Invasive species threaten plants like the keladi (foreground), discovered only 2 years ago and found only on Batu Caves, says limestone specialist Ruth Kiew. (SL Wong)

When Invaders Move In On Batu Caves (And They Have)

From flowering plants to butterflies , invasive species are taking over Batu Caves. This alarming threat to the fragile limestone ecosystem needs addressing.

WE HAD trekked up Batu Caves for about 10 minutes when botanist Dr Ruth Kiew turned to me and asked, “Can you see the difference in the vegetation?”

“Between limestone and non-limestone vegetation, you mean?”


I scanned the plants before me. This was pre-pandemic times and I had been researching limestone species from lists provided by Kiew.

(Photo: Invasive species threaten plants like the keladi (foreground), discovered only 2 years ago and found only on Batu Caves, says limestone specialist Ruth Kiew. ~ pic by SL Wong)

Creeping upwards

Being with one of Malaysia’s foremost limestone plant experts made me feel compelled to recall in a silly, frantic and ultimately hopeless way, what I had learned.

I finally admitted, “No, I can’t see it.”

And of course the difference was ridiculously obvious once Kiew pointed it out. There was almost a line demarking the advance of the non-limestone, invasive plants growing up the hill towards the native plants.

What shocked me was how far up the 329-metre tall hill the invaders had gone.

But then, invasive species are called that for good reason. A global problem, invasive species are organisms that are spread by humans and tend to decimate native species and damage food crops and human health (see below).

In fact, a recent study found that worldwide, invasions incur an annual mean cost of USD26.8 billion (RM110.6 billion). Invasions have also been increasing threefold every decade since 1970.

The latest global assessment report on biodiversity and ecosystem services stated that invasive alien species deal “particularly severe” effects in places with more endemic species (Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, 2019).

Highly sensitive

Limestone hills like Batu Caves are particularly vulnerable as they are arks of endemic species – plants and animals that are found nowhere else.

This is because limestone is so inhospitable that plants and animals have had to evolve specialised traits to survive there. Many are so far changed that they cannot survive in any other ecosystem.

In terms of plants alone, Batu Caves has 6 endemic species and 31 type species – newly-discovered plants that were first described in that location.

A single cave, Dark Cave, is the type locality for over 50 animal species, while various parts of the hill house 9 endemic species, including the famous Batu Caves bent-toed gecko (Cyrtodactylus metropolis).

In fact, on that day that Kiew quizzed me, we were looking for the newest entry to the plant list.

This was a new keladi, at that time un-named. Kiew was looking to collect the flowers of this plant, without which it could not be scientifically described. The plant grows only in deep, damp valleys.

Keladi carpet

After a wrong turn, we found it. Kiew had mentioned that it grew abundantly in that spot. But I was unprepared for the breathtaking sight of hundreds of keladi carpeting the slopes.

Nonetheless, the wonder did wear off when, after what felt like hours turning over the leaves of virtually every single plant, we could not find a single flower.

Kiew and her team did eventually lay hands on the inflorescence, but not easily. “We got the keladi flowers by growing it in the Forest Research Institute of Malaysia Herbarium Nursery, but even then we had to wait more than a year for it to flower!”

Hidden treasures

The Schismatoglotiis guabatuensis is now on the Batu Caves checklist. Its name references both the hill (‘Gua Batu’ in Malay) and ‘split tongue’ after the shape of the leaves.

This find typifies what Kiew and all other Batu Caves scientists say: that this single hill holds untold possibilities of new discoveries.

Unfortunately, one invasive alien plant has already become a problem on the hill. This is the spike pepper (Piper aduncum), originally from South and Central America.

“The Piper now dominates around the base,” said Kiew. “It is beginning to invade near cave mouths.”

Piper aduncum (PT Ong)
Piper aduncum - A big threat to native species (PT Ong)

Its tiny seeds are dispersed by bats which go in and out of the caves, and the plant is already crowding out native species. “It is one of the biggest threats to Batu Caves’ biodiversity,” said Kiew.

Completely gone

At the same time, scientists reported in a 2020 symposium of a seminal 2019 scientific expedition that plants, mosses, the Sumatran serow (Capricornis sumatrensis) and the cave racer (Elaphe taeniura) have disappeared.

Once species disappear from the fragile limestone ecosystem, they might never return. For example, Batu Caves’ south-western section, the site of a century of quarrying, remains bereft of animals and plants even though quarrying stopped 40 years ago.

Buffer zone missing

A buffer zone around the hill would have helped keep invasive species at bay. The vegetation there would have prevented invasive species from encroaching into the limestone area.

However, except for a tiny patch, Batu Caves is girdled by infrastructure, from temples and shops to houses and farms.

The proximity of human dwellings led to a massive fire in 2016 that engulfed a large section of the hill, right up to the summit.

It was the extent of the fire that “drew attention to the fact that we had to make an effort to get Batu Caves protected,” said Kiew at the symposium.

'The Great Burn', Batu Caves, 2016 (PT Ong)
'The Great Burn' decimated a large chunk of Batu Caves' biodiversity, 2016 (PT Ong)

Though arguably less devastating than the quarrying, the so-called ‘Great Burn’ has made it difficult for the natural vegetation to recover.

One reason is that invasive species were established first, said botanist Dr Siti Khadijah Rambe at the symposium. She researched the estimated 17-hectare burnt area with the volunteer Malaysian Nature Society Flora Special Interest Group.

The group found that three years after the fire, mature stands of non-limestone pioneer species dominated. Pioneers are plants that are able to take root in poor conditions and generally enrich the soil for other species to take over.

Uncertain recovery

However, while one dominant tree – Macaranga tanarius – was allowing the understorey to grow, another – the thorny, bushy Pterolobium densiflorum – was stifling other species.

“It’s important to have subsequent surveys to see the vegetation succession of pioneer species,” said Khadijah at the symposium. “When they disappear from the area, then we can say that the area has recovered.”

Even Batu Caves’ butterfly population comprises alien invasive species. Naturalist Dr Rosli Omar found 2 of these among the 51 species he recorded for the site’s first-ever butterfly census in 2019.

Dominant alien

Particularly plentiful was the Julia butterfly Dryas iulia which hails from the Americas. Rosli said it is commonly released in Phuket for good luck but he is not sure how these insects got to Batu Caves.

However, he said at the symposium that “they out-compete [other butterflies] in terms of the nectar … They were really dominant there, like a gang of gangsters.”

This is worrying because Rosli’s checklist also includes butterflies that are rarely found outside of very specific sites.

Rare finds

For example, in his 8 years of photographing butterflies in Malaysia, Rosli had only encountered the smaller wood nymph (Ideopsisgaura perakana) in highlands  1,000 m above sea level. However, he spotted it in Batu Caves 4 times in 17 trips.

Meanwhile, the Ceylon blue glassy tiger (Ideopsis similis persimilis) is rarely found south of Kedah and Terengganu and yet, he has at least 6 records of it in the hill.

Ceylon blue glassy tiger (Rosli Omar)
smaller wood nymph (Rosli Omar)
Julia Butterfly (Rosli Omar)
Clockwise from top left: Rare finds, the Ceylon blue glassy tiger and the smaller wood nymph could be in danger of being overwhelmed by the invasive Julia butterfly (Rosli Omar)

“There’s something about Batu Caves’ climate or limestone, where unique species that shouldn’t be here, are here”, Rosli said. “That’s why it’s important to conserve it.”

There is however, a glimmer of hope that native populations can recover from an invasion, and that glimmer comes from underground.

When a walkway was built in the recreational cave of Dark Cave in the 1970s, the native cave cockroach (Pycnoscelus striatus) almost got overwhelmed.

American invader

The culprit was the American cockroach (Periplaneta americana), common in Malaysian homes.

The lighted walkway gave passage to it to invade and overpower the native residents. “The American cockroach follows human activity,” said biologist Dr Lim Teck Wyn at the symposium.

Luckily, the walkway was completely dismantled by 2012, and Lim confirmed in 2019 that the native cave cockroach population had recovered.

Ultimately, the best defence against further invasions of Batu Caves is to turn it into a protected area and draw up a management plan for it. This will also protect it from all the other threats.

While the Selangor state government recently earmarked one tiny section of the hill as a geo-site – which would restrict development there – the entire hill has to be protected as a unit and managed.

Management imperative

“Management is crucial. Batu Caves cannot be left alone as it is not sufficiently resilient,” said Kiew. “Because (the site) is very small, populations of many species are also small.

“Therefore any change to their habitat or environment will push the population to extinction on Batu Caves or if it is endemic to Batu Caves, will result in the extinction of the species.”

[Edited by Law Yao Hua]

Alistair Hay & Yuzammi Yuzammi (2000). Schismatoglottideae (Araceae) in Malesia I – Schismatoglottis. Telopea. 9. 1-177. 10.7751/telopea20002008.

IPBES (2019). Global assessment report on biodiversity and ecosystem services of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. IPBES secretariat, Bonn, Germany.

Malaysian Cave and Karst Conservancy (2020). Batu Caves: Malaysia’s Majestic Limestone Icon (2nd ed), Kuala Lumpur, Malaysia.

Rosli Omar (2020, September 26). Butterflies of Batu Caves [Symposium presentation]. Malaysian Cave and Karst Conservancy 2020 Batu Caves Scientific Expedition Symposium, Kuala Lumpur, Malaysia. 

Ruth Kiew, Rafidah Abdul Rahman, Mohamad Aidil Noordin & Ong Poh Teck (2020, September 26). The Changing Flora of Batu Caves – 1890 to 2020. [Symposium presentation]. Malaysian Cave and Karst Conservancy 2020 Batu Caves Scientific Expedition Symposium, Kuala Lumpur, Malaysia.

Siti Khadijah Rambe, Azmir Mustapha, Kevin Choong & Lim Khoon Hup (2020, September 26). Flora Survey on Affected Forest Fire Area of Batu Caves [Symposium presentation]. Malaysian Cave and Karst Conservancy 2020 Batu Caves Scientific Expedition Symposium, Kuala Lumpur, Malaysia.

Teo Eng Wah, Wong Siew Por & Zarris Kem (2020, September 26). Revisiting the Herpetofauna Checklist of Batu Caves, Selangor. [Symposium presentation]. Malaysian Cave and Karst Conservancy 2020 Batu Caves Scientific Expedition Symposium, Kuala Lumpur, Malaysia.

Related story: Discoveries Support Urgent Protection for Batu Caves. Also check out our Karst Ecosytem section featuring limestone natural history and human uses.

Disclaimer: SL Wong was involved in an early draft of the guidebook based on the 2019 Batu Caves Scientific Expedition.




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