| Kurzbeschreibung | Dispersal is a key process in the life cycle of all organisms, determining, among others, colonization, large scale migration, local and meta-population dynamics, the long term survival of species as well as the spatial and genetic structure of populations and taxa.
Having in mind that there is hardly any ecological or evolutionary process not effected by dispersal, it is not surprising that two of the most striking organs of higher plants, which are sessile organisms, attend to enhance spatial dispersion: Flowers enhance gene flow via pollination whereas fruits (seeds, in effect: diaspores) ensure spatial dispersion of offspring which also includes gene flow. Here, it is necessary to distinguish between abiotic dispersion (e.g. dispersal by wind) and dispersion by biotic vectors (e.g. dispersal by birds). The development of the flamboyant and costly organs (flowers and seeds/fruits), which even gave name to the whole taxonomic group (flowering plants or seed plants, respectively) can only be understood as a consequence of a coevolution between plants and their biotic dispersal vectors.
Most of the existing studies addressing coevolution between plant and animals are about pollination whereas relatively few studies deal with the evolutionary ecology of seed dispersal. Here, ornithochory, i.e. dispersal by birds, is by far the best studied system. These studies resulted in a well established and useful theoretical framework, elaborating e.g. costs, benefits or successful strategies for the involved partners. For instance, it is undisputed that the main reward of the birds is nutrition in form of the nutrient rich fruits. The fruits appeal mainly by colours, whereas odour plays only a minor role for attracting birds. In contrast, the evolutionary ecology of seed dispersal by mammals, especially ruminants has only been studied rarely, although this system may be even more important and interesting from scientific as well as from applied points of view: - In some systems (e.g. dispersal of fruits of tropical trees by monkeys) the co-adaptation between plants and their seed dispersers is obvious; in other systems (e.g. dispersal of seeds of grassland species by ruminants) the nature of the interaction is less obvious. This issue is recently subject of an intense ongoing scientific debate in ecology, but was hardly discussed from an evolutionary point of view.
- A theoretical concept addressing epizoochory, i.e. dispersal of seeds attached to fur or claws, has not been developed at all. For instance, the costs and benefits of epizoochory were not yet quantified.
- Seed dispersal by mammals, especially ruminants, is predominant in natural and semi-natural grassland (covering vast dimensions worldwide) but not well understood.
- Coevolution between plants (especially grasses) and mammals (especially ruminants) is discussed as one reason for the radiation of both species groups during the Pliocene and may have played an important role for the first rise of savanna-ecosystems during that time. However, comprehensive studies on the evolutionary ecology of seed dispersal (intrinsically tied to herbivory) are still lacking.
- In many developing countries in the African tropics, utilisation of savannas tributes substantially to nutrition, health and survival of local human populations. Such utilisation, especially grazing by livestock, may fundamentally change the interactions between savanna plants and their seed dispersers with serious consequences for long term stability and development of the whole ecosystem.
The obvious lack of interdisciplinary studies and general theories dealing with seed dispersal by mammals, especially ruminants, in a co-evolutionary context can be attributed to the fact that the overwhelming importance of seed dispersal by mammals in grassland ecosystems has been completely overlooked until some years ago. Additionally, methodological limitations in quantifying seed dispersal in general, and especially to assess costs, benefits, and trade-offs for plants as well as for animals impeded such studies until now. |
