Will black rats benefit from cat control on Christmas Island with negative consequences for endemic birds?

(Previous title: The ecological impacts of invasive species and invasive species control on Christmas Island)

Informing monitoring of rat populations in the presence of land crabs

·       In complex multi- invaded systems, where apex predator control is occurring but mesopredators are also present, monitoring non-target impacts will minimize the potential for surprise outcomes and increase the capacity of managers to respond if a system moves towards an undesirable state

·       The choice of monitoring methods is important: testing the capacity of methods to meet the monitoring objectives; assessing their suitability in the specific environment and for the specific species that are to be used for; and determining their cost to implement will help managers make decisions around implementing monitoring that is meaningful and achievable

·       We provide an example of what needs to be taken into consideration and the tradeoffs involved to design a robust monitoring program for detecting population change in an invasive rat population

Key results

·       We compared the suitability of a relative activity index (RAI) and a density estimation method to detect changes in an invasive rat population in the context of a cat control program for a tropical island ecosystem with unique constraints

·       Methods were assessed on their i) levels of crab interference, concomitant rat detection probability and precision, ii) the effort required to minimise uncertainty in trend estimates (or power to detect change) and iii) their cost to implement.

·       We found that RAI methods have less crab interference than density estimation methods except at disturbed sites, and higher rates of rat detection except at coastal sites. Rat detection becomes compromised when crab interference reaches 20% for RAI and 25% for density, but can be overcome by repeating or extending surveys, although at additional cost.

·       There was a weakly positive relationship between RAI and density however with a large range of uncertainty, to the point that a small change in RAI (±50%) could represent a large range of change in density.

·       To detect population changes of 50% or more, the RAI monitoring method is significantly cheaper. However, it cannot be used to detect change in disturbed habitats, as the method is already saturated there, and precision is limited – there is an increased chance of type 1 (increases to RAI that don’t reflect increases in density) and type 2 error (no change in RAI when there is a change in density) using this method.

·       The choice of monitoring method will depend on how sensitive the trigger for action needs to be. If small changes in the rat population are meaningful and therefore important to detect, than density estimation methods, while more expensive, will be the only appropriate method to use.

·       RAI may be suitable for use if managers are only concerned about detecting greater magnitudes of change and are comfortable with accepting the risk of type 2 error occurring. Type 1 error can be minimized by following up with density estimation once change in RAI reaches a pre-determined threshold (i.e. greater than what has been observed seasonally).

·       Setting triggers for action and choosing monitoring methods based on the required sensitivity requires knowledge of what measured increases would mean in the context of increased impact.

Assess spatial and temporal patterns of rat distribution and relative abundance across Christmas Island in relation to environmental and biotic covariates

·       Whether rats are limited by the productivity of the Christmas Island ecosystem and competition with crabs, or by top-down predation pressure from cats or crabs are factors that will determine whether mesopredator release is likely to occur when cat densities are reduced.

·       If rat populations are mainly regulated by the productivity of the Christmas Island ecosystem (Russell et al., 2011), then rat population release is unlikely. This has been the case on other tropical islands (Harris & Macdonald 2007; Russell et al. 2011; James et al., 2020).

·       There is no previous quantified or comprehensive empirical basis for a spatial understanding of rat population dynamics for Christmas Island.

·       We describe the spatial and temporal patterns of activity and density for rats on Christmas Island and thus contextualize potential rat impacts.

·       We investigate how abiotic and biotic drivers of rat activity and density combine to produce the observed spatial patterns and test competing hypotheses i.e. rats are limited by the top-down (by cat predation) using temporal variation in cat density or driven by bottom up processes (productivity and/ or competition with crabs) using spatial variation in environmental covariates

·       This knowledge will increase our insight into whether changes in the cat population may affect the population of rats, to help guide decision making around whether complementary rat control is required as cat densities are reduced.

Key results

·       Rats are widespread across Christmas Island. Their average estimated density is 21.38 ha-1 which is low – moderate compared to other tropical islands (1.2 – 119 per rats hectare−1; Harper et al., 2015).

·       Rat activity and density is spatially variable and is related, however there is a large range of uncertainty as detailed in Chapter 1A.

·       There is no strong evidence that a rat population release has occurred as cat have been reduced.

·       There is some evidence that rats are limited by resources, and by interactions with disturbance.

·      Disturbed and dry habitats (i.e. mostly associated with mining) have higher levels of rat activity while wet areas or areas of high crazy ant abundance have lowest activity.

·      High rat density is associated with disturbed vegetation types including in close proximity to the settlement area.