The assault on the invaders
Scenic, picturesque beaches are a common sight in Western Australia, and I was lucky enough to be involved in some conservation work along the beautiful coast of Sorrento during the mid-semester break when I returned home. Over two days, under the enthusiastic guidance from Karis, the project co-ordinator, myself and a small group of keen volunteers were armed with gloves, bags, a trusty shovel and, one of the best tools for the job, our hands; and we were ready to hand weed the coastal dunes of Sorrento. Our main targets were dune onion weed (Trachyandra divaricata), a native of South Africa, sea spurge (Euphorbia paralias), a native of Europe, Northern Africa and Western Asia, and rose pelargonium (Pelargonium capitatum), endemic to South Africa. These three species are all alien to Australia, and, more importantly, unwanted and threatening to Australia’s coastal habitats.
My conservation work continued back in Canberra, where I volunteered with the passionate project co-ordinator Tim for a day to continue the battle against invasive plant species on the other side of Australia. This time, I was high up in Isaacs Ridge Nature Reserve in South Canberra, where I had a beautiful view of the green hills in the distance. It would have been a perfect view, except for one thing… there were countless sweet briar (Rosa rubiginosa), lining the horizon and were alarmingly widespread throughout the area we were in.
Sweet briar is not easy to take down, and proved to be trickier to remove than the coastal species in WA, as the plant is painfully covered with needle-like thorns (equipping very thick gloves and clothing is advised!), and after cutting through the stem, glyphosate chemical must be applied to the exposed stem almost immediately, as after 15 seconds, sweet briar will begin to heal the cut, foiling the chance to kill it. This method of stem injection relies on the active uptake and growth of the plant to move the chemical through its tissue to ultimately bring it down for good.
Casualties of war
Along Sorrento coast in Western Australia, the native plant species that were threatened by dune onion weed, sea spurge and rose pelargonium were mainly Spinifex spp. and goat’s foot convolvulus (Ipomoea pes-caprae). The invasive coastal weeds compete aggressively for resources, and can rapidly replace the native coastal flora, and establish dense infestations, with sea spurge believed to be able to alter the natural shape and structure of beach and dunes, and has adverse effects on the nesting habitat of a range of threatened shorebirds.
Back in Canberra, while sweet briar looks quite pretty when it flowers, it absolutely does not belong in Australia’s native grasslands. Native to Europe and Western Asia, sweet briar can rapidly take over areas as it aggressively competes with native species in the area, such as bulbine lily (Bulbine bulbosa), sheep’s burr (Acaena echinata) and Australian trefoil (Lotus australis). Isaacs Ridge Nature Reserve is also home to patches of the critically endangered White Box-Yellow Box-Blakely’s Red Gum Grassy Woodland in both woodland and open grassland forms. Sweet briar also impedes the movement of native grazing animals, and also shelters rabbits and other pest animals.
Two very different locations, one very common problem: native species under threat from weeds. It is clear that biodiversity all across Australia is threatened by invasive plant species, as these foreign invaders often compete with native flora species for resources such as nutrients, water and light (Grice 2006). Invasive plants can also have bottom-up impacts in the structure of ecosystems and fauna communities (Vila et al. 2011), as there might be negative changes in the breeding or foraging habits of various endemic fauna that rely on the native species for habitat or as a food source (Vila et al. 2011). Some invasive plant species, such as buffel grass (Cenchrus ciliaris), can even change fuel properties of native ecosystems (Brooks et al. 2004), which in turn can affect fire behaviour and, ultimately, alter fire regime characteristics such as intensity, seasonality, frequency, extent and type of fire (Brooks et al. 2004).
These alien invaders definitely pose severe and formidable threats to the whole of Australia, with weeds representing the second greatest threat to biodiversity in New South Wales, and there has been a correlation between invasive species and threatened species across Australia (Figure 1).
Therefore, it is evident that there are many significant benefits in working with the volunteer groups to remove the offending alien plants, as it would help the endemic flora, especially species with vulnerable or endangered statuses, to take back their native ecosystem so that they may flourish and thrive and protect biodiversity in Australia.
Invasion of the habitat snatchers
So we’ve all seen them. Invasive species are fairly ubiquitous in Australia’s grasslands and bushlands but most people don’t always know that they’re invasive and alien to this country. Sometimes all that we see is green, and we think that must be a good thing! But greenery doesn’t always mean healthy. So, how do invasive plants, like the ones I helped to remove, arrive in Australia, and why do they thrive in a place that is not their home?
Many invasive plants have been introduced to Australia since European settlement, where they were brought over for ornamental purposes. In modern times, international travel and trade have seen a large increase, that has subsequently intensified the frequency and extent of species transfer around the world, and shows no sign of slowing down (Catford, Jansson & Nilsson 2009). There are three main factors that contribute to invasion success – Propagule pressure (P), abiotic characteristics (A) of the recipient environment, and biotic characteristics (B) (Catford, Jansson & Nilsson 2009) of the invading plant species and the potential environment to be invaded (Figure 2). The PAB must be accommodating for the invader plants for the actual invasion to be successful, and favourable, if the invasive plants are to thrive in their new environment (Catford, Jansson & Nilsson 2009).
Alien invasion defense strategies
Under the Australian Constitution, the Biosecurity Act 2015 enables the Australian Government to physically prevent the introduction of weeds through the inspection of incoming luggage, cargo, mail, animals and plants and their products. Each state and territory has legislation covering the control of noxious weeds, and there are currently 32 Weeds of National Significance (WoNS) identified by the Australian Government to help focus national efforts to control the impact of the WoNS at the state level.
My personal experiences with weed removal involved hand weeding and also chemical control using glyphosate, however the type of weed removal strategy is wholly dependent on the invasive plant species, and also the ecosystem it resides in, as the strategy should not also negatively impact on the native flora and fauna.
Overall, being able to take part in different weed removal projects has been admittedly not easy, and quite demanding, but it is definitely extremely fulfilling and I encourage anyone who is interested in any conservation project to volunteer with Conservation Volunteers Australia for a rewarding experience!
Li-Ann Koh (u5673835)
Australian Government 2011, State of the Environment 2011. Available from: http://olr.npi.gov.au/soe/2011/report/biodiversity/3-9-invasive-species-and-pathogens.html. [10 October 2016].
Brooks, ML, D’Antonio, CM, Richardson, DM, Grace, JB, Keeley, JE, DiTomaso, JM, Hobbs, RJ, Pellant, M & Pyke, D 2004, ‘Effects of invasive alien plants on fire regimes’, BioScience, vol. 54, no. 7, pp. 677-688.
Catford, JA, Jansson, R & Nilsson, C 2009, ‘Reducing redundancy in invasion ecology by integrating hypotheses into a single theoretical framework’, Diversity and Distributions, vol. 15, no. 1, pp. 22-40.
Commonwealth of Australia 2016, Weeds in Australia – Legislation. Available from: http://www.environment.gov.au/biodiversity/invasive/weeds/government/legislation.html. [10 October 2016].
Grice, AC 2006, ‘The impacts of invasive plant species on the biodiversity of Australian rangelands’, The Rangeland Journal, vol. 28, no. 1, pp. 27-35.
Vila, M, Espinar, JL, Hejda, M, Hulme, PE, Jarosik, V, Maron, JL, Pergl, J, Schaffner, U, Sun, Y & Pysek, P 2011, ‘Ecological impacts of invasive alien plants: a meta-analysis of their effects on species, communities and ecosystems’, Ecology Letters, vol. 14, no. 7, pp. 702-708.