Passive dispersal the seed rain

Most seeds fall close to the parent and their density declines with distance from that parent. This is the case for wind-dispersed seeds and also for those that are ejected actively by maternal tissue (e.g. many legumes). The eventual destination of the dispersed offspring is determined by the original location of the parent and by the relationship relating disperser density to distance from parent, but the detailed microhabitat of that destination is left to chance. Dispersal is nonexploratory; discovery is a matter of chance. Some animals have essentially this same type of dispersal. For the meanings of 'dispersal' and 'migration'

Figure 6.1 Spring densities of the winged form of the aphid, Aphis fabae, in large part reflect their carriage on the wind. (a) A. fabae eggs are found on spindle plants and their distribution in the UK over winter reflects that of the plants (log10 geometric mean number of eggs per 100 spindle buds). (b) But by spring, although the highest densities are in spindle regions, the aphids have dispersed on the wind over the whole country (log10 geometric mean aerial density). (After Compton, 2001; from Cammell et al., 1989.)

example, the dispersal of most pond-dwelling organisms without a free-flying stage depends on resistant wind-blown structures (e.g. gemmules of sponges, cysts of brine shrimps).

The density of seeds is often low immediately under the parent, rises to a peak close by and then falls off steeply with distance (Figure 6.2a). However, there are immense practical problems in studying seed dispersal (i.e. in following the seeds), and these become increasingly irresolvable further from the source. Greene and Calogeropoulos (2001) liken any assertion that 'most seeds travel short distances' to a claim that most lost keys and contact lenses fall close to streetlights. Certainly, the very few studies of long-distance dispersal that have been carried out suggest that seed density declines only very slowly at larger distances from the original source (Figure 6.2b), and even a few long-distance dispersers may be crucial in either invasion or recolonization dispersal (see Section 6.3.1).

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