POLLUTANTS AND TROPHIC LEVEL
- Pollutants especially non-degradable ones move through the various trophic levels in an ecosystem. Nondegradable pollutants mean materials, which cannot be metabolized by the living organisms.
- Example: chlorinated hydrocarbons.
- We are concerned about these phenomena because, together they enable even small concentrations of chemicals in the environment to find their way into organisms in high enough dosages to cause problems.
Movement of these pollutants involves two main processes:
- Bioaccumulation
- Biomagnification.
Bioaccumulation
- It refers to how pollutants enter a food chain.
- In bioaccumulation there is an increase in concentration of a pollutant from the environment to the first organism in a food chain.
Biomagnification
- Biomagnification refers to the tendency of pollutants to concentrate as they move from one trophic level to the next.
- Thus in biomagnification there is an increase in concentration of a pollutant from one link in a food chain to another.
- In order for biomagnification to occur, the pollutant must be: long-lived, mobile, soluble in fats, biologically active.
- If a pollutant is short-lived, it will be broken down before it can become dangerous. If it is not mobile, it will stay in one place and is unlikely to be taken up by organisms. If the pollutant is soluble in water, it will be excreted by the organism. Pollutants that dissolve in fats, may be retained for a long time.
- It is traditional to measure the amount of pollutants in fatty tissues of organisms such as fish. In mammals, we often test the milk produced by females, since the milk has a lot of fat in it are often more susceptible to damage from toxins (poisons). If a pollutant is not active biologically, it may bio-magnify, but we really don’t worry about it much, since it probably won’t cause any problems Examples: DDT.
BIOTIC INTERACTION
Organisms living in this earth are interlinked to each other in one way or other. The interaction between the organisms is fundamental for its survival and functioning of ecosystem as a whole.
Types of biotic interaction:
Mutualism:
Both species benefit. Example: in pollination mutualisms, the pollinator gets food (pollen, nectar), and the plant has its pollen transferred to other flowers for cross-fertilization (reproduction).
Commensalism:
One species benefit, the other is unaffected.
Example: cow dung provides food and shelter to dung beetles. The beetles have no effect on the cows.
Competition:
Both species are harmed by the interaction.
Example: if two species eat the same food, and there isn’t enough for both, both may have access to less food than they would if alone. They both suffer a shortage of food
Predation and parasitism:
One species benefit, the other is harmed.
Example: predation—one fish kills and eats
parasitism: tick gains benefit by sucking blood; host is harmed by losing blood.
Amensalism:
One species is harmed, the other is unaffected.
Example: A large tree shades a small plant, retarding the growth of the small plant. The small plant has no effect on the large tree.
Neutralism:
- There is no net benefit or harm to either species. Perhaps in some interspecific interactions, the costs and benefits experienced by each partner are exactly the same so that they sum to zero. It is not clear how often this happens in nature.
- Neutralism is also sometimes described as the relationship between two species inhabiting the same space and using the same resources, but that have no effect on each other. In this case, one could argue that they aren’t interacting at all.
- Pollutants and trophic level, Biotic Interaction & Ecological Succession
ECOLOGICAL SUCCESSION
- Succession is a universal process of directional change in vegetation, on an ecological time scale. Succession occurs when a series of communities replace one another due to large scale destruction either natural or manmade. This process continues - one community replacing another community, until a stable, mature community develops.
- Succession is a progressive series of changes which leads to the establishment of a relatively stable climax community.
- The first plant to colonise an area is called the pioneer community. The final stage of succession is called the climax community. The stage leading to the climax community are called successional stages or series.
- Succession is characterised by the following: increased productivity, the shift of nutrients from’ the reservoirs, increased diversity of organisms with increased niche development, and a gradual increase in the complexity of food webs.
Primary Succession
- In primary succession on a terrestrial site the new site is first colonized 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.
- These new conditions may be conducive to the establishment of additional organisms that may subsequently arrive at the site. 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 the substratum releasing nutrients to the substratum. Organic debris accumulates in pockets, providing soil in which seeds can become lodged and grow.
- As the community of organisms continues to develop, it becomes more diverse and competition increases, but at the same time new niche opportunities develops.
- 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
- Secondary succession occurs when plants recognize an area in which the climax community has been disturbed. Secondary succession is the sequential development of biotic communities after the complete or partial destruction of the existing community. 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 farmland 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 farmland over a period becomes dominated by trees and is transformed into a forest.
- The differences between primary and secondary succession, the secondary succession starts on a well-developed soil already formed at the site. Thus secondary succession is relatively faster as compared to primary succession which may often require hundreds of years.
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.
Autotrophic and Heterotrophic succession
- Succession in which, initially the green plants are much greater is quantity is known as autotrophic succession; and the ones in which the heterotrophs are greater in quantity is known as heterotrophic succession.
- Succession would occur faster in area existing in the middle of the large continent. This is because, here all seeds of plants belonging to the different series would reach much faster, establish and ultimately result in climax community.
