B. COMPETITION AND SEGREGATION
1. Plant Migration, Competition, and Segregation[188]
Invasion is the complete or complex process of which migration, ecesis (the adjustment of a plant to a new home), and competition are the essential parts. It embraces the whole movement of a plant or group of plants from one area into another and their colonization in the latter. From the very nature of migration, invasion is going on at all times and in all directions.
Effective invasion is predominantly local. It operates in mass only between bare areas and adjacent communities which contain species capable of pioneering, or between contiguous communities which offer somewhat similar conditions or contain species of wide range of adjustment. Invasion into a remote region rarely has any successional effect (effect tending to transform the character of a plant community), as the invaders are too few to make headway against the plants in possession or against those much nearer a new area. Invasion into a new area or a plant community begins with migration when this is followed by ecesis. In new areas, ecesis produces reaction (the effect which a plant or a community exerts upon its habitat) at once, and this is followed by aggregation and competition, with increasing reaction. In an area already occupied by plants, ecesis and competition are concomitant and quickly produce reactions. Throughout the development migrants are entering and leaving, and the interactions of the various processes come to be complex in the highest degree.
Local invasion in force is essentially continuous or recurrent. Between contiguous communities it is mutual, unless they are too dissimilar. The result is a transition area or ecotone which epitomizes the next stage in development. By far the greater amount of invasion into existing vegetation is of this sort. The movement into a bare area is likewise continuous, though it is necessarily not mutual, and hence there is no ecotone during the earlier stages. The significant feature of continuous invasion is that an outpost may be repeatedly reinforced, permitting rapid aggregation and ecesis, and the production of new centers from which the species may be extended over a wide area. Contrasted with continuous invasion is intermittent or periodic movement into distant regions, but this is rarely concerned in succession. When the movement of invaders into a community is so great that the original occupants are driven out, the invasion is complete.
A topographic feature or a physical or a biological agency that restricts or prevents invasions is a barrier. Topographic features are usually permanent and produce permanent barriers. Biological ones are often temporary and exist for a few years or even a single season. Temporary barriers are often recurrent, however. Barriers are complete or incomplete with respect to the thoroughness of their action. They may affect invasion either by limiting migration or by preventing ecesis.
Biological barriers comprise plant communities, man and animals, and parasitic plants. The limiting effect of a plant community is exhibited in two ways. In the first place, an association acts as a barrier to the ecesis of species invading it from associations of another type, on account of the physical differences of the habitats. Whether such a barrier be complete or partial will depend upon the relative unlikeness of the two areas. Shade plants are unable to invade a prairie, though the species of open thickets or woodland may do so to a certain degree. Closed communities (one in which all the soil is occupied) likewise exert a marked influence in decreasing invasion by reason of the intense and successful competition which all invaders must meet. Closed associations usually act as complete barriers, while more open ones restrict invasion in direct proportion to the degree of occupation. To this fact may be traced the fundamental law of succession (the law by which one type of community or formation is succeeded by another) that the number of stages is determined largely by the increasing difficulty of invasion as the area becomes stabilized. Man and animals affect invasion by the destruction of germules. Both in bare areas and in seral stages the action of rodents and birds is often decisive to the extent of altering the whole course of development. Man and animals operate as marked barriers to ecesis wherever they alter conditions unfavorably to invaders or where they turn the scale in competition by cultivating, grazing, camping, parasitism, etc. The absence of pollinating insects is sometimes a curious barrier to the complete ecesis of species far out of their usual habitat or region. Parasitic fungi decrease migration in so far as they affect seed production. They restrict or prevent ecesis either by the destruction of invaders or by placing them at a disadvantage with respect to the occupants.
By the term reaction is understood the effect which a plant or a community exerts upon its habitat. In connection with succession, the term is restricted to this special sense alone. It is entirely distinct from the response of the plant or group, i.e., its adjustment and adaptation to the habitat. In short, the habitat causes the plant to function and grow, and the plant then reacts upon the habitat, changing one or more of its factors in decisive or appreciable degree. The two processes are mutually complementary and often interact in most complex fashion.
The reaction of a community is usually more than the sum of the reactions of the component species and individuals. It is the individual plant which produces the reaction, though the latter usually becomes recognizable through the combined action of the group. In most cases the action of the group accumulates or emphasizes an effect which would otherwise be insignificant or temporary. A community of trees casts less shade than the same number of isolated individuals, but the shade is constant and continuous, and hence controlling. The significance of the community reaction is especially well shown in the case of leaf mold and duff. The leaf litter is again only the total of the fallen leaves of all the individuals but its formation is completely dependent upon the community. The reaction of plants upon wind-borne sand and silt-laden waters illustrates the same fact.