Plant Colonization
Colonization is one way in which plants can change the ecology of a site. Colonization is a process with two components: invasion and survival. The rate at which a site is colonized by plants depends on both the rate at which individual organisms (seeds, spores, immature or mature individuals) arrive at the site and their success at becoming established and surviving. Success in colonization depends to a
great extent on there being a site available for colonization – a safe site where disturbance by fire or by cutting down of trees has either removed competing species or reduced levels of competition and other negative interactions to a level at which the invading species can become established. For a given rate of invasion, colonization of a moist, fertile site is likely to be much more rapid than that of a dry, infertile site becau of poor survival on the latter. A fertile, plowed field is rapidly invaded by a large variety of weeds, whereas a neighboring construction site from which the soil
has been compacted or removed to expo a coarse, infertile parent material may remain
virtually free of vegetation for many months or even years despite receiving the same input of seeds as the plowed field.
Both the rate of invasion and the rate of extinction vary greatly among different plant species. Pioneer species – tho that occur only in the earliest stages of colonization – tend to have high rates of invasion becau they produce very large numbers of reproductive propagules (seeds, spores, and so on) and because they have an efficient means of dispersal (normally, wind)
If colonizers produce short-lived reproductive propagules, then they must produce very large numbers unless they have an efficient means of dispersal to suitable new habitats. Many plants depend on wind for dispersal and produce abundant quantities of small, relatively short-lived seeds to compensate for the fact that wind is not always a reliable means of reaching the appropriate type of habitat. Alternative strategies have evolved in some plants, such as tho that produce fewer
but larger seeds that are dispersed to suitable sites by birds or small mammals or tho t
hat produce long-lived seeds. Many forest plants seem to exhibit the latter adaptation, and viable seeds of pioneer species can be found in large numbers on some forest floors. For example, as many as 1,125 viable seeds per square meter were found in a
100-year-old Douglas fir/western hemlock forest in coastal British Columbia. Nearly all the seeds that had germinated from this seed bank were from pioneer species. The rapid colonization of such sites after disturbance is undoubtedly in part a reflection of the large seed bank on the forest floor.
An adaptation that is well developed in colonizing species is a high degree of variation in germination (the beginning of a seed’s growth). Seeds of a given species exhibit a wide range of germination dates, increasing the probability that at least some of the seeds will germinate during a period of favorable environmental conditions. This is particularly important for species that colonize an environment where there is no existing vegetation to ameliorate climatic extremes and in which there may be great climatic diversity.
Species succession in plant communities, i.e., the temporal sequence of appearance and
disappearance of species is dependent on events occurring at different stages in the life history of a species. ■ Variation in rates of invasion and growth plays an important role in determining patterns of succession, especially secondary succession. ■ The species that are first to colonize a site are tho that produce abundant seed that is distributed successfully to new sites. ■ Such species generally grow rapidly and quickly dominate new sites, excluding other species with lower invasion and growth rates. The first community that occupies a disturbed area
therefore may be composed of species with the highest rate of invasion, whereas the community of the subsequent stage may consist of plants with similar survival rates but lower invasion rates. ■
1. According to paragraph 1, how does disturbance of a site influence its colonization by a plant species?
A) Disturbance reduces or eliminates competition by other species.
B) Disturbance increases negative interactions with other organisms on the site. C) Disturbance prevents a plant species from colonizing a new site.
D) Disturbance reduces the fertility of a site.
2. The word “virtually” in the passage is closest in meaning to
A) almost totally
B) unusually
C) consistently
D) unnaturally
3. Why does the author mention a plowed field and a construction site in the passage?
A) To argue that sites that have been affected by human activity tend to be colonized slowly
B) To illustrate the kind of sites that may be invaded by weeds
C) To contrast sites in terms of their suitability for colonization
