Plectranthus amboinicus (Lour.) Spreng., Plectranthus monostachyus (P.Beauv.) B.J.Pollard, and Plectranthus scutellarioides (L.) R.Br. are exotic herbs in Singapore. A Weed Risk Assessment (WRA) was conducted for each species in order to assess its potential for invasiveness in Singapore. No spontaneous growth of Plectranthus amboinicus is known in Singapore as it rarely flowers in Southeast Asia, although it has a WRA score of 9, which indicates weediness, since scores above 6 indicate high risk. Plectranthus monostachyus, first recorded in 2009 and now naturalised in Singapore, received a high WRA score of 14 so should be considered as a locally high-risk weed and be removed, especially from the edges of the nature reserves. The WRA score of Plectranthus scutellarioides is 13, indicating that it also has a high risk of becoming an agricultural or environmental weed. Although spontaneous growth of Plectranthus scutellarioides is unknown in Singapore, its cultivation should be avoided to be cautious. This article is our second attempt at using the modified Australia Weed Risk Assessment system for assessing exotic plants in Singapore. We recommend its use for any new exotic species imports.

Although Stegastes obreptus is known from Singapore waters (Sin et al., 1994: 53), it does not appear to be often noticed, and specimens from there do not seem to have been illustrated in publications. Adult fish are brown with dark scale margins and a black spot at the front of the dorsal fin. The juveniles are yellow with blue spots on the head and have a large ocellus at the front of the dorsal fin. The western gregory grows to 14.5 cm in total length, lives as solitary individuals or in loose groups, and is distributed from Sri Lanka eastwards to New Guinea, and north to southern Japan (Allen & Erdmann, 2012: 629). At around 3 cm, the example featured here appears to be a juvenile that is yellow only at the posterior and lacks the blue spots on the head. It seems to be transitioning to adult colour pattern.

The presence of “avian sentinels” in bird waves (mixed-species foraging flocks), which mob or alarm-call in response to predators, is widely recognised. Yet in the highly threatened lowland deciduous forests of South-east Asia, a mammal, the western striped squirrel Tamiops mcclellandii, which usually accompanied bird-waves, was a more obtrusive sentinel than any bird. It called most often in response to predators (40 out of 70 observations) and was the first species to emit alarm calls on 24 occasions (60%) where studied in Huai Kha Khaeng Wildlife Sanctuary, western Thailand. The squirrels fed in close proximity to drongos (Dicrurus spp.), suggesting they may benefit from the drongos’ tendency to mob predators. Additionally, the drongos actively followed the bark-foraging squirrels in order to capture flushed arthropods. The presence of the western striped squirrel both helped reduce the need for vigilance among avian flock members and contributed to flock cohesion.