Mobile Mis-Behaving ‘Weeds’: detection of rat eradication survivors and reinvaders

1. Mobile Mis-Behaving ‘Weeds’: detection of rat eradication survivors and reinvaders James Russell, Rachel Fewster, Mick Clout, Steven Miller University of Auckland Dave Towns Department of Conservation

2. Issues • Invasive rats are a major conservation issue • On islands eradication is possible • How do we detect survivors from eradications for mop-up? • How do we detect reinvaders to rat-free islands for prevention • How do we distinguish the two? • On the mainland only rat control is possible • Same questions but more ‘gray’

3. Eradications • For detecting eradication survivors DOC has usually assumed eradications are 100% successful, and had a ‘lay-down’ period of two years. • Monitoring after two years detects rat presence (widespread i.e. failure) or absence (success) • This is loosely based on anecdote and basic modelling which suggests even a large island will be completely reinvaded from survivors within two years • In most eradications no survivors have been detected after two years i.e. never been a need for better system • Moving in to trickier eradications e.g. very large areas, what system should be used?

4. Reinvaders • For detecting reinvaders DOC invested funding into a recently completed PhD project (James Russell) specifically for addressing the detection and elimination of reinvaders after arriving (incursions) • Of particular importance if the behavioural shift from high-density to low-density (e.g. N = 1) populations • How does this affect detection rates?

5. Alternatives to Ecology • Genetic methods provide an alternate way to access data on phenomena which can not be easily observed ecologically • e.g. inter-island migration rates • Application to assigning the source of new individuals • e.g. assign newly detected individuals to previous population (i.e. survivors) or source population (i.e. reinvader) • Issues with ‘ghost population’ i.e. populations which have not been sampled

6. Alternatives to Ecology • Genetic methods provide an alternate way to access data on phenomena which can not be easily observed ecologically • e.g. inter-island migration rates • Application to assigning the source of new individuals • e.g. assign newly detected individuals to previous population (i.e. survivors) or source population (i.e. reinvader) • Issues with ‘ghost population’ i.e. populations which have not been sampled

7. Eradication // Reinvasion //Genetics

10. Weed Modelling • Detecting rats is probably similar (mathematically) to detecting weeds except between sampling periods the individuals can change location, in addition to having their inherent detectability change (behaviour) • Occupancy problem (or RSF) • Can search-effort models for weed eradication and follow-up monitoring for ‘survivors’/’reinvaders’ (i.e. resurgences) be applied to animal modelling?

11. Cost - Benefit • Can detection models feedback into cost-benefit models for large-scale eradication operations? • e.g. for very large sites with prohibitive large-scale costs in both labour for eradication and follow-up monitoring, as well as the cost of failure, would a ‘rolling front’ approach be more cost-effective? • i.e. likelihood of survivors determines the size of stages, and when you move onto the next stage?

12. Applications • Results would have important and immediate application to pending large-scale rodent eradication operations, where detecting both survivors and reinvaders is vital • Maungatautari - ~3000ha targeted rats and mice 2006 • Gough Island - ~5000ha target mice in the near future • Great Barrier - ~27,000ha target ship rats in the distant future