Gambia grass - A study in unintended consequences

Gamba grass (Adropogon gayanus) is a tall grass species native to a number of regions in Africa. It was deliberately introduced from Africa into other countries on the basis of it value as feed for cattle. It is a classic case of action without considering consequences.

The Misplaced

Structure and Reproduction

Adropogon gayanus is a perennial grass with individual tussocks growing up to 4m high and almost 1 m wide. There may be as many as 400 stems per plant. The leaves are hairy.

Each plant produces large numbers of light, fluffy seeds (up to a quarter of a million seeds each year, with 65% fertilised and viable). These 'seeds' (approx. 7-9 mm long) are quite hairy and have a large, bent and twisted, awn (1-4 cm long) as can be seen in the photo below.

These seeds may be dispersed via wind, animals, humans on foot or operating machinery, although many of them land with a few metres of the parent plant.

Ecology

Gamba grass is broadly suited to areas with an annual rainfall above 500mm, experiencing a 3-6 month dry season. The specific habitats that gamba grass is found in include savanna woodlands plus the margins of wetlands, rainforest and creeks. It is also found in disturbed areas and roadsides.

This grass is drought tolerant and can grow on a wide variety of soils.

Its growth is restricted where mean minimum temperature of the coldest month is below 4 ºC

The Displaced

It grows vigorously and displaces native plant species. Since its introduction, the plant has led to ecosystem degradation, habitat loss and species decline. Gamba grass  transforms biodiverse savanna ecosystems as the native grasses, which grow to around 1 metre high, are replaced by monocultures of gamba towering up to 4 metres high.

Not only does gamba grass outcompete natives due to it vigorous use of space, it is also changing the nature of nitrogen flows in the soils in which it is growing to make them less suitable for natives.¹

Dense stands of ungrazed, dry gamba grass constitute fuel reserves that can yield intense fires in the late dry season in Australia and damage native plant communities.  Fuel loads  from Gamba grass biomass can easily be 3 times more than that associated with the same area of native grasses. Also due to their height, the intense fires that the grass supports are particularly dangerous to trees as they set off crown fires and remove a high % of trees in woodland areas.

Gamba tussocks recover well after fire, thus suppressing native species re-establishment²

The Consequences

Gamba grass now has an established presence in northern areas of Australia, India and Brazil. Map is from CABI³

Case study - Australia

Gamba grass (Andropogon gayanus Kunth.) was intentionally introduced to Northern Australia from Africa in the 1930s and 1940s as a pasture grass species, primarily for cattle feed.  It was planted extensively in the 1980s. It is now recognised as a environmental weed in the Northern Territory, Queensland and Western Australia. By 1993, it had been declared a weed of national significance by the Australian Government.

Some direct and indirect examples of species being displaced by the invasion of Gamba grass

• the endangered Atlas moth (Attacus wardi). This species is found only in the Darwin region, where its larvae survive on certain seeds in remnant grasslands
• Cycas armstrongii and Ptychosperma bleeseri are majorly threatened by the increased intensity of fires due to the fuel load gamba imposes
• the yellow-snouted ground gecko (Diplodactylus occultus) and the endangered yellow chat (Epthianura crocea tunneyi)
• the vulnerable eastern partridge pigeon (Geophaps smithii smithii). The most important threats to this species are species loss which will affect the diversity, timing and abundance of seeds available as a food source.
• the endangered Helicteres sp.. which attracts birds and butterflies is susceptible to smothering from perennial weedy grasses such as gamba grass

Current Distribution

Gamba grass now occurs from Cape York (in Queensland), across the Northern Territory, to the Kimberly (in WA). In the NT, it covers an area of approximately 1.5 million hectares, but has it has been predicted that it could potentially eventually invade up to 40 million hectares.

Remediation efforts

In 2020, the Northern Territory Government released Weed Management Plan: Gamba Grass 2020-2030, outlining a roadmap to tackle the issue. However funding for the program has been sporadic, when it needs to be long term and reliable. The Pew Charitable Trust has been contributing to removal efforts, but it is ultimately the duty of Australian federal and state governments to manage this endeavour.

Example at a local level from NT resident Denise Lawungkurr Goodfellow
Goodfellow criticises the conventional control method of spraying the plant with glyphophate on the basis of its reported toxicity to non targeted entities and the potential for glyphosate resistance to develop.

So, for all the above reasons I began experimenting, some years ago, with different ways of getting rid of gamba that didn’t involve glyphosate or pulling the stuff out. But the most important thing was to remove tit from the tree canopy and so my first move was to push the plant over and stamp on the base so it stayed flat.

As a child I’d heard that vinegar killed weeds. But while it did away with small gamba plants it did not affect the large tussocks. That was until I flattened them, and crushed the lower stems. I then just poured vinegar on the base. The clumps, no matter their size, died, sometimes within hours (the biggest weeds took two days). ...

Goodfellow also notes that gamba is unable to grow under certain native trees due to the saponins (chemical compounds) in their leaves.

Comment

Aboriginal people had worked out fire regimes that allowed people to be fed without major disruption to the savannah ecosystem.

When Europeans colonised the regions under discussion, they decided to cultivate a beef industry,  and introduced cattle into the region.

Their cattle could not thrive on native vegetation, so they then introduced an invasive species which has seriously degraded the stability of the ecosystem, and has lead to high burn intensity risks.

What if the had asked the local indigenous people about burning? What if they had decided to explore alternative food sources that did not cause high levels of damage?

Reaping what you sow!

Dig Deeper

• Australia’s Fight to Stop Invasive Gamba Grass Is Noble—But Not Enough
• Field of nightmares: gamba grass in the Top End
• Andropogon gayanus
• Gamba grass - a synthesis of research