Table of Contents
- Ecological succession or ecosystem development
- Homeostasis (or cybernetic) or feedback control mechanisms
- Energy flow through the food chain (next post)
- Nutrient cycling (biogeochemical cycles) (next post)
- The process by which communities of plant and animal species in an area are replaced or changed into another over a period of time is known as ecological succession.
- Succession is a universal process of directional change in vegetation, on an ecological time scale.
- Succession occurs due to large scale changes or destruction (natural or manmade).
- The process involves a progressive series of changes with one community replacing another until a stable, mature, climax community develops.
Primary Succession (Picture Credits: NIOS Environment)
- The first plant to colonize an area is called the pioneer community.
- The final stage of succession is called the climax community.
- A climax community is stable, mature, more complex and long-lasting.
- The stage leading to the climax community is called successional stages or seres.
- Each transitional community that is formed and replaced during succession is called a stage in succession or a seral community.
- Succession is characterized by the following: increased productivity, the shift of nutrients from the reservoirs, increased diversity of organisms, and a gradual increase in the complexity of food webs.
- Succession would occur faster in area existing in the middle of the large continent. This is because here seeds of plants belonging to the different seres would reach much faster.
- Primary succession takes place an over where no community has existed previously.
- Such areas include rock outcrops, newly formed deltas and sand dunes, emerging volcano islands and lava flows, glacial moraines (muddy area exposed by a retreating glacier), etc.
- In primary succession on a terrestrial site, the new site is first colonised by a few hardy pioneer species that are often microbes, lichens and mosses.
- The pioneers over a few generations alter the habitat conditions by their growth and development.
Q. Lichens, which are capable of initiating ecological succession even on a bare rock, are actually a symbiotic association of
- algae and bacteria
- algae and fungi
- bacteria and fungi
- fungi and mosses
- Lichen are plant-like organisms that consist of a symbiotic association of algae (usually green) or cyanobacteria and fungi.
- Fungi provide shelter, water and minerals to the algae and, in return, the alga provides food.
Answer: b) Algae and Fungai
- The pioneers through their death any decay leave patches of organic matter in which small animals can live.
- The organic matter produced by these pioneer species produce organic acids during decomposition that dissolve and etch the substratum releasing nutrients to the substratum.
- Organic debris accumulates in pockets and crevices, providing soil in which seeds can become lodged and grow.
- The new conditions may be conducive to the establishment of additional organisms that may subsequently arrive at the site.
- As the community of organisms continues to develop, it becomes more diverse, and competition increases, but at the same time, new niche opportunities develop.
- The pioneer species disappear as the habitat conditions change and invasion of new species progresses, leading to the replacement of the preceding community.
- Secondary succession is the sequential development of biotic communities after the complete or partial destruction of the existing community.
Secondary Succession (Picture Credits: NIOS Environment)
- A mature or intermediate community may be destroyed by natural events such as floods, droughts, fires, or storms or by human interventions such as deforestation, agriculture, overgrazing, etc.
- This abandoned land is first invaded by hardy species of grasses that can survive in bare, sun-baked soil.
- These grasses may be soon joined by tall grasses and herbaceous plants. These dominate the ecosystem for some years along with mice, rabbits, insects and seed-eating birds.
- Eventually, some trees come up in this area, seeds of which may be brought by wind or animals.
- And over the years, a forest community develops. Thus, an abandoned land over a period becomes dominated by trees and is transformed into a forest.
Difference Between Primary and Secondary Succession
- Unlike in the primary succession, the secondary succession starts on a well-developed soil already formed at the site. Thus, secondary succession is relatively faster.
Autogenic and Allogenic Succession
- When succession is brought about by living inhabitants of that community itself, the process is called autogenic succession, while change brought about by outside forces is known as allogenic succession.
- Autogenic succession is driven by the biotic components of an ecosystem.
- Allogenic succession is driven by the abiotic components (fire, flood) of the ecosystem.
Q. In the grasslands, trees do not replace the grasses as a part of an ecological succession because of
- insects and fungi
- limited sunlight and paucity of nutrients
- water limits and fire
- None of the above
- Grasses have one good trick to monopolise a place. In the dry season the grasses dry up and cause fires which destroy other plant species and their seeds.
- Also, grasslands develop in regions with scanty rainfall where plant growth cannot be achieved.
- Though forests form the climax community in most of the ecosystems, in the grassland ecosystem grasses form the climax community. Thanks to fire and lack of water.
- Grasslands are almost irreversible once deforestation in water-scarce areas gives way to grasslands.
Answer: c) water limits and fire
Autotrophic and Heterotrophic succession
- Succession in which, initially the green plants are much greater in quantity is known as autotrophic succession; and the ones in which the heterotrophs are greater in quantity is known as heterotrophic succession.
Succession in Plants
- Succession that occurs on land (dry areas) where moisture content is low for e.g. on a bare rock is known as xerarch.
- Succession that takes place in a water body, like ponds or lake is called hydrarch.
- Both hydrarch and xerarch successions lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric).
- With time the xerophytic habitat gets converted into a mesophyte (plat needing only a moderate amount of water).
Succession in Water
- In primary succession in water, the pioneers are the small phytoplankton, and they are replaced with time by free-floating angiosperms, then by rooted hydrophytes, sedges, grasses and finally the trees.
- The climax again would be a forest. With time the water body is converted into land.
- Another important fact is to understand that all succession whether taking place in water or on land, proceeds to a similar climax community – the mesic.
Homeostasis in Ecosystem
Why these conformers had not evolved to become regulators?
- In ecology, the term homeostasis applies to the tendency for a biological system to resist changes.
- Ecosystems are capable of maintaining their state of equilibrium.
- They can regulate their own species structure and functional processes.
- This capacity of the ecosystem of self-regulation is known as homeostasis.
- For example, in a pond ecosystem, if the population of zooplankton increases, they consume a large number of the phytoplankton and as a result, food would become scarce for zooplankton.
- When the number of zooplanktons is reduced because of starvation, the phytoplankton population start increasing.
- After some time, the population size of zooplankton also increases, and this process continues at all the trophic levels of the food chain.
- Note that in a homeostatic system, negative feedback mechanism induced by the limiting resource (here its scarcity of food) is responsible for maintaining stability in an ecosystem.
- However, the homeostatic capacity of ecosystems is not unlimited as well as not everything in an ecosystem is always well regulated.