|dc.description.abstract||Functional responses describe the relationship between an animal’s consumption rate of food and the abundance of the food resource. They illustrate how consumers cope with declining food resources and provide a greater understanding of plant-herbivore relationships. These play a central role in modelling the dynamics of ecosystems. Modelling helps guide management decisions by allowing predictions about the effects of changes in parts of the ecosystem. Despite the prolonged and significant impacts of rabbits on both conservation values and agriculture in the dry grasslands of Central Otago, there have been no attempts to measure rabbit functional responses in New Zealand. This study is the first to empirically estimate the functional responses of New Zealand rabbits on improved, semi-improved, and ‘rough’ semi-improved pastures in the dry grasslands of New Zealand.
Functional response studies of rabbits in Australia have fitted relationships known as ‘type II’ (asymptotic curve) and ‘type II with an ungrazed residue’ responses. In the current study, rabbits exhibited very different patterns of intake between pasture types, with an unsaturating type I response (represented by a linear relationship) on improved pasture, and a saturating (inverted exponential Ivlev relationship) type II response on semi-improved pasture, both with very high intake rates and large ungrazed residues. Intake of green vegetation showed a type V response (although similar in shape to a type II response, the equations differ between the two, with a type V curve including a function quantifying the ungrazed residue). No functional response relationships could be fitted to the data from the ‘rough’ semi-improved pasture. Rabbits on this pasture type showed very high intake rates – far greater than in previous studies. This was probably a result of the high error associated with daily estimates of vegetation biomass on ‘rough’ semi-improved pasture.
To help understand the differences in patterns of intake between pasture types the nutritional quality of the pasture was assessed by measuring the acid detergent fibre (ADF) and crude protein (CP) of the vegetation at the time of the trails. Improved pasture had the lowest ADF, and highest CP, and ‘rough’ semi-improved pasture the highest ADF and lowest CP. Reported minimum levels of ADF and maintenance protein requirements for wild rabbits in the literature are wide ranging and often conflicting. It is generally accepted, however, that rabbits are unable to process forage with ADF levels greater than 40%. ‘Rough’ semi-improved pasture had very high ADF levels; well above the levels rabbits are able to survive on, and also had very low CP levels, below minimum estimates of CP in rabbit diets. ADF and CP of semi-improved pasture and improved-pasture were within the limits of the fibre and protein that rabbits require to maintain health. Although food quality is known to influence an animal’s daily food intake rate, few functional response studies have examined the effect of food quality on an animal’s daily food intake rate, even though it potentially has considerable impact. Functional response experiments may therefore be far more informative by including a term for food quality for describing the shape of the curve.
Functional responses are highly specific to the location of experiments because an animal’s intake rate is influenced by a number of location-specific factors. These include physiological condition of the animal, characteristics of the vegetation that may aid or hinder grazing, and climatic conditions (Crawley, 1983). The current study illustrates how the shape of a functional response relationship changes with different locations and pasture types – therefore, caution must be used when extrapolating the results of functional response experiments. Additionally, the shape of the relationship changed when unpalatable or inaccessible food was excluded. Understanding an animal’s food source is important in functional response experiments, because what a researcher considers to be available food may not in fact be accessible to the animal. Theoretically the inclusion or exclusion of this unpalatable food can have important consequences for dynamic ecosystem models, especially if the unpalatable food is an important resource for other animals within the system of interest.||