Geoscience Reference
In-Depth Information
CHAPTER 27
An Individual-Based Model of
Swan-Macrophyte Conflicts on a
Chalk River
Kevin A. Wood
1,2
, Richard A. Stillman
2
, Francis Daunt
1
and Matthew T. O'Hare
1
1
Centre for Ecology and Hydrology, Edinburgh, UK
2
School of Conservation Sciences, Bournemouth University, Poole, UK
Introduction
trout (
Salmo trutta
) may be degraded (Trump,
1996; O'Hare
et al.
, 2007). The reduction in
Ranunculus
can also affect water levels in chalk
rivers (Dawson, 1989). Successful chalk river
management in these circumstances therefore
requires a good understanding of the movement
and behaviour of mute swans and the likely
intensity of grazing on
Ranunculus
at different
localities. Mathematical modelling of ecological
interactions and animal-plant relationships can
be applied to river management. For example,
O'Hare
et al
. (2007) developed a simple 'resource-
consumer' model that predicted the depletion of
Ranunculus
and the mute swan carrying capacity
of a short section of chalk river. However, the
relevance of this type of mathematical model to
river management is limited because of several
assumptions, notably: (i) swans do not have energy
reserves; (ii) they feed at a fixed rate; (iii) swans
leave a feeding area when
Ranunculus
biomass falls
below a threshold level, regardless of food quantity
outside the study site; and (iv) the river is the
only available foraging habitat used. Given what
is known about swan behaviour on chalk rivers
these assumptions are unlikely to be valid (Trump,
1996). Individual-based models (IBMs) are also
'resource-consumer' models but do not make these
unrealistic assumptions. Instead, they predict the
Mathematical models are a useful way of predicting
the outcomes of ecological change on animal
populations; in so doing, they can facilitate the
testing of management options for conservation
management problems (Stillman and Goss-
Custard, 2010). One such problem is the grazing of
aquatic macrophytes by mute swans (
Cygnus olor)
in
chalk rivers in southern England. Many chalk rivers
are highly valued for their salmon and trout fishing
and for their nature conservation interest and
several are protected under the European Habitats
Directive (Council of the European Communities,
1992). Their naturally high diversity of aquatic
macrophyte and macroinvertebrate communities
is maintained by careful management of the
river channel and banks for nature conservation
and fisheries purposes (Environment Agency,
2004). Water crowfoot (
Ranunculus penicillatus
ssp
. pseudofluitans
) is a particularly important
macrophyte in these rivers, supporting a high
secondary production of macroinvertebrates and
fish (Berrie, 1992). However, grazing by flocks of
non-breeding mute swans can severely reduce the
biomass of
Ranunculus
, raising concerns among
fisheries interests in particular that habitat and
cover
for other
river
species, notably brown
