If we give up the fight with weeds in our garden or a farm is abandoned, the freshly exposed soil will soon be colonized by a variety of plants, most of which are annuals. Within a few years, these will have been joined by perennial weeds and, unless the climate is very arid, it will not be long before woody plants make their appearance either shrubs or trees. This is called ecological succession.
The process of change by which the biotic communities replace each other and by which the physical environment becomes altered over a period of time is called sere. The various communities that together make up a sere are called seral stages.
It can be distinguished:
- Primary succession: it begins in environments that lack organic matter and which have not yet been altered in any way by living organisms (a fresh rock surface exposed by a landslide…).
- Secondary succession: succession starts in an environment which has already been altered by a period of occupancy by living organisms (Forest clearcuts, abandoned agricultural fields…)
The driving force behind succession, the reason why change occurs, is not always the same:
- Autogenic succession: the replacement of one community by the next results from changes in the physical environment that have been produced by the resident organisms. These changes tend to render the site less optimal for the organisms producing the change and more optimal for those organisms that replace them.
- Allogenic succession: it occurs when geological processes cause changes in the physical environment, which in turn lead to changes in the biota.
- Biogenic succession: it occurs when there is a sudden interference with an autogenic or allogenic succession by a living organism which becomes the major agent of successional change, at least temporally. A sudden change in herbivore pressure on the plant community or the sudden removal of a segment of the plant community by a pathogen could be two good examples.
Although, change in the composition of the biota over time is a fundamental characteristic of all ecosystems, the rate of changes varies widely in different seres and between the different stages of a single sere. In most areas, change does not continue indefinitely. Communities development in which rates of change become exceedingly slow, or in which the composition of the biota remains approximately constant for a long period of time, are called CLIMAX. These stable communities represent either the final or an indefinitely prolonged stage of a sere.
Most of above successional terminology refers to a progressive, forward development of the ecosystem toward a climax condition. By contrast, successional retrogression refers to the effects of disturbance in altering the seral condition of an ecosystem back to an earlier stage. For example, when fire destroys a climax community and leads to the development of a pioneer primary or secondary successional community, depending of the severity of the fire.
Why does change or succession occur?
It occurs as the result of either autogenic processes (associated with the living community) or allogenic processes (associated with the physical environment).
Examples of allogenic succession can be observed along the banks of a meandering river in a valley floodplain, where succession starts on recently deposited sandbars. Such examples are less common than the products of autogenic succession, which is the result of three major biotic mechanisms: 1) Colonization, 2) alteration of the physical characteristics of the site and 3) displacement of species by competition or antibiosis.
It is a process with two components: invasion and survival. The rate at which a site is colonized depends on both the rate (numbers per unit time) at which individual organisms (seed, spores, immature or mature individuals) arrive at the site, and on their success at becoming established and surviving.
Thus, 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 because of poor survival on the latter.
A fertile, plowed field is quickly invaded by weeds, whereas a neighboring construction site from which the soil has been removed to expose a coarse may remain free of vegetation for many months, in spite of receiving the same input of seeds as the plowed field.
Pioneer or fugitive species (those which only occur in the earliest seral stages) tend to have high rates of invasion due to their large numbers of reproductive propagules and efficient means of dispersal (wind). Many plants rely on wind and produce abundant quantities of small, relatively short-lived seeds to compensate for the fact that wind is not a reliable means of reaching the appropriate type of habit. Those which produce fewer but larger and long-lived seeds are dispersed to suitable sites by birds or small mammals.
Variation in rates of invasion and growth is considered, thus, a major factor in succession, especially secondary succession. Early seral species are those that produce abundant seed which is successfully distributed to new sites. Such species grow very quickly and fully occupy such sites, excluding other species with lower invasion and growth rates. The first community occupying a disturbed area may reflect species with the highest rate of invasion, while the community of the subsequent seral stage may consist of plants with similar survival rates but slower invasion rates. In fact, the environmental alteration by pioneer species is frequently a prerequisite for successful establishment of later successional species. In other words, each seral stage prepares the site for the next stage.
2.Alteration of the Physical Characteristics of the Ecosystem
Occupying the site, a species inevitably changes the site conditions, and the changes are frequently not favorable to the continued occupancy of the site by that species. The changes may either reduce the competitive abilities of the resident species or increase those of the invading species, or both. The net result is the replacement of one group of species by another group. For example, shade-intolerant pioneer species create so much shade as their community develops that their own seedlings either cannot survive or the grow poorly. In this way, the pioneers are replaced by the subsequent seral community.
Similarly, the change in soil pH accompanying the accumulation of tree litter and the development of a forest floor generally favors the nutrition of climax tree species. Temperature and moisture changes beneath the developing vegetation are undoubtedly driving mechanisms in the succession. On the other hand, the successional development of vegetation produces significant alterations in microclimate: light density and wind speed are reduced, temperature extremes are moderated, relative humidity is increased and the evaporative power of the air beneath the vegetation canopy is reduced.
Not only do plants alter the microclimate and the physical and inorganic chemical characteristics of the soil, they also alter their organic chemical environment. The plants produce a wide variety of chemicals to inhibit germination and/or growth of other species (allelopathic substances). In fact, allelopathy may serve to accelerate succession, whereas in others it impedes it.
On the other hand, any factor influencing the availability of nitrogen would thus affect the rate of succession. Several of the pioneer plants which are tolerant of low nitrogen availability have adapted to prolong their occupancy of the site by producing allelochemicals that inhibit nitrogen-fixing and nitrifying bacteria and in consequence, they impair the growth of later seral plants.
Heather, for instance, does provide us with an interesting case of indirect allelopathy that impedes the progress of succession. It produces chemicals that inhibit mycorrhizal fungi and thereby prevent the invasion of the site and the displacement of the heather by trees. Another example, which involves allelopathic mechanisms, could be that of the ericaceous shrub Kalmia angustifolium in eastern Canada.
If this species becomes well-established, such as after a fire, it can form a dense shrub community that resists tree invasion for extremely prolonged periods. It was found that Kalmia heath had a raw humus layer weighing 293 t/ha in comparison with weights of 87 t/ha in the Picea forest and 65 t/ha in the Abies forest. These accumulations represented 78, 21 and 14 times the annual litterfall, respectively, implying very slow litter decomposition on the Kalmia heath. Thus, Kalmia heaths inhibit mineralization and accumulate nutrients in unavailable form, which in turn, could provide insufficient nutrients for the tree invasion.
Allelochemicals serve to modify the competitive relationships of species, but competition itself, particularly for light, is also important. Early successional plants are generally shade intolerant and small in stature, whereas later successional species are generally shade tolerant and taller in stature. Seeds of shade tolerant, late successional species are often larger and the seedlings often have lower growth and death rates in the shade than shade intolerant, early successional species. However, early successional species have a much higher growth potential than shade tolerant when they are fully illuminated. Thus, successional replacement is closely involved with competition for light.
The explanation for the successional replacement of species varies from one situation to the next. Various combinations of invasions rate, shade tolerance, allelopathy and autotoxicity, environmental modification, nutrient and moisture competition, old age and senescence, seed longevity and availability of seed sources, and ability to sprout from subterranean organs are responsible for the general patterns of species replacement.
What does the Rate of Successional Change depend on?
It depends upon:
- The degree of environmental change that must occur before one community can be replaced by another: the greater the change, the more prolonged the stage.
The typical primary hydrosere or xerosere involve extensive change. In a xerosere, one starts with a completely unmodified microclimate, and little soil or unconsolidated soil parent material. There is little or no soil moisture storage capacity, nutrient retention capacity or available nutrient capital, therefore the degree of environmental change is enormous. In case of hydrosere, it starts from a nutrient-poor aquatic environment through semiterrestrial condition to a forested condition. The ecological conditions between the start and finish of the sere are enormously different.
They typical mesosere involves little environment change, since there may be not a very great change in the soil. The major changes are in microclimate and surface soil conditions. As a result of these differences, xerarch and hydrarch succession tend to be slow, while mesarch succession tends to be rapid.
2. The productivity of the organisms and the efficiency with which they produce environmental change: more productive and efficient the organisms, the shorter the duration of the seral stages.
The living community in early xerarch and hydrarch is composed of small organisms that either grow very slowly or are very short-lived and accumulate very little biomass. The extensive ecological changes that are necessary early in xerarch and hydrarch succession are associated with diminutive and frequently slow-growing life forms. Consequently, rates of autogenic succession are very slow in these seral stages.
The situation in mesarch succession is quite different, with very much less environmental alteration involved and a productive, rapidly growing community of plants involved in causing it. Most mesarch seral stages involve a single generation of plants, whereas most xerarch and hydrarch seral stages involve many generations.
Succession generally proceeds faster in humid, mesothermal climates which favor the fast-growing plants that produce autogenic change quickly.
3. The longevity of the organisms dominating each seral stage: the longer lived the organisms, the longer the stage may last.
In mesarch succession, the length of a seral stage is partly determined by the longevity of the organisms involved, especially in midseral and subclimax stages. The subclimax Douglas-fir stage in the western hemlock zone of British Columbia can last for many centuries before it is replaced by the climax western hemlock-western red cedar, simply because of the great age reached by Douglas-fir.
4. The degree to which communities at any particular stage occupy and dominate the site and resist invasion by other species: the better developed the community, the more resistant to invasion and the longer lasting.
The climax community persists because it fully occupies and dominates the site and resists the invasion of nonclimax species. It may do this by competing efficiently for light, by making nutrients unavailable to all but appropriately adapted climax species (direct nutrient cycling), by competing for moisture and/or inhibiting germination of seeds of other species by allelochemical mechanisms.
Similarly, earlier seral communities can become very long lasting if they ever become so well established that they are able to dominate the site and resist invasion. Shrubs stages, once established, may become extremely resistant to invasion and colonization by the tree species of the subsequent seral stage.
Variable combinations of these four determinants of the rate succession and duration of particular seral stages precludes any reliable general statement about rates and durations. Anyway, there are some points to be considered:
- Rates of succession are generally slower in primary than in secondary succession because of the greater degree of environmental alteration that is involved.
- Rates of succession are much faster in mesarch.
- Rates of succession in the earlier stages of xeroseres and hydroseres are slower than in later stages. The opposite is true for mesarch succession.
- The duration of any particular stage will be greatly influenced by the timing and rate of invasion of the site by reproductive propagules of individuals of the subsequent seral stage. Where such invasion is slow or delayed, a seral plant community may become very well stablished, resist invasion and consequently, lasts very much longer than where such invasion is rapid and immediate (relay floristics & initial vegetative composition).
- Succession will be much faster in climates that promote high rates of NPP and biomass accumulation than in climates that limit plant growth.