The following points highlight the top four types of habitat. The types are: 1. Marine Habitat 2. Fresh-Water Habitat 3. Estuarine Habitat 4. Terrestrial Habitat.
Type # 1. Marine Habitat:
The marine habitat is the largest of all habitats. The seas, oceans and bays have occupied about 70% of the earth’s surface. The physical features of the marine habitat are relatively stable.
The depth varies from intertidal zone (a zone covered by water only part of the time) to depth as great as 35,400 feet or 6.7 miles. The average depth is about 12,500 feet. In tropical seas, the usual temperature is about 32°C. and that in the arctic region is about—2.2°C. In a given area this temperature rarely varies more than 5°C. during the year (Fig. 3.1).
The different salts dissolved in sea-water vary slightly in concentration. The average variation in the concentration of various salts is about 3’5% by volume (NaCl = 2.35, MgCl2 = 0.5, Na2SO4 = 0.4, CaCl2= 0.11). Various gases, namely, O2, CO2 and N2 remain dissolved in sea-water.
The amount of gas dissolved is dependent upon depth and temperature. So far as the concentration of oxygen is concerned it is more in surface water as also in cold water at a certain depth. Penetration of light into the sea-water is dependent upon factors like turbidity and surface motion.
On an average light can penetrate a depth up to 6000 feet and below it there is permanent darkness (Fig. 3.2). Pressure at different depths of the sea is different. It is known that pressure increases at the rate of one atmosphere, i.e., 14.7 lbs per square inch per 10 m of water.
This means that organisms in the depth of the sea are exposed to tremendous pressure. It has been estimated that an organism residing at a depth of 3500 feet experiences a pressure of 257 tons per square inch on the body.
Living organisms exhibit much stratification in their distribution in marine water. Producer organisms are limited up to the region where light penetration ends. But consumers occur in decreasing number from surface to bottom. Even the deepest portion of the sea is inhabited by living organisms. All the phyla (excepting amphibia) have representatives present in the sea.
Ecological classification of marine organisms:
The marine biota is varied. The animal communities are grouped into two vertical components—Pelagic and Benthic forms. The pelagic forms include the communities inhabiting the open sea, while the benthic forms are the bottom dwellers. All the communities are grouped into two heads—Producers and Consumers.
Pelagic organisms:
Pelagic organisms are found in the open sea. Pelagic organisms are classified ecologically on the basis of their means of locomotion and the depth at which they live. The recognised groups are listed below. They are divided into two groups: Plankton and Nekton. Plankton includes organisms that lack the power of locomotion by their own.
They float in water and move passively from one place to another with the help of wind or water current. Most of them are microscopic in size and are represented by protozoa and larval forms of crustacea, helminths, coelenterates and molluscs.
Both producers and consumers are present among the plankton.
The plankton bio- mass are classified into:
(i) Nannoplankton: Minute phytoplanktonic organisms (2.25 n), bacteria, protozoa, etc., which pass through the smallest meshes of a plankton net. The nannoplanktonic forms are also known as microplankton.
(ii) Macroplankton: The planktonic forms which are caught by the planktonic net.
The planktonic forms belonging to plant kingdom are called the Phytoplankton while the animal forms are called the Zooplankton.
The Zooplanktons are subdivided into:
(a) Holoplankton: organisms which are permanent planktons and
(b) Meroplarikton: organisms which are temporary planktons.
Most of the larval forms, viz., pilidium larva, zoea larva, planula larva, veliger larva, etc., are the meroplanktonic forms. Nekton organisms can swim about freely in water. They are mainly consumers and are represented by squids, whales, seals, fishes, turtles and many sea birds.
Benthic organisms:
Marine benthos or bottom dwellers are characterised by numerous sessile or relatively inactive animals. The benthonic forms exhibit marked zonation and they are quite distinct from each other in three primary regions—supratidal, intertidal and subtidal zones.
The benthonic animals are divided into two classes:
(i) Epifaunna: inhabiting the bottom-surface either attached or moving freely on the surface,
(ii) Infaunna: dig into the substrate or construct tubes or burrows.
The epifaunna reaches maximum development in the intertidal zone.
The infaunna is more fully developed in the subtidal zone and below. The distribution of benthonic sub-communities depends largely on the type of bottom, whether sandy/rocky/muddy. A sandy beach is better populated. Most of the large animals are specialised burrowers. The diatoms, amphipods and other infaunna live amongst the sand grains.
The rocky coasts show three distinct zones:
(a) Periwinkle zone or high-tide area;
(b) Barnacle or mussel zone and
(c) Sea-weed zone or low-tide area.
Zonation in sea:
The sea is regionated into different zones, primarily depending on the penetration of light rays (Fig. 3.3).
The main regions are:
(A) Neritic or shallow-water, zone,
(B) Oceanic or open deep-water zone.
Neritic zone:
It is the region of shallow water above the continental shelf. This region receives enough light and as a result producer organisms are present in profuse number. This zone has been divided into three subdivisions.
(i) Intertidal zone:
Intertidal or Littoral zone is alternately exposed to air and covered by water usually twice daily. This region is most favourite of all habitats with much light, oxygen, carbon dioxide and minerals. All the major phyla from Protozoa to Chordata are found here. Some of the inhabitants are motile and some are sessile.
(ii) Subtidal zone:
This zone extends between low tide mark and down up to a depth of 150 feet. This region is populated by both producers and consumers.
(iii) Lower neritic zone:
This zone extends on to the edge of the continental shelf and is characterised by the presence of some light but lesser organisms.
Oceanic zone:
The region of the open sea beyond the continental shelf embraces the oceanic division. It is further subdivided into bathyal, abyssal and hadal on the basis of depth.
Bathyal zone:
This zone extends from the edge of the continental shelf (600 feet) to a depth of 6,000 feet. The water of this region is quiet and becomes progressively colder with depth. There is little or no light and fewer number of organisms are present there.
Abyssal zone:
This zone extends from 6000 feet level to the deeps. The water is cold, quiet and has little oxygen concentration. The zone is permanently dark. Very few animals live there. The organisms living there usually have dark body colours. Eyes, when present, are large. Many of the inhabitants have the power of light production.
Hadal zone:
The hadal zone includes the deep areas of the ocean lying bottom- wards.
Besides the zonation, the sea can be divided into two regions depending on the penetration of light.
The zones are:
(i) Euphotic zone:
Light zone extending from the surface of sea up to the depth of about 150-200 m.
(ii) Photic zone:
It is the lightless zone of open sea including the bathyal, abyssal and hadal subdivisions.
Type # 2. Fresh-Water Habitat:
Fresh-water habitat is the smallest of the three primary environments. The habitat is extremely variable. In depth such habitats vary from shallow rain puddles to Lake Baikal of U.S.S.R. that reaches a depth of 5712 feet. Temperature of these habitats varies from frozen to boiling condition.
Dissolved salts vary widely in concentration. Dissolved gases vary from almost none to about 7 c.c. per litre. Though the freshwater habitat is small in area and less varied than terrestrial habitat it contains many more kinds of animals than does the terrestrial habitat.
Type # 3. Estuarine Habitat:
Estuarine or Brackish zone is formed in those regions where a river meets a sea. The composition of water in this zone undergoes constant change. The concentration of dissolved substances in such habitats is unstable. At high tides such habitats experience maximum salinity. Salinity decreases during low tide and periods of heavy rain.
The rate of flow of the water or current varies greatly in different estuaries, also in various regions of any one. The constant turnover in the water of estuaries brings about considerable changes in temperature of comparatively short duration. Thus the conditions prevailing in different estuaries vary greatly depending upon topography and other factors.
Type # 4. Terrestrial Habitat:
Of the three major types of habitats, terrestrial habitat is the most variable. In altitude it ranges from below sea-level (Death-valley) to mountain peaks more than 28,000 feet in height. A considerable variation of temperature is encountered in terrestrial habitat.
The lowest recorded temperature is —60°C. and the highest recorded temperature is 60°C. in certain deserts. The chemical and physical nature of the soil, sand and surface rocks varies greatly. The amount of moisture or relative humidity and rate of precipitation are extremely variable in different- regions of the globe.
The major subdivisions of the terrestrial habitat are given below:
(i) Tundra:
The arctic region of North America, Europe and Asia is known as Tundra region. This treeless region is characterised by long-cold winter with no or little sunlight. The summer is cool, short and is with long daylight hours. Frosting is very common and may appear at any time in this region.
Soil is never thawed beyond a depth of few inches from the surface. Bogs, marshes and ponds are the common features of this region. Mosses, Lichens and low herbs are the common vegetation. Migrant birds, like water fowls make their summer nesting grounds in the region.
Resident birds of North America are Snowy owls and Ptarmigan. Amongst the mammals, Lemmings, Arctic hares, Arctic foxes, Caribou, Musk-oxen and Weasels live in this region.
The alpine community on the top of many high mountains of both temperate and tropical regions resemble the tundra biomes in many respects. The plant and animal species of tundra and alpine regions are almost identical.
(ii) Taiga:
The broad band just south of the tundra region of Eurasia and North America is known as taiga region. The taiga region in the southern hemisphere is poorly developed because of the absence of land mass. The region bears evergreen coniferous forests.
Climatic condition of this region is represented by bleak winter and cool summer. Precipitation is of moderate type. The summer lasts from three to six months. Red foxes, Lynx, Caribou, few reptiles and birds are found here. Spruce, Firs, Pines and Cedars are the common vegetation.
(iii) Temperate deciduous forest:
This region is characterised by warm summer and cold winter. There is about 40 inches of rainfall throughout the year and the rainfall is uniform throughout the year. The dominant trees of this region bear broad leaves. The leaves are shed in summer and new leaves develop in the following spring.
Beech, Maples, Oaks, Walnuts are the common plants of this region. Deer, Gray foxes, Racoons, flying squirrels, many snakes and amphibia are the major vertebrates present in this region.
(iv) Grass lands:
Large areas in both temperate and tropical regions do not support trees but remain covered with heavy growth of grass. This is because of lack of sufficient moisture. All the grass lands are ecologically similar in appearance.
These grass lands are also known as Steppes, Prairies, Savanas etc. The rainfall in these regions is usually 12 to 40 inches per year. The rainfall is limited to few weeks in the year. Dominant plants of this region are Blue stem and Grama grasses. Characteristic grass land vertebrates are Bison, Pronghorn antelope, Coyotes, Prairie dogs, Rabbits, Larks, several snakes and lizards.
(v) Deserts:
Deserts occur both in temperate and tropical regions. Precipitation of moisture is the controlling factor in all deserts. The average rainfall in a desert is less than 10 inches per year and that too is very erratic with half or more than half of the annual rainfall occurring in one or two heavy downpour. Summer temperature is very high. The rate of evaporation too is very high.
The plants of the deserts are highly modified for the purpose of conservation of water. The leaves are reduced or absent or modified into thorns. The roots are long and go deep inside the soil. Many plants possess internally spongy tissue which collect and store water obtained during rainy season.
Cacti, Yucca are the dominant plants. Kangaroo, rats, Foxes, Coyotes, many lizards, several snakes and toads are the common vertebrates present in the desert.
Deserts may be snowy too. The sage brush area of North America and the Sierra-Cascade mountain system are deserts of this type. The climate in these deserts is very dry. The deserts are hot in summer and cool in winter. Pronghorn antelope, Coyote, ground squirrels and many reptiles are found in these snowy deserts.
(vi) Rain forests:
Abundant rainfall in the tropics, subtropics and few regions in the temperate zone results in the formation of luxuriant vegetation of broad-leaved evergreen trees. Vines, Orchids and Epiphytes are plentiful in these regions. Characteristic vertebrates of these regions are: Monkeys, Sloths, Ant -eaters, Bats, many colourful birds, frogs and salamanders, turtles and snakes.
A close similarity between terrestrial zonation and the zonations in mountains of high altitude is found. The zonation in a high mountain consists of a number of vegetational belts located a:
various altitudes along a mountain slope.
Temperature and rainfall play a role on the distribution of these vegetational belts. In a high mountain temperature decreases correspondingly at higher altitudes and thus produces communities of both animals and plants located along the slopes similar to more spacious biomes that occupy large areas within certain latitudes from the equator to the poles (Fig. 3.4).
