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

16 introduction
the fact that the total number of individuals in the community
is often limited by other factors such as abiotic resources or
simply space. In chapter 3, we employ a modified version of
the model developed in chapter 2 to investigate the effect of the
presence of pollinators on species richness and evenness of a
plant community. In accordance with Bastolla et al. (2009), we
compare plant species richness for fully connected, nested and
diagonal interaction networks. Unlike this earlier study, we examine
plant communities that fill nearly all available habitat in
the absence of pollinators as well as communities with a total
population size far below the habitat capacity in the absence of
pollinators. In addition, we study the effect of a trade-off in effectiveness
of generalised pollinators on different plant species
(see section 1.4.2). This analysis clarifies the conditions under
which interactions with pollinators may promote plant diversity.
1.5.2 Stability of plant-pollinator networks in the face of anthropogenic
disturbances
According to current knowledge, the anthropogenic increase
of greenhouse gas emissions has two main effects on the global
climate: A long-term rise in mean global temperature and an increase
in the frequency and intensity of extreme climatic events
(IPCC, 2007). In chapter 4, we examine the stability of multispecies
plant-pollinator systems in the face of temporary disturbances
such as those caused by extreme climatic events. Since
stability is a generic term for system responses to different
types of perturbations (see section 1.3.3), we compare results
for four criteria of stability that differ in the the state variable
under consideration and relate to different intensities of disturbances.
The main aim of this study is to understand the relationship
between specialisation and stability of plant-pollinator
systems. Many studies have emphasised the destabilising effect
of specialisation through an increased extinction risk (see
section 1.4.3). However, as a form of resource partitioning specialisation
may also have a stabilising effect by reducing the
risk of competitive exclusion (see section 1.3.2). Using a variant
of the model presented in chapter 2, this chapter examines the
interplay between the two contrasting effects of specialisation.
Chapter 5 is concerned with the second aspect of climate
change, the worldwide increase in mean temperatures. There
is now sufficient evidence that rising temperatures are causing
a shift in the phenology of many plant and animal species in
temperate and arctic regions (Parmesan, 2006; Cleland et al.,