Merino sheep are the most numerous domestic livestock in Australia and feral
goats are wide-spread and locally abundant in many of the regions where sheep
are grazed. Foot and mouth disease (FMD) is a disease of ungulates that causes
severe economic hardship to countries where outbreaks occur or where it has
become endemic. In India, Africa, Greece and recently the United Kingdom and
Eire, sheep and goats have been implicated in the spread and maintenance of
FMD. In Australia, there are contingency plans (AUSVETPLANS) for dealing with
an outbreak of FMD. Included in those plans are strategies for control of the
disease in feral ungulates including feral pigs and feral goats. Modelling has
provided assistance in developing strategies to combat outbreaks in feral pigs and
for controlling outbreaks in domestic livestock. No models have been constructed
to aid decisions about controlling FMD in feral goats where they co-occur with
merino sheep.
In Australia, the greatest densities of free-ranging feral goats and domestic
livestock occur in the high rainfall zone (> 500 mm mean annual rainfall) along the
eastern tablelands and adjacent slopes of the Great Dividing Range. Previous
studies of feral goat biology, population dynamics and behaviour in Australia have
concentrated on arid and semi-arid zones or on islands. Interactions between free
ranging feral goats and merino sheep have not previously been studied in the high
rainfall zone.
My study investigated the ecological and behavioural characteristics of feral goats
and their interactions with sympatric merino sheep at a high rainfall site in central
eastern New South Wales. The population dynamics, biological and behavioural
parameters of feral goats and sheep were then used to model FMD in such an
environment. Deterministic temporal models and a new spatial stochastic model
were used. Of particular interest were the rates of contact within and between
subgroups of feral goats (termed herds and mobs), within and between subgroups
of merino sheep (termed flocks and mobs), and between subgroups of the two
species.
Feral goats at the study site were found to be numerous (mean density = 34.94
goats km-2, from aerial surveys), in good condition, fecund and had high adult
survival and low annual adult mortality (survival= 0.81�1.00) in the absence of
harvesting and hunting. They had an observed instantaneous rate of increase of
0.112 per year. Annual rate of increase was similar to other sites in Australia
without sustained harvesting pressure. Home ranges were small for both males
(3.754 km², s.e. = 0.232, n = 116 goats) and females (2.369 km², s.e. = 0.088, n =
241 goats). From this and other Australasian studies, an inverse power function
was found to be an excellent descriptor of the relationship between mean annual
rainfall and female home range size.
A resource selection function was fitted in a geographic information system to
observational data of feral goats. The habitat selection of feral goats included a
preference for wooded vegetation on eastern and north eastern aspects at higher
elevations. The resource selection function was also used to set the probabilities of
occurrence of feral goats in 1 ha areas of the landscape and these probabilities
were used to generate heterogeneity in a spatial model of foot and mouth disease
virus (FMDV) transmission.
Daily per capita contact rates were estimated from observed contacts in the field
where a contact between individuals was recognised when one approached within
1 body length ([approx] 1m) of another. The contacts between feral goats within herds
were frequent and occurred at a rate of 6.96 (s.e. = 1.27) goat�goat contacts day-1.
Sheep to sheep contacts were slightly less frequent (4.22 sheep�sheep contacts
day-1, s.e. = 0.65) but both estimates were most likely negatively biased because of
observer errors in estimating the number of individual animals coming in contact
with observation subject (focal) animals. Contacts between herds of feral goats
were not common and those between adjacent populations were fewer than 1 per
year. In sheep, flock to flock contact was largely governed by husbandry practices
and occurred at a mean daily rate of 0.0014 flock�flock contacts. Contacts
between sheep and feral goats were less frequent but nonetheless common (2.82
goat�sheep contacts day-1, s.e. = 0.40). In feral goats the size of the mob in which
focal goats were observed was found to be the most important factor in
determining contact rates between individuals and a counter-intuitive inverse
relationship was identified. Contacts were heterogeneous and density was not an
important determinant of contact rates implying that, because of the uniformly high
densities at the site, saturation had occurred.
The temporal models of FMDV transmission showed that the rate of contact within
and between species was such that FMD was predicted to spread rapidly
throughout an infected herd or flock. Control strategies of intense culling of feral
goats at the population level were predicted to allow the disease to persist at low
prevalence, with a small peak corresponding to the annual lambing pulse in sheep.
However, the same level of control (>90% reduction) at the herd level was
predicted to eliminate FMD and allow the safe reintroduction of sheep. Extreme
control that left very small groups (<3 individuals) may be counter productive
because such small groups are likely to join the reintroduced sheep in an effort by
the goats to meet gregarious urges.
The spatial model was more reassuring. It predicted that FMD would die out in a
mixed sheep and feral goat population in less than 90 days because of the low rate
of herd to herd contact and herd to flock contact. For similar environments, the
contingency planning consequences are that an outbreak of FMD introduced into
feral goats from sympatric sheep is likely to be containable by removing all the
sheep, determining the extent and likely range of the feral goats, then removing a
substantial proportion of or eradicating each herd. Feral goats, being relatively
sedentary, are unlikely to spread to adjacent populations and the disease will die
out through lack of contact between herds and populations. Because feral goat
home ranges overlap and are centred on one or two small catchments, a
containment ring of feral goat control, set to encompass the home range of a target
herd and that of adjacent herds, should be adequate to limit spread of FMD.
| Identifer | oai:union.ndltd.org:ADTP/218653 |
| Date | January 2004 |
| Creators | Fleming, Peter J S, n/a |
| Publisher | University of Canberra. Resource, Environmental & Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Peter J S Fleming |
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