Biological dispersal refers to those processes by which a species maintains or expands the distribution of a population. Dispersal implies movement—movement away from an existing population (population expansion) or away from the parent organisms (population maintenance). In the latter case, dispersal may simply involve replacement of the parent generation by the new generation, with only minor changes in geographic area occupied. In either case, dispersal is important because new life must replace old, and the two generations cannot easily occupy the same physical space during the transition. More significantly, dispersal enables the species population to occupy much of the available habitat, thereby maximizing resources in its favor and providing a hedge against local adverse events.
In most cases, organisms (plants and especially sedentary animals) have evolved adaptations for dispersal that take advantage of various forms of kinetic energy occurring naturally in the environment: water flow, wind, falling (response to gravity).
Dispersal in plants
Unlike animals, plants are limited in their ability to seek out favorable conditions for life and growth. Consequently, plants have evolved many ways to disperse and spread a population through their seeds or spores (see also vegetative reproduction). Those properties or attributes that promote the movement of the next generation away from the parent plant may involve the fruit more so than the seeds themselves.
Dispersal is a universal biological need, and it is to be expected that most higher plants have solved the problem in one way or another through adaptations involving their fruit or seed. Examine the fruit of any species and it is likely, with perhaps a bit more knowledge about the ecosystem, to at least intelligently speculate on what these adptations are in that plant. However, realize that particularly where plant-animal interactions are central to the dispersal mechanism, seeing a plant outside of its native ecosystem may not reveal so much about the adaptations present in the fruit and seed. Indeed, in many instances of plants introduced into areas where they are not native, it is the failure of the dispersal mechanism that accounts for the species not becoming established beyond the garden.
The effect of gravity on the dispersal of seeds and spores is straight forward. Heavier seeds will tend to drop downward from the parent plant, and not by themselves travel very far. Spores, being much lighter, are more easily impacted by motions in the environment, especially those caused by wind and water, and therefore less strictly subject to the law of gravity (see examples below). Gravity may be sufficient agent for plants growing on steep slopes, but upslope movement of a population can be a problem. The naked seeds of gymnosperms are largely dependent upon gravity for dispersal. Most extant conifers are long-lived large shrubs or tall trees, thus taking full advantage of gravitational dispersal and allowing for gradual upslope movement of a population. Dispersal of seeds "strictly" by gravity should not overlook storm effects: seeds from a deteriorating cone placed high on a tall, narrow tree will get spread widely during a wind storm (see "Wind" below).
Encasing seeds in a rounded fruit promotes gravity driven movement away from the parent.
Numerous species have evolved mechanical means to overcome the tendency of a seed to drop close to its parent. Seedpods are often shaped so that the seeds are flung away from the parent plant with considerable force as the seedpod matures.
Examples of fruit with mechanical dispersal mechanisms:
Dandelion "clock," showing brown achenes and attached pappuses
For non-aquatic, terrestrial plants, the wind is an obvious supplier of energy of movement, and many plant adaptations exist that clearly take advantage of this fact. Perhaps most familiar are the feather-light fibre parachutes with attached achenes that are produced by a number of species of Asteraceae, a well-known example being the dandelion (see right).
Plants that grow in water (aquatic and obligate wetland species) are likely to utilize water to disperse their seeds. For example, all mangroves disperse their offspring by water. Rhizophora demonstrates an unorthodox method of propagation called vivipary: the embryo is retained on the plant until after germination; in essence, a dry seed is not produced. The hypocotyl of the germinating seedling (now called a propagule) bursts through the fruit and hangs, poised for continued growth. In R. mangle, the hypocotyl can reach a length of 20 to 25 cm; and in R. mucronata lengths up to 1 m have been recorded. Eventually, the seedling separates from the fruit, leaving its cotyledons behind, and—floating horizontally on the water surface—is carried away by tidal or river flow. After a month or two, the propagule turns vertical in the water. Once the hypocotyl of a propagule "feels" bottom or strands, roots start to develop and leaves appear at the upper end (Hogarth, 1999).
Adaptations commonly seen in littoral plants are those that promote floatation of the fruit, allowing the seed to be carried away on the tide or ocean currents. Examples would be:
- Cocos nucifera – the coconut produces a large, dry, fiber-filled fruit (a fibrous drupe) capable of a long survival adrift at sea.
- Calophyllum inophyllum – Alexandrian laurel or kamani produces a globose fruit that is almost cork-like.
The co-evolution of plants and animals is a fascinating story in itself. And a very significant aspect of this co-evolution involves plant adaptations that take advantage of animal abilities to locomote. Some fruit have prickly burrs or spikes that attach themselves to a passing animal's fur so that the animal will carry them away. Seeds are contained within a soft fruit that "invites" animals to consume it. These seeds have a tough protective outer-coating so that while the fruit is digested, the seeds will pass through their host's digestive tract intact, and grow wherever they fall. Such fruit attractive to birds is perhaps the most successful of fruit adaptations related to plant dispersal.
Examples of fruits with attachment hairs and structures:
- Bidens spp. – Many species of this beggartick genus produce achenes with awns that are barbed.
Dispersal in animals
Most (but not all) animals are capable of locomotion and the basic mechanism of dispersal is movement from one place to another. Locomotion allows the organism to "test" new environments for their suitability, although movement is usually guided by inherited behaviors.
There are numerous animal forms that are non-motile, such as sponges, bryozoans, tunicates, sea anemones, corals, and oysters. In common, they are all either marine or aquatic. It may seem curious that plants have been so successful at stationary life on land, while animals have not, but the answer lies in the food supply. Plants produce their own food from sunlight and carbon dioxide—both generally more abundant on land than in water. Animals fixed in place must rely on the surrounding medium to bring food at least close enough to grab, and this occurs in the three-dimensional water environment, but with much less abundance in the atmosphere. However, that such a life form might be possible is at least suggested by the orb-weaver spiders.
All of the marine and aquatic invertebrates whose lives are spent fixed to the bottom (more or less; anemones are capable of getting up and moving to a new location if conditions warrant) produce dispersal units. These may be specialized "buds", or motile sexual reproduction products, or even a sort of alteration of generations as in certain cnidaria.
Corals provide a good example of how sedentary species achieve dispersion. Corals reproduce by releasing sperm and eggs directly into the water. These release events are coordinated by lunar phase in certain warms months, such that all corals of one or many species on a given reef will release on the same single or several consecutive nights. The released eggs are fertilized, and the resulting zygote develops quickly into a multicellular planula . This motile stage then attempts to find a suitable substratum for settlement. Most are unsuccessful and die or are fed upon by zooplankton and bottom dwelling predators such as anemones and other corals. However, untold millions are produced, and a few do succeed in locating spots of bare limestone, where they settle and transform by growth into a polyp. All things being favorable, the single poplyp grows into a coral head by budding off new polyps to form a colony.
Although motile animals can, in theory, disperse themselves by their locomotive powers, a great many species utilize the existing kinetic energies in the environment. Dispersal by water currents is especially associated with the physically small inhabitants of marine waters known as zooplankton. The term plankton comes from the Greek, πλαγκτoν, meaning "wanderer" or "drifter".
- Hogarth, Peter J. 1999. The Biology of Mangroves. Oxford Univ. Press. 228 p. ISBN 0198502222