The relationship between the rate of food intake by a herbivore, and the biomass of available food (the functional response) is of ecological significance because it provides a mechanism linking the animal’s distribution to its food supply (Holling 1959; Fryxell 1991, 1992; Sutherland 1996; Illius & Gordon 1999). The ideal free distribution predicts that animals in a population distribute themselves in direct proportion to, or according to some increasing function of their attainable intake rate (Fretwell & Lucas 1970; Farnsworth & Beecham 1997). Because the functional response is expected to map directly onto the distribution of the animals, its precise form is crucial to the prediction of animal distribution.
In general, the rate of food intake of vertebrate herbivores is an asymptotic function of bite mass, a relationship that holds for grazing and browsing animals, regardless of the biomass of available forage (Spalinger & Hobbs 1992). However, for herbivores grazing simple grassy swards there is a close relationship between sward biomass and bite mass, and despite some compensation because of greater rates of biting on low sward biomasses, the functional response relationship of intake rate with biomass is also asymptotic (Laca et al. 1994). This arises because on short swards, the mouth is not filled at each bite, but as sward height increases, bite sizes increase until a point is reached, above which further increases in sward biomass result in no further increase in bite dimensions or short-term intake rate. Although the relationships between bite mass and intake rate of lagomorphs have been established (Gross et al. 1993), there is surprisingly little published empirical evidence on the relationship between intake rate and biomass on grass-dominated swards, the main habitat of the European Rabbit (Oryctolagus cuniculus L.). Attempts have been made to measure daily intake rate of rabbits by sampling vegetation biomass before and after grazing periods lasting several days (Short 1985; Armstrong 1988). Short (1985) fitted an asymptotic functional response relationship. However, the fit of the asymptotic model was poor at high biomasses, the fitted asymptote lying between the alternative extremes of the intake rate continuing to increase at high biomasses or declining at high biomasses to give a unimodal form to the functional response. If the functional response of rabbits follows the asymptotic form, then we would expect their distribution also to increase monotonically with biomass. Alternatively, it has been suggested for several intermediate-sized herbivores, including rabbits, that foraging efficiency is impaired both at low standing crops, by limited food availability, but also at high standing crops, by the requirement for selection among the structurally more complex food environment (Van de Koppel et al. 1996). Under these circumstances, the postulated resultant unimodal functional responses were hypothesized to underlie the observed patterns of habitat selection, which were biased towards intermediate standing crops (Van de Koppel et al. 1996).
We adopt an experimental approach with captive European Rabbits to quantify their functional response on simple Ryegrass (Lolium perenne L.) swards. We also test the extent to which the observed functional response predicts the habitat selection by foraging, free-living wild rabbits, on similar swards in the field situation.
It is apposite to investigate the generality of the rabbit’s functional response and therefore assess its applicability to prediction of distribution in contrasting situations. There is considerable evidence that the morphology of plants and the structure of the sward also influence vertebrate herbivores’ intake rate and therefore their functional response (Gross et al. 1993; Rowcliffe, Sutherland & Watkinson 1999; Bergman, Fryxell & Gates 2000). Greater sward complexity at high biomass was the hypothesized cause of reduced foraging efficiency, and the reduced selection of high standing crop communities by intermediate-sized herbivores (Van de Koppel et al. 1996). But this effect is likely to be more profound in selectively foraging, small and medium-sized herbivores, than in larger grazing herbivores, whose biting characteristics are determined by whole-sward characteristics, rather than the morphology of individually prehended plants or plant parts. Therefore we also compare the functional response of rabbits on the relatively simple broad-leaved Ryegrass swards, with that on a Sheep’s Fescue (Festuca ovina L.) sward. Sheep’s Fescue is a narrow-leaved grass with lower biomass per leaf length than Ryegrass (Illius et al. 1995; Tallowin, Brookman & Santos 1995), and greater leaf and tiller-densities than the broader-leaved grasses (Illius et al. 1995). We test the prediction that greater overall rates of intake can be achieved by rabbits on the broader-leaved grass.