- Last updated:
- 06 Jun 2022
Article and images by Dr Julie O'Connor, Senior Conservation Partnerships Officer
“The cities of the 21st Century are where human destiny will be played out, and where the future of the biosphere will be determined. There will be no sustainable world without sustainable cities” Herbet Girardet
In 2020, a little over 56% of the world’s human population were living in urban areas. In Australia, that figure nudged just over 86%. We might be tempted to think that our opportunity to contribute to biodiversity in urban environments is limited. But just changing what you plant in your garden or on your apartment balcony can bring a major boost to biodiversity in your own backyard.
Australia is home to more than 24,000 native plant species, but we are now also home to 27,500 plant species that evolved elsewhere. While not all introduced plants become well established or naturalised, an estimated 10% do, and many of those have either become invasive or have the potential to do so.
The impact of an invasive plant species extends beyond just taking up space that could be occupied by a comparable native species. The most serious impact relates to the use of plants by phytophagous invertebrates, i.e. those invertebrates who use a plant or its products for food. Numerous studies have recorded reduced phytophagous invertebrate abundance and species richness in communities dominated by non-native species compared to those dominated by native species. This reduction of species abundance and diversity at lower trophic levels can reverberate through an entire ecosystem to negatively impact predatory invertebrates, reptiles, amphibians, birds and mammal species who depend on invertebrates for their survival.
You will have noticed that most introduced plant species look great. Their leaves haven’t been nibbled by phytophagous insects and they often seem to be free of disease. So why do introduced plants generally support a depauperate invertebrate assemblage? Firstly, in most cases, plants arrive at their new location without their entire suite of pathogens and predators that they co-evolved with. Secondly, and importantly, the native suite of predators and pathogens in the plant’s new home are likely to ignore the newcomer.
Plants produce a range of compounds (e.g. phenols, terpenes, and alkaloids) to help them defend themselves against phytophagous insects. The chemical arsenal the plant produces can be either toxic or repellent to insects, thus protecting the plant from insect attack. While plants were spending thousands of years developing these defences, the insects living alongside them devoted their time figuring out ways to get around those defences. So when a new plant with its own unique set of chemical defences arrives on the scene, particularly where it has few close relatives, a native insect is likely to recognise the plant as non-palatable, toxic, or in other ways unsuitable, and leave it alone. The flow on effect of this has huge implications for the ecosystem.
In a terrestrial ecosystem green plants are the productive base of the food chain. The phytophagous invertebrates that feed on those plants then act as a food source for a range of consumers, including other invertebrates, amphibians, reptiles, birds, and mammals. That, of course, is a very simplified depiction of more complex feeding interactions. For some species, its trophic level (i.e. the position it occupies in a food chain) is not fixed. Many organisms feed at several trophic levels for all or part of their lives. For example, many birds can be nectar feeders for part of the year but will include insects such as caterpillars and other arthropods while raising young.
An example of how dominance of introduced plants can have a flow on effect through an ecosystem can be seen in the butterflies and moths. For example, there are 416 known butterfly species on the Australian mainland, only 105 of which are known to utilise introduced plant species as larval food plants. In almost all cases, however, the introduced plant is a close relative of the butterfly’s native host plant.
The common grass blue uses at least 11 known genera of introduced plants, but all within the Family Fabaceae. The long-tailed pea-blue also utilises a wide range of introduced plants (at least 16) as larval hosts, but they also all fall within the Family Fabaceae. The native larval host plants of both butterflies also belong to Fabaceae.
Similarly, the evening brown and the leafwing have each been recorded utilising six introduced plant genera as larval hosts, but all within their native host plant Families Poaceae and Acanthaceae respectively.
The process for native invertebrates to adapt to an introduced plant species is almost always a very slow one. For example, in Australia the native Melaleuca quinquenervia is known to be a food plant for at least 409 phytophagous arthropods. But in Florida, where it was introduced over 100 years ago, it is still a food source for only eight known species. Examples of faster adaptations to introduced plants do exist, but they are the exception rather than the rule.
Most terrestrial birds rear their young on insects. And as insects go, caterpillars are the crème brulee of baby bird food – soft fat little packages of protein, lipids and other nutrients essential to growth. If you want to boost urban biodiversity, start with gradually replacing the introduced plants in your garden with locally native species. And don’t worry about the chewed leaves - they are a sign that you are doing something right. The plant will recover, and birds will thank you for it.
If you would like to glimpse the productive relationship that exists between native plants and insects, have a look at the wonderful Xanthorrhoea flower spike filmed recently by council’s Waterways Technical Officer, Gordon Agnew.
For more information on choosing the best native plants for your area, visit one of your local native nurseries.
- Barung Landcare Nursery
- Florabunda Bushcare Community Native Nursery
- Mooloolah River Landcare native plant nursery
- Noosa and District Landcare - Native Nursery
- Coolum Community Native Nursery
- Forest Heart Eco-Nursery
You may also find it beneficial to access the Land for Wildlife website - https://www.lfwseq.org.au/ - which will give you access to a range of resources relating to planting, weed management and a host of other interesting and relevant material.
Barung Landcare’s Gardens for Wildlife program is also designed to assist residents to create gardens for wildlife.
- Braby, M. (2016). The complete field guide to butterflies of Australia. CSIRO Publishing.
- Costello, S. L., Pratt, P. D., Rayamajhi, M. B., & Center, T. D. (2003). Arthropods associated with above-ground portions of the invasive tree, Melaleuca quinquenervia, in south Florida, USA. Florida Entomologist, 86(3), 300-322.
- Orr, A., & Kitching, R. (2010). The butterflies of Australia. Allen & Unwin.
- Tallamy, D. W. (2007). Bringing nature home: How you can sustain wildlife with native plants.