Understanding dispersal and dispersion of wild ungulates for their better management
In the mid-1990s, Wayne Fraser and colleagues reviewed the geographic distribution of 11 species of wild ungulates in New Zealand, including the various species of deer, Himalayan tahr, chamois, feral goats and feral pigs (J. Roy. Soc. NZ, 2000, 30: 419–437). The researchers identified 258 new population records and concluded that all species except Himalayan tahr were more widespread than previously documented and all were continuing to extend their ranges. Twenty years on, Dave Latham and his colleagues report that anecdotal information and limited survey data suggest that this process is continuing to occur for some of these species. For example, a survey in 2011 of selected landowners in Southland indicated that red deer, fallow deer and feral pigs are all spreading into previously unoccupied areas of the Southland Plains. Clearly if wild ungulates are continuing to expand their ranges and increase their numbers, this has implications for biosecurity and the spread of wildlife diseases, native biodiversity, and agriculture and forestry. Although wild deer and pigs are not maintenance hosts of bovine tuberculosis (TB) in New Zealand, they can sometimes spread this disease through either humanassisted translocation or long-distance dispersal. Where this results in a new outbreak of TB far from affected areas (as in the Wilberforce River valley in 2011), it can increase the cost and slow the currently strong progress toward local TB freedom being achieved by TBfree New Zealand (formerly the Animal Health Board).
More drastically, if foot and mouth disease (long seen as a threat to local livestock) entered the country, widespread contiguous populations of wild ungulates, particularly when adjacent to farmland, would make it difficult to manage the impacts of this disease on livestock. Further, it has been inferred (although not yet proven) that feral pigs could spread the micro-organism that causes dieback in kauri, so the occurrence of new pig populations in kauri forests is cause for concern. Similarly, many of the habitats that wild ungulates have dispersed or been liberated into are naturally occurring rare ecosystems (such as coastal wetlands and sand dunes) that may not be resilient to ungulate browsing or trampling, or rooting by pigs.
Some hunters think the spread of wild ungulates into new areas is desirable because of increased hunting opportunities. Where new populations occur on private land and landowners are amenable to hunting, this supposition appears valid. However, some landowners view wild ungulates as undesirable because they damage agricultural crops, orchards and plantation forests. Further, many new populations of wild ungulates occur on public land not gazetted for hunting (at least of large game). Often these areas are popular recreational areas for the nonhunting public or occur close to residential areas. Consequently, hunting may never be acceptable in these areas because of the risks to human safety.
Dave and his colleagues believe the current extent of wild ungulate dispersion and the occurrence of new populations need to be revised using new approaches and technologies. For example, a systematic grid survey (similar to that used in the atlas of bird distribution in New Zealand) could be useful to depict presence/absence of wild ungulates, have high repeatability and produce quantifiable changes in distributions.
Similarly, the mechanisms responsible for the ongoing spread of wild ungulates need to be reassessed. In the past, most new populations have resulted from farm escapes, illegal liberations and natural dispersal. Accurate information about how new populations are establishing is important because different mechanisms can have vastly different implications for management. For example, the best strategy for preventing deer escapes from farms is to educate farmers about the importance of sound deer fences, and to report and (if possible) recapture escapees promptly. Conversely, the clandestine nature of illegal liberations makes it more difficult to prevent the establishment of new populations. For example, we know that fallow deer have low dispersal rates and yet there are now many new populations of fallow deer, often considerable distances from the nearest source population. These types of illegal actions are likely best addressed through hunter education programmes coordinated by the Game Animal Council.
Relative to farm escapes and illegal liberations, natural dispersal has resulted in far fewer new populations in recent decades. However, understanding the dispersal process can be crucial for wild ungulate management strategies. For instance attempted eradication of animals from a specific site is likely to be hampered by reinvasion. This might occur because neighbouring populations are able to interact with each other despite being separated by artificially or naturally fragmented habitats. Alternatively, survivors of an eradication programme might disperse to adjacent habitat following disturbance, establish viable populations and subsequently pose a reinvasion threat to the area from which they were initially removed.
Dispersal studies can provide information about the prevalence of dispersal within populations, the attributes of dispersing individuals, and the corridors, habitats or other landscape features that most favour dispersal. Recent GPS technology, coupled with environmental data in a GIS framework, can be an excellent method of quantifying dispersal. However, this approach is only useful if some radio-tagged individuals disperse from a population. Where anticipated dispersal is low, more animals need to be tagged to quantify this process. That said, the cost, both of GPS units and of deploying them, is likely to be prohibitively high.
Revising the work of Fraser et al. will allow managers to estimate the numbers of new populations and changes in the distributions of species. Regionally, much of this information already exists but it needs to be collated and summarised to provide a nationwide picture. This would enable managers to identify new locations and likely sources of new populations, determine which populations must be removed to mitigate unwanted impacts, and so develop species management plans. Similarly, understanding how and why animals disperse, particularly around the interface of areas where wild ungulates are permitted and areas where they are not, will also be key for effective management and biosecurity.
This work was funded by the Ministry of Business, Innovation and Employment.