Invasive Species Threat Summary
The Rarotonga Monarch Pomarea dimidiata is classified as ‘Vulnerable (VU)’ in the IUCN Red List of Threatened Species. It is endemic to Rarotonga, Cook Islands, where its distribution is chiefly confined to the Totokoitu, Turoa and the western Avana valleys. The global population is believed to be stable, and numbers are estimated at 310 mature individuals (BirdLife, 2012). Threats to the population include predation pressures by introduced mammals such as black rats ( Rattus rattus) and cats ( Felis catus). Other threats include habitat modification or destruction due to weed invasion, forest clearance, and cyclone damage (BirdLife, 2012).
Invasive Species Management Summary
The population of P. dimidiata declined rapidly, and in 1989, numbers were estimated at just 29 individuals (Robertson 2000; Robertson et al., 2009). In 1989 The Kakerori Recovery Plan was drafted and recommending a cost-effective recovery programme for P. dimidiata by targeting its predators, as well as scientific study aimed at evaluating the effectiveness of the work (Hay and Robertson, 1988 in Robertson et al., 2009). Bait stations were set up in the Totokoitu Valley in 1989, where the majority of the population was situated. The bait station network was later expanded into the Turoa and Avana valleys. Poisoned baits (contained brodifacoum) were placed in specific areas from September through December to decrease rat numbers during the breeding season. Breeding success rates in poisoned and unpoisoned areas were recorded and compared (Robertson et al., 2009).
In 1995, the Recovery Plan was updated to include recommendations to investigate the feasibility of initiating and maintaining an insurance population on an island in the southern Cook Islands (Atiu or Aitutaki), which was free of R. rattus
(Robertson et al., 2009).
In 1996, the 155-ha Takitumu Conservation Area (TCA), which is also the habitat of P. dimidiata
, was adopted as a conservation area as part of the South Pacific Biodiversity Conservation Programme (SPBCP). In 1998, a population census was carried out, where as many young individuals of the species as possible were colour banded (Robertson, 2000).
In 2002, the TCA received funding from the Pacific Initiative of the Environment (PIE) to shift the focus from species recovery to long-term sustainable management of the population at about 250 individuals on Rarotonga. Intensity of poisoning efforts were reduced in order to determine the levels of predator control that can be maintained.
Between 2001 and 2003, 30 individuals were translocated to Atiu in a series of three transfers as part of the aim to establish an ‘insurance’ population (Robertson et al., 2009).
The objectives of the sustainable management programme included:
1. Mist-netting and colour-banding as many individuals as possible in order to determine breeding productivity and to record annual survival rate
2. Decreasing annual rat poisoning effort by replacing bait in each bait station fortnightly
3. Transferring a further 10 juveniles to Atiu in 2003 and
4. Monitoring survival of individuals on Atiu
The August 1998 population census in the Takitumu Conservation Area estimated numbers at 160 individuals, and a 5% population increase from 1997 to 1998 was recorded (Robertson, 2000). Annual adult survival was quantified at 92.2%, but out of the minimum 39 fledglings raised in the 1997/1998 period, only 22 yearlings were positively identified (Robertson, 2000).
The population increased each year from 1989, with 100 individuals in 1995, 200 by 2000 and 282 individuals by 2002 (Robertson et al., 2009). In 2000, the species was downlisted by the IUCN Red List from ‘Critically Endangered (CR)’ to ‘Endangered (EN)’ (BirdLife International, 2008; Robertson et al., 2009). In 2003, the population increase of the species in Rarotonga was recorded at 8%, growing from 272 birds (10 were transferred to Atiu) to 293 (Robertson et al., 2009).
Monitoring of the population on Atiu found that breeding was successful, and in the period 2006/2007, a minimum of 43 adult individuals of P. dimidiata
was estimated. 11 fledglings were known to have been raised during the period, though the total is thought to be higher (Robertson et al., 2009). By 2007, the global population of the species had risen to 314 individuals, with 271 on Rarotonga, and 43 on Atiu.
In 2012, the species was further downlisted by the IUCN Red List from ‘Endangered (EN)’ to ‘Vulnerable (VU)’, and the existing global population is estimated at 380 individuals, though continuing conservation management is required to sustain the level of success (Roth and Fowlie, 2012).