City Life: Urban Areas as Sinks, Sources and Refuges for Plants and Animals

Catherine J. Tait & Assoc. Prof. Chris Daniels – School of Earth and Environmental Sciences, Discipline of Environmental Biology, The University of Adelaide

Introduction

Urban ecosystems are where people live at high densities (equal to or greater than 186 people/ km²).  Currently, slightly less than half the world’s population lives in dense urban areas, particularly cities. In industrialised countries this percentage is closer to 80%.  Nearly 85% of Australians live in towns with 1000 or more inhabitants and 2/3^rd of the population live in the eight capital cities.  Urban systems are dynamic environments in which the assemblages of the constituent flora and fauna are a direct result of the human influence.  Two particular concerns for biological diversity in urban environments include the maximisation of native biodiversity within the city and the management of problem species (e.g. introduced and native weeds).  As the world’s population increasingly includes people with an urban lifestyle, it is predominantly in this urban environment that they will gather their views on nature.  If all the plants and animals are portrayed in a negative light because of a lack of scientific knowledge, the average city dweller will not appreciate the richness and diversity of life around them and won’t be able to transfer this environmental awareness to areas outside of the city.

Adelaide: A Unique City

Adelaide is unique among Australian capital cities due to its position in the South Australian landscape, between the hills and the sea, isolating it from other regions.  The Adelaide metropolitan region spans eighty kilometres, from Sellicks Beach in the south to Gawler River in the north, and is bound by the sea of Gulf St Vincent in the west and the escarpment of the Mt Lofty Range in the east and south (the Development Act 1993).  The Adelaide region includes a wide range of vegetation types and is characterised by its unique combination of biota.  The Adelaide plains are situated within plant and animal species ranges from both the western mediterranean-type ecoregions and the southeastern ecoregions of southern Australia (often the extreme east or west of these home ranges).  •Historically, Adelaide has supported a diverse range of natural habitats.  The dominant land use for the Adelaide area now is cropping and pasture (slightly greater than 2/5ths), urban habitats (slightly less than 2/5ths), and the remaining 1/5th is composed of remnant vegetation, orchards, irrigated areas, water bodies, quarries, swamp, hardwood plantations, and pine plantations.  Before 1836, the Adelaide Plains supported approximately 1130 species of native vascular, garden escape plants, approximately 290 species of bird (including migratory and nomadic species), 40 mammals, 56 reptiles and 7 amphibians.

Original Biodiversity

Adelaide probably had the richest source of biodiversity in South Australia before European settlement.  Most of the area was originally lightly wooded, with grey box (Eucalyptus microcarpa), peppermint box (E. odorata) and river red gum (E. camaldulensis var. camaldulensis) as the dominant species.  Xanthorrhoea spp. (grass trees) were common along the foothills, and trees such as the quandong (Santalum acuminatum) and native cherry (Exocarpus cupressiformis) were also present.  The understorey was species-rich.  Extensive grasslands were located on the lower plains, dominated by Austrostipa and Lomandra spp..  The coastal environments contained Allocasuarina spp., Eucalyptus spp. and Acacia spp., found growing among the sand dunes.  Beyond the dunes were areas of swamp and reedy marsh that drained into the estuaries of the Port and Patawalonga Rivers, a mangrove forest environment (Avicennia marina var. resinifera, grey mangrove).  There has been heavy vegetation clearance for the development of townships, grazing and agricultural practice.  For example, the Black Forest (Eucalyptus microcarpa, grey box woodland) south of the central districts, the eastern woodlands and the mallee woodlands in the northern areas were cleared by 1880.  The Reedbeds in the western section of the plains, the Pinery at Grange (named after the native pine Callitris preissii), and the coastal dune area remained intact until the rapid urban and economic development predominantly after World War II.  Less than 4% of the natural vegetation now remains on the Adelaide plains.  The larger patches of remnant vegetation are in National, Conservation and Recreation parks, in vacant blocks, roadside verges, ponds and rivers, railway embankments, and untended/rarely tended suburban parks.  The major threat to the remaining remnant vegetation is through the spread of weedy species.

Adelaide’s Biodiversity: Changes through Time

A database on both the native and introduced species present in Adelaide between 1836 and 2002, was created using close to 200 historical and recent sources of information.  The data was collated to produce graphs showing the changes in species richness through time.  Some of the overall patterns of change were:

• 30% increase in the total number of all species, with around 132 native species lost and 648 introduced.
• The plants increased by 524 species (46% increase over the original).
• The native plant species have decreased by approximately 8% (89 species).
• The vertebrates experienced an overall decline of 12 species, or 3%.
• 50% (20 out of 40) of the original native mammal species were lost.
• The amphibians did not lose or gain any species.

The increase in total species numbers due to introductions, as a percentage of the original species numbers, is 54% in plant species from 1836 to today, 22.5% in mammals, 7% in birds, 3.5% in reptile species.  The high numbers of introduced species in Adelaide may be a result of the high levels of many soil nutrients in the urban area, especially phosphorus. Concentrated nutrient supplies promote invasion of exotic plants, and also result in changes to the composition of plant communities.

Plants

These three graphs were created from my database results.  The bottom axis is the year, the left axis is the proportion of species present, and the right axis is the total number or count of species present.  The total plant species graph is a combination of the changes to both native and introduced species.  There was relatively little change in species numbers from 1960 to 1996.

Native plants

Looking at the native plant species graph, it can be seen that the number of species remained relatively constant from 1836 (1136) to 1995 (1125).  The drop in 1996 was because of the recording of a substantial number of natives as extinct (mostly Darrell Kraehenbuehl’s data), whereas previously no one had actually said that they were extinct although they hadn’t been seen for many years.  The real loss of native plants is not actually known because there may still be some viable seeds in the seed bank.  The plant community most destroyed by agricultural and grazing practice was the box grassy woodlands that used to dominate the Adelaide plains.  Due to their location in areas of reliable rainfall, these woodlands were targeted for agricultural development, and as open systems they were preferred grazing areas.

Plant extinctions could have resulted from the invasion of exotic species, which can out-compete some of the winter-growing native species by either germinating earlier or surviving over summer, and then growing rapidly (often after the first rains of the season) while relying on tubers for energy and water.  The very few remaining examples of box grassy woodlands in the Adelaide area are highly susceptible to weed invasion, especially by soursobs (Oxalis pes-caprae).  Agriculture and grazing were especially important mechanisms of native vegetation removal within the Adelaide area from the early years of settlement.

Some other reasons for the loss of native species include the fact that the River Torrens was polluted by as early as the mid-1840s, and that large vegetation removal for roads and railway lines enhanced the fragmentation of Adelaide, by further increasing the proportion of edge to center habitat, as well as encouraging the spread of weedy species and exotics.

Introduced plants

Looking at the introduced plant species graph, there were two periods of rapid species increase: 1836-1838 and 1856-1858.  Initially, three species were introduced (Olea europea (olive), Medicago sativa (lucerne), and Chloris truncata (windmill grass)).  There were 105 new species in Adelaide by 1858.  The large numbers of introduced plant species arriving so early on resulted from the rapid and intentional modification of the South Australian landscape.  By 1851, there were weeping willows, olives, roses, fuchsias, vines, and lemon and pear trees growing freely around Waterfall Gully.  Of the more than 600 introduced species of plants, some were intentionally released as:

• ornamentals
• for food
• erosion control
• fodder for animals.

Others were accidentally introduced as:

• contaminants of agricultural of horticultural seeds,
• in earth attached to footwear, livestock and agricultural machinery,
• seeds stuck on clothing, fleece and fur of animals, or on sacking and other materials.

Some of these introduced plants have become environmental weeds (e.g. Echium plantagineum (salvation jane), Chrysanthemoides monilifera (boneseed) and Pennisetum clandestinum (kikuyu)).

Introduced trees

Over time, the central Adelaide plains has effectively gone from an open woodland environment with grassy understorey and low native shrubs, to a dense forest habitat of fewer species (mostly introduced), almost no understorey and lawns/introduced grasses (Kikuyu).  The highest levels of introductions occurred within the annual, herbaceous perennial and tree groups, with a 160% increase for the trees (with no native tree extinctions).  It is possible that the longevity of tree species, in comparison to the annual and herbaceous perennial groups, has led to the phenomenon of high rates of introductions with no loss of native species richness.  We may just be experiencing a longer time lag between introduction and extinction, so that it is probably very important to include monitoring of tree populations, as well as species richness.

View from Windy Point 1895 (above) and 1985 (below)

Source of Introduced Plants

The majority of introduced plant species have come from Europe, the Mediterranean, America, and Southern Africa.  The spread of plant species from Europe and the Mediterranean has reduced/ stopped, while species from America and Southern Africa are still continuing to be introduced into Adelaide (possibly a result of changes in trade, exports/imports, and tourism).  The high proportion of introduced Mediterranean species in the Adelaide area, as well as the presence of native species with Mediterranean ecological characteristics (flower and fruit during raining months) has led to the potential co-existence between introduced species (flower and fruit during dry summer time) with native species that flower and fruit at different times of year (usually winter).  These ecological characteristics of many of the introduced species possibly explain why so many introduced species have survived in Adelaide along with twice as many native species.

Birds

The total number of bird species present in 1836 was 286 species; by 2002 it was 283 species.  21 native species of birds have been lost, while 20 introduced species have arrived.  The greatest rate of extinction has occurred over the past 30 years.  The major decline in species richness began in 1959.  From 1959 to 2002, 15 species were lost.  Before 1959 the long-tailed finch and ground parrot were lost.

The Eurasian skylark, European goldfinch and common starling were the first introduced birds to be recorded in Adelaide, and this was in 1868.  Members of the South Australian Acclimatisation Society were releasing exotic bird species into the Adelaide area mostly during 1879-1887.  Several species of granivorous birds were introduced (starling, skylark, blackbird, goldfinch and greenfinch) increasing competition and affecting the habitat resources.  The crested pigeon, mallard and Indian mynah were first recorded in 1952-1953.  •The azure kingfisher disappeared by 1969.

The following graphs show changes to specific groups of bird species:

The drop in insectivorous birds in 1878 was due to the loss of the inland thornbill, Horsefield’s bushlark, variegated fairy-wren and Gilbert’s whistler.

Four native nectarivorous birds are no longer recorded in Adelaide: the swift parrot, and the regent, blue-faced and striped honeyeaters.  These absences are a recent phenomenon all occurring after 1969, and in fact the swift parrot, regent and striped honeyeaters have only not been recorded since 1990.  They can’t actually be called extinctions since a minimum of 50 years without sitings needs to occur before ‘extinction’ is official.  However, because of the extensive activity of professional and amateur ornithologists in the Adelaide region, it is more than likely that birds no longer seen today are indeed locally extinct.

Granivorous species, such as the little corella and galah, have increased their distribution into the Adelaide area (doubling the total number of granivore species in Adelaide) with the advent of more watering points for grazing, and spilt grain from grain production providing reliable food sources.

The species commonly found in woodland habitat (e.g. the owls, many of the lorikeets, cockatoos and parrots, songlarks, brown-headed cisticola, scarlet and flame robins, and many of the other small- and medium-sized birds) have experienced the most substantial decline in species richness to date.  There was a loss of 11% in the proportion of woodland bird species present in the Adelaide area from 1836 to 2002.  12 native species were lost. The period of greatest species decline occurred from 1958 to 1975, with a loss of approximately two species every five years.  Of these woodland birds, the ground-dwelling or ground-foraging species, such as the painted button-quail, hooded robin, Jacky winter, restless flycatcher, southern whiteface and diamond firetail among others, particularly showed declines in distribution.  Woodland habitats were disproportionately affected by the clearance of the Adelaide area.

The small and medium sized bird species (e.g. many waterbirds, thornbills and honeyeaters) are showing a substantial recent decline in the number of species still present in Adelaide.  This is very worrying and indicates that of the species still remaining, their population sizes are more than likely decreasing.  The major reasons for the loss of these species is under investigation and is probably a combination of factors such as: decline and then loss of critical understorey plant habitat, decline in abundance and local availability of insect and nectar resources, and so on.

Reasons for the Loss of the Native Birds

There are several major reasons for the loss of native birds.  These include the fact that Australian birds are thought to be relatively long-lived, implying that small but non-viable (non-reproducing) populations of species may have persisted in urban vegetation remnants for long periods before the last remaining individuals died out.  This could explain the patterns of extended time without loss of bird species, followed by the recent rapid decline in species numbers.  This project only focussed on the numbers of species, not population sizes, thus changes along these lines were difficult to assess.  Other contributing factors include the loss of relatively unfragmented habitat outside the city, changes in urban predators (such as foxes and cats) and changes in the nature of the habitat within the city but outside the bushland reserves (e.g. changes to private and public gardens and changes to street trees).  The loss of native understorey plants, as well as increases in particular exotic plants in urban areas, such as berries, have led to the change in species composition.  An increase in resource competition between bird species and individuals, due to dwindling resources, possibly also contributed to population declines and subsequent extinction.  For example, if the movement of nectarivorous birds was on a seasonal basis, then the loss of particularly important seasonal resources may have caused the local extinction of the regent honeyeater and swift parrot from the Adelaide area.

Mammals

The total number of mammal species declined from 40 to 29 species present, (27.5%).  Initially, there was an increase, with two jumps at 1836-1840 (three species were introduced, the feral goat, the feral cat and the pig) and 1858-60 (a further two species arrived, European rabbit and red fox).  Between 1900-1930 there was a loss of thirteen species.  The first three native mammals recorded as absent (which occurred in 1909), were the euro, brush-tailed bettong, and the spotted-tailed quoll.  After 1960, a slow decrease in species numbers occurred.  From 1960 to 2002, a further two species (wild dog and feral goat) were lost and five species (fat-tailed dunnart, common wombat, feathertail glider, dingo, and white-footed tree-rat) were recorded as absent.

The introduced mammals had mostly arrived by around 1870.  Looking at the native species graph, you can see a time lag of approximately 30 years between the arrival of most of the introduced mammal species in 1870, and the beginning of the decline of native species richness in 1900.  There is a slow decrease in species numbers after 1960 where the fat-tailed dunnart, common wombat, feathertail glider, and white-footed tree-rat, were recorded as absent.  Overall, there has been a large decline in mammal species numbers.  This was probably the result of a number of ecological disturbances, such as:

• the introduction of hunting by Europeans,
• clearing and fragmentation of native vegetation for agriculture and urban development,
• grazing by sheep, cattle, rabbits, and other stock (both feral and domestic),
• altered fire regimes,
• introduction and spread of exotic diseases, and
• increased predation from introduced species such as foxes and cats.

The impact of hunting was probably not as important to the observed species decline as the invasion of introduced rabbits and foxes, and the changes in land use and land management practices within the urban and suburban areas of Adelaide.

Most of the recorded extinctions were marsupials and the rodents.  The bats have experienced few, if any, extinctions.  The platypus has been lost from the Adelaide area.  Almost all the medium-sized ground-dwelling marsupials (e.g. bettongs and quolls in the weight range 35.0g to 5.5kg) have become extinct.  The decline of the medium-sized ground-dwelling species has been attributed in part to the effects of grazing reducing the availability and constancy of food resources and shelter from predation (by removing the understorey layer).

No mammal habitats have been completely cleared, and none of the extinctions can be attributed entirely to the clearing of native vegetation.  The invasive mammals, principally rabbits, foxes and cats, effectively altered physical and ecological characteristics of the habitat, thus leading to the local extinction of native fauna, such as the burrowing and brush-tailed bettongs.  Rabbits potentially enhance the negative impacts of introduced predators by supporting large populations of these predators.  During the period of rabbit (and fox) introduction and spread, there was also large-scale clearing of native vegetation in Adelaide for agricultural and grazing purposes.  The impact of these introduced mammals on fauna species richness was thus compounded by the reduction and fragmentation of native habitat occurring simultaneously.

The Adelaide mammalian fauna originally consisted of two ecological groups: the plains group and the hills group.  The plains to the east and west of central Adelaide were predominantly grassy woodlands and the mammals living in this habitat were mostly wide-ranging animals adapted to living in open habitat, or restricted to the immediate vicinity of water, such as the water and swamp rats.  These species included the kangaroo, wallaby, bettong, bandicoot, quoll, echidna, and possum.  The greater bilby and brush-tailed bettong survived in the northern areas of the plains until rabbits and foxes finally eliminated them by 1900.  The hills supported species with smaller home ranges, such as the yellow-footed antechinus and the bush rat, in the dense vegetation and gullies of the western face of the Mount Lofty Ranges, which, with lower annual rainfall than the plains, did not initially encourage European settlement or agriculture.  These reasons, together with the ability of many of the hill-dwelling mammals to live in relatively small areas, have allowed the survival of many of the original hill species.  The plains were cleared, thus decreasing habitat area as well as introducing predators, while the arboreal, hill-dwelling species were less impacted by clearing and agriculture.  The smaller home ranges of the hill species allowed the Conservation and National parks in the region to be potentially big enough to support viable populations.

Reptiles and Amphibians

The total reptile species richness did not change from 1836 to 2002.  Two species were introduced and two native species (out of a total of 55) became extinct.  The two species introduced were the common long-necked tortoise (an amphibious, nocturnal species) and the water dragon (an arboreal, diurnal species).  The two species that went locally extinct were both terrestrial species.  One was nocturnal (common death adder) and the other diurnal (pygmy bluetongue).  The common death adder is a recent loss, while the pygmy bluetongue has not been recorded in the Adelaide area since 1952.  Changing the vegetation-type of the Adelaide area through the extinctions and introductions of plant species may not have impacted on the reptile fauna as much as on the birds and mammals, resulting in the minimal change to reptile species numbers.

None of the seven native amphibian species became extinct (although it is now debateable whether the southern bell frog was actually a native species, or has only been introduced into the Adelaide area at Salisbury and then into the Onkaparinga in the late 1950s/60s).  The other native frogs found in Adelaide are: the common froglet, short-legged (Bibron’s) toadlet, painted (trilling) frog, marbled (spotted grass) frog, eastern banjo (bullfrog) frog and the brown tree frog.  It is amazing that no amphibian species have been lost from Adelaide since it would be impossible to find a creek in pristine condition because, apart from direct pollution such as litter, the indirect impact of air pollution has permeated the whole region.

Future

BioCity: The Centre for Urban Habitats, a partnership between the leading environmental institutions in the State, was formed to promote urban environmental awareness and establish environmental initiatives in the urban area.  The role of BioCity is to interact with local and state governments, industry and the community to undertake urban research and to develop strategies that will hopefully enhance urban living and promote the communication and education of the importance of our local environment.  BioCity is aiming to demonstrate that the development of the city can be undertaken in a manner that, allowing for multiple-use issues, maximises environmental quality and preserves the heterogeneous nature of our garden city, but which minimises the introduction of pest species and allows for the conservation of native fauna and flora.

Total plant, mammal, bird, reptile and amphibian species numbers in the Adelaide metropolitan area during 1836, 1900, 1950 and 2002.