Avian biodiversity across Auckland’s volcanic cone reserves

ABSTRACT

Auckland, a city with a population of approximately 1.7 million, is located directly on the Auckland Volcanic Field, a late Quaternary-era monogenetic field. There are at least 53 volcanoes across the field, many of which are of geological, cultural and ecological significance, such as for being reserves for native species; however, few assessments of the richness of avian biodiversity across the volcanoes have been made. To address this data shortfall, we conducted avian biodiversity surveys using stationary point counts within nine of Auckland's volcanic cone reserves. Thirty-eight species were detected across the sites, of which 18 were native. Our estimates of relative species abundances and detection probabilities revealed that the most common native birds within these reserves were silvereyes, tui and southern black-backed gulls, while common mynas, house sparrows, Eurasian blackbirds and eastern rosellas were the most common introduced species. In addition to tui and silvereyes, the presence of other natives critical to the functioning of native ecosystems, such as New Zealand fantails, grey warblers and New Zealand pigeon, suggest that the volcanoes possess a diverse native avifauna supported by native flora that warrant continued and intensified restoration efforts. We discuss several feasible strategies for improving faunal and floral biodiversity across the volcanic cone reserves. Continued avian biodiversity surveys are also of critical importance as they will enable us to further evaluate and prioritise restoration projects within Auckland's multitude of diverse volcanic cone reserves.

KEYWORDS:

• Acridotheres tristis
• bird counts
• detectability
• maunga
• New Zealand

Introduction

Auckland's urban area, a city with a human population of approximately 1.7 million (Statistics New Zealand 2017), has been built and developed directly on the late Quaternary Auckland Volcanic Field (Kermode 1992). This field comprises at least 53 scattered, small-volume, basaltic, monogenetic volcanoes, locally known as ‘maunga’, that are still potentially active (Hayward et al. 2011). The Auckland Volcanic Field ranges from Wiri in the south through the Auckland Isthmus to Rangitoto Island and Lake Pupuke at its northern limits. The highest and youngest volcano within the field is Rangitoto (260 m above sea level) and the highest volcano within Auckland city is Maungawhau (Mount Eden, 196 m, Edbrooke et al. 2003). These volcanoes are distinctive landmarks of the city, and many are recreational reserves managed by Auckland Council that harbour a variety of important native wildlife (Burns et al. 2013).

Urban landscapes are gaining recognition for their importance in supporting a variety of biodiversity (Savard et al. 2000; Germaine and Wakeling 2001; Chace and Walsh 2006; Hall et al. 2017), and also for their significance as opportunities for urban dwellers to experience nature and wildlife (Miller 2005; Shanahan et al. 2015; Soga and Gaston 2016; Müller et al. 2018). Birds have long been a common subject of biodiversity surveys as bio-indicators (Temple and Wiens 1989; Monks et al. 2013; Tulloch 2015), including more recently in urban avifaunal studies (Savard et al. 2000; Chace and Walsh 2006; Strohbach et al. 2009; Murgui and Hedblom 2017). For example, count-based avian surveys conducted in Dunedin by van Heezik et al. (2008) found roughly half of all bird species were native, thus identifying the importance of urban areas for indigenous avifauna. Auckland's volcanoes may support ecologically significant populations of non-native and native avian species and so are potential areas of special interest within the urban environment; however, little is known of the avian biodiversity across these volcanoes.

Auckland's volcanoes have long held cultural and historical human significance for Māori, descendants of the Polynesians that first settled New Zealand c. ad 1250–1300 (Esler and Esler 2010). Early Māori inhabitants modified the slopes and summits of the volcanic cones, and more recent modifications to these volcanoes have also occurred through quarrying for scoria, water reservoir construction, and the building of defensive military positions during the Second World War (Burns et al. 2013). However, large-scale human modification of Auckland's volcanoes has been largely restricted amid recognition of the values that they have to Auckland residents (Burns et al. 2013). Many of Auckland's volcanoes are important recreational reserves; Maungawhau (Mount Eden), for example, is an important ‘urban green space’ regularly visited for a variety of recreational activities. This, and other volcanoes within the Auckland Volcanic Field, such as Maungakiekie (One Tree Hill), are also key urban tourism locations (Lawton and Page 1997; Piñeros Arenas 2010).

Previous human modification has led to significant changes in the vegetation of Auckland's volcanoes, which has probably affected the avifauna. Although these areas would have originally been covered in native broadleaf forest (Smale and Gardner 1999), currently they mostly support exotic grassland with some remaining remnants of native forest. Extant vegetation of the volcanoes primarily is determined by slope, fertility and historical land-management practices: flatter, more fertile cone reserves are generally dominated by exotic herb and grass species such as white clover (Trifolium repens) and kikuyu (Pennisetum clandestinum); moderately steep reserves are dominated by exotic grasses, whereas steeper, less-fertile reserves are dominated by native weeping grass Microlaena stipoides (Burns et al. 2013).

In August 2014 ownership of reserves covering parts of 14 of Auckland's volcanoes was transferred to the Tamaki Collective, who now co-govern these maunga in association with Auckland Council. The Tamaki Collective consists of 13 iwi and hapū, each of which have historical Treaty claims to the Auckland Volcanic Field (New Zealand Government 2012). The co-governance body is known as the Tūpuna Maunga o Tāmaki Makaurau Authority (Tūpuna Maunga). Tūpuna Maunga has recently initiated several programmes for the management, maintenance and restoration of the volcanic cone reserves. To provide baseline data against which to measure progress towards restoration goals, the biodiversity of the volcanoes needed to be assessed, of which the richness of avian biodiversity within the volcanic cones has scarcely been quantified. To address this data shortfall, we conducted bird surveys across nine of these reserves to obtain baseline avian biodiversity data that can be integrated into the long-term management of the reserves by the Auckland Council and Tūpuna Maunga.

Methods

Nine volcanoes in Auckland, New Zealand were chosen as study sites based on the interest of the Auckland Council in measuring the biodiversity values of the Tūpuna Maunga reserves. Resources only allowed for these nine of the 14 reserves to be surveyed. The sites were chosen in terms of their potential to support indigenous bird species as they were the largest reserves and also contained the greatest area of forest, treeland and/or scrub habitats.

A total of 47 bird-surveying stations were established across the nine volcanoes (Figure 1). More survey stations were used at the larger reserves (Table 1). Stations were situated at least 100 m but no more than 200 m apart, which equated to an average of one station per 4.9 ha at each site. Stations for each volcano were established by digitising polygons around non-grass vegetation and then randomly selecting points within these areas using ARCMAP 10 and the Create Random Points tool (ESRI 2011).

Figure 1. Map of the nine volcanic (maunga) cone reserves that were the subject of the avian biodiversity surveys conducted within this study. Locations of the volcanoes are shown in relation to Auckland Central Business District (CBD).

Table 1. An overview of the nine Auckland volcanic (maunga) cone reserves surveyed in this study, indicating the area of the study site (including the mean ± standard error of the mean) and the number of bird counts conducted at each site.

Volcanic cone reserve Tūpuna Maunga	Reserve area (ha)	Bird count stations	No. of replicates at each station	Total no. of counts	Area (ha) per station
Maungakiekie – One Tree Hill	48.5	6	3	18	8.1
Maungawhau – Mount Eden	27.5	6	3	18	4.6
Maungarei – Mount Wellington	26.7	6	3	18	4.5
Ōtāhuhu – Mount Richmond	20.8	3	3	9	6.9
Ōhinerau – Mount Hobson	14.5	3	3	9	4.8
Ōwairaka/Te Ahi-kā-a-Rakataura – Mount Albert	9.5	3	3	9	3.2
Pukewīwī/Puketāpapa – Mount Roskill	9.1	2	3	6	4.6
Te Tātau a Riukiuta – Big King	7.3	2	3	6	3.7
Takarunga – Mount Victoria	6.8	2	3	6	3.4
		Total: 33		Total: 99	Mean: 4.9 ± 1.6

The percentage cover of different general land-cover types within each reserve was mapped in a GIS desk-top assessment using 2015 ortho-aerial imagery, aerial photographs, Auckland Council ecosystem spatial data and local knowledge by drawing polygons around each continuous land-cover type (Table 2). Five different general land-cover types were mapped: (1) Native ‘forest’ included areas with a continuous canopy cover of native trees or shrubs – in some cases, these ‘forest’ areas had partially developed understorey, shrub and ground tiers, but in many cases, these were absent; (2) exotic or mixed ‘forest’ was similar to native ‘forest’ but the canopy was dominated by exotic trees, or a mix of native and exotic; (3) treeland comprised pasture with a moderate-to-sparse canopy cover of native or exotic trees and shrubs; (4) pasture was dominated by low-lying grass species with little or no structural diversity from trees and shrubs – this class included both sports fields and areas of grassland used for passive recreation; (5) the final class included impermeable cover from roads, car parks and buildings (but not footpaths, which were included within the other vegetated classes).

Table 2. Percentage cover of different general land-cover types of the nine Auckland volcanic cone reserves (maunga) in this study.

Volcanic cone reserve Tūpuna Maunga	Native ‘forest’	Exotic or mixed ‘forest’	Exotic or native treeland	Pasture	Road, car park and buildings
Maungakiekie – One Tree Hill	6%	2%	40%	45%	7%
Maungawhau – Mount Eden	26%	0%	26%	36%	12%
Maungarei – Mount Wellington	1%	18%	0%	79%	2%
Ōtāhuhu – Mount Richmond	1%	18%	36%	34%	11%
Ōhinerau – Mount Hobson	1%	13%	14%	70%	2%
Ōwairaka/Te Ahi-kā-a-Rakataura – Mount Albert	18%	13%	21%	42%	6%
Pukewīwī/Puketāpapa – Mount Roskill	2%	7%	17%	67%	7%
Te Tātau a Riukiuta – Big King	14%	14%	23%	49%	0%
Takarunga – Mount Victoria	9%	5%	37%	42%	7%

Three 10 min bird counts were conducted at every survey station from 0700 to 1300 h between 28 January and 3 February 2016. For each count, every new bird detected during the first 5 min was recorded (Dawson and Bull 1975), with a second 5 min period recording only species that had not been detected in the first 5 min. This stationary point-count method requires the observer to be familiar with bird calls, because many identifications are audial, especially in forested areas. The method has been widely used in New Zealand for avian biodiversity surveys and to monitor population dynamics/trends (Anderson and Ogden 2003; Murphy and Kelly 2003; Greene et al. 2013). Although this method does not provide a population estimate, a readily obtainable and comparable index for each species, such as relative abundance, can be inferred (Anderson and Ogden 2003). Relative species abundance was calculated by dividing the total number of observations for each species respectively by the total number of individual bird observations. We also calculated detection probabilities for all species. This was achieved by dividing the number of sites in which each species was detected by the total number of sites surveyed.

Results

In total, 38 avian species (18 native and 20 non-native) were detected across the nine volcanoes. The highest total numbers of native species were detected at Maungarei (Mount Wellington, 12 species) and Takarunga (Mount Victoria, 12 species), followed by Mount Eden and Rakataura (Mount Albert), each with 11 species. Native species with the highest mean relative abundances were silvereyes (Zosterops lateralis), tui (Prosthemadera novaeseelandiae) and southern black-backed gulls (Larus dominicanus) (Table 3). Native species with the highest detection probabilities and that were detected at every site were silvereyes, tui, sacred kingfishers (Todiramphus sanctus) and grey warblers (Gerygone igata). Non-native species with the highest mean relative abundance were common mynas (Acridotheres tristis), house sparrows (Passer domesticus), Eurasian blackbirds (Turdus merula) and eastern rosellas (Platycercus eximius) (Table 3).

Table 3. Bird species recorded at nine surveyed volcano (maunga) sites within the Auckland Metropolitan area.

Bird species recorded	Relative species abundance	Detection probability (%)
Silvereye (Zosterops lateralis)	0.0173 ± 0.0043	100
Common myna (Acridotheres tristis)*	0.0146 ± 0.0035	100
Tui (Prosthemadera novaeseelandiae)	0.0125 ± 0.0032	100
House sparrow (Passer domesticus)*	0.0124 ± 0.0019	100
Eurasian blackbird (Turdus merula)*	0.0054 ± 0.0009	100
Eastern rosella (Platycercus eximius)*	0.005 ± 0.0017	89
European goldfinch (Carduelis carduelis)*	0.0043 ± 0.0008	100
Common starling (Sturnus vulgaris)*	0.0039 ± 0.0019	100
European greenfinch (Carduelis chloris)*	0.0031 ± 0.001	89
Southern black-backed gull (Larus dominicanus)	0.0031 ± 0.0017	89
Rock pigeon (Columba livia)*	0.003 ± 0.0013	100
Chaffinch (Fringilla coelebs)*	0.0028 ± 0.0007	89
New Zealand fantail (Rhipidura fuliginosa)	0.0025 ± 0.0008	78
Sacred kingfisher (Todiramphus sanctus)	0.0024 ± 0.0004	100
Grey warbler (Gerygone igata)	0.0023 ± 0.0006	100
Welcome swallow (Hirundo neoxana)	0.002 ± 0.0011	67
Yellowhammer (Emberiza citrinella)*	0.0019 ± 0.0008	67
Spotted dove (Streptopelia chinensis)*	0.0011 ± 0.0004	67
Song thrush (Turdus philomelos)*	0.0011 ± 0.0003	78
Eurasian skylark (Alauda arvensis)*	0.0011 ± 0.0008	33
California quail (Callipepla californica)*	0.001 ± 0.001	11
Spur-winged plover (Vanellus miles)	0.0009 ± 0.0005	67
Australian magpie (Gymnorhina tibicen)*	0.0008 ± 0.0003	56
Red-billed gull (Larus novaehollandiae)	0.0006 ± 0.0003	33
Chicken (Gallus gallus domesticus)*	0.0004 ± 0.0003	22
Pukeko (Porphyrio melanotus)	0.0004 ± 0.0001	56
New Zealand pigeon (Hemiphaga novaeseelandiae)	0.0003 ± 0.0001	56
Variable oystercatcher (Haematopus unicolor)	0.0003 ± 0.0001	11
White-fronted tern (Sterna striata)	0.0002 ± 0.0002	11
Common pheasant (Phasianus colchicus)*	0.0002 ± 0.0001	33
Shining cuckoo (Chrysococcyx lucidus)	0.0002 ± 0.0001	33
Caspian tern (Hydroprogne caspia)	0.0001 ± 0.0001	11
Peafowl (Pavo cristatus)*	0.00004 ± 0.00004	11
Black shag (Phalacrocorax carbo)	0.00004 ± 0.00004	11
Dunnock (Prunella modularis)*	0.00004 ± 0.00004	11
Mallard (Anas platyrhynchos)*	0.00004 ± 0.00004	11
Pied shag (Phalacrocorax varius)	0.00004 ± 0.00004	11
White-faced heron (Egretta novaehollandiae)	0.00004 ± 0.00004	11

Notes: Species are listed in decreasing order of total relative species abundance across all sites (mean ± standard error of the mean) per 10 minute count (total number of individuals detected = 2389). Detection probabilities are also shown.

*Non-native species.

Discussion

In total, 18 native and 20 introduced bird species were detected within the nine volcanic cone reserves that were surveyed. Native species included silvereyes, tui, shining cuckoos (Chrysococcyx lucidus), grey warblers, as well as other comparatively rarer species such as Caspian terns (Hydroprogne caspia) and white-fronted terns (Sterna striata). Introduced species such as common mynas, house sparrows, Eurasian blackbirds and European goldfinches (Carduelis carduelis) were detected at all nine sampled reserves.

Silvereyes and tui were found in high relative abundance at all sites. From an ecological perspective, these avian species and other natives found on Auckland's volcanoes, including New Zealand pigeon (Hemiphaga novaeseelandiae), are critical for maintaining healthy ecosystems. For example, tui and silvereyes are important pollinators of native trees such as pōhutukawa (Metrosideros excelsa) (Anderson 2003). Tui are also important pollinators of native species such as flax (Phormium tenax), kōwhai (Sophora microphylla) and puriri (Vitex lucens) (Craig and Stewart 1988; Anderson 2003). Silvereyes and tui are important dispersers of the small fruits of native plants such as karamu (Coprosma robusta), red matipou (Myrsine australis), and kohekohe (Dysoxylum spectabile). New Zealand pigeon eat and disperse larger native tree fruits such as that of tawa (Beilschmiedia tawa), puriri, karaka (Macropiper excelsa), fivefinger (Pseudopanax arboreus) and pigeonwood (Hedycarya arborea) (Kelly et al. 2006; Wotton 2007; Wotton and McAlpine 2015). These native plants are currently found, with varying degrees of abundance, on Auckland's volcanoes. The high relative abundance of native birds such as silvereye and tui found in this study suggests that the volcanic reserves continue to retain some remnants of their former ecological functions, e.g. the dispersal, pollination and recruitment of native plants by native birds. These reserves contain important biodiversity, which would benefit from continued and intensified restoration projects, and thus become managed for their biodiversity as well as their recreational values.

Numbers of exotic birds, such as common starlings (Sturnus vulgaris) and European goldfinches, were high across the nine reserves. These species favour open agricultural land, so this finding reflects the large amount of open habitat/pasture on the volcanoes. Future ecological restoration at these sites, such as the regeneration or planting of native bush, may support an increase in the proportion of native to non-native avifauna, as well as other indigenous biodiversity. Indeed, recommendations for future management efforts have suggested that some native forest and more herbaceous native plant species should be established at these reserves (Burns et al. 2013). Limiting the usage of fertiliser within the reserves is recommended to reduce soil nutrient levels down to more natural levels (Burns et al. 2013). This would in turn reduce the abundance of exotic grasses, such as kikuyu, that can out-compete native plants.

Planting of presently uncommon native plants such as maidenhair vine (Muehlenbeckia complexa), large-leaved muehlenbeckia/pohuehue (Muehlenbeckia australis), Austral bracken (Pteridium esculentum) and ring fern/mātata (Paesia scaberula) will increase groundcover biodiversity within the reserves (Burns et al. 2013). Groundcover plants help protect soil, retain moisture and keep the roots of taller plants cool in the heat (Bird et al. 1992; Fish 2002). Collectively, these efforts would probably support native avifaunal populations in the reserves by increasing the amount of native plants and forest available for foraging and nesting. Varying plantings across the reserves so as to include areas of both forest and open-habitat species would likely help support a wider range of native biodiversity such as lizards and invertebrates.

Control programmes for mammalian predators are currently in place on the volcanic cones; however, an area of concern within these volcanic cone reserves is the high relative abundance of common mynas (the second most abundant species we observed overall). Mynas are classified as a pest in the current Auckland Council regional pest management strategy (Auckland Regional Council 2007), and can harm native and non-native bird populations by predating eggs and chicks (Byrd 1979; Byrd et al. 1983; Hughes et al. 2008; Canning 2011). Mynas also compete with native species for territories and nest cavities (Feare and Craig 1998; Dhami and Nagle 2009). Evidence of the serious threat that myna can pose to native species is clearly demonstrated for the island endemic Seychelles black flycatcher (Terpsiphone corvina, where egg and chick predation has decreased their reproductive success; Canning 2011). The common myna is also known to negatively affect the abundance of bird species smaller than itself including fantails (Grarock et al. 2012, 2014). Myna eradication programmes have previously been successfully implemented in New Zealand, and many different myna control strategies can be employed. For example, on Moturoa Island, Bay of Islands, a combination of poisons and decoy birds in traps was used. Consequently, a significant increase in native bird species was reported following the extirpation of mynas from the island (Tindall et al. 2007). In this study, trapping was found to be the most effective technique to control myna numbers (Tindall et al. 2007).

As restoration projects are executed at Auckland's volcanoes in the future, further avian biodiversity surveys will be useful to assess and monitor their ecological impact. Bird survey stations have now been established within all nine volcanic cone reserves. Data collected at these stations will provide baseline data for monitoring biodiversity changes in these reserves over time. Further volcanic cone reserves may be added to the list of reserves designated for restoration. Hence, we recommend establishing similar surveys to enable us to evaluate and prioritise these novel restoration activities.

Acknowledgements

We would like to thank Jade Khin at Auckland Council for providing technical support to this project.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

We are grateful to Auckland Council Parks for their financial support.