6.3. Dispersal potential

Dispersal potential is defined as the proportion of dispersules produced by one individual that travelz a certain distance, which can be chosen arbitrarily depending on the question. The dispersules may be seeds or fruits or vegetative propagules. In contrast to dispersal syndrome, dispersal potential allows the assessment of dispersability of a seed in relation to distance. It varies not only among species, but also strongly among species with the same dispersal syndrome. Therefore, it is a crucial variable when asking if dispersal is limiting the occurrence of a species in suitable habitats or species richness of plant communities, or if fragmentation is a threat to the survival of species or populations. The capacity to survive in disturbed habitats or in fragmented landscapes is often correlated with a high dispersal potential. Both seed production and also seed characters may be correlated with dispersal potential. The more seeds are produced, the higher the probability that one seed spans larger distances. The seed characters such as e.g. mass, form and structure of seed surface responsible for a high dispersal potential depend on the dispersal vector. There may be a trade off between dispersal potential (in space) and maximum plant lifespan as well as seed-bank persistence (dispersal in time). Long-lived species often exhibit a low dispersal potential, as do species with a long-term persistent seed bank.

How to record?

Dispersal potential is a continuous variable and may be recorded either by direct measurements in the field or can be identified by measurements of traits related to the dispersal potential, or by modelling approaches. Wind-dispersal potential is correlated with dispersule-releasing height and terminal velocity, dispersal potential by water to buoyancy of the dispersules and animal-dispersal potential to either attachment potential or survival after digestion. Dispersal by humans, machines or vehicles is very complex. Measuring dispersal potential, therefore, requires studies adapted to the specific question.

Measurements should be carried out on the intact dispersule, i.e. seed or fruit with all the structures, such as e.g. pappus and awns, that are still attached when it is released. Releasing height should be measured during dispersule release and is the difference between the highest elevation of the seed or fruit and the base of the plant. Terminal velocity is measured on freshly collected air-dry dispersules and, most simply, by the actual rate of fall in still air. Floating capacity (proportion of dispersules floating after a defined time) is measured by putting dispersules in glass beakers that are placed on a flask shaker moving with a frequency of 100 min–1. Attachment capacity (proportion of dispersules still attached after a defined time) is measured by putting seeds on the respective animal fur, which is then shaken by a shaking machine. Survival after digestion is measured either by digestion experiments with the respective animals or by simulating ingestion by a standardised mechanical treatment and digestion by a standardised chemical treatment, which have to be calibrated by digestion experiments.

To assess animal-dispersal potential, field studies should be added where possible, because the behaviour of animals (e.g. selection of species by grazing animals) strongly influences dispersal potential. Predicting animal-dispersal potential requires process-based models with the ability to predict over a range of scenarios.

Special cases or extras

(1) For water plants, seed releasing height is the distance between the highest point of seeds or fruits and water surface.

(2) Secondary process, e.g. dispersal by wind on the ground, may strongly affect dispersal potential. Such processes are often obvious only from field studies and may require the establishment of additional new methods.
 
References on theory and significance: Bruun and Poschlod (2006); Poschlod et al. (1998, 2005) Tackenberg (2003); Tackenberg et al. (2003); Schurr et al. (2005); Will and Tackenberg (2008); Cousens et al. (2010).

More on methods: Fischer et al. (1996); Römermann et al. (2005a, 2005b, 2005c).