The ability of feral herbivores to reduce the amount of food available to domestic livestock has
rarely been quantified. This thesis seeks to examine the degree to which feral goats (Capra
hircus) reduce the availability of pasture and shrub biomass for other herbivores. The
interaction between feral goat populations and their food supply will be explored using a
modelling approach. In addition to this it will also examine the cost of controlling goats and
attempt to identify the cost efficient target densities for control operations. The implications of
this information for management will be discussed.
There are two ways an animal population can interact with its food supply, through: (1)
intrinsic food shortages, and (2) extrinsic food shortages. Intrinsic food shortages occur when a
negative feed-back loop exists between the animal population and their food supply. This
means the animals affect the availability of their food and their food supply affects the
dynamics of the animal population. Since the animals are affecting their own food supply it
could be said that they will also affect the availability of that food to other herbivores if they
consume the same species. Extrinsic food shortages occur when there is no feed-back from the
animals to their food supply. Food availability is determined by extrinsic factors such as
rainfall and is unaffected by the animal population. To determine how feral goats interact with
their food supply several models will be examined, and these include: (1) single species models
which use data from the animal population only. These have historically been used to identify
density dependence which is commonly caused by the animal population being regulated
through their food supply in the case of large herbivores, and (2) trophic models which
incorporate data from at least two trophic levels in an ecosystem, in this case those being the
animal population and the vegetation they are thought to consume. These models allow a more
direct examination of the relationship between the feral goat population and their food supply.
The various models were fitted to data collected on the field sites and the following results
were obtained:
(1) the dynamics of the feral goat populations could not be represented by single-species
models. This was most probably due to the stochastic environment in which they lived
causing the level of density dependence experienced by the goats to vary greatly masking its
presence.
(2) the rate of increase of the goat population could be predicted by the numerical response of
rate of increase to pasture biomass. This demonstrated that food availability influenced the
dynamics of the goat population.
(3) goat density affected the availability of some species of shrub biomass. There was,
however, no response seen in the availability of pasture species to changes in goat density.
Since the study was conducted during a drought this is in agreement with other studies
which indicated that goats will primarily browse during dry spells but switch to pasture
species when conditions improve following rainfall.
These results indicate that a negative feed-back loop does exist between feral goat populations
and their food supply since the goats affected the availability of some shrub species and so they
suffer intrinsic food shortages. This means goats have the ability to reduce the availability of
food to other herbivores providing both are eating the same species.
Cost-efficiency analysis showed that the cost of removing individual feral goats increases
exponentially as density decreases because the search time per animal becomes greater. This
relationship was used to construct a model that predicted the cost of achieving a target density.
The model describing cost of control over density was also combined with a productivity
model based on the numerical response of feral goats to pasture biomass to predict the cost of
maintaining target densities under different environmental conditions. Using these models the
most cost-efficient density identified was 11 goats/km2.
From this study we can make the following conclusions:
(1) feral goats have the ability to reduce the amount of shrub biomass available to other
herbivores during dry conditions
(2) their ability to influence the availability of pasture species remains unknown
(3) given the cost of initial and ongoing control and the minimal benefits that result it would be
difficult to justify controlling goats during a drought on the field sites.
| Identifer | oai:union.ndltd.org:ADTP/219169 |
| Date | January 1997 |
| Creators | Maas, Sylvana, n/a |
| Publisher | University of Canberra. Resource, Environmental & Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Sylvana Maas |
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