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

2 introduction
specialisation (section 1.4). Moreover, the pollination mutualism
plays an important part both in natural ecosystems and in
agricultural production. According to a recent estimate, approximately
87.5% of all angiosperm plant species are pollinated
by animals (Ollerton et al., 2011). Since flowering plants
fulfil an essential function as primary producers in all terrestrial
ecosystems, the integrity of these ecosystems depends on
the pollination services provided by insects and other animals.
However, the degree of dependence on animal pollination varies
among plant species and ecoregions. For example, whereas
wind-pollinated plants always constitute a minority in terms
of species numbers, they dominate with respect to biomass in
most arctic and temperate plant communities (Schemske et al.,
2009). Furthermore, many animal-pollinated plant species are
able to reproduce in the absence of pollinators, either asexually
or through self-fertilisation, thereby reducing the risk of reproductive
failure (Bond, 1994; Busch & Delph, 2012). The dependence
of these plants on animal pollination is more difficult to
assess than for obligatory mutualists. It requires knowledge
about the fitness of selfed versus outcrossed offspring and the
importance of genetic diversity for the continued existence of
populations.
By offering nectar and pollen in exchange for pollination services,
flowering plants sustain a diverse guild of flower-visiting
animal species, mostly insects, but also vertebrates such as birds,
bats and primates. To my knowledge, no estimates of species
numbers of pollen and nectar feeding insects in relation to
species with other feeding strategies exist, but numerous flowervisiting
species are found in major insect orders such as Coleoptera,
Diptera, Hymenoptera and Lepidoptera (Kevan & Baker,
1983). As with flowering plants, the degree of dependence of
these insects on the pollination mutualism varies from occasional
flower visits to complete dependence of all life stages on
floral resources.
Beside the ecological importance of plant-pollinator interactions,
the pollination services of animals also constitute a considerable
economic value. In a recent review article, Klein et al.
(2007) estimated that animal pollination has a positive effect
on the production of 70% of the world’s most important crops.
The global economic value of pollination services to all crops
directly used as human food has been estimated as AC153 billion
or 9.5% of the value of worldwide agricultural production
(Gallai et al., 2009). While the production of most staple
food such as cereals does not depend on animal pollination,
other essential ingredients of a balanced human diet such as