Experiment 1: the functional response of rabbits
For both the Lolium and the Festuca swards the relationship between the sward height and the biomass was approximately linear, although in both cases the mass per height declined at the tallest sward heights (Fig. 2). We henceforth describe swards in terms of their height (cm) from which mass can be calculated from the following polynomial relationships:
Figure 2. The relationship between sward height and standing crop biomass of mini-swards of (a) Lolium perenne and (b) Festuca ovina, offered to captive rabbits to determine functional respones.
Download figure to PowerPoint
- mass (g DM m−2) = −20·5 + 34·7 Sward Height − 1·2 Sward Height2, eqn 2
F2,133 = 410·1, P < 0·001, r2 = 85·8%.
- mass (g DM m−2) = −119·6 + 52·7 Sward Height − 1·9 Sward Height2, eqn 3
F2,116 = 295·2 P < 0·001, r2 = 83·3%.
The addition of the quadratic term significantly improved the fit of the relationship for both species (additional r2 due to added quadratic term: Lolium= 13·7%, F1,133 = 21·1, P < 0·001; Festuca= 16·4%, F1,116 = 22·8, P < 0·001).
The mean NDF concentration in the ungrazed control samples was 60·45% DM (SE 0·456) and did not vary significantly between the two grass species (F1,46 = 0·0, NS). The NDF concentration declined as the overall height increased (cm) in the Lolium control swards (slope = −0·487, SE 0·185, F1,28 = 6·9, P < 0·05, r2 = 17%), but did not vary with height in the Festuca controls (F1,18 = 1·49, NS). The nitrogen concentration in the control swards did not vary significantly with changing sward height (F1,51 = 1·06, NS) but was significantly higher in the Festuca than the Lolium swards (Festuca: mean 3·143% DM; Lolium: mean 1·975% DM, SED 0·1592, F1,51 = 53·4, P < 0·001).
The mean wet mass eaten by rabbits was 7·5 g trial−1 (SE 0·56), which did not differ between the two grass species (F1,195 = 3·8, NS). Rabbits spent less time foraging during the trials with Festuca swards than with Lolium (Festuca: 578·5 s, Lolium: 828·5 s; F1,195 = 29·5, P < 0·001) and they ate correspondingly less dry plant material (Festuca: 1·68 g DM/trial, Lolium: 2·32 g DM/trial, F1,195 = 16·6, P < 0·001), but the rate of intake did not differ between the two grass species (F1,195 = 3·8, NS).
The functional responses of the five individual rabbits grazing Lolium swards were typically asymptotic in form (Fig. 3a, Table 1). Those on the Festuca swards were variable but definitely not described by an asymptotically increasing relationship; they tended to be negative relationships (see Fig. 3b).
Figure 3. The functional responses of captive rabbits grazing on (a) Lolium perenne and (b) Festuca ovina. Intake rate is expressed as g dry matter min−1 of actual foraging time.
Download figure to PowerPoint
Table 1. Estimates of the parameters and their standard errors of the asymptotic curves y = A + [B/(1 + Dx)], fitted to the functional responses of intake (g DM min−1) against the height (cm) of Lolium swards for individual rabbits
|Rabbit no.||Parameter estimates (standard errors)||F (df)||Probability|
| ||(0·647)||(0·189)||(0·075)|| || (2,19)|| |
|2||−0·475||0·006|| 0·132||34·8|| 6·08||<0·01|
| ||(0·064)||(0·007)||(0·008)|| || (2,19)|| |
| ||(7·69)||(3·08)||(0·035)|| || (2,18)|| |
| ||(0·022)||(0·010)||(0·019)|| || (2,19)|| |
|5||12·0||−2·8|| 0·217||37·0|| 6·87||<0·01|
| ||(362·0)||(81·2)||(0·046)|| || (2,18)|| |
After fitting functional response curves for each rabbit on Lolium swards, and fitting the overall mean rates of intake for each individual on the Festuca swards, the residuals about these fitted relationships were found to be weakly negatively correlated with the number of hours elapsed since removal of pelleted food (F1,199 = 4·18, P < 0·05, r2 = 1·6%). The height of the areas of grazed swards remaining after each trial, was unrelated to the time since removal of pelleted food (F1,194 = 0·17, NS) but was positively related to the initial height of the sward (F1,197 = 795·45, P < 0·001, r2 = 91·3%), the slope of the relationship being significantly greater in Festuca (b = 0·351, SE 0·0167) than in Lolium (b = 0·24, SE 0·0131: grass species × initial sward height, interaction F1,195 = 26·7, P < 0·001). After adjusting for these effects, the height remaining after grazing was significantly greater on the Festuca than the Lolium swards (Festuca: 2·70 cm, Lolium: 2·31 cm, SED 0·064, F1,195 = 14·08, P < 0·001).
Experiment 2: habitat selection by wild rabbits
The characteristics and analysis of the experimentally mown swards are given in Table 2. The intended sward conditions were achieved and maintained, although there was a slightly lower mean sward height during Period 2 due to drought.
Table 2. Characteristics of the experimentally mown swards in Experiment 2, in relation to their target experimental treatment sward heights and in each experimental period (1–3). Unlike superscripts denote significant differences, LSD test (P < 0·05)
| ||Experimental sward height (cm)||SED||F (df 3,36)||F probability|
|Measured height (cm)|| 2·36a|| 4·29b|| 6·65c|| 11·35d||0·28||387·5||< 0·001|
|Biomass (g DM m−2)||48·8a||130·1b||217·9c||353·0d||2·8||172·1||< 0·001|
|NDF (% DM)||55·84||53·86||54·64|| 55·47||1·19||0·4||NS|
|Nitrogen (% DM)|| 2·00ae||2·32b||2·08a|| 1·82de||0·09||11·0||< 0·001|
| ||Experimental period||SED||F (df 2,36)||F Probability|
|Measured height (cm)|| 6·30a|| 5·76b|| 6·42a||0·24|| 4·2||< 0·05|
|Biomass (g DM m−2)||36·9ab||34·2a||41·4b||2·4|| 4·5||< 0·05|
|NDF (% DM)||54·48||56·04||54·34||1·2|| 1·7||NS|
|Nitrogen (% DM)|| 2·27a|| 2·13a|| 1·76b||0·08||24·2||< 0·001|
The mean number of grazing rabbit observations per plot per day was 10·47 (range 0–101), and these data were highly skewed. REML analysis using the specified model showed progressively fewer observations of foraging rabbits as the sward height increased with significantly more observations of foraging rabbits on the two shorter sward heights (Table 3). The mean residence time of foraging rabbits in each plot tended to increase with sward height (Table 3), and was greater when group sizes were equal to three as compared to smaller or larger group sizes (Group size 1, 250·0 s; 2, 254·2 s; 3, 398·5 s; > 3: 290·0 s, SED 59·7, F3,1422 = 17·9, P < 0·001).
Table 3. Results of habitat selection experiment: number of daily feeding observations per plot and mean residence times of foraging rabbits in each plot. Unlike superscripts indicate significant differences (P < 0·05, LSD test)
| ||Experimental sward height (cm)||SED||F (df)||F probability|
|Daily no. of feeding obs/plot. Log(n + 1)||1·842a||1·688a||1·150b|| 0·703b|| 0·262|| 7·9 (3,12)||< 0·01|
|Daily no. of feeding obs/plot (untransformed)||19·3||15·2||5·5|| 2·2||–||–||–|
|Mean residence time of feeding rabbits (s)||260·0ab||172·1a||349·3bc||411·4c||67·0||11·0 (3,9)||< 0·01|