Will reinvasion stymie large-scale eradication of invasive mammals in New Zealand?
Cartoon - Susan Marks.
So far, eradication of invasive species has mostly been achieved in relatively small areas. For terrestrial invasive species, the coast of a small island or the fence around a reserve is often used to set the spatial limit to eradication, but these boundaries are just special cases of a more general selection of areas, including portions of large land masses, where managers might decide to aim for eradication. So, is eradication of invasive mammals, such as possums, mustelids and rodents, over very large areas of New Zealand just a matter of scaling up from what has been achieved on near-shore islands or in fenced sanctuaries?
If a control programme is rolled out across the country, the problem of pest reinvasion will become a more important issue than that encountered in local eradication programmes for two reasons. Firstly, the boundary between treated and untreated areas is likely to be very long and will often not coincide with a physical barrier (such as a river or the ocean) expected to block or slow dispersing animals. This means there will be a large pool of potential invaders immediately outside treated areas. Secondly, it will be difficult to confirm that all individuals of an invasive species have been removed from a large treated area. In such a case, it may not be obvious whether any resurgence of a pest population is due to recruitment from survivors or from immigration, and sometimes determining the source of reinfestation might be impractical or too costly.
Peter Banks and Chris Dickman from the University of Sydney, with Andrea Byrom and Roger Pech from Landcare Research, have been examining how the problem of ‘reinvasion’ can differ from an initial invasion by an exotic species, and why this might matter. They defined reinvasion as: ‘re-establishment of a species in a location it had previously invaded, but was controlled or eradicated to manage unwanted impacts’. Essentially three factors can change the management of reinvasion, compared to in an initial invasion. These are (1) changes in the invader itself, (2) changes in the invaded environment, and (3) interactions among invaders and with other local species. These three factors are illustrated with the following examples.
Changes in the invader: Various pest control techniques have had a long history of use in New Zealand and susceptibility of invasive mammals to these techniques can change over time, as for example through development of resistance to poisons or disease. There is some evidence of this in the declining effectiveness of rabbit haemorrhagic disease as a biocontrol agent (Bruce Warburton et al; Kararehe Kino, Issue 18) and with the use of sodium monofluoroacetate (1080) for rabbits in parts of Australia. Also individuals that survive a control programme may be bait-shy or trap-shy. Consequently new methods 6of control may be required to manage reinvasion due either to in situ recruitment from survivors or immigration from outside populations that have a history of exposure to conventional control techniques.
Changes in the environment: Many exotic plants have become established in New Zealand since the first introductions of rodents (kiore, ship rats, Norway rats and house mice). Grant Norbury and colleagues have shown that in the grasslands of the central South Island, mouse populations are generally larger in areas with introduced pasture grasses that regularly produce high volumes of seed compared with populations in indigenous tussock lands that mast less frequently (Fig; see also Kararehe Kino, Issue 18). Clearly these dryland environments have changed in a way that benefits rodents, which will increase the likelihood of populations resurging after control.
Species interactions: There is now substantial evidence that interactions among invasive species need to be taken into account in control programmes. For example, in North Island forests control of ship rats leads to greatly increased abundance of house mice (Ruscoe et al., Kararehe Kino, Issue 13). And Chris Jones and colleagues have shown that stoats are specialist predators of mice, which suggests that with high mouse populations following local eradication of rats and mustelids, conditions are likely to be ideal for re-establishment of stoat populations through reinvasion.
These examples illustrate why managing reinvasion could be more difficult than dealing with an initial invasion. However, Peter and his colleagues note that in many cases better knowledge of the biology of invasive species and their impacts should increase the ability of managers to block reinvasion. The review has highlighted some key areas of research that would improve the chances of achieving large-scale eradication of invasive mammals across New Zealand. These include understanding: the population dynamics of reinvaders at ultra-low population density; genetic changes in invader traits; how to exploit changes in ecological processes such as predation, competition and disease; biophysical factors that enhance or suppress reinvasion; and interactions between reinvasion and other drivers of global change such as climate change and land-use change.
Andrea Byrom was supported by a Caughley Fellowship, and all authors by an Australian Research Council Linkage Grant (L100100600: ‘The return of the native: reintroductions, reinvasions, and a new paradigm in restoration ecology’).
Andrea Byrom
Peter Banks, Chris Dickman (University of Sydney)
Roger Pech
Fig. Grasslands dominated by introduced pasture species provide a more reliable supply of seed for mice than those dominated by indigenous tussock.
Grasslands dominated by introduced pasture species. Image - Grant Norbury.
Grasslands dominated by indigenous tussock. Image - Grant Norbury.
