In this article we will discuss about the stratification of typical forest and lentic water.
Stratification of a Typical Forest:
A forest community is a typical example of terrestrial stratification because here a number of strata both above and below the soil can be recognised.
A forest bears an over story stratum. This stratum is composed of trees which are forty or more feet in height. These trees often form a canopy. Relative proximity of the trees determines whether the canopy will be open or thick-set.
Below this lies the understory stratum extending from twenty feet in height to a little below the over- story. The trees of this stratum are usually shade-tolerant and never grow to the height of the over story.
Below this stratum lies the transgressive stratum extending from four feet in height to a short distance below the understory stratum. The trees of this stratum are also shade-tolerant and have the potentiality to grow to increase the density or to replace those species making up the over story or understory strata.
A seedling stratum ranges between the soil level and the lower limit of the transgressive stratum. These seedlings are hardwood species. Finally, there is a herbaceous stratum containing non-woody plants (Fig. 3.22).
Animals residing in the wood are also stratified but the stratification is not so rigid because of the mobility of the animals. Animals move from one stratum to another stratum in search of food or in response to other abiotic factors. Insects, birds, squirrels and many other animals travel between seedling to upper strata.
In the herbaceous stratum rabbit, deer, wolves and foxes are found. Spring tails, beetles, fly larvae, millipeds, centipeds, mites, spiders, annelids, snails, etc., are found on the soil surface.
Stratification in Lentic Water:
Quiet water has been termed lentic water and flowing water is called lotic water. Ponds, lakes and shallow pools are examples of lentic water.
The strata in a lentic water fall under the following categories (Fig. 3.23):
(a) Neuston:
It is the faunal group found on the surface film covering a body of water. Those animals which are found on the upper surface of the film are called supraneuston while those that are found below the water surface are called infraneuston. Aquatic birds, water boatman and egg rafts of mosquito are the fauna of supraneuston. Larvae and pupa of mosquito, cladocera and aquatic snails are inhabitants of infraneuston.
(b) Plankton and nekton:
Plankton is a group of organisms that float aimlessly or swim feebly beneath the neuston. The plankton is a collective name for phyto- and zooplankton. Phytoplankton is represented by bacteria and algae. The zoo- plankton is represented by protozoa, shrimp larvae, rotifers, etc.
Another group of organisms that occupy the same stratum is the Nekton. These organisms are active swimmers and are represented by many fishes, shrimps, diving beetles, molluscs, etc.
(c) Pedon:
The lowest stratum in the faunal composition in lentic water is called pedon. It includes all the bottom dwelling animals or the burrowing types that live within the substrate. Protozoans, annelids, arthropods, dragon fly larvae, etc., make the faunal composition.
Fresh-Water Zonation:
Zonation of lakes and ponds is made on the basis of water depth and the type of vegetation.
The following ones are recognised:
(a) Supralittoral zone:
It is the zone lying just above the edge of standing water. This zone is not submerged but is exposed
to splash and current action. Life is sparse in this zone and is represented by bulrushes, sedges, insects and earthworms.
(b) Littoral zone:
The littoral zone extends between water’s edge and to a depth of six metres. The rooted hydrophytes are limited in this zone. Animal representatives are directly or indirectly dependent upon the vegetation present. The vegetation present serves as anchorage for egg masses of snails and insects.
(c) Sublittoral zone:
The sublittoral zone extends between the lower limit of the rooted vegetation to a depth of ten metres. The lower limit of this zone marks the compensation level. It is the level where the rate of photosynthesis is equal to the rate of respiration of the flora. The sublittoral zone is often called shell zone because large number of molluscs live here (Fig. 3.24).
(d) Profoundal zone:
It is the deepest zone in any fresh-water habitat. Light penetration is very low in this zone. Water in this region contains high hydrogen sulphide and is acidic in nature. Molluscs and larvae of insects belonging to the orders Diptera, Odonata, Ephemeroptera, etc., are found here.
Marine zonation:
The sea and ocean are subdivided into two basic zones—a nerilic province and an oceanic province. The neritic province is of limited extent and extends between high-tide marks and the outer mark of continental shelf (a depth of 200 metres). The oceanic province is of considerable magnitude. The neritic zone is further subdivided in a number of more restricted zones.
Subdivisions of Neritic Zone:
(a) Intertidal zone:
The zone is limited between high and low tide marks. As this zone is readily accessible to ecologists, it has been worked out thoroughly. This is a complex one and the complexity is due to the different types of organisms and to the diverse nature of the habitat. Tides periodically cover and uncover this zone twice daily. This makes the temperature, light, salinity and rate of evaporation extremely variable.
The intertidal zone may be made up of loose or stable rocks or may be sandy. The biota in the loose rock area are Collembola, Gammarus, Orchestia, Nereis, Hermit crabs, Starfish and Sea-urchin. Large and stable rocks are inhabited by Boring mollusks, Barnacles, Annelids, Sea- urchins. In sandy zones Tellina (Lamellibranch), burrowing copepods and mole crabs form the principle biota.
(b) Sublittoral zone:
The sublittoral zone extends from the low-tide mark down to a depth of 200 metres. The substrate of this zone is of soft consistency being formed by sand, clay, small rocks and broken shells. Numerous animals are found in this habitat.
The important ones are Foraminifera, Clione (sponge), Sipunculida, Chaetopterus, Nemertine worms, Sea-fan, Sea-pen, Maia (spider crab) Palincerus, Pecten, Dentalium and many echinoderms.
Subdivisions of the Oceanic Zone:
The oceanic zone is subdivided into the following zones (Fig. 3.25):
(a) Archibenthic zone:
The zone ranges from the continental shelf to a adepth of 1000 metres. The drop off of this zone is very sharp. At the lower level of this zone the temperature of water remains fairly constant throughout the year. Pressure of water on an average is 400 lbs. per sq. inch and a little amount of blue-violet light is available in the zone.
Animal biota in this zone is sparse and is represented by Sponge (Geodia), Sea- cucumber, Starfish, Brittle star, Brachio- pods, etc.
(b) Abyssobenthic zone:
From the lower level of the archibenthic zone down to the very depth of the ocean extends the abyssobenthic zone. This zone lies beyond the continental shelf. The characteristics of this zone are: no seasonal change in temperature, little dissolved oxygen, no light, pressure more than a ton per square inch and little, gross movement of water.
The substrate of the abyssobenthic zone is loose and is made up of deposits, oozes and clays. Animal life is sparse with one or two organisms per meter. Scattered forests of sponges (Cladorhiza) are found between the depths from 1600 to 2000 metres.
As the substrate is of loose consistency, the bottom dwellers are mostly burrowers or are with long stalks. Annelids, long stemmed sea- pen, crinoid, bryozoans and tunicates are the sessile forms found in this zone.
The active forms are represented by broad- footed Meopallina, sea-cucumber, pycnogonids and the largest of the crustacean crab, Kaempfferia with a leg expanse of more than five meters. Animals living below 1000 metres depth lose their body pigmentation.
All the deep-sea fishes and invertebrates in general possess wide mouth opening. Bioluminescent organisms are very common. The light emitted by these organisms is used to lure the prey or to recognise the individuals of the species.