Chapter 1 – Environment & Ecology Continued,
Chapter -2 – Functions of Ecosystem
- An ecotone is a zone of junction or a transi- tion area between two biomes (diverse ecosys- tems).
- Ecotone is the zone where two communities meet and integrate.
- For e.g. the mangrove forests represent an eco- tone between marine and terrestrial ecosystem.
- Other examples are grassland (between forest and desert), estuary (between fresh water and salt water) and riverbank or marshland (be- tween dry and wet).
Characteristics of Ecotone
- It may be narrow (between grassland and forest) or wide (between forest and desert).
- It has conditions intermediate to the adjacent ecosystems. Hence it is a zone of tension.
- Usually, the number and the population density of the species of an outgoing community de- creases as we move away from the community or ecosystem.
- A well-developed ecotone contains some organ- isms which are entirely different from that of the adjoining communities.
- Ecocline is a zone of gradual but continuous change from one ecosystem to another when there is no sharp boundary between the two in terms of species composition.
- Ecocline occurs across the environmental gradi- ent (gradual change in abiotic factors such as al- titude, temperature (thermocline), salinity (halo- cline), depth, etc.).
Edge Effect – Edge Species
- Edge effect refers to the changes in population or community structures that occur at the boundary of two habitats (ecotone).
- Sometimes the number of species and the pop- ulation density of some of the species in the eco- tone is much greater than either community. This is called edge effect.
- The organisms which occur primarily or most abundantly in this zone are known as edge spe- cies.
- In the terrestrial ecosystems edge effect is espe- cially applicable to birds.
- For example, the density of birds is greater in the ecotone between the forest and the desert.
- Niche refers to the unique functional role and position of a species in its habitat or ecosystem.
- The functional characteristics of a species in its habitat is referred to as “niche” in that common habitat.
- In nature, many species occupy the same habitat, but they perform different functions:
- habitat niche – where it lives, food niche – what is eats or decomposes & what species it competes with,
- reproductive niche – how and when it repro- duces,
- physical & chemical niche – temperature, land shape, land slope, humidity & another requirement.
- Niche plays an important role in the conservation of organisms. If we have to con- serve species in its native habitat, we should have knowledge about the niche requirements of the species.
Difference between niche and habitat
- The habitat of a species is like its ‘address’ (i.e. where it lives) whereas niche can be thought of as its “profession” (i.e. activities and responses specific to the species).
- A niche is unique for a species while many species share the habitat.
- No two species in a habitat can have the same niche. This is because of the competition with one another until one is displaced.
- For example, a large number of different species of insects may be pests of the same plant, but they can co-exist as they feed on different parts of the same plant.
Q. Which one of the following terms describes not only the physical space occupied by an organism but also its functional role in the com- munity of organisms?
- Ecological niche
- Home range Explanation:
- Ecotone – zone of transition between two eco- systems. E.g. grasslands, mangroves etc.
- Habitat – surroundings in which an organism lives (home).
- Home Range – A home range is an area in which an animal lives and moves on a daily or periodic basis (a little bigger than habitat – home → office → home).
- The function of an ecosystem includes:
1. Ecological succession or ecosystem devel- opment
- Homeostasis (or cybernetic) or feedback control mechanisms
- Energy flow through the food chain
- Nutrient cycling (biogeochemical cycles)
- 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 de- velops.
- The first plant to colonize an area is called the pi- oneer 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: in- creased productivity, the shift of nutrients from the reservoirs, increased diversity of or- ganisms, 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 vol- cano islands and lava flows, glacial mo- raines (muddy area exposed by a retreating glac- ier), 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 develop- ment.
Q. Lichens, which are capable of initiating ecolog- ical succession even on a bare rock, are actually a symbiotic association of
- algae and bacteria
- algae and fungi
- bacteria and fungi
- fungi and mosses Explanation
- 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 decompo- sition that dissolve and etch the substratum re- leasing nutrients to the substratum.
- Organic debris accumulates in pockets and crev- ices, providing soil in which seeds can become lodged and grow.
- The new conditions may be conducive to the es- tablishment of additional organisms that may subsequently arrive at the site.
- As the community of organisms continues to de- velop, it becomes more diverse, and competi- tion 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 develop- ment of biotic communities after the complete or partial destruction of the existing commu- nity.
- A mature or intermediate community may be de- stroyed 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 eco- system 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 ani- mals.
- And over the years, a forest community devel- ops. Thus, an abandoned land over a period be- comes 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 al- ready formed at the site. Thus, secondary succes- sion is relatively faster.
Autogenic and Allogenic Succession
- When succession is brought about by living in- habitants of that community itself, the process is called autogenic succession, while change brought about by outside forces is known as al- logenic 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 be- cause of
- limited sunlight and paucity of nutrients
- water limits and fire
- None of the above Explanation:
- 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 eco- system grasses form the climax community. Thanks to fire and lack of water.
- Grasslands are almost irreversible once defor- estation in water-scarce areas gives way to grass- lands.
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
- Homeostasis is the maintenance of stable equilibrium, especially through physiological (through bodily part functions. E.g. Cooling your body through sweating processes.
Organisms try to maintain the constancy of its internal environment despite varying external environmental conditions that tend to upset their homeostasis
- Some organisms can maintain homeostasis by physiological (sometimes behavioural – mi- grating to tree shade)
means which ensures constant body temperature, constant osmotic
- All birds and mammals and a very few lower vertebrate and invertebrate species are indeed capable of such regulation (thermoregula- tion and osmoregulation).
- The ‘success’ of mammals is largely due to their ability to maintain constant body tem- perature and thrive whether they live in Ant- arctica or the Sahara Desert.
- Plants, on the other hand, do not have such mechanisms to maintain internal tempera- tures.
- An overwhelming majority of animals and nearly all plants
cannot maintain a constant in- ternal environment. Their body
temperature changes with the ambient
- In aquatic animals, the osmotic concentration of the body fluids changes with that of the am- bient water osmotic concentration. These ani- mals and plants are simply conformers.
Why these conformers had not evolved to be- come regulators?
- Thermoregulation is energetically expensive for many organisms. This is particularly true for small animals like shrews and hummingbirds.
- Heat loss or heat gain is a function of surface area. Since small animals have a larger sur- face area relative to their volume, they tend to lose body heat very fast when it is cold out- side; then they have to expend much energy to generate body heat [a lot of food goes into heat generation] through metabolism.
- This is the main reason why very small ani- mals are rarely found in polar regions.
- 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
- This capacity of the ecosystem of self-regula- tion is known as homeostasis.
- For example, in a pond ecosystem, if the popula- tion of zooplankton increases, they consume a large number of the phytoplankton and as a result, food would become scarce for zooplank- ton.
When the number of zooplanktons is reduced because of starvation, the phytoplankton popu- lation start increasing.
Energy Flow Through an Eco- system – Trophic Levels
(Trophe = Nourishment)
- A trophic level is the representation of energy flow in an ecosystem.
- The trophic level of an organism is the position it occupies in a food chain.
- Trophic level interaction deals with how the members of an ecosystem are connected based on nutritional needs.
|Autotrophs||Green plants (Producers)|
|Heterotrophs||Herbivore (Primary consumers)|
|Heterotrophs||Carnivores (Secondary consumers)|
|Heterotrophs||Carnivore (Tertiary consumers)|
|Heterotrophs||Top carnivores (Quaternary consumers)|
- Energy flows through the trophic levels from producers to subsequent trophic levels is unidi- rectional.
Energy level decreases from the first trophic level upwards due to loss of energy in the form of heat at each trophic level
- This energy loss at each trophic level is quite sig- nificant. Hence there are usually not more than four-five trophic levels (beyond this the energy available is negligible to support an organism).
- The trophic level interaction involves three con- cepts namely
- Food Chain
- Food Web
- Ecological Pyramids
- Transfer of food energy from green plants (pro- ducers) through a series of organisms with re- peated eating and being eaten link is called a food chain. E.g. Grasses → Grasshopper → Frog
→ Snake → Hawk/Eagle.
- Each step in the food chain is called trophic level.
- A food chain starts with producers and ends with top carnivores.
- The trophic level of an organism is the position it occupies in a food chain.
- Types of Food Chains: 1) Grazing food chain and 2) Detritus food chain
Grazing food chain
The consumers which start the food chain, utilising the plant or plant part as their food, constitute the grazing food chain.
For example, in a terrestrial ecosystem, the grass is eaten by a caterpillar, which is eaten by lizard and lizard is eaten by a snake.
In Aquatic ecosystem phytoplankton (primary producers) are eaten by zooplanktons which are eaten by fishes and fishes are eaten by pelicans.