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Revegetating for Habitat in Parks, Gardens and Roadsides
Does it matter if we use exotic, Australian or locally native plants?
It is often suggested that we need to plant native, especially locally native, plants in our public and private landscapes to provide our native animals with the food, shelter and breeding sites they require. I was interested to see if there were any studies to support the proposition that native animals prefer native plants.
The Australian fauna has evolved using Australian plants. There is now a great deal of information on the various uses made of native plants by our animals and the dependence of many of those plants on our native birds, mammals and insects for pollination, seed dispersal and pest control.
However, are Australian animals able to adapt to using exotic species for all their needs?
Most of the relevant work concentrates on birds and I've looked at a number of studies and one survey to try to find some answers.
A Melbourne suburban study (Green, 1986) carried out in the late 1970s found that native birds as a group depended on native vegetation. This was found to be partly due to the greater number of insects amongst native vegetation. All the common exotic birds were mainly ground foragers while many native birds found their food on foliage and bark.
A survey carried out in 1991 (Wilson) by the Bird Observers Club of Australia attempted to determine the most important factors which attract native birds to gardens. They looked at exotic, mixed and native gardens. The survey concluded that:
A Brisbane study carried out in 1991/92 (Sewell) suggests that, to encourage bushland bird communities back into suburbs, we need to create shrub layers with similar proportions of plant species as those found in undisturbed areas, as well as increase the size of plantings, reduce human disturbance and predators.
- the presence of tall eucalypts or other tree species appropriate to the region was more important than "mainly native planting";
- the presence of dense middle and ground level shrubs planted beneath and adjacent to trees was more influential than the choice between natives and exotics; and
- the presence of permanent water would increase the number of bird species visiting a garden, whatever the garden type.
|"....to attract the greatest variety of birds to a vegetated area you must provide a well developed shrub and herb layer as well as a range of tree species...."|
Despite some differences in the importance given to mixed or all native plantings, all writers in this area agree that to attract the greatest variety of birds to a vegetated area you must provide a well developed shrub and herb layer as well as a range of tree species so as to offer as much surface area for wildlife use as possible. This is known as structural diversity and is considered to be much more important than floristic diversity (or the range of plant species).
There are, however, habitat problems which may be caused by planting species exotic to your area. Some of these problems are:
It would appear that, with native birds at least, the problem is not that no native birds use exotic plants, but rather that exotic (or alien native) plants cater for only a small number of unspecialised native birds who are often territorial and quite aggressive and once in large numbers, appear to harass and predate on the smaller birds which are already under great pressure.
- Invasion of nearby natural areas by planted species, from wind or water or ant dispersed seed or dumped garden refuse (eg. golden wreath wattle or orange wattle, Acacia saligna, from Western Australia has naturalised in bushland around Sydney; Impatiens; etc).
- Invasion of distant natural areas by berry-bearing planted species from seed dispersed by fruit-eating birds (eg. cotoneaster; Indian hawthorn; African olive; camphor laurel, etc).
- Declining numbers of native animals with very specialised diets. For example, glossy black cockatoos eat only the seeds of various species of Casuarina and they need large, old eucalypts with deep hollows for nesting. Their populations are declining everywhere as land clearing removes their required habitat. An appeal is being mounted to educate landowners and encourage them to to provide the species needed by these birds.
Also, a number of small possums (sugar and squirrel gliders and the rare Leadbeter's possum) have been shown to depend on Acacia gum as an important source of carbohydrate during winter, when other sources of energy-rich food such as nectar and some insects are scarce. The quality and quantity of gum produced by different wattles are highly variable and it is the abundance of suitable wattles (and the availability of nest hollows) which determines the numbers of these possums.
The beautiful birdwing butterflies of north-eastern coastal Australia have larvae which feed exclusively on species of Aristolochia which are native rainforest vines. If they lay their eggs on the introduced garden plant Aristolochia elegans (Dutchman's pipe), the caterpillar fails to survive.
- Declining numbers of insectivorous and seed-eating animals. Only a small range of insects and spiders found on native vegetation are found on exotic plants (eg. privet is eaten only by the larvae of a native hawk moth) and most exotic plants do not produce seeds that can be eaten by seed-eating birds.
|"....the problem is not that no native birds use exotic plants, but rather that exotic plants cater for a smaller number of unspecialised native birds...."|
- Increased numbers of some native bird species which are able to make use of a wide variety of exotic plants. These birds, such as pied currawongs, noisy miners, crested pigeons, rainbow lorikeets, red wattlebirds, Australian magpies, welcome swallows, pee wees and willie wagtails are able to make use of more open areas (where the native shrub layer has been removed) or exotic fruit (like cotoneaster), or exotic flowers (like coral trees), or exotic seed (unmown lawns). While some increases in population seem harmless, others appear to apply great pressure to the smaller birds whose populations are already under threat from clearing of habitat, reduction in food and shelter trees, etc. The larger honeyeaters (like noisy miners and red wattlebirds), who have quite unspecialised diets and who are also favoured by the often unnaturally high occurrence of planted flowering eucalypts and grevilleas which are heavy nectar-bearers, are very aggressive to smaller native birds and will chase them out of their territories (and away from food plants). Pied currawongs will eat small birds and nestlings and appear to be having quite an impact on Sydney's populations of small birds.
There are, of course, other components besides vegetation which are important in habitat creation. These include access to clean water and safe breeding, roosting, basking and display sites, the shelter provided by exposed rocks, fallen timber and leaf litter, and the availability of prey such as insects, fish, mammals, reptiles and frogs.
- Berry, S. (1992) Wattles and Wildlife. In "Land for Wildlife Note No. 17", Dept. of Conservation & Environment, Vic.
- Buchanan, R. (1992) Birds, Beasts and Seed Dispersal. In proceedings from "Urban Bushland Management Conference", Ku-ring-gai Municipal Council, NSW.
- Catterall, C.P., Green, R.J., & Jones, D.N. (1991) Habitat use by birds across a forest - suburb interface in Brisbane: implications for corridors. In Saunders D.A.& Hobbs R.J.(eds.), "Nature Conservation 2: The Role of Corridors", Surrey, Beatty & Sons, Aust.
- Chafer, C. (1994) Project Currawong - The Results. In "Cumberland Bird Observers Club Newsletter", Vol 15, No. 4.
- Fox, M.D. & Adamson, D. (1986) The ecology of invasions. In Recher H.F., Lunney D. & Dunn, I. (eds.), "A Natural Legacy : Ecology in Australia", Pergamon Press, Aust.
- Gilmore, A.M. (1985) The influence of vegetation structure on the density of insectivorous birds. In Keast A., Recher H.F., Ford H.& Saunders D.(eds.), "Birds of the Eucalypt Forests and Woodlands", Surrey Beatty & Sons, Aust.
- Green, R.J. (1986) Native and exotic birds in the suburban habitat. In Ford H.A.& Paton D.C.(eds.), "The Dynamic Partnership : Birds and Plants in Southern Australia", Handbooks Committee of S.A. Government.
- Gullan, P.K. & Norris K.C. (1981) Floristic classifications, small mammals and birds. In Gillison A.N.& Anderson D.J. (eds.), "Vegetation Classification in Australia", CSIRO, Canberra, ACT.
- Johnson, I. (1994) Australian Museum Nest Test. In "Cumberland Bird Observers Club Newsletter", Vol 15, No. 3.
- Low, T. (1994) Invasion of the Savage Honeyeaters. In "Australian Natural History", Spring 1994.
- Loyn, R.H. (1987) Effects of patch area and habitat on bird abundances, species numbers and tree health in fragmented Victorian forests. In Saunders D.A., Arnold G.W., Burbidge A.A., Hopkins A.J.M.(eds.), "Nature Conservation : The Role of Remnants", Surrey Beatty & Sons, Aust.
- Majer, J. (1990) The Greening of Australia: Taking the Animals into Account. In proceedings from "Sowing the Seeds : Direct Seeding & Natural Regeneration Conference", Greening Australia, ACT.
- Majer, J. & Recher, H.F. (1994) Revegetation in Urban Areas: An Opportunity for Wildlife Conservation. In proceedings from "A Vision for a Greener City: The Role of Vegetation in Urban Environments Conference", Greening Australia, ACT.
- Sewell, S. (1992) The effects of residential development, forest fragmentation and loss of understorey on bushland bird communities in a subtropical city. Griffith University Hons. Thesis, Qld.
- Wilson, Z. (1991) Birds and Gardens Survey Report No. 3. Bird Observers Club of Australia, Vic.
- Young, D. (1993) Glossy Black Cockatoo Habitat Decline. In "The Bird Observer", No. 735.
Updated by Danie from her article in the November 1995 issue of the newsletter of SGAP's Garden Design Study Group.
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Australian Plants online - March 1997
The Society for Growing Australian Plants