24 November 2019

Relative Abundance Indicators

Butterflies : Relative Abundance Indicators
From Common to Rare

A Malay Lacewing feeding on the flower of Lantana camara at Central Catchment Area. Circa 2005

A friend recently asked me, "so has your butterfly survey project concluded?". My answer was yes and no. Firstly, surveys to "inventorise" our local butterfly fauna are always "ongoing". Although each official project may last 1-2 years, like the Bukit Timah Nature Reserve Comprehensive Biodiversity Survey, and the current Pulau Ubin Biodiversity Survey commissioned by the National Parks Board, many local butterfly watchers and enthusiasts continue to observe and keep records of butterfly activity in Singapore.

A Magpie Crow feeding on a sweaty backpack strap at MacRitchie Nature Trail. Circa 2005

With social media and digital photography, it is much easier to collect records and collate information on butterflies today. Apps such as Biome and iNaturalist are relatively good repositories of sightings and documentation of species diversity, abundance and location. Data collected can henceforth be analysed and may give a better overview of butterfly activity across Singapore the years.

In the Red Data Book, the convention used to describe the relative abundance of species is crafted after the International Union for Conservation of Nature (IUCN) guidelines. The Red List Categories and Criteria were basically categorised by "threat to extinction". For the layman, it may be rather scientific and hard to relate to.

Hence, when I was working on the Butterflies of Singapore, I adopted a less scientific approach  and used a more simple "Relative Abundance Indicator" table and layman-friendly 6-category abundance indicators from "Common to Very Rare". The final category of "Seasonal Migrant" covers all the occasional appearances of species that are non-resident in Singapore.

The annual variability and irregularity in the sightings of certain species over the years continue to affect their relative abundance. It is more pronounced in some species compared to others. Hence what was listed as "common" or "moderately common" may be come "rare" and conversely, what was "rare" or "moderately rare" may become "common".

Examples of some of the more obvious species that may require a re-classification of their abundance, are highlighted here. These are certainly not exhaustive nor definitive and the relative abundance indicators will continue to evolve and change over the years, as more data and observations are collected.

1. The Magpie Crow (Euploea radamanthus radamanthus)

A Magpie Crow puddling at a damp streambank near Rifle Range Trail. Circa 2008

The Magpie Crow was widely distributed across Singapore in the past decade and often spotted mainly in the nature reserves. It was most regularly seen at the Bukit Timah Visitor Centre before the upgrading works, but has been spotted at Rifle Range Road Trail, Sime Forest, MacRitchie Reservoir Park, Upper Seletar Reservoir Park and many other localities.

A trio of Magpie Crows puddling at a muddy spot at Upper Seletar Reservoir Park in 2005

A Magpie Crow puddling on a concrete water pipe

Considered "Moderately Common" between 2005 and 2015, the Magpie Crow appears to be rarely sighted in recent years. Records from recent social media posts seem to validate this observation. Have there been any ecological changes that made the species become much rarer? Has its caterpillar host plant been affected in some way and become less common? We will need to keep looking out for this species again in the coming years to see whether its population rebounds.

2. The Malay Lacewing (Cethosia hypsea hysina)

The Malay Lacewing, once a regularly-seen species in the nature trails, appears to be much rarer these days

A favourite amongst butterfly watchers and photographers, the Malay Lacewing features intricate patterns on black and orange coloured wings and is one of the prettiest butterflies in Singapore. The species was also categorised as "Moderately Common" as it was often seen flying around in the Central Catchment Nature Reserves along forest trails and feeding on flowering plants.

A Malay Lacewing perched on a leaf at Ulu Sembawang Park Connector, Mandai. Circa 2013

Again, after 2015, records of sightings of this species have decreased significantly. The non-native Leopard Lacewing, discovered in 2005, was not known to aggressively compete with the Malay Lacewing for its host plant, Adenia macrophylla, as its caterpillars prefer to feed on Passiflora foetida found abundantly in urban areas. Could something have changed and the "invader" has edged the native species out? This is something for the environmental ecologists to study further.

3. The Common Tree Nymph (Idea stolli logani)

Will the Common Tree Nymph soar amongst the treetops as commonly as before?

The large and spectacular Danainae is always a crowd pleaser whenever it appears high up in the forest canopy, soaring and gliding amongst the treetops. It is a forest-dependent species and the Common Tree Nymph has almost exclusively been sighted only within the forested nature reserves of Singapore. Also categorised as "Moderately Common" between 2005 and 2015, sightings of this species have become rarer these days.

Three Common Tree Nymphs frolicking at tree-top level at Upper Seletar Reservoir Park in 2008

In the past decade, when the Spicate Eugenia is in full bloom, the flowers attract a myriad of butterflies.  The Common Tree Nymph is one of the species that makes an appearance.  In recent years, when this tree is in flower, the Common Tree Nymph is sadly absent.

At the forest fringes of the nature reserves, when the Spicate Eugenia (Syzygium zeylanicum) trees flower, there are many species of butterflies that are instantly attracted to feed on the nectar of the flowers. The Common Tree Nymph was one of the species that turns up regularly - sometimes even up to 3-4 individuals. In the past few years, when the Spicate Eugenia trees are in bloom, the Common Tree Nymphs have failed to show up. Has anything changed in the environment to make this species become "Rare"?

4. The Common Three Ring (Ypthima pandocus corticaria)

The Common Three Ring, the largest of the Ypthima species, is no longer as common as before

This "unattractive" butterfly was described in the reference book, Butterflies of the Malay Peninsula to "almost dispute the claim of the Common Grass Yellow to be regarded as the commonest butterfly in Malaya". Usually seen along forest fringes amongst grassy patches, this species was categorised as "Common". It flies close to the ground, and in the past, several individuals can be encountered at its favourite locations.

Today, the species is not as often encountered. Could it be due to the reduction of its grass host plant, Ischaemum muticum? For a common butterfly, its life history is interestingly long - between 30-35 days as a caterpillar. Did something happen in its habitats to interrupt the life history development? Once again, should the Common Three Ring, which was previously "Common" may now have to be reclassified as "Moderately Rare"?

The abundance of various species of butterflies will continue to change and evolve, as the environment, habitats and ecology changes over the years. Climate change may have a strong influence on these factors. Nature groups and researchers with the resources to undertake projects to sustain butterfly populations in Singapore may need to widen their scope to look at some of these previously "common" species and target species recovery programmes to include these butterflies.

Text and Photos by Khew SK