Geoscience Reference
In-Depth Information
Table 27.1
Derivation of model parameters. Plant biomass and intake rate values are expressed as dry mass.
Parameter
Value
Derivation
Number of swans
41
The initial number of swans that were present on the study site
5842 kJ d
−1
Swan energy requirements
O'Hare
et al.
(2007)
Swan energy store
145 690 kJ
Energy content of swan tissue (34.28 kJ g
−1
) multiplied by the
difference between mean (10 750 g) and starvation (6500 g) mute
swan mass (Wood, unpublished data)
184.5 g m
−2
Initial
Ranunculus
biomass
in study area
O'Hare
et al.
(2007)
171.1 g m
−2
Initial
Ranunculus
biomass
outside study area
O'Hare
et al.
(2007)
0.0 g m
−2
d
−1
Ranunculus
growth rate
Growth rate under swan grazing pressure as swans remove growth
tissues (O'Hare
et al.
, 2007)
16.5 kJ g
−1
Ranunculus
energy content
O'Hare
et al.
(2007)
Ranunculus
digestibility
0.523
O'Hare
et al.
(2007)
Swan intake rate when
feeding on
Ranunculus
0.00-0.03 g s
−1
Intake rate, I
=
(0.0031
x
)/(1
+
(0.0934
x
)), where
x
=
Ranunculus
biomass (Wood, unpublished data)
Initial grass biomass
203.8 g m
−2
This study
0.0 g m
−2
d
−1
Grass growth rate
This study
Grass energy content
17.6 kJ g
−1
Value for pasture grasses foraged by Bewick's swans (
Cygnus
columbianus
) (van Gils
et al
., 2008)
Grass digestibility
0.269
Mean value for digestibility of pasture grasses consumed by Bewick's
swans (van Gils
et al
., 2008)
Swan intake rate when
feeding on grass
0.01 g s
−1
Bewick's swan functional response for grass (van Gils
et al.
, 2007) for
a sward height of 7.2 cm (this study)
The model was run 10 times and results
compared with field observations. As all 10
simulations were based on identical parameter
values, predictions do not typically vary and thus
the standard deviation (SD) for model predictions
was zero unless stated otherwise.
by a time budget study in May 2010 for the
River Frome (mean 27.2%
±
19.2 SD; Wood,
unpublished data), and for the nearby River Avon
which is a similar chalk river (mean 25.3%
±
15.1
SD) (Trump, 1996). The mean percentage of time
spent by swans on river patches was predicted to
be 96.0% (
0.2 SD); this was very close to the
observed value of 97.8%.
To assess how robust these predictions were,
the sensitivity of the model was examined when
subjected to an increase and a decrease of 25%. In
general, these changes to the parameter values did
not alter the predictions of the model (Figure 27.2).
The total number of swan days,
Ranunculus
biomass
after grazing and the percentage of time spent on
the river patches were all sensitive to changes in
Ranunculus
biomass, both in the model patches and
outside the model area, but not to changes in any
other parameter.
±
Results
The MORPH model predicted 181.1 swan days
occupancy, compared with 215 swan days observed
in the field and the 181-193 swan days predicted
by the mathematical model of O'Hare
et al.
(2007).
Predicted mean
Ranunculus
biomass after grazing
(i.e. depletion) was 171.0 g m
−
2
,whichclosely
matched the 171.1 g m
−
2
observed. The proportion
of predicted time that swans spent foraging during
daylight (38.5%) was within the range calculated
