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Monitoring provides important evidence to evaluate if invasive mammal eradications on islands achieved conservation goals and to inform future allocation of conservation funds. However, monitoring can be costly, and compete with management actions for resources, so it is important that efforts are targeted. It can be difficult to obtain results of monitoring from previous projects, which puts future projects at a disadvantage when projecting possible conservation outcomes. In this short paper we discuss practical considerations for designing monitoring associated with an eradication programme. We focus on the ecological outcomes of invasive species eradication, as opposed to speci?cally monitoring if an eradication was successful or not. We identify major motivations for undertaking monitoring and present a decision tree intended to improve the efficiency of monitoring by supporting project managers determining when and what to monitor, and how to incorporate monitoring into project planning.

A successful ground-based eradication of black rats (Rattus rattus) was undertaken on the remote, uninhabited Shiant Isles of north-west Scotland over winter (14 October–28 March) 2015–16. The rat eradication was carried out as part of the Shiants Seabird Recovery Project, which aims to secure long-term breeding habitat for protected seabirds and to attract European storm petrels and Manx shearwaters to nest on the Shiants. Throughout the eradication operation, teams were stationed on two of the three main Shiant islands (Eilean an Tighe, Eilean Mhuire), with access to the third (Garbh Eilean) via a boulder causeway from Eilean an Tighe. Bait (Contrac® blocks containing the anticoagulant bromadiolone 0.005% w/w), was deployed in a grid of 1,183 bait stations covering all areas of the islands and sea stacks. Bait stations were set 50 m apart, with intervals reduced to 25 m in coastal areas of predicted high rat density. Difficult areas were accessed by boat and cliff s of ~120 m in height were accessed by abseiling down ropes made safe using either bolted anchors or ground stakes. The team of staff and volunteers worked through difficult conditions, deploying bait and monitoring intensively for any surviving rats using a combination of tools. The islands were declared rat free in March 2018. This ambitious and challenging project has greatly enhanced UK capacity in rodent eradications for the purposes of conservation.

Rodent predation on eggs and chicks is one of the main threats to procellariiform species in the Mediterranean, where the black rat (Rattus rattus) and brown rat (R. norvegicus) have been present on many islands for centuries. The yelkouan shearwater (Puffinus yelkouan) is an endemic Mediterranean seabird species classified as vulnerable. Malta holds up to 10% of the global population; the largest colony, Rdum tal-Madonna (RM), protected as a Natura 2000 site, hosts around 500 breeding pairs. This colony has been monitored since its discovery in 1969. A very low reproductive success due to rat predation was noticed in the late 1990s to early 2000s. In 2007 a seasonal rodent control programme was established during the breeding season of yelkouan shearwater to reduce rat predation on eggs and chicks. Rodent control took place between 2007 and 2010 and was reviewed and continued from 2012 to 2017. Breeding success since 2007 has been higher than 80%. In two other colonies with rat presence and where rodent control did not take place, the breeding success in 2016 and 2017 was substantially lower than in the colony with the rodent control programme. The European storm-petrel (Hydrobates pelagicus melitensis) only breeds in rat-free areas like remote sea caves or islets around the Maltese islands. In 2014, the first breeding attempt by European storm-petrel was recorded on the Maltese mainland at RM with a chick fledging successfully for the first time in 2016. The ongoing LIFE Arcipelagu Garnija project is assessing rat predation in all Maltese yelkouan shearwater colonies in order to establish predator control in the most important yelkouan shearwater breeding sites in 2018.

Rat eradication techniques developed in New Zealand are a proven method for removing invasive rodents from islands worldwide. This technology moved rapidly from ground-based bait station operations to aerial application of rodenticides. Rat eradications on tropical islands using similar methods, have not always been as successful as those in temperate regions. As most previous eradications in the Caribbean have been on islands smaller than 50 ha, the eradication of black rats (Rattus rattus) from 207 ha Dog Island was a significant increase in size. Reptile and seabird populations on Dog Island had been in decline for a number of years and black rats were identified as the most likely factor. Following the feasibility study in 2007, the Dog Island Recovery Project was launched in 2011. This was a multiple-year project incorporating a ground-based eradication with establishment of biosecurity procedures to prevent reinvasion, alongside long-term monitoring of native species. Bait stations with cereal-based wax blocks containing brodifacoum at 0.005% w/w were established on a 30–50 m grid over the island. Interference with bait stations by non-target invertebrates, particularly crabs, was high and bait stations required moving or elevating to avoid this. However, there was no evidence of any non-target animals being killed or injured by the bait. Eradication success was confirmed in 2014.

An invasive alien species are known to cause significant economic and environmental damage. Islands are much more vulnerable to the invasion of invasive alien species, and have higher rates of extinction. In the islands of the South Pacific, there are several species Polynesian land birds which are threatened with extinction due to invasive alien species and human interference. Without any intervention from humans, it is likely that these birds will suffer extinction in the next few decades. The island of Atiu in the Cook Islands (South Pacific) supports a range of avian fauna. In 2007 the Rimitara lorikeet (Vini kuhlii), also known locally as the ‘Kura’ was introduced to Atiu, to make a reserve population, as there were only approximatley 1000 left on the island of Rimitara, French Polynesia. There is concern for this species on Atiu due to the existence of the common myna (Acridotheres tristis). The myna is an aggressive invasive which is one of the world’s worst 100 invasive species. It was introduced to the Cook Islands by the government, with the intention to control the coconut-stick insect (Graeffea crouani), but is now a pest itself. A myna control programme on Atiu is being coordinated and controlled by Gerald McCormack (Cook Islands Natural Heritage Trust), working through George Mateariki, and using funding from various conservation organisations. Poisioning is being carried out by George in order to reduce myna numbers, and a bounty has been set on the birds or for every right foot so that the locals can help as much as possible. This investigation aimed to estimate the total population size on the island to use as a starting count to determine whether the programme is successful or not. Two methods were used which included roost counts to find an estimate of the population, and a transect method using a Distance programme to estimate the number of birds per hectare as well as the level of abundance in various habitat types. The two methods had overlapping results, giving a total estimated range of 3250 to 8460 birds on the island, although it is more likely to be at the higher end of this estimate at around 6000 to7000 birds. Further investigation will need to determine any change in the population size, any changes in habitat composition as well as the impact on the native species, especially the effect on the Kura and its population size.