Linking Specialisation and Stability of Plant ... - OPUS Würzburg

4.5 discussion 81
termine the range of starting conditions and the appropriate
time scale.
Although the model used in this study contains a fair amount
of mechanistic detail, it still makes a number of simplifying assumptions
that may impact the results of our analyses. In the
following, we discuss potential caveats and limitations of our
modelling approach, and suggest directions for future research.
First, the results presented in this paper to some degree depend
on the choice of parameter values for numerical simulations,
specifically on the assumption of an equal number of plant and
animal individuals in the system at equilibrium. Choosing a
lower animal-plant ratio results in a narrower range of specialisation
and niche overlap values allowing qualitative stability of
the coexistence equilibrium, while a higher animal-plant ratio
extends that region (see Benadi et al., 2012b). However, a different
animal-plant ratio does not qualitatively affect the patterns
presented in this study for the region allowing stable coexistence.
As discussed in Benadi et al. (2012b), the few available
empirical studies indicate that the amount of floral resources
needed to sustain one animal varies widely, but at least for pollination
mutualisms it seems that the ratio of animal to plant
individuals is usually below one. On related terms, the choice
of equal competition coefficients for both plant species, constant
sums of trait matching values for all mutualistic interactions
and equal values for all other parameters in this study
was made in order to concentrate on the effects of communitywide
specialisation. Since real mutualistic systems always deviate
from the perfect symmetry assumed in this study, future
work should explore the consequences of asymmetric interaction
strengths and demographic parameters. For example, it
would be interesting to study the consequences of a trade-off
in plant competitive ability with respect to abiotic resources
and attractiveness towards pollinators. Furthermore, future research
should target larger mutualistic communities containing
a mixture of various degrees of specialisation, and elucidate the
effect of different distributions of specialisation levels on community
stability. Some attempts in this direction have already
been made (see e.g. Okuyama & Holland, 2008; Bastolla et al.,
2009), but much remains to be understood.
The current study is to some extent a theoretical exercise
whose predictions are difficult to test in the field. Its main
result, the finding that no single relationship between specialisation
and stability of mutualistic systems exists, is bad news
for conservationists and managers in search of rules of thumb
for assessing the fragility of ecological systems. In order to