the impact of alien mammal exclusion on invertebrate food ... · 62 hunting by humans and...
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The impact of alien mammal exclusion on invertebrate food resources for 1
native birds in New Zealand 2
Paul S. EDDOWES 3
Centre for Ecology and Conservation, University of Exeter, Cornwall Campus, Penryn, 4
UK, TR10 9EZ 5
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The impact of alien mammal exclusion on invertebrate food resources for 22
native birds in New Zealand 23
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Abstract 25
Invertebrate sampling was carried out in late summer and autumn at six treatment and 26
control sites on the North Island of New Zealand to assess the impact of alien mammal 27
exclusion has on invertebrate abundance, diversity and biomass. The aim was to assess 28
whether increased food resources or reduced predation allows recovery of native bird 29
species within fenced reserves and ‘Mainland Islands’. Across all six sites invertebrate 30
abundance was only significantly higher in the areas of mammal exclusion compared to 31
control sites when sampling with the portable light trap. In contrast invertebrate biomass 32
was significantly higher in mammal-present areas when sampling with the beating tray, 33
sweep net, malaise trap and pitfall traps. If invertebrate resources throughout the year show 34
comparable patterns of abundance, then recovery of populations of avian insectivores 35
within fenced reserves seems likely to benefit more from reduced predation than greater 36
food availability. 37
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Keywords 39
New Zealand conservation, Invertebrate biomass, Mainland Island, Invertebrate sampling, 40
Introduced mammals. 41
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1. Introduction 45
1.1. Overview 46
Like many other island archipelagos, New Zealand has an evolutionary history that 47
diverged markedly from the rest of the world about 65-80 million years ago (Cooper and 48
Milliner, 1993) when it separated from the southern continent of Gondwanaland. New 49
Zealand totals 26 million ha over 3 main islands, plus another 700 smaller islands greater 50
than 5 ha. These stretch from the subtropics to the sub Antarctic (29oS to 52
oS) across two 51
tectonic plates, leading to a diverse landscape (Craig et al., 2000). 52
New Zealand’s biota evolved completely free of the influence of terrestrial mammals, 53
excluding two bat species, and over the past 10,000 years birds were the largest animals in 54
all terrestrial ecosystems. The ratites were very common, were large in size and often 55
flightless (Atkinson and Millener, 1991). The reptiles that evolved on the island include 56
tuatara, geckos and skinks but no snakes or crocodiles. 57
New Zealand was the last major land mass to be colonised by humans. The predecessors 58
of the Maoris arrived 700-1000 years ago and the Europeans around 200 years ago (Craig et 59
al., 2000). Birds and reptiles are the two groups of animals that have suffered the most from 60
this anthropogenic presence. The cause of this has been ecosystem loss and fragmentation, 61
hunting by humans and depredation by introduced alien species. Temperate rainforests have 62
been reduced from an original 78% of land area to just 23% and wetlands have been 63
reduced by over 90% of their pre-European area (Ministry for the Environment, 1997). 64
Native grasslands have decreased greatly through over-sowing with European pasture 65
grasses and poor land management (Craig et al., 2000). Maori hunting eliminated 26 species 66
(30%) of endemic land birds including many Moa species, and 4 species (18%) of sea birds. 67
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Also, tuatara, many lizards and many invertebrates were eliminated from the main islands 68
(Craig et al., 2000). European colonisation increased ecosystem destruction due to increase 69
demand for timber and pastoral agricultural land, which in turn saw the extinction of a 70
further 16 land birds as well as bat, fish and invertebrate species (Ministry for the 71
Environment, 1997). Nationally, bird, bat, lizard and invertebrate species are characterised 72
by low population densities and severe population fragmentation (Towns and Daugherty, 73
1994). 74
The consequences of this make conservation in New Zealand a key issue, and there are 75
many considerations to be made by the Department of Conservation (DOC). Preservation 76
versus sustainable management, amount of land represented by conservation areas, 77
reintroductions of native species, the maintenance of whole ecosystems and the control of 78
introduced alien species are the main considerations that are of importance in New Zealand 79
conservation. 80
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1.2. Effects of introduced alien species on native terrestrial flora and fauna 82
New Zealand now has 34 species of land mammal. The introduced mammals include 83
predators such as rats, dogs, cats, stoats, ferrets and weasels and browsers like red deer, 84
rabbits and horses (Atkinson, 2001). Nearly all the mammals were intentionally introduced 85
and they affect a wide range of organisms including the kaka (Nestor meridionalis) 86
(Moorhouse et al., 2003), shorebirds like oystercatchers and snipe (Dowding and Murphy, 87
2001), the long-tailed bat (Chalinolobus tuberculatus) (O’Donnell, 2000) and tuatara and 88
the lizards, geckos and skinks (Towns et al., 2001). Stoats and cats have caused the 89
extinction of 9 endemic bird species in the last 150 years (Mooney and Hobbs, 2000) and 90
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introduced mammals such as possums, red deer and goats have altered the structure and 91
composition of the native forests of New Zealand (Nugent et al., 2001). Little is known 92
about the effects that mammals have on invertebrates though the few studies that have been 93
conducted indicate that mammal presence has no direct association with invertebrate 94
activity and abundance (Watts, 2004; Hunt et al., 1998). 95
The reasons for these effects on the native species vary depending on the organisms 96
involved. In many cases it is predation by the mammals that is having the detrimental effect. 97
The pacific rat (Rattus exulans), that probably accompanied the first travellers to New 98
Zealand, spread quickly from sea level to the sub-alpine zone. Native invertebrates, frogs, 99
skinks, geckos, tuatara and smaller sea birds and forest birds would have been naïve to new 100
introduced ground dwelling mammalian predators (Holdaway, 1989). The three mustelids, 101
stoats (Mustela erminea), ferrets (M. furo) and weasels (M. nivalis) were introduced to 102
control rabbits in the 1880’s (Atkinson, 2001). Stoats have been shown to have a negative 103
impact on the kaka by predation (Moorhouse et al., 2003; Wilson et al., 1998). Ferrets and 104
dogs (Canis familiaris) are the main predators of the adult kiwi (Apteryx spp.), stoats and 105
cats of young kiwi and possums (Trichsurus vulpecula) and ferrets are the main egg 106
predators (Mclennan et al., 1996). The long-tailed bat has also suffered at the hands of 107
introduced mammals in the form of predation by feral cats (Felis catus) (Daniel and 108
Williams, 1984), and competition from ship rats (Rattus rattus) and even introduced 109
starlings (Sturnus vulgaris) (Sedgeley and O’Donnell, 1999). The situation maybe 110
exacerbated when weed invasion is increased by introduced small mammals like mice (Mus 111
musculus), ship rats and possums (Williams et al., 2000). 112
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While alien mammals are widely recognised as inimical to native species, the effects of 113
invasive plant species are less well known. Almost half of all the vascular plants growing in 114
New Zealand are introduced. 2,068 out of the 19,000 species introduced are now considered 115
naturalised and the DOC recognises 240 species of invasive weeds (Owen, 1998). Old mans 116
beard (Clematis vitalba), wild ginger (Asarum canadense) and pampus grass (Cortaderia 117
selloana) are just a few of the invasive species threatening New Zealand’s native species. 118
The survival of 61 native vascular plant species is threatened (Owen, 1998). Negative 119
effects can be seen in the seedling species richness and abundance in podocarp/broad leaved 120
forest remnants in the presence of the invasive weed Tradescantia fluminensis (Standish et 121
al., 2001). Invasive weeds like T. fluminensis can also have impacts on invertebrates. It is 122
known that epigaeic invertebrates suffer reduced abundance in the presence of T. 123
fluminensis (Standish, 2004). However, the effects of invasive weeds may not always be 124
negative, as is shown by the wide range of impacts that Senecio jacobaea on pasture 125
ecosystems (Wardle et al., 1995). Nevertheless, with predictions that invasive weeds will 126
threaten areas covering more than 580,000 ha over the next 10-15 years (Owen, 1998), it 127
seems that failure to manage this problem could lead to the further loss of native species. 128
Introduced invertebrates in New Zealand are also of concern to the survival of native 129
wildlife. The common wasp (Vespula vulgaris (L.)), is a common predator of Diptera, 130
Lepidoptera and Araneida (Harris, 1991). Orb-web spiders (Eriophora Pustulosa) are 131
known to suffer from predation by the common wasp and it has been shown that poisoning 132
of wasps can increase the survival of orb-web spiders, although wasp abundance would 133
need to be reduced by up to 90% in order for the spider population to survive (Toft and 134
Rees, 1998). The effect of the common wasp may not be confined to direct predation of the 135
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aforementioned invertebrate orders. Many native birds in New Zealand rely on such 136
invertebrate resources for food. The estimated biomass intake of the common wasp on the 137
South Island of New Zealand is thought to be similar to that of the entire insectivorous 138
avifauna (Harris, 1991). Hence, the common wasp may be acting as a competitor with 139
native insectivorous birds for invertebrate food resources as much as introduced mammals 140
do. 141
Introduced birds are well established in New Zealand, especially in modified landscapes, 142
but are also commonly found within large tracts of native forests. Censuses have shown that 143
five introduced European passerines, chaffinch (Fringilla coelebs), blackbird (Turdus 144
merula), song thrush (Turdus philomelos), dunnock (Prunella modularis) and red poll 145
(Acanthis flammea), represented 18% of all bird individuals, where as all the native forest 146
passerines represented only 64% of all bird individuals. It has been suggested that the ability 147
of the introduced birds to colonise, is increased in heavily browsed forests with 148
impoverished native bird communities (Diamond and Veitch, 1981). 149
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1.3. Methods of pest control 151
In New Zealand there have been two routes taken to conserve the native wildlife. The 152
first is restoration of populations and communities of native species on offshore islands, and 153
the second is restoration of sites on the mainland, often referred to as Mainland Islands. 154
Approximately 150 of New Zealand’s offshore islands above 5 ha in size have been 155
colonised by introduced mammals. However, since 1920, 53 of these islands have had one 156
or more mammal species removed and 36 are now completely free of mammals. Out of the 157
16 islands that are greater than 50km away from the mainland, 8 have been cleared of at 158
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least one or more mammal species with 5 now completely free (Atkinson, 2001). Successful 159
campaigns include the removal of feral cats from Stephens Island (Nogales et al., 2004) and 160
the removal of goats (Capra hircus), cats and brushtail possums from Kapiti (Atkinson, 161
2001). 162
There are currently 46 projects (Fig. 1) on the North and South islands of New Zealand 163
as well as Stewart Island, that are making a serious attempt at controlling introduced 164
mammals as well as other pest animal and plant species (White, 2007). Six of these are 165
funded by the DOC (Saunders and Norton, 2001). These Mainland Island projects can be 166
fenced or unfenced and in most cases pest control is achieved through intensive poisoning 167
and trapping regimes. Several different poisons are used in New Zealand which include 168
1080, brodifacoum and feracol. 1080 (Sodium Monofluroacetate) is of particular use in New 169
Zealand as New Zealand has no other native large land mammals that could be put at risk 170
from 1080, so aerial drops can be made without adversely affecting any native species 171
(Green, 2004). Despite difficulties in maintaining such areas, significant successes have 172
been recorded. Possum and rat population densities have been reduced and maintained at 173
low levels for more than 12 months at several Mainland Island sites. Feral goat populations 174
at Boundary Stream have been reduced by 90%, and cattle have been excluded at Hurunui 175
for the first time in 125 years (Saunders and Norton, 2001). At Mapara, control of predators 176
has allowed the population of North Island Kokako (Callaeas cinerea wilsoni) to be 177
successfully protected (Pryde and Cocklin, 1998). At Rotoiti, stoat control has resulted in 178
successful fledging of kaka nestlings at sites previously not viable (Wilson et al., 1998; 179
Paton et al., 2004). Additionally, at Trounson there has been a dramatic increase in the 180
numbers of native pigeon (Hemiphaga novaeseelandiae) (Saunders and Norton, 2001). 181
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It appears that the DOC and many private enterprises have had considerable success in 182
certain areas of New Zealand in restoring native wildlife (Atkinson, 2001, Wilson et al., 183
1998; Nogales et al., 2004; Saunders and Norton, 2001; Paton et al., 2004), and it seems that 184
in most of these cases removing predation by the alien species is the main benefit to the 185
native species. Some work has focused on the impact that mammal removal has on 186
invertebrate abundance and diversity (Watts, 2004; Hunt et al., 1998); however, a little 187
researched aspect of the management strategies mentioned above, is the role of increased 188
invertebrate food supplies as opposed to reduced predation on the recovery of native fauna. 189
There is considerable evidence that many mammals’ diets are rich in invertebrates in New 190
Zealand. In one study at Boundary Stream reserve on the North Island the larvae of the 191
Tortricid moth were found in 31% of all guts sampled (Jones and Toft, 2006). Other 192
invertebrates commonly detected in the guts of mice include beetles, weta, spiders and other 193
Lepidoptera larvae (Fitzgerald, 1996). Other mammals like hedgehogs (Berry, 1999) also 194
have very invertebrate rich diets. So it is expected that the presence of mammals has an 195
impact on the local invertebrate population. This paper aims to ascertain the extent to which 196
invertebrate food resources respond to mammal exclusion and what impact this has on 197
native fauna. The three hypotheses that will be tested are as follows. 1. There will be no 198
difference in invertebrate biomass between mammal-absent and mammal-present sites. This 199
might occur if neither mammal predation nor native vertebrates affect invertebrate 200
abundances. Equally, any effect of mammal removal might be fully compensated by the 201
recovery of bird populations. 2. There will be a higher invertebrate biomass in the mammal-202
free sites. This situation would arise if invertebrate populations are otherwise depressed by 203
mammal predation. 3. There will be a higher invertebrate biomass in the mammal present 204
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sites. If avian impacts on invertebrates are paramount, the suppression of bird populations 205
by predation may benefit invertebrates. These hypotheses will be discussed further below. 206
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2. Methods 227
2.1. Site locations 228
The study was carried out at a total of six sites on the North Island of New Zealand (Fig. 229
1) from March 16th to May 25th. There were three fenced sites - Karori Wildlife Sanctuary, 230
Tawharanui Open Sanctuary and Bushy Park and three non fenced sites - Boundary Stream 231
Reserve, Mapara Reserve and Mount Bruce National Wildlife Centre. At each site there was 232
a treatment area which was within the reserves and received some form of intensive 233
mammal control, and a control site, matched with the treatment site for vegetation type, 234
which was outside the reserves and had received no intensive mammal control. The control 235
areas for each of the sites were as follows; for Karori the control area was Birdwood 236
Reserve, at Boundary Stream it was Bellbird Bush, at Tawharanui it was Hubbard’s Bush. 237
At Mapara it was Aratoro Reserve, at Bushy Park it was the area of bush just outside the 238
fence and for Mount Bruce the control area was the W.A. Miller Reserve. 239
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2.2. Invertebrate sampling 241
Invertebrate sampling methods involved sweep netting, beating trays, portable light 242
traps, malaise traps and pitfall traps. Sweep netting was carried out over three days in each 243
of the treatment and control sites. Fifty sweeps a day were carried out over five patches, ten 244
sweeps over each patch. Sweep net sampling was carried out only on dry days and on dry 245
vegetation with stature. Invertebrates were extracted with a pooter and identified. Sampling 246
patches were chosen by walking along the path into the reserve, stopping every 50m and 247
walking 10m into the bush away from the path, to ensure that the effects of disturbance 248
from the path was minimised. 249
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The beating trays were also used over three days in each of the treatment and control 250
sites. Fifty bushes/trees were sampled on each day and the beating tray was held under the 251
tree while it was shaken for 30 seconds. Once again this method was only used on dry days 252
and on dry vegetation. There were 5 sampling patches, each with ten bushes/trees. Sampling 253
patches were chosen using the same methods as the sweep netting. 254
A portable light trap was deployed from dusk till dawn over two nights in each of the 255
treatment and control sites. The light trap was set out at 200m and 400m intervals along the 256
path into the reserve and 10m in from the path to avoid the effects of disturbance from the 257
path. 258
A single malaise trap was set out for 24 hours on three days in each of the treatment and 259
control sites. It was moved to a new position each day at 300m, 500m and 700m intervals 260
along the path and 10m in from the path. 261
Ten pitfall traps set out for 24 hours on three days in each of the treatment and control 262
sites. Each trap lay on a transect 5m apart, which began 10m into the bush from the edge of 263
the path. On the first day the first trap was set 50m in along the path into the reserve and the 264
transect line moved along another 50m on each following day. A small plastic cup with a 265
diameter of 8cm served as the trap, with care taken to ensure that the soil was level with the 266
rim of the cup. A plastic plate with two nails in it was propped over the trap to ensure rain 267
and intruding mammals were kept out while still allowing invertebrates to be trapped. 268
The portable light trap, malaise trap and pitfall traps were used regardless of weather 269
conditions. The regime used in order to ensure that all sampling was carried out as 270
efficiently as possible is outlined in Table 1. This regime ensured enough time to travel 271
between and move equipment from the control to treatment, and gave enough time to move 272
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the static sampling methods (malaise trap, pitfall traps and portable light trap) to a new 273
position each day. Where rain postponed sweep net and beating tray sampling, they were 274
carried out on the first subsequent dry day. 275
Invertebrate specimens were identified to family level and where possible to genus level 276
using Crowe (1999) and Grant (1999). Invertebrate biomass (dry weight (mg)) was worked 277
out using methods described in Collins (1992) for Gastropoda, Sage (1982) for Orthoptera 278
and Araneida and for the remaining invertebrates Sample et. al. (1993) was used. Where 279
biomass models were not available for certain invertebrates, the general insect model used 280
in Sample et. al. (1993) was applied. 281
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2.3. Bird sampling 283
Five minute bird counts were carried out on two suitable mornings with settled and still 284
weather in each of the treatment and control areas. There were five stops made at 200m 285
intervals along the path through the study site. At each stop bird calls were identified by 286
sound and recorded if within a 200m radius. No bird was knowingly identified more than 287
once. The target species that were identified were the insectivorous Grey Warbler, Fantail, 288
Tomtit and Robin. Presence of other native birds such as the nectivorous Silvereye and 289
Bellbird and the herbivorous Tui, Kaka and Kereru were also recorded. This would enable 290
comparisons to be made between the insectivorous and non-insectivorous birds which 291
depending on the results of the invertebrate biomass, may indicate if it is reduced predation 292
or increased invertebrate food resources that are allowing bird populations to recover. 293
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2.4. Statistical analyses 295
There were four levels of analysis carried out on the invertebrate data. Firstly, a linear 296
mixed effects model was used to analyse the effect that treatment had on the invertebrate 297
biomass and invertebrate abundance categorised by sampling method. Site was added as the 298
random effect allowing correction for between site variations. Secondly, a generalized linear 299
model was used to analyse the effect that treatment had on invertebrate biomass and 300
abundance for each sampling method and each site in order to find any significant 301
differences within each site. The third analysis was a linear mixed effects model looking at 302
the effect of fenced exclusion as opposed to intensive poisoning and trapping, on the 303
invertebrate biomass and abundance in the areas receiving treatment. Once again site was 304
added as the random effect to allow for between site variations. All three models were run 305
with Poisson response distributions and identity as the link function to allow for the non-306
normal distribution of the data. Date, temperature, altitude, latitude, and other weather 307
factors such as presence of rain, Beaufort scale and Oktas were introduced in to the models 308
but removed if no significant effect (P>0.05) was found. The final analysis was to look at 309
species diversity indices of invertebrates at the control and treatment sites and run T-tests of 310
the results from the treatment and control sites. The bird data served to indicate what impact 311
the invertebrate biomass had on the native birds so Chi-squared tests were carried out on the 312
abundance of insectivorous, non-insectivorous and total birds. 313
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3. Results 318
3.1. Invertebrate data 319
A total of 4613 invertebrates were collected and classified into 69 separate invertebrate 320
ID groups at family and genus level. Of the 4613 individuals, 2235 were collected from the 321
control sites and 2378 were collected from the treatment sites. Invertebrate biomass totalled 322
90.2 g from the control sites and 106.3 g from the treatment sites (Fig.2, Table 2). Notable 323
invertebrate families included the Lepidoptera families, Noctuidae, Geometridae and 324
Tortricidae which were high in abundance in the portable light traps in both the control and 325
treatment sites. There were 203 geometridae caught from the control sites and 337 from the 326
treatment sites. Due to the large size of these invertebrate families they constituted a large 327
portion of the overall invertebrate biomass. Another family that was high in abundance was 328
the black fly (Simulidae) which is generally common on the North Island of New Zealand. 329
There were 228 caught in the control sites and 184 caught in the treatment sites, however, 330
due to the small size of the black fly the contribution to the overall invertebrate resource 331
(65.659 mg for control and 52.988 mg for treatment) was not as much as with the 332
Lepidoptera families such as geometridae (15212.643 mg for control and 25254.486 mg for 333
treatment). Other significant invertebrate ID groups contributing to the overall invertebrate 334
resource include therididae, landhoppers and geometridae larvae. An anomaly that should 335
be pointed out is the high abundance of termites that were found in the control site at 336
Boundary Stream, 340 compared to 1 found at the treatment site and none at any other site. 337
This was due to the heavy rain that was experienced during the sampling period at the 338
control site. Accordingly, termites were treated as an outlier and excluded from subsequent 339
analysis. 340
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When comparing the invertebrate abundances at the control and treatment sites from 341
each sampling method across all six sites, invertebrate abundance was higher in the 342
treatment sites than the control for four out of the five methods (Fig. 3). However, there was 343
weak statistical support for these differences for all methods but the portable light trap 344
(beating tray, N=364 P=0.052; sweep net, N=262 P>0.1; pitfall trap, N=133 P>0.1) where 345
N=sample size. The difference in the abundance seen in the portable light trap was 346
significant (light trap, N=157 P<0.01). The malaise trap was the only method that had a 347
higher invertebrate abundance in the control than the treatment sites, but once again the 348
statistical support for this difference was weak (malaise trap, N=364 P>0.1). 349
Invertebrate biomass was significantly higher in the control than the treatment sites for 350
four of the sampling methods (beating tray, N=364 P<0.01; sweep net, N=262 P<0.01; 351
malaise trap, N=158 P<0.01; pitfall trap, N=133 P<0.01) (Fig. 4). However, the invertebrate 352
biomass was significantly higher in the treatment sites for the portable light trap (light trap, 353
N=195 P<0.01). 354
There was considerable variation in the differences of invertebrate biomass and 355
abundance from the control and treatment areas from site to site (Tables 3 and 4). Treatment 356
abundance was significantly higher than control abundance with the portable light trap at 357
Karori, Boundary Stream and Mount Bruce, with the pitfall traps at Tawharanui and with 358
the malaise trap at Karori (P<0.01). Where control abundance was higher than treatment 359
abundance, significant results were found with the beating tray at Karori, the portable light 360
trap at Boundary Stream, the pitfall trap at Mapara and the malaise trap at Boundary Stream 361
(P<0.01) and with the beating tray at Mapara and the malaise trap at Tawharanui (P<0.05). 362
All the differences seen in the invertebrate biomass data were significant (Table 5), with the 363
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exception of one (i.e. malaise trap at Mapara). From the beating tray data the control 364
biomass was significantly higher than the treatment biomass at Karori and Mapara. At the 365
other four sites the treatment biomass was significantly higher than the control biomass. 366
When the sweep net data were analysed, treatment biomass was higher than control biomass 367
at Boundary Stream and Tawharanui and at Karori, Mapara, Bushy Park and Mount Bruce 368
control biomass was higher. The portable light trap was the only method with which all sites 369
had significantly higher biomass in the treatment than the control areas. With the malaise 370
trap the only site that had a significantly higher invertebrate biomass in the treatment site 371
was Bushy Park, Karori, Boundary Stream, Tawharanui and Mount Bruce all had 372
significantly higher invertebrate abundance in the control sites. Finally, from the pitfall trap 373
data, Karori and Mount Bruce were the only sites where treatment biomass was significantly 374
higher, at the other four sites control biomass was significantly higher. 375
Evidence that there was no difference in invertebrate abundance at treatment areas 376
between fenced and unfenced (Fig. 5) was supported by statistical results. When all the 377
methods were analysed no significant differences were seen (P>0.1), however pitfall traps 378
were close to being significant (P=0.057), for invertebrate biomass no significant difference 379
was seen with any of the methods (P>0.1). 380
Diversity indices for each site indicate that there is not a large difference between the 381
control and the treatment sites and the T-test results (P>0.1) support this, with none of the 382
species diversity index values being significantly different between the control and 383
treatment sites. 384
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3.2. Bird data 387
A total of 190 individual birds of 11 different species were recorded from five minute 388
bird counts during the project. There were 69 individuals from the control sites and 121 389
individuals from the treatment sites (Table 6). The three most common species detected by 390
sight or sound were the Tui, Fantail and Grey Warbler which were abundant in both the 391
treatment and control sites (Fig. 6). When the bird data were split up into insectivorous and 392
non-insectivorous bird species (Fig. 7) it was still clear that there were more individuals of 393
both sub-groups detected in the treatment as opposed to the control sites (Table 6). With the 394
insectivorous birds there were 33 recorded in the control and 54 observed in the treatment. 395
The non-insectivorous birds show a similar pattern, with 39 being from the control sites and 396
67 being from the treatment sites. However, Chi-squared tests reveal that the differences 397
seen in the total bird abundance, and with both the insectivorous and non-insectivorous bird 398
abundance, are not significant (P>0.1). 399
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4. Discussion 410
So conclusions that can be drawn from these results are that control sites have a 411
significantly higher invertebrate biomass than the treatment sites when using the beating 412
tray, sweep net, malaise trap and pitfall traps. Conversely, invertebrate biomass is 413
significantly higher in the treatment sites both collectively and individually when sampling 414
using the portable light trap. Indeed, the biomass from the light trap (68.5 g from treatment 415
sites and 32.9 g from control sites) is so large, that with out it, the overall total invertebrate 416
biomass would be much higher in the control than in the treatment sites (37.8 g in treatment 417
sites and 57.3 g in control sites). 418
Why was total invertebrate biomass exclusive of the light trap data much higher in the 419
control sites? 420
There are many possible reasons for this higher biomass of ground and vegetation 421
dwelling invertebrates in the control sites. Invertebrate abundance and therefore biomass, is 422
something that is accounted for by many different factors, not just the presence or absence 423
of alien mammals. As mentioned earlier the weed Tradescantia fluminensis can have a 424
negative effect on epigaeic invertebrates (Standish et al., 2001; Standish, 2004). The 425
presence of such weeds in the areas sampled may alter the number of invertebrates caught. 426
Although, given that invertebrate diversity has been shown to be positively correlated with 427
plant diversity (Crisp et al., 1998), it would be expected that the treatment sites, recovering 428
from browsing by mammals, would contain a higher plant diversity and abundance. Many 429
of the protected reserves are weeded native plant species are re-established. This increased 430
plant diversity would be expected to increase invertebrate diversity and abundance. 431
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However, this does not explain the lower invertebrate biomass in the treatment sites seen 432
during this study. 433
Another possible reason for the higher invertebrate biomass at the control sites excluding 434
the portable light trap is that although every effort is made to eradicate mammals in these 435
protected areas, it is very difficult to ensure complete mammal eradication. A lot of the 436
reserves still have a problem with mice and other small rodents (Ward-Smith et al., 2005; 437
White, 2007), which are very hard to completely eradicate. At reserves such as Boundary 438
Stream small mammals such as mice and hedgehogs are not directly targeted due to them 439
being of secondary conservation importance in the presence of larger mammals like feral 440
cats, mustelids, rats and possums (Jones and Toft, 2006). This kind of management strategy 441
could be a factor contributing to the low invertebrate biomass in the treatment sites, with the 442
main predators of invertebrates being relatively abundant due to reduced predation from 443
larger mammals. Small rodents are natural prey of mustelids (Martinoli et al., 2001) and 444
cats, so they will be naturally kept in check by the presence of these larger mammals. 445
Without this natural predation, poisoning and trapping of small rodents may not be enough 446
to keep populations at a level low enough to keep them from having an adverse impact on 447
invertebrate biomass. 448
At Karori Wildlife Sanctuary, a study on ground dwelling beetles before and after 449
mammal eradication revealed no change in beetle abundance or species number (Watts, 450
2004). In another study by Hunt et al., (1998), there was no clear relationship between rat 451
numbers and invertebrate abundance at Karioi Rahui, Ohakune. So it is not a novel finding 452
that there was no significant difference observed in four out of five methods in invertebrate 453
abundance and diversity at the sites studied during this research. A reason suggested by 454
21
Watts, 2004 for the lack of significant differences seen in invertebrate abundance and 455
activity is that although mammal numbers have been greatly reduced in these reserves, 456
many of them have received translocations of insectivorous birds such as Weka, North 457
Island Robin and Kiwi. Perhaps the impact of these birds is simply replacing the impact the 458
alien mammals once had. Indeed, the effect of the new bird residents may be going beyond 459
the effect mammals once had, which may explain the increased invertebrate biomass at the 460
control sites. 461
Although there was higher abundance of both insectivorous and non-insectivorous birds 462
at the treatment sites, these differences were not significant. This may be due to the 463
generally higher invertebrate biomass (excluding portable light trap) at the control sites, or it 464
may just be due to overspill of birds from the reserves to areas of bush in close proximity to 465
the reserves. The reserves may act as a sanctuary for the birds to nest in, allowing them to 466
forage outside of the reserve. 467
In addition to the impact that mammal removal may have on invertebrate abundance and 468
biomass, the impact that the fence, as opposed to intensive poisoning and trapping regimes, 469
may have on invertebrate abundance and biomass inside the reserves was analysed. 470
Interestingly, there was no difference observed between the fenced and unfenced sites. This 471
may be due to the fact that this study did not have the power to detect and effect. However, 472
unfenced reserves can be just as successful at restoring native wildlife as fenced ones are. 473
At Ark in the Park, in the Waitakere ranges, possum, rat, stoat and feral cat populations 474
have been reduced sufficiently enough for whiteheads, North Island robins and even 60 hihi 475
to be released (White, 2007). 476
22
As more and more of these Mainland Island project arise, so does the opportunity for 477
research. Invertebrates are understudied in New Zealand and there can be much more done 478
in order to understand the distribution and diversity of them (McGuiness, 2001). Future 479
work should focus on continuous studies of sites before and after mammal control has 480
started. This way will allow a better analysis of how invertebrates are affected by mammal 481
exclusion. 482
In conclusion, based on three studies to date it is apparent that the differences in 483
invertebrate biomass and abundance between treatment and control sites are absent or 484
modest. The hypothesis that the removal of alien mammals will release invertebrates from 485
predator control is incorrect. More likely, their control is taken over by other organisms 486
which could be birds, other invertebrates or a mixture of both. 487
488
489
490
491
492
493
494
495
496
497
498
499
23
Acknowledgements 500
This research was carried out as part of a master’s programme in conservation and 501
biodiversity at the University of Exeter, Cornwall campus. Thanks to David Bryant and 502
Murray Williams for project coordination. Thanks to Raewyn Empson, Denise Fastier, Phil 503
Bradfield, Phil Brady, Kate O’Neill, Allan Anderson, Terry O’Conner and Matt Maitland 504
who all helped to provide access to the study sites. 505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
24
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639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
30
Tables 657
Table 1 658
Sampling regime carried out at each site a 659
Day One Two Three Four Five Six
Sampling
Methods
Control
Treatment
BT
SN
BC
MT
PT
PLT
BT
SN
MT
PT
PLT
BT
SN
MT
PT
MT
PT
PLT
BT
SN
BC
MT
PT
PLT
BT
SN
MT
PT
BT
SN
a BT= Beating Tray, SN= Sweep Net, BC= Bird Count, MT= Malaise Trap, PT= Pitfall 660
Trap, PLT= Portable Light Trap. 661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
31
Table 2 679
Invertebrates collected from all sampling methods at all sites from March 16th to May 25
th 2007
a 680
Invertebrate Order (Sub-Order) Invertebrate ID Length
(mm)
Individual Dry
Weight (mg)
Control
Abundance
Control Dry
Weight (mg)
Treatment
Abundance
Treatment Dry
Weight (mg)
Lepidoptera Aegeriidae 15 30.525 2 61.049 7 213.672
Lepidoptera Geometridae 20 74.939 203 15212.643 337 25254.486
Lepidoptera Geometridae Larvae 15 8.200 180 1475.980 70 573.992
Lepidoptera Noctuidae 20 74.939 71 5320.678 161 12065.200
Lepidoptera Noctuidae Larvae 35 100.612 4 402.449 1 100.612
Lepidoptera Pterophoridae 6 1.747 0 0.000 1 1.747
Lepidoptera Pyralidae 10 8.608 53 456.214 57 490.646
Lepidoptera Saturnidae 40 652.416 0 0.000 1 652.416
Lepidoptera Tortricidae 20 74.939 48 3597.078 116 8692.939
Orthoptera Anistostomatidae 25 3040.820 9 27367.382 4 12163.281
Orthoptera Gryllidae 22 1315.908 3 3947.725 2 2631.817
Orthoptera Tettigoniidae 20 752.862 1 752.862 2 1505.725
Diptera (Brachycera) Calliphoridae 15 11.822 3 35.466 0 0.000
Diptera (Brachycera) Drosopholidae 2 0.153 89 13.605 120 18.344
Diptera (Brachycera) Muscidae 10 4.928 6 29.569 12 59.139
Diptera (Brachycera) Stratiomyidae 12 7.304 12 87.649 27 197.209
Diptera (Brachycera) Syrphidae 10 4.928 1 4.928 2 9.856
Diptera (Nematocera) Anisopodidae 5 0.891 18 16.046 22 19.612
Diptera (Nematocera) Chironomidae 9 3.272 41 134.133 78 255.180
Diptera (Nematocera) Culicidae 5 0.891 34 30.309 40 35.657
Diptera (Nematocera) Mycetophilidae 10 4.130 46 189.988 35 144.556
Diptera (Nematocera) Simulidae 3 0.288 228 65.659 184 52.988
Diptera (Nematocera) Tipulidae 10 4.130 52 214.769 65 268.461
Acarina Mite 1 0.027 79 2.099 50 1.328
Hemiptera Aphididae 2.5 0.140 1 0.140 1 0.140
Hemiptera Aradidae 10 9.939 1 9.939 0 0.000
Hemiptera Cercopidae 10 9.939 3 29.816 0 0.000
Hemiptera Cicadidae 30 291.396 11 3205.352 54 15735.365
Continued on next page
32
Invertebrate Order (Sub-Order) Invertebrate ID Length
(mm)
Individual Dry
Weight (mg)
Control
Abundance
Control Dry
Weight (mg)
Treatment
Abundance
Treatment Dry
Weight (mg)
Hemiptera Flatidae 8 5.004 4 20.017 1 5.004
Hemiptera Lygaeidae 18 60.576 1 60.576 1 60.576
Hemiptera Nabidae 15 34.579 29 1002.799 33 1141.116
Hemiptera Pentatomidae 8 5.004 4 20.017 11 55.046
Hemiptera Reduviidae 10 9.939 4 39.755 0 0.000
Hemiptera Ricaniidae 10 9.939 1 9.939 0 0.000
Coleoptera Carabidae 25 115.289 8 922.309 17 1959.906
Coleoptera Cerambycidae 8 6.739 13 87.612 8 53.915
Coleoptera Chrysomelidae 12 18.511 1 18.511 0 0.000
Coleoptera Coccinllidae 4 1.198 4 4.792 33 39.534
Coleoptera Curculionidae 10 11.752 19 223.292 22 258.549
Coleoptera Elateridae 10 11.752 2 23.504 9 105.770
Coleoptera Lucanidae 30 181.596 0 0.000 1 181.596
Coleoptera Scarabaeidae 8 6.739 28 188.703 13 87.612
Coleoptera Staphylinidae 10 11.752 7 82.266 10 117.522
Coleoptera Tenebrionidae 5 2.089 10 20.891 12 25.069
Blattodea Blatidae 13 15.942 2 31.885 7 111.597
Hymenoptera Bracconidae 3 0.267 11 2.932 9 2.399
Hymenoptera Formicidae 4 0.579 26 15.053 34 19.684
Hymenoptera Ichneumonidae 18 33.396 36 1202.263 97 3239.432
Hymenoptera Vespidae 12 11.193 1 11.193 1 11.193
Trichoptera Conoesucidae 15 37.838 2 75.675 5 189.188
Araneida Corinnidae 10 146.759 20 2935.175 27 3962.486
Araneida Green spider 10 146.759 11 1614.346 2 293.517
Araneida Lycosidae 8 58.581 65 3807.763 32 1874.591
Araneida Miturgidae 15 1457.882 0 0.000 1 1457.882
Araneida Salticidae 10 146.759 14 2054.622 10 1467.587
Araneida Theridiidae 5 14.774 148 2186.489 220 3250.186
Araneida Thomisidae 5 14.774 37 546.622 52 768.226
Pseudoscorpiones False Scorpion 5 0.015 0 0.000 1 0.015
Continued on next page
Table 2 (continued)
33
Invertebrate Order (Sub-Order) Invertebrate ID Length
(mm)
Individual Dry
Weight (mg)
Control
Abundance
Control Dry
Weight (mg)
Treatment
Abundance
Treatment Dry
Weight (mg)
Chilopoda Garden centipede 35 188.489 0 0.000 1 188.489
Gastropoda Small Native Land Snail 4 5.487 4 21.947 12 65.842
Neuroptera Hemerobiidae 3 0.190 42 7.989 62 11.794
Amphipoda Land hopper 6 2.318 129 299.005 163 377.813
Archaeognatha Meinertellidae 12 13.057 1 13.057 10 130.574
Isopoda Oniscidae 10 8.287 1 8.287 2 16.573
Diplopoda Sphaerotheridae 50 458.787 0 0.000 8 3670.294
Diplopoda Spirobollelidae 50 458.787 10 4587.867 0 0.000
Isoptera Termite 13 15.942 340 5420.424 1 15.942
Thysanoptera Thripidae 1 0.027 1 0.027 0 0.000
Collembola Tomoceridae 5 1.471 0 0.000 1 1.471
Total 2235 90231.134 2378 106362.358 a Invertebrate identified to family level and where possible to genus level, length was measured according to the protocols 681
set out in Collins (1992), Sage (1982) and Sample et. al. (1993). 682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
Table 2 (continued)
34
Table 3 698
Mean (SE±) invertebrate abundance for each method at the control and treatment areas of each site a
699
Site Karori Boundary Stream
Reserve
Tawharanui Mapara Bushy Park Mount Bruce
Control 6.9±4.0 3.9±1.5 2.2±0.7 5.2±1.9 3.5±1.3 3.0±1.0
BT
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
SN
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
Method PLT
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
MT
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
PT
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 a BT= Beating Tray, SN= Sweep Net, MT= Malaise Trap, PT= Pitfall Trap, PLT= Portable Light Trap. 700
701
702
703
704
705
35
Table 4 706
Mean (SE±) invertebrate dry weight (mg) for each method at the control and treatment areas of each site a 707
Site Karori Boundary Stream
Reserve
Tawharanui Mapara Bushy Park Mount Bruce
Control 137.651±62.201 65.115±40.553 56.698±23.487 634.444±527.115 179.005±126.091 37.117±21.346
BT
Treatment 92.606±54.780 156.986±77.707 282.041±252.474 152.827±66.849 179.393±125.734 65.042±33.458
Control 70.197±39.142 28.207±11.434 8.250±4.011 341.166±319.023 94.918±80.003 24.450±14.743
SN
Treatment 28.922±14.027 114.232±67.341 13.241±6.444 23.179±9.879 30.745±12.580 20.345±16.839
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
Method PLT
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
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
MT
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±21.166 543.866±384.281 488.300±455.733 63.818±57.636
PT
Treatment 282.980±135.244 345.134±207.180 56.654±42.732 71.153±42.207 164.665±91.909 109.690±14.608 a BT= Beating Tray, SN= Sweep Net, MT= Malaise Trap, PT= Pitfall Trap, PLT= Portable Light Trap. 708
709
710
711
712
713
714
715
36
Table 5 716
Probability values for differences in biomass (dry weight (mg)) between control and 717
treatment areas at each site using each method a 718
Site 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*
Method 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** a BT= Beating Tray, SN= Sweep Net, MT= Malaise Trap, PT= Pitfall Trap, 719
PLT= Portable Light Trap. 720
* Significantly higher control biomass 721
** Significantly higher treatment biomass 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
37
Table 6 752
Abundance of insectivorous and non-insectivorous birds at the 753
control and treatment areas of each site 754
Bird Species Control Treatment
Insectivorous Fantail 17 18
Grey Warbler 13 20
Robin 0 9
Saddleback 0 3
Tomtit 3 4
Sub-Total 33 54
Non-Insectivorous Bellbird 6 6
Kokako 1 2
Kaka 0 8
Kereru 9 16
Silvereye 1 0
Tui 19 35
Sub-Total 39 67
Total 69 121
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
38
Figures 770
771 772
Fig. 1. A map of New Zealand showing 46 projects currently controlling for alien 773
mammals. Underlined sites indicate study sites (White, 2007). 774
39
775
776
777
0
500
1000
1500
2000
2500
Control Treatment
Invertebrate Abundance
Mount Bruce
Bushy Park
Mapara
Tawharanui
Boundary Stream
Karori
778
0
20000
40000
60000
80000
100000
120000
Control Treatment
Invertebrate Dry W
eight (m
g)
779 Fig. 2. Total abundance (mg) (a) and dry weight (mg) (b) of invertebrates 780
caught from all sampling methods across all treatment and control sites 781
from March 16th to May 25
th 2007. 782
783
784
785
786
787
788
789
790
a)
b)
40
791
792
793
794
0
50
100
150
200
250
300
350
400
Control Treatment
Invertebrate abundance
0
100
200
300
400
500
600
700
Control Treatment
Invertebrate abundance
Mount Bruce
Bushy Park
Mapara
Tawharanui
Boundary Stream
Karori
795
0
100
200
300
400
500
600
700
800
900
1000
Control Treatment
Invertebrate abundance
0
50
100
150
200
250
300
Control Treatment
Invertebrate abundance
796 797
0
50
100
150
200
250
300
Control Treatment
Invertebrate abundance
798 799
Fig. 3. Invertebrate abundance at the control and treatment sites from the sweep net (a), 800
beating tray (b), light trap (c), malaise trap (d) and pitfall trap (e). 801
802
803
804
805
b) a)
d) c)
e)
41
0
2000
4000
6000
8000
10000
12000
Control Treatment
Invertebrate dry weight (m
g)
0
5000
10000
15000
20000
25000
30000
Control Treatment
Invertebrate dry weight (m
g)
Mount Bruce
Bushy Park
Mapara
Tawharanui
Boundary Stream
Karori
806 807
0
10000
20000
30000
40000
50000
60000
70000
80000
Control Treatment
Invertebrate dry weight (m
g)
0
1000
2000
3000
4000
5000
6000
Control Treatment
Invertebrate dry weight (m
g)
808 809
0
2000
4000
6000
8000
10000
12000
14000
16000
Control Treatment
Invertebrate dry weight (m
g)
810 811
812
813
Fig. 4. Invertebrate dry weight (mg) at the control and treatment sites from the sweep net (a), 814
beating tray (b), light trap (c), malaise trap (d) and pitfall trap (e). 815
816
817
818
819
820
821
822
823
a)
e)
c) d)
b)
42
0
200
400
600
800
1000
1200
1400
Fenced Unfenced
Invertebrate abundance
PT
MT
PLT
SN
BT
824 825
0
10000
20000
30000
40000
50000
60000
70000
Fenced Unfenced
Invertebrate dry weight (m
g)
826 Fig. 5. Invertebrate abundance (a) and dry weight (mg) (b) 827
recorded in treatment sites at fenced and unfenced reserves.
828
BT= Beating Tray, SN= Sweep Net, MT= Malaise Trap, 829
PT= Pitfall Trap, PLT= Portable Light Trap. 830
831
832
833
834
835
836
837
838
839
a)
b)
43
840
841
842
0
5
10
15
20
25
30
35
40
Fantail
Grey Warbler
Robin
Saddleback
Tomtit
Bellbird
Kokako
Kaka
Kereru
Silvereye
Tui
Bird A
bundance
Control
Treatment
843 844
Fig. 6. Total bird abundance observed at the control and treatment sites using 845
the five minute bird count method. 846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
44
869
870
871
0
10
20
30
40
50
60
Control TreatmentTreatment
Insectivorous bird abundance
Mount Bruce
Bushy Park
Mapara
Tawharanui
Boundary Stream
Karori
872
0
10
20
30
40
50
60
70
80
Control TreatmentTreatment
Non-insectivorous bird abundance
873 874
Fig. 7. Total number insectivorous (a) and non-insectivorous (b) 875
birds observed at control and treatment sites using the five 876
minute bird count method. 877
878
b)
a)