Blog 9 - Ecological Succession

Ecological Succession refers to the process of orderly changes in the composition (which may be biological, physical, and chemical) or species structure of an ecological community over time. It describes the development of an ecological community through a series of stages that are interdependent with each other; in addition, each stage prepares itself for the next. These stages are the pioneer, moss, grass/prairie, shrub, tree and climax stages. The several types of succession varies by disturbance/human interference, fluctuating species interactions/recurring events, soil changes caused by organisms, soil changes caused by external environmental influences, and many others. Such types of succession are primary, secondary, cyclic, autogenic and allogenic.

Ecological succession is understood as a fundamental concept in ecology and categorized as a habitat terminology. It shows the growth and development [and possibly destruction and re-establishment] of an ecosystem with recognition to the biotic components, i.e. lichens and mosses, that shape the abiotic components, i.e. rock surfaces, and elevate the ecological community to the next stage. It helps explain the abundance and distribution of biodiversity (which increases as much as a tenfold until climax stage) in the environment. Since ecological succession can also explain how some species can become less or more abundant in a community at a certain interval, it explains the distribution and abundance of organisms, which is one idea that ecologists seek to explain.

            Succession may initiate due to natural disaster/disturbance that destroy the existing community, i.e. forest fires and severe wind throws, or by the formation of an unoccupied habitat, i.e. a cooled lava flow and glacial tills. If the area develops initially without human interference, the first stage in ecological succession would be primary succession, which is the so-called Pioneer stage that is characterized by lichens, algae, and fungi that facilitate paedogenesis, or the formation of soil, which is vital to vegetation. This pioneer stage prepares the habitat for the moss stage, which prepares the forest area/rock surface with vegetation. Soon after, insects start to appear.

            If the land had been previously modified by man, which means the soil has already been conditioned with higher nutrient content, or the land was an already established ecosystem reduced by natural events, secondary succession happens instead. Secondary succession occurs on preexisting soil, as opposed to primary succession. It needs not pioneer colonial species (lichens, algae, or fungi) and may still contain vegetative organs of plants to help start developing an area with vegetation; thus, it is faster than primary succession.

With regard to primary succession, the organic matter will gradually accumulate (in this stage) and support the growth of more complex plants that live in thin, mineral-based soil, with such plants being grasses and some shrubs. This stage is the grass/prairie stage. It brings about more animal types ranging from insects to organisms that eat insects such as bird insectivores. Then more complex plants like shrubs grow, this is the shrub stage. Some of these plants produce seeds that scatter around and more and more nutrients accumulate in the soil throughout time. Until more complex shrubs and trees grow, it still has yet to reach the Climax stage.

The climax stage or climax vegetation is characterized by type of vegetation best suited (soil type, precipitation levels, moisture, nutrient availability, or temperature.) At this stage, the ecological community should reach stability (which is measured by the Photosynthesis Respiration ratio or PR ratio, and it should be equal to one) and there should also be no more net accumulation of organic matter. Trees will grow and eventually limit the sunlight available to the species below it; eventually, the dark-tolerant plants will remain and there will be a reduction or increase in the abundance of other species in the climax forest, until the species that can grow most efficient and produce the most viable offspring will become the most abundant organisms.

Changes in microclimatic conditions (like wind patterns, shade, or evaporation from soil) and available resources can change the biotic structure by fluctuation or abundance.  Here we see how abiotic factors can affect biotic components. Trees, for example are known to frequently moderate the microclimate below them. The more there are trees that block sunlight, the less the chance of extreme temperature fluctuations, which is good for animals that tolerate less extreme temperature and moisture, i.e. insects living under a log. If this microclimatic condition is disturbed, it may result to the disappearance of the animal species it helps regulate. When these animal species disappear, it possibly causes a disturbance, this time, in the microclimatic conditions. The animal species that begin to disappear also disappears along with its contribution to the abiotic components/microclimatic conditions it controls.

MECHANISMS OF SUCCESSION

The general process of ecological succession starts with Nudation, which is the development of a bare site without any life form. The three types of causes of nudation are topographic, climatic, or biotic. It is topographic if it is due to soil erosion by gravity, wind or water, with examples of these being soil erosion, landslide, and earthquakes. The cause of nudation is climatic when dry periods, glaciers, storm and hail and the like destroy the previously existing community. It is biotic when human activity is involved and man is responsible for the destruction; also, epidemics caused by bacteria, etc destroy the whole population. Whatever the cause of nudation, soon after, a new surface in the area will be exposed.

            The mechanism that follows nudation is Invasion, the successful establishment of species in a bare area. Invasion usually happens in three steps, namely: migration, ecesis, and aggregation. Migration happens when various migration agents like water and wind make seeds, spores, and/or other propagules of species reach and disperse the bare site. Ecesis, the process of successful establishment of a plant or animal species in a habitat, is the following step after the propagules or seeds successfully adjust with the prevailing condition of the bare site. Colonization may also follow ecesis; it is also the initial growth of the vegetation, or when the seeds or propagules begin to grow a start to reproduce. In Aggregation, the established organisms in the bare area increase in number due to colonization by successive offspring and new migrants. Those plants first to colonize are called pioneers.

            The next mechanism is Competition and Coaction. As the aggregation of the number of individuals increases, competition arises as a result of natural resistance like the need for the acquisition of nutrition, light and the limited space. The said competition can either be interspecific or intraspecific. The two differ in terms of the species involved. Interspecific competition involves different species competing for the same source, while intraspecific competition involves organisms belonging to the same species competing for the same source. Coaction on the other hand is defined as ‘any of the reciprocal actions or effects, such as symbiosis, that can occur in a community’ or simply any interaction or relationship among organisms within a community. Examples of coaction in an ecological community are amensalism, commensalism, parasitism, mutualism, among others.

            Following competition and coaction is the mechanism of Reaction. This significant stage involves the modification of the environment by the influence of the living organisms in the existing community. The modification discussed are changes that take place in abiotic factors such as soil, water, light conditions, temperature, etc. As the environment changes, it becomes unsuitable for the existing community and so sooner or later it will be replaced by another community, or seral community. Here one might be able to observe the existence of seral stages or developmental stages, which involve seres, a sequence of communities which replaces one another in the given area. These seres are made up of different seral communities.

            The last mechanism in succession is Stabilization. This is the climax community or final terminal community. Coming from the name itself, the community becomes stabilized and sustains itself in equilibrium with the climate of the area. The climax community becomes established for a longer period of time and can maintain itself in harmony with the climatic conditions. The stage is called Climax stage.

Ecological succession has never appeared to me completely (with myself being able to witness all the stages and mechanisms in one area), but only gradually and by stage. I used to observe ecological succession in abandoned plots or farming areas nearby my parents’ ancestral house in La Union. The area being highly urbanized, it would be quite rare to still see land allotted for farming. In our area, there are lands that have been left uncultivated and some previously cultivated. I also noticed the abandoned land areas that only had soil with little vegetation. Here I observed the grass stage developing into the shrub stage. Only a few trees were able to grow in the meadow-like field across our old house, which was, observably, in the shrub stage. My observations follow a time span of around 15 years. The lands had observable yet few changes every year.

As we moved to Los Banos, I am finally able to observe a climax stage and a climax forest. I noticed the sudden abundance and diversity of plants and animals (especially insects) when I went hiking once to Peak 3 at Mt. Makiling, UPLB. I also noticed the plant diversity on the different hills as we drove along PCCARD road. Different populations (such as coconut trees) aggregate in one area and become observable to the naked eye even from afar.

I also observed the climax stage in my trip to Palawan last May 2010. I could easily compare the great diversity in flora and fauna along the mountains near Cleopatra’s Needle (the highest mountain there) and near St. Peter’s mountain (in which the underground river is found) to the agriculturally-developed mountains found in Bay. The mountains near here in Bay were probably in the “young forest” stage or somewhere still near shrub stage. The areas on the mountain are still not fully occupied, unlike the forest area on Cleopatra’s needle where every square meter of the land was occupied by vegetation or forest area. Knowing this, there was interspecific competition between the different plant species for sunlight and animal species for food.

Fig. 1. Mouth of the Cave

I was able to characterize most of the stages I have seen with the amount of vegetation I observed and animal species that are likely to be present.