Seed and pollen dispersal Remember from the first lecture that seeds may be dispersed on wind, by water, and by animals. Animal-borne seeds may be carried externally or may be transported until defecated from the gut. The basic spatial pattern for dispersal is very similar for all mechanisms of dispersal. All show a peak in abundance of dispersed propagules at a short distance from the source plant, then a decline with increasing distance. The rate of decline differs, but all distributions are skewed, with a more-or-less extended tail. Think of a wind-dispersed seed as one example. Seeds are broken free of the parent (abscised) by a sufficiently strong velocity (a gust possibly) of wind.
The force required to break the seed free explains why the peak of the distribution occurs at some small distance from the parent. The remainder of the distribution is dependent on what type and size of dispersal accessory structure the seed carries. Some seeds have no dispersal accessory. They are sometimes called ploppers, and the number dispersed drops very rapidly with distance.
Other seeds (particularly a number of common weeds) have parachutes that make possible relatively long distance dispersal. A dandelion, in case you didnt recognize it.
The distance that animals disperse seeds may be determined by how far the animal moves during gut passage time, or how far it moves before its behaviour (scratching, rubbing against something) releases the seed. Yet these radically different kinds of dispersed propagules have similar curves of the number of propagules dispersed differing distances.
The figure on the left represents microsatellite markers that identify distances of saplings of bur oaks from the parents. It may not estimate acorn dispersal accurately. The figure on the right shows dispersal distances for propagules of prairie fugitive species. Note that here peaks are not at 0 distance.
Pollen movement follows a similar pattern. This figure shows pollen dispersal distance for a number of species. Note that the x-axis is log transformed there would be a more-or-less long tail on each of these distributions, and that many peak meters from the parent. However, also note that pollen can travel much further, on average, than seeds.
Pollen may be moved by insect or bird pollinators. Then dispersal distance is determined by the foraging behaviour of the pollinator, and may be much more local. The peaked distribution (filled circles) are interplant flight distances for the pollinator. The open symbols, a much flatter distribution, are the distribution of interplant distances and that for realized pollen dispersal. The plant, Asclepias exultata, uses pollinia (packages of pollen grains).
The picture is of an flower of Asclepias sullivantii. The lumps in the middle are pollinia, each about 3mm long. A pollinator moves whole pollinia from this flower onto another one. Pollinia on the leg of a carpenter bee Copyright 2004 Beatriz MoissetBeatriz Moisset
What is important about all these distributions is the relatively long tails the skewness of the distributions. Long distance movements are what produce new population isolates, what produce gene flow in continuous populations to reduce subdivision, and, in general, what determines the amount of genetic structure in populations. Without reference to mechanism, we can also look at the distance between mating parent plants (determined by molecular markers that reveal parentage). These data are for a dioecious lily.
At the opposite end of pollen dispersal is the exchange of pollen among flowers of the same plant (or genet). This kind of pollen exchange is called geitonogamy. It can occur in species that are self-fertile, but it can also gum up receptive surfaces if the species is self-incompatible. The amount of geitonogamous pollen transfer apparently depends in part on the number/density of flowers on a plant/in an area. In the musk mallow, Malva moschata, selfing rate varied with the number of flowers on plants. More selfing occurs on plants with larger numbers of flowers (since bees probably spend longer moving among flowers on those plants).
One of the best examples that show pollen and seed dispersal have separate impacts is the classic example of the genetics of mine spoil grass, Agrostis tenuis, that grows on and near Welsh lead mines. The mines are relatively recent (