Secondly, a generalized linear
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model was used to analyse the effect that treatment had on invertebrate biomass and
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abundance for each sampling method and each site in order to find any significant
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differences within each site. The third analysis was a linear mixed effects model looking at
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the effect of fenced exclusion as opposed to intensive poisoning and trapping, on the
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invertebrate biomass and abundance in the areas receiving treatment. Once again site was
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added as the random effect to allow for between site variations. All three models were run
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with Poisson response distributions and identity as the link function to allow for the non-
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normal distribution of the data. Date, temperature, altitude, latitude, and other weather
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factors such as presence of rain, Beaufort scale and Oktas were introduced in to the models
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but removed if no significant effect (P>0.05) was found. The final analysis was to look at
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species diversity indices of invertebrates at the control and treatment sites and run T-tests of
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the results from the treatment and control sites. The bird data served to indicate what impact
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the invertebrate biomass had on the native birds so Chi-squared tests were carried out on the
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abundance of insectivorous, non-insectivorous and total birds.
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3. Results
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3.1. Invertebrate data
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A total of 4613 invertebrates were collected and classified into 69 separate invertebrate
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ID groups at family and genus level. Of the 4613 individuals, 2235 were collected from the
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control sites and 2378 were collected from the treatment sites. Invertebrate biomass totalled
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90.2 g from the control sites and 106.3 g from the treatment sites (Fig.2, Table 2). Notable
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invertebrate families included the Lepidoptera families, Noctuidae, Geometridae and
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Tortricidae which were high in abundance in the portable light traps in both the control and
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treatment sites. There were 203 geometridae caught from the control sites and 337 from the
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treatment sites. Due to the large size of these invertebrate families they constituted a large
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portion of the overall invertebrate biomass. Another family that was high in abundance was
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the black fly (Simulidae) which is generally common on the North Island of New Zealand.
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There were 228 caught in the control sites and 184 caught in the treatment sites, however,
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due to the small size of the black fly the contribution to the overall invertebrate resource
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(65.659 mg for control and 52.988 mg for treatment) was not as much as with the
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Lepidoptera families such as geometridae (15212.643 mg for control and 25254.486 mg for
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treatment). Other significant invertebrate ID groups contributing to the overall invertebrate
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resource include therididae, landhoppers and geometridae larvae. An anomaly that should
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be pointed out is the high abundance of termites that were found in the control site at
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Boundary Stream, 340 compared to 1 found at the treatment site and none at any other site.
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This was due to the heavy rain that was experienced during the sampling period at the
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control site. Accordingly, termites were treated as an outlier and excluded from subsequent
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analysis.
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When comparing the invertebrate abundances at the control and treatment sites from
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each sampling method across all six sites, invertebrate abundance was higher in the
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treatment sites than the control for four out of the five methods (Fig. 3). However, there was
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weak statistical support for these differences for all methods but the portable light trap
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(beating tray, N=364 P=0.052; sweep net, N=262 P>0.1; pitfall trap, N=133 P>0.1) where
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N=sample size. The difference in the abundance seen in the portable light trap was
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significant (light trap, N=157 P<0.01). The malaise trap was the only method that had a
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higher invertebrate abundance in the control than the treatment sites, but once again the
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statistical support for this difference was weak (malaise trap, N=364 P>0.1).
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Invertebrate biomass was significantly higher in the control than the treatment sites for
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four of the sampling methods (beating tray, N=364 P<0.01; sweep net, N=262 P<0.01;
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malaise trap, N=158 P<0.01; pitfall trap, N=133 P<0.01) (Fig. 4). However, the invertebrate
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biomass was significantly higher in the treatment sites for the portable light trap (light trap,
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N=195 P<0.01).
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There was considerable variation in the differences of invertebrate biomass and
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abundance from the control and treatment areas from site to site (Tables 3 and 4). Treatment
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abundance was significantly higher than control abundance with the portable light trap at
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Karori, Boundary Stream and Mount Bruce, with the pitfall traps at Tawharanui and with
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the malaise trap at Karori (P<0.01). Where control abundance was higher than treatment
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abundance, significant results were found with the beating tray at Karori, the portable light
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trap at Boundary Stream, the pitfall trap at Mapara and the malaise trap at Boundary Stream
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(P<0.01) and with the beating tray at Mapara and the malaise trap at Tawharanui (P<0.05).
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All the differences seen in the invertebrate biomass data were significant (Table 5), with the
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exception of one (i.e. malaise trap at Mapara). From the beating tray data the control
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biomass was significantly higher than the treatment biomass at Karori and Mapara. At the
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other four sites the treatment biomass was significantly higher than the control biomass.
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When the sweep net data were analysed, treatment biomass was higher than control biomass
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at Boundary Stream and Tawharanui and at Karori, Mapara, Bushy Park and Mount Bruce
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control biomass was higher. The portable light trap was the only method with which all sites
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had significantly higher biomass in the treatment than the control areas. With the malaise
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trap the only site that had a significantly higher invertebrate biomass in the treatment site
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was Bushy Park, Karori, Boundary Stream, Tawharanui and Mount Bruce all had
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significantly higher invertebrate abundance in the control sites. Finally, from the pitfall trap
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data, Karori and Mount Bruce were the only sites where treatment biomass was significantly
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higher, at the other four sites control biomass was significantly higher.
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Evidence that there was no difference in invertebrate abundance at treatment areas
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between fenced and unfenced (Fig. 5) was supported by statistical results. When all the
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methods were analysed no significant differences were seen (P>0.1), however pitfall traps
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were close to being significant (P=0.057), for invertebrate biomass no significant difference
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was seen with any of the methods (P>0.1).
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Diversity indices for each site indicate that there is not a large difference between the
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control and the treatment sites and the T-test results (P>0.1) support this, with none of the
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species diversity index values being significantly different between the control and
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treatment sites.
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3.2. Bird data
A total of 190 individual birds of 11 different species were recorded from five minute
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bird counts during the project. There were 69 individuals from the control sites and 121
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individuals from the treatment sites (Table 6). The three most common species detected by
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sight or sound were the Tui, Fantail and Grey Warbler which were abundant in both the
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treatment and control sites (Fig. 6). When the bird data were split up into insectivorous and
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non-insectivorous bird species (Fig. 7) it was still clear that there were more individuals of
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both sub-groups detected in the treatment as opposed to the control sites (Table 6). With the
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insectivorous birds there were 33 recorded in the control and 54 observed in the treatment.
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The non-insectivorous birds show a similar pattern, with 39 being from the control sites and
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67 being from the treatment sites. However, Chi-squared tests reveal that the differences
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seen in the total bird abundance, and with both the insectivorous and non-insectivorous bird
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abundance, are not significant (P>0.1).
400
401
402
403
404
405
406
407
408
409
19
410
411
4. Discussion
So conclusions that can be drawn from these results are that control sites have a
412
significantly higher invertebrate biomass than the treatment sites when using the beating
413
tray, sweep net, malaise trap and pitfall traps. Conversely, invertebrate biomass is
414
significantly higher in the treatment sites both collectively and individually when sampling
415
using the portable light trap. Indeed, the biomass from the light trap (68.5 g from treatment
416
sites and 32.9 g from control sites) is so large, that with out it, the overall total invertebrate
417
biomass would be much higher in the control than in the treatment sites (37.8 g in treatment
418
sites and 57.3 g in control sites).
419
420
421
Why was total invertebrate biomass exclusive of the light trap data much higher in the
control sites?
There are many possible reasons for this higher biomass of ground and vegetation
422
dwelling invertebrates in the control sites. Invertebrate abundance and therefore biomass, is
423
something that is accounted for by many different factors, not just the presence or absence
424
of alien mammals. As mentioned earlier the weed Tradescantia fluminensis can have a
425
negative effect on epigaeic invertebrates (Standish et al., 2001; Standish, 2004). The
426
presence of such weeds in the areas sampled may alter the number of invertebrates caught.
427
Although, given that invertebrate diversity has been shown to be positively correlated with
428
plant diversity (Crisp et al., 1998), it would be expected that the treatment sites, recovering
429
from browsing by mammals, would contain a higher plant diversity and abundance. Many
430
of the protected reserves are weeded native plant species are re-established. This increased
431
plant diversity would be expected to increase invertebrate diversity and abundance.
20
432
However, this does not explain the lower invertebrate biomass in the treatment sites seen
433
during this study.
434
Another possible reason for the higher invertebrate biomass at the control sites excluding
435
the portable light trap is that although every effort is made to eradicate mammals in these
436
protected areas, it is very difficult to ensure complete mammal eradication. A lot of the
437
reserves still have a problem with mice and other small rodents (Ward-Smith et al., 2005;
438
White, 2007), which are very hard to completely eradicate. At reserves such as Boundary
439
Stream small mammals such as mice and hedgehogs are not directly targeted due to them
440
being of secondary conservation importance in the presence of larger mammals like feral
441
cats, mustelids, rats and possums (Jones and Toft, 2006). This kind of management strategy
442
could be a factor contributing to the low invertebrate biomass in the treatment sites, with the
443
main predators of invertebrates being relatively abundant due to reduced predation from
444
larger mammals. Small rodents are natural prey of mustelids (Martinoli et al., 2001) and
445
cats, so they will be naturally kept in check by the presence of these larger mammals.
446
Without this natural predation, poisoning and trapping of small rodents may not be enough
447
to keep populations at a level low enough to keep them from having an adverse impact on
448
invertebrate biomass.
449
At Karori Wildlife Sanctuary, a study on ground dwelling beetles before and after
450
mammal eradication revealed no change in beetle abundance or species number (Watts,
451
2004). In another study by Hunt et al., (1998), there was no clear relationship between rat
452
numbers and invertebrate abundance at Karioi Rahui, Ohakune. So it is not a novel finding
453
that there was no significant difference observed in four out of five methods in invertebrate
454
abundance and diversity at the sites studied during this research. A reason suggested by
21
455
Watts, 2004 for the lack of significant differences seen in invertebrate abundance and
456
activity is that although mammal numbers have been greatly reduced in these reserves,
457
many of them have received translocations of insectivorous birds such as Weka, North
458
Island Robin and Kiwi. Perhaps the impact of these birds is simply replacing the impact the
459
alien mammals once had. Indeed, the effect of the new bird residents may be going beyond
460
the effect mammals once had, which may explain the increased invertebrate biomass at the
461
control sites.
462
Although there was higher abundance of both insectivorous and non-insectivorous birds
463
at the treatment sites, these differences were not significant. This may be due to the
464
generally higher invertebrate biomass (excluding portable light trap) at the control sites, or it
465
may just be due to overspill of birds from the reserves to areas of bush in close proximity to
466
the reserves. The reserves may act as a sanctuary for the birds to nest in, allowing them to
467
forage outside of the reserve.
468
In addition to the impact that mammal removal may have on invertebrate abundance and
469
biomass, the impact that the fence, as opposed to intensive poisoning and trapping regimes,
470
may have on invertebrate abundance and biomass inside the reserves was analysed.
471
Interestingly, there was no difference observed between the fenced and unfenced sites. This
472
may be due to the fact that this study did not have the power to detect and effect. However,
473
unfenced reserves can be just as successful at restoring native wildlife as fenced ones are.
474
At Ark in the Park, in the Waitakere ranges, possum, rat, stoat and feral cat populations
475
have been reduced sufficiently enough for whiteheads, North Island robins and even 60 hihi
476
to be released (White, 2007).
22
477
As more and more of these Mainland Island project arise, so does the opportunity for
478
research. Invertebrates are understudied in New Zealand and there can be much more done
479
in order to understand the distribution and diversity of them (McGuiness, 2001). Future
480
work should focus on continuous studies of sites before and after mammal control has
481
started. This way will allow a better analysis of how invertebrates are affected by mammal
482
exclusion.
483
In conclusion, based on three studies to date it is apparent that the differences in
484
invertebrate biomass and abundance between treatment and control sites are absent or
485
modest. The hypothesis that the removal of alien mammals will release invertebrates from
486
predator control is incorrect. More likely, their control is taken over by other organisms
487
which could be birds, other invertebrates or a mixture of both.
488
489
490
491
492
493
494
495
496
497
498
499
23
500
Acknowledgements
501
This research was carried out as part of a master’s programme in conservation and
502
biodiversity at the University of Exeter, Cornwall campus. Thanks to David Bryant and
503
Murray Williams for project coordination. Thanks to Raewyn Empson, Denise Fastier, Phil
504
Bradfield, Phil Brady, Kate O’Neill, Allan Anderson, Terry O’Conner and Matt Maitland
505
who all helped to provide access to the study sites.
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
24
523
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639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
30
657
Tables
658
659
Table 1
Sampling regime carried out at each site a
Day
Control
One
BT
SN
BC
Two
BT
SN
Three
BT
SN
Four
MT
PT
PLT
Five
MT
PT
PLT
Six
MT
PT
MT
PT
PLT
MT
PT
PLT
MT
PT
BT
SN
BC
BT
SN
BT
SN
Sampling
Methods
Treatment
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
a
BT= Beating Tray, SN= Sweep Net, BC= Bird Count, MT= Malaise Trap, PT= Pitfall
Trap, PLT= Portable Light Trap.
31
679
680
Table 2
Invertebrates collected from all sampling methods at all sites from March 16th to May 25th 2007 a
Invertebrate Order (Sub-Order)
Lepidoptera
Lepidoptera
Lepidoptera
Lepidoptera
Lepidoptera
Lepidoptera
Lepidoptera
Lepidoptera
Lepidoptera
Orthoptera
Orthoptera
Orthoptera
Diptera (Brachycera)
Diptera (Brachycera)
Diptera (Brachycera)
Diptera (Brachycera)
Diptera (Brachycera)
Diptera (Nematocera)
Diptera (Nematocera)
Diptera (Nematocera)
Diptera (Nematocera)
Diptera (Nematocera)
Diptera (Nematocera)
Acarina
Hemiptera
Hemiptera
Hemiptera
Hemiptera
Invertebrate ID
Aegeriidae
Geometridae
Geometridae Larvae
Noctuidae
Noctuidae Larvae
Pterophoridae
Pyralidae
Saturnidae
Tortricidae
Anistostomatidae
Gryllidae
Tettigoniidae
Calliphoridae
Drosopholidae
Muscidae
Stratiomyidae
Syrphidae
Anisopodidae
Chironomidae
Culicidae
Mycetophilidae
Simulidae
Tipulidae
Mite
Aphididae
Aradidae
Cercopidae
Cicadidae
Length
(mm)
15
20
15
20
35
6
10
40
20
25
22
20
15
2
10
12
10
5
9
5
10
3
10
1
2.5
10
10
30
Individual Dry
Weight (mg)
30.525
74.939
8.200
74.939
100.612
1.747
8.608
652.416
74.939
3040.820
1315.908
752.862
11.822
0.153
4.928
7.304
4.928
0.891
3.272
0.891
4.130
0.288
4.130
0.027
0.140
9.939
9.939
291.396
Control
Abundance
2
203
180
71
4
0
53
0
48
9
3
1
3
89
6
12
1
18
41
34
46
228
52
79
1
1
3
11
Control Dry
Weight (mg)
61.049
15212.643
1475.980
5320.678
402.449
0.000
456.214
0.000
3597.078
27367.382
3947.725
752.862
35.466
13.605
29.569
87.649
4.928
16.046
134.133
30.309
189.988
65.659
214.769
2.099
0.140
9.939
29.816
3205.352
Treatment
Abundance
7
337
70
161
1
1
57
1
116
4
2
2
0
120
12
27
2
22
78
40
35
184
65
50
1
0
0
54
Treatment Dry
Weight (mg)
213.672
25254.486
573.992
12065.200
100.612
1.747
490.646
652.416
8692.939
12163.281
2631.817
1505.725
0.000
18.344
59.139
197.209
9.856
19.612
255.180
35.657
144.556
52.988
268.461
1.328
0.140
0.000
0.000
15735.365
Continued on next page
32
Table 2 (continued)
Invertebrate Order (Sub-Order)
Hemiptera
Hemiptera
Hemiptera
Hemiptera
Hemiptera
Hemiptera
Coleoptera
Coleoptera
Coleoptera
Coleoptera
Coleoptera
Coleoptera
Coleoptera
Coleoptera
Coleoptera
Coleoptera
Blattodea
Hymenoptera
Hymenoptera
Hymenoptera
Hymenoptera
Trichoptera
Araneida
Araneida
Araneida
Araneida
Araneida
Araneida
Araneida
Pseudoscorpiones
Invertebrate ID
Flatidae
Lygaeidae
Nabidae
Pentatomidae
Reduviidae
Ricaniidae
Carabidae
Cerambycidae
Chrysomelidae
Coccinllidae
Curculionidae
Elateridae
Lucanidae
Scarabaeidae
Staphylinidae
Tenebrionidae
Blatidae
Bracconidae
Formicidae
Ichneumonidae
Vespidae
Conoesucidae
Corinnidae
Green spider
Lycosidae
Miturgidae
Salticidae
Theridiidae
Thomisidae
False Scorpion
Length
(mm)
8
18
15
8
10
10
25
8
12
4
10
10
30
8
10
5
13
3
4
18
12
15
10
10
8
15
10
5
5
5
Individual Dry
Weight (mg)
5.004
60.576
34.579
5.004
9.939
9.939
115.289
6.739
18.511
1.198
11.752
11.752
181.596
6.739
11.752
2.089
15.942
0.267
0.579
33.396
11.193
37.838
146.759
146.759
58.581
1457.882
146.759
14.774
14.774
0.015
Control
Abundance
4
1
29
4
4
1
8
13
1
4
19
2
0
28
7
10
2
11
26
36
1
2
20
11
65
0
14
148
37
0
Control Dry
Weight (mg)
20.017
60.576
1002.799
20.017
39.755
9.939
922.309
87.612
18.511
4.792
223.292
23.504
0.000
188.703
82.266
20.891
31.885
2.932
15.053
1202.263
11.193
75.675
2935.175
1614.346
3807.763
0.000
2054.622
2186.489
546.622
0.000
Treatment
Abundance
1
1
33
11
0
0
17
8
0
33
22
9
1
13
10
12
7
9
34
97
1
5
27
2
32
1
10
220
52
1
Treatment Dry
Weight (mg)
5.004
60.576
1141.116
55.046
0.000
0.000
1959.906
53.915
0.000
39.534
258.549
105.770
181.596
87.612
117.522
25.069
111.597
2.399
19.684
3239.432
11.193
189.188
3962.486
293.517
1874.591
1457.882
1467.587
3250.186
768.226
0.015
Continued on next page
33
Table 2 (continued)
Invertebrate Order (Sub-Order)
Chilopoda
Gastropoda
Neuroptera
Amphipoda
Archaeognatha
Isopoda
Diplopoda
Diplopoda
Isoptera
Thysanoptera
Collembola
Invertebrate ID
Garden centipede
Small Native Land Snail
Hemerobiidae
Land hopper
Meinertellidae
Oniscidae
Sphaerotheridae
Spirobollelidae
Termite
Thripidae
Tomoceridae
Total
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
a
Length
(mm)
35
4
3
6
12
10
50
50
13
1
5
Individual Dry
Weight (mg)
188.489
5.487
0.190
2.318
13.057
8.287
458.787
458.787
15.942
0.027
1.471
Control
Abundance
0
4
42
129
1
1
0
10
340
1
0
Control Dry
Weight (mg)
0.000
21.947
7.989
299.005
13.057
8.287
0.000
4587.867
5420.424
0.027
0.000
Treatment
Abundance
1
12
62
163
10
2
8
0
1
0
1
Treatment Dry
Weight (mg)
188.489
65.842
11.794
377.813
130.574
16.573
3670.294
0.000
15.942
0.000
1.471
2235
90231.134
2378
106362.358
Invertebrate identified to family level and where possible to genus level, length was measured according to the protocols
set out in Collins (1992), Sage (1982) and Sample et. al. (1993).
34
698
699
Table 3
Mean (SE±) invertebrate abundance for each method at the control and treatment areas of each site a
Site
Karori
Control
6.9±4.0
Boundary Stream
Reserve
3.9±1.5
Tawharanui
Mapara
Bushy Park
Mount Bruce
2.2±0.7
5.2±1.9
3.5±1.3
3.0±1.0
Treatment
5.1±1.5
6.1±1.9
2.3±0.7
4.7±1.3
4.3±1.2
5.3±1.9
Control
4.0±1.9
3.3±1.1
1.9±0.7
3.1±1.4
2.4±0.9
3.8±1.8
Treatment
4.9±2.4
4.2±1.4
1.9±0.4
2.0±0.7
2.4±0.8
4.7±2.2
Control
4.6±2.0
19.9±16.0
4.2±1.6
3.2±1.8
11.6±4.3
8.5±3.6
Treatment
13.3±4.6
8.7±4.1
4.2±1.6
7.1±3.1
14.3±7.0
17.3±7.7
Control
2.2±0.5
7.4±3.5
3.8±2.4
3.6±1.3
1.2±0.4
2.5±0.8
Treatment
3.1±0.8
4.0±1.7
0.6±0.2
2.5±0.9
2.6±1.2
1.8±1.0
Control
1.5±1.1
3.9±1.8
4.2±2.7
7.2±5.3
3.8±1.9
3.2±2.0
Treatment
4.9±3.1
4.6±2.5
7.2±4.8
2.5±1.1
5.5±3.4
5.4±2.2
BT
SN
Method
PLT
MT
PT
700
701
702
703
704
705
a
BT= Beating Tray, SN= Sweep Net, MT= Malaise Trap, PT= Pitfall Trap, PLT= Portable Light Trap.
35
706
707
Table 4
Mean (SE±) invertebrate dry weight (mg) for each method at the control and treatment areas of each site a
Site
Karori
Control
137.651±62.201
Boundary Stream
Reserve
65.115±40.553
Treatment
92.606±54.780
156.986±77.707
Control
70.197±39.142
Treatment
28.922±14.027
Tawharanui
56.698±23.487
Mapara
Bushy Park
Mount Bruce
634.444±527.115 179.005±126.091
37.117±21.346
282.041±252.474
152.827±66.849
179.393±125.734
65.042±33.458
28.207±11.434
8.250±4.011
341.166±319.023
94.918±80.003
24.450±14.743
114.232±67.341
13.241±6.444
23.179±9.879
30.745±12.580
20.345±16.839
BT
SN
Control
254.959±158.844
492.235±273.057 245.977±124.420 202.894±144.732 570.695±333.057 472.087±273.083
Treatment
1464.877±977.399 608.115±311.496 473.496±250.997 386.212±243.656 822.946±543.052 935.571±554.307
Method PLT
Control
37.137±17.472
19.202±12.715
53.011±46.709
40.055±33.656
10.040±7.452
401.393±377.498
Treatment
21.908±7.922
10.700±5.396
6.636±4.675
32.402±23.040
36.110±16.019
27.926±19.372
Control
19.616±11.093
430.106±248.153
57.742±21.166
282.980±135.244
345.134±207.180
56.654±42.732
MT
543.866±384.281 488.300±455.733
63.818±57.636
PT
Treatment
708
709
710
711
712
713
714
715
a
71.153±42.207
BT= Beating Tray, SN= Sweep Net, MT= Malaise Trap, PT= Pitfall Trap, PLT= Portable Light Trap.
164.665±91.909
109.690±14.608
36
716
717
718
Table 5
Probability values for differences in biomass (dry weight (mg)) between control and
treatment areas at each site using each method a
Site
Method
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
a
Karori
Boundary Stream
Tawharanui
Mapara
Bushy Park
Mount Bruce
BT
P<0.01*
P<0.01**
P<0.01**
P<0.01*
P<0.01**
P<0.01**
SN
P<0.01*
P<0.01**
P<0.05**
P<0.01*
P<0.01*
P<0.01*
PLT
P<0.01**
P<0.01**
P<0.01**
P<0.01**
P<0.01**
P<0.01**
MT
P<0.01*
P<0.01*
P<0.01*
P>0.1
P<0.01**
P<0.01*
PT
P<0.01**
P<0.01*
P<0.01*
P<0.01*
P<0.01*
P<0.01**
BT= Beating Tray, SN= Sweep Net, MT= Malaise Trap, PT= Pitfall Trap,
PLT= Portable Light Trap.
* Significantly higher control biomass
** Significantly higher treatment biomass
37
752
753
754
Table 6
Abundance of insectivorous and non-insectivorous birds at the
control and treatment areas of each site
Control
17
13
0
0
3
33
Treatment
18
20
9
3
4
54
Sub-Total
6
1
0
9
1
19
39
6
2
8
16
0
35
67
Total
69
121
Insectivorous
Bird Species
Fantail
Grey Warbler
Robin
Saddleback
Tomtit
Sub-Total
Non-Insectivorous
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
Bellbird
Kokako
Kaka
Kereru
Silvereye
Tui
38
770
Figures
771
772
773
774
Fig. 1. A map of New Zealand showing 46 projects currently controlling for alien
mammals. Underlined sites indicate study sites (White, 2007).
39
775
776
777
2500
a)
Mount Bruce
Bushy Park
Invertebrate Abundance
2000
Mapara
Tawharanui
1500
Boundary Stream
Karori
1000
500
0
Control
Treatment
778
Invertebrate Dry Weight (mg)
120000
b)
100000
80000
60000
40000
20000
0
Control
779
780
781
782
783
784
785
786
787
788
789
790
Treatment
Fig. 2. Total abundance (mg) (a) and dry weight (mg) (b) of invertebrates
caught from all sampling methods across all treatment and control sites
from March 16th to May 25th 2007.
40
791
792
793
794
400
700
a)
Mount Bruce
Bushy Park
Mapara
Tawharanui
Boundary Stream
Karori
600
Invertebrate abundance
350
Invertebrate abundance
b)
300
250
200
150
100
500
400
300
200
100
50
0
0
Control
Treatment
Control
Treatment
Control
Treatment
795
c)
900
300
800
250
Invertebrate abundance
Invertebrate abundance
1000
700
600
500
400
300
200
d)
200
150
100
50
100
0
0
Control
Treatment
Control
Treatment
796
797
300
e)
Invertebrate abundance
250
200
150
100
50
0
798
799
800
801
802
803
804
805
Fig. 3. Invertebrate abundance at the control and treatment sites from the sweep net (a),
beating tray (b), light trap (c), malaise trap (d) and pitfall trap (e).
41
a)
b)
30000
Invertebrate dry weight (mg)
Invertebrate dry weight (mg)
12000
10000
8000
6000
4000
2000
Mount Bruce
Bushy Park
Mapara
Tawharanui
Boundary Stream
Karori
25000
20000
15000
10000
0
5000
0
Control
Treatment
Control
Treatment
806
807
6000
c)
Invertebrate dry weight (mg)
Invertebrate dry weight (mg)
80000
70000
60000
50000
40000
30000
20000
10000
d)
5000
4000
3000
2000
1000
0
0
Control
Treatment
Control
Treatment
Control
Treatment
808
809
Invertebrate dry weight (mg)
16000
e)
14000
12000
10000
8000
6000
4000
2000
0
810
811
812
813
814
815
816
817
818
819
820
821
822
823
Fig. 4. Invertebrate dry weight (mg) at the control and treatment sites from the sweep net (a),
beating tray (b), light trap (c), malaise trap (d) and pitfall trap (e).
42
1400
a)
PT
Invertebrate abundance
1200
MT
PLT
1000
SN
800
BT
600
400
200
0
Fenced
Unfenced
824
825
Invertebrate dry weight (mg)
70000
b)
60000
50000
40000
30000
20000
10000
0
Fenced
826
827
828
829
830
831
832
833
834
835
836
837
838
839
Unfenced
Fig. 5. Invertebrate abundance (a) and dry weight (mg) (b)
recorded in treatment sites at fenced and unfenced reserves.
BT= Beating Tray, SN= Sweep Net, MT= Malaise Trap,
PT= Pitfall Trap, PLT= Portable Light Trap.
43
840
841
842
40
Control
35
Treatment
Bird Abundance
30
25
20
15
10
5
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
i
Tu
K
ak
a
K
er
er
u
Si
lv
er
ey
e
Fa
nt
G
re
ai
l
y
W
ar
bl
er
Ro
bi
Sa
n
dd
le
ba
ck
To
m
tit
Be
llb
ird
K
ok
ak
o
0
Fig. 6. Total bird abundance observed at the control and treatment sites using
the five minute bird count method.
44
869
870
871
60
Mount Bruce
a)
Insectivorous bird abundance
Bushy Park
50
Mapara
Tawharanui
40
Boundary Stream
Karori
30
20
10
0
Control
Treatment
Treatment
Control
Treatment
Treatment
872
80
b)
Non-insectivorous bird abundance
70
60
50
40
30
20
10
0
873
874
875
876
877
878
Fig. 7. Total number insectivorous (a) and non-insectivorous (b)
birds observed at control and treatment sites using the five
minute bird count method.