Rainforest weeds and their ways:the need for vigilance

May 2001

Amidst the mosaic of colour andpattern that forms the tropicalrainforest landscape, it can be difficultto discern which plants belong there,and which don’t. Environmentalweeds can establish self-sustainingpopulations in the Wet Tropicsbioregion capable of causing greatharm to the natural values for whichthe area is renowned. An audit under-taken by Garry Werren of the Centrefor Tropical Freshwater Researchfound more than 500 environmentalweeds species have become natural-ised in the Wet Tropics bioregion. Thisrepresents about 11% of total plants inthe region, with a further 29 exoticspecies under suspicion since the lastofficial list was published by theQueensland Herbarium in 2000.

Eradication of environmental weedsoften requires back-breaking work anddemands on-going vigilance. Whendealing with over 500 species, it iscrucial that regular investigations ofthe types and numbers of weeds areundertaken to help determine whichweeds pose the greatest threats, whichparts of the rainforest are at most risk,how weeds are getting into the naturalenvironment and how to stop themspreading.

The most threatening weedsThe adjacent box lists the twelveweeds, in order of severity, which theaudit identified as posing the mostserious threats to the environmentalintegrity of the Wet Tropics bioregion.Two of these species are considered tobe Weeds of National Significance#.

The most threatened ecosystemsThe wet tropical environment isideally suited to exploitation byinvasive and opportunistic species ofplants.The warm moist climateconditions are highly conducive torapid growth which continues formost of the year, factors that greatlyaccelerate the invasion process.

The Wet Tropics bioregion contains awide range of regional ecosystemsthat sustain a wealth of biodiversityacross cool highland forests, wetsclerophyll zones, hot, dry tropicalwoodlands and very wet tropicallowland forests and communities.The environment, therefore, offers richterritories for the establishment of awide array of alien plants. Accordingto risk assessment figures quoted inthe audit, 24 regional ecosystemswithin the Wet Tropics are endangeredby weed invasion and a further 17 areof some concern.

Left: A mature stand of Pond Apple (Annona

glabra) which shows the extreme level of

infestation this weed is capable of.

Above: Pond Apple fruit and seeds which are

attractive to and easily dispersed by animals.

(photographs: Peter van Haaren)

• Pond Apple# (Annona glabra)• Leucaena (Leucaena leucocephala)• Siam Weed (Chromolaena odorata)• Singapore daisy (Sphagneticola

trilobata)• Hymenachne # (Hymenachne

amplexicaulis)• Miconia (Miconia calvescens)• Guava (Psidium guajava)• Laurel vines (Thunbergia spp.)• Mile-a-minute (Mikania

micrantha)• Pará grass (Brachiaraia mutica)• Guinea grass (Panicum maximum)• Cucumber tree (Parmentiera

aculeata)

How do weeds get into the forest andwhy do they succeed?There are many ways weeds can turnup in natural ecosystems, althoughthey are rarely considered pests in allAustralian settings. Leucaena, ParáGrass and Guinea Grass wereoriginally introduced as a stock fodderand continue to be cultivated anddistributed and Hymenachne waspromoted for use as a ponded pasturespecies until recent years. Ornamentaland cultivated plants have alsobecomes weeds. These include theMango (Mangifera indica), variouspassionfruits (Passiflora spp.), AfricanTulip (Spathodea campanulata) andBrazilian Nightshade (Solanumseaforthianum).

Even in the natural rainforest environ-ment, plants constantly compete withother plants and with animals. Manyfactors determine which species willtriumph. Imported species oftenovercome natural species due tounnatural advantages, succeedingsimply because their natural enemiesare absent from the new environmentor because of special growth forms thatenable infiltration into new territory indifferent ways. Such strategies includereplacing the natural herb or shrublayer, forming dense mats over nativeflora, or having fruit and seeds whichare easily dispersed by wind, water oranimals. A more insidious category ofweeds known as sleepers reinforces theneed for constant vigilance in thenatural environment. Species likeBauhinia (Bauhinia monandra) forexample, appear as minor weeds now,but can develop into a major pests inthe future.

Stopping the spreadThe audit identified several importantfactors in management of environ-mental weeds in the Wet Tropicsregion:Prevention of EntryAs intentionally introduced weedsoutnumber those that are uninten-tionally introduced, a combination ofconsultation with the nursery industry,wider public awareness campaignsand properly constituted quarantineexclusion systems are necessary toprevent any future incursions ofpotential weed species.Early interventionEfforts should be focused onidentifying sleeper weeds before theybecome widespread and whileeradication or control is still feasible.

Integrated weed managementAn improved understanding of theecology and dispersal habits of weeds,how control methods may impact onother aspects of the environment, andcoordination between various controlagencies will ensure best results forleast effort in the most appropriatelocations.Containment of existing weedsWhere weed species are so wellestablished that eradication is nolonger possible, efforts should befocused on critical areas of highconservation value which containendangered regional ecosystems orspecies.

For further information contact:Mr Garry WerrenACTFR, James Cook UniversityPO Box 6811, Cairns QLD 4870Email: Garry.Werren@jcu.edu.au

Left: The flowering head of Siam Weed

(Chromolaena odorata), a weed which

ranks 23rd on a list of the world’s worst

alien invaders.

Above: An infestation of Hymenachne

(Hymenachne amplexicaulis) or ponded

pasture grass, an aggressive weed with the

ability to invade wetland ecosystems.

(photographs: Peter van Haaren)

To obtain a full list of weeds that have

naturalised in the Wet Tropics:

http://www.rainforest-crc.jcu.edu.au/

downloads/WeedList.doc