The following points highlight the three ecological factors that influence plants and animals. The ecological factors are: 1. Light 2. Temperature 3. Rainfall.
Ecological Factor # 1. Light:
Light is an important ecological factor. Direct exposure of protoplasm to light causes death. Still, without light, life would not exist, as it is the ultimate source of energy. Moreover, the evolution of the biosphere has chiefly evolved from the taming of incoming solar radiation the useful wavelengths exploited and the harmful ones shielded out.
Light, thus, is a kind of radiation, the visible wavelength of electromagnetic radiation. This part of the spectrum runs from violet (shorter wavelength) to red (longer wavelength). Wavelengths shorter than violet are ultraviolet radiation, visible to fishes and insects but invisible to man.
Wavelengths longer than red are infrared and microwave radiation. The energy of the solar radiation reaching the earth’s surface consists of one half in the visible range and another half in the near infrared. The ozone layer of the atmosphere prevents the ultraviolet rays, except a few, to reach the earth’s surface. Ecologically, the important aspects of light are the quality (wavelength), the intensity (energy), and the duration (length of day).
ADVERTISEMENTS:
(i) Quality of Light:
Animals and plants both respond to different wavelengths. For example, the rate of photosynthesis varies somewhat with different wavelengths. Among mammals, colour vision is well-developed only in primates.
(ii) Intensity of Light:
The intensity of light controls the entire ecosystem through its influence on primary production. Photosynthesis follows linear increase up to an optimum or light saturation level. Individual plants and communities adopt to different light intensities by becoming shade adapted (i.e. reaching saturation at low intensities) or sun adapted.
ADVERTISEMENTS:
Phytoplankton, other than diatoms, are shade adapted and photosynthesis in them are very much inhibited by light intensities (peak production in the sea occurs below rather than at the surface). Diatoms, on the other hand, are sun adapted and show maximum rate of photosynthesis when light intensity is less than 5 percent of full sunlight.
(iii) Duration of Light:
The duration of light or photoperiodicity refers to the length of the light and dark portions of the 24-hour day. Change in day length provides organisms information about the advance of seasons, so that they can schedule their life history events, breeding conditions and other activities at the appropriate season.
In case of the plants, photoperiod may be responsible for the timing of many events including flowering. Short-day plants requires lesser length days to come into flower (Examples: soybean, chrysanthemum, violet etc.). Long-day plants need day lengths longer to come into flower (Example: evening primrose, spinach, wheat grass etc.).
ADVERTISEMENTS:
Light in relation to water:
Water layers affect light intensity. The intensity of light decreases gradually with increasing depth. Light penetration in water depends upon the turbidity, solute content, motion of water, plankton growth etc. Thus, submerged plants receive weaker illumination than the exposed plants, floating on the surface of water.
Functions of Light in Plants:
Light affects plant life directly and indirectly.
Thus it serves the following functions:
1. Production of chlorophyll:
Most plants require sunlight for chlorophyll formation. Light is thus, essential for the production of food and for the existence of life forms as a whole.
2. Transpiration:
Plants exposed to light, raise their temperature and the rate of transpiration increases. Water absorption is thus, affected due to corresponding effect on transpiration rates. This may result in formation of dry habitat.
3. Stomatal regulation:
ADVERTISEMENTS:
Light regulates the opening and closing of stomata and is thus related to transpiration and absorption.
4. Plant distribution:
The distribution of light intensity varies with the latitude and it is probably one of the reasons for the varied vegetations at different parts of the world. In case of a water body the phytoplankton get concentrated at areas where the radiation of the sun is strongest. It is one of the reasons for the phytoplankton to get concentrated at the surface of the water during daytime.
5. Plant classification:
Plants can be classified based on the requirement of light:
(a) Heliophytes grow best in full sunlight.
(b) Sciophytes grow best at lower light intensity.
In comparison to the sciophytes, heliophytes show thicker stems, more frequent branching, smaller chloroplasts, roots longer, high respiration rate, low water content, higher concentration of salts and sugar, high osmotic pressure etc.
6. Photoperiodism:
Photoperiod — the total length of daylight period to which plants are exposed — plays an important part in local distribution of plants by affecting stem elongation, flowering, fruit development etc. On the basis of photoperiod plants may be (i) short-day plants (Example: Salvia, Datura, Cannabis, Cosmos etc.) and (ii) long-day plants (Example: Brassica, Secala, Sorghum etc.).
7. Photosynthetic efficiency:
The photosynthetic efficiency varies with the diurnal light intensity. The peak efficiency is attained at 2:00 pm, when light intensity is most pronounced.
Functions of Light on Animals:
Light plays an effective role on the activity of animals which are:
1. Metabolism:
The metabolic activities of animals are affected by light through its heating effect on tissues. It results in an increase in enzymatic activity and in degree of solubility of salts and minerals. Animals residing in caves show slow metabolic activities.
2. Locomotion:
Locomotion in some lower animals is regulated by light — a phenomena known as photo kinesis. For example, blind larvae of mussel crab (Pinnotheres) moves at a faster rate when exposed to an increased light intensity. Locusts stop their flight when the sun gets hidden by the clouds.
The movement of animals in response to light is called photo taxis, and such movements are called phototactic. Some animals like Euglena etc. are said to be positively phototactic as they move towards the light source. Others like earthworm, slugs, copepods etc. move away from the light source and are thus known as negatively phototactic. Some animals like the polyps of many coelenterates show movement of only a part of their body in response to light and are thus termed as phototropism.
3. Reproduction:
The gonads in some birds become active with increased light intensity. Thus, light initiates breeding activities. Some animals (sheep, goat, deer etc.) are short-day animals and they can be brought to sexual act by decreasing the length of exposure to daylight. Animals like starlings, turkeys etc. are long-day animals, while, squirrels, guinea pigs etc. are indifferent towards the length of light exposure.
4. Development:
The larva of Salmon undergoes normal development when exposed under sufficient light conditions. However, rapid mortality takes place in the absence of light. Smolt transformation in fishes is initiated by the rate of change of daily photo-period. The larva of Mytilus develops better in darkness than in light.
5. Eyes:
In case of cave dwelling, Proteus anguinus, and deep sea fishes, the eyes are totally absent or rudimentary. This occurs due to the absence of light in these habitats. Some deep sea fishes do have larger eyes than the normal size to capture any ray of light that might be present. However, there is total absence of cones in these eyes.
6. Vision:
Most animals can see objects and differentiate colours only in the presence of light. However, vision is reduced in nocturnal animals when they are exposed to sufficient light conditions.
7. Synthesis of calcium:
Calcium synthesis in bones require sunlight. The bones of deep sea fishes are brittle as they are unable to synthesise calcium due to the total absence of sunlight.
8. Pigmentation:
The process of pigmentation in animals are influenced by light. Colouration and mimicry are best exemplified in the presence of light.
9. Photoperiodism:
Photoperiodism on the total light period has profound effect on animals through such processes as gonadal activity, reproduction, metamorphosis, migration etc. Migration of birds and fishes are known to be affected by photoperiodism. Early maturation in certain cat fishes seems to be induced by longer period of day.
10. Vitamin D formation:
Ultraviolet radiation of the sun plays a role in vitamin D production in animals.
Ecological Factor # 2. Temperature:
Life on this earth exist within a narrow range of 300 degree Celsius (from -200°C to 100°C). In fact, most species activities are restricted to an even narrower band of temperature. Some organisms during the resting stage can survive through very low temperatures, while a few microorganisms (bacteria) can tolerate and even reproduce in hot springs, where the temperature is close to the boiling point (80°C for cyanobacteria). The range of temperature variation is less in water than on land. Thus, aquatic organisms have narrower range of temperature tolerance than land animals.
Temperature, thus, is a limiting factor. It affects organisms by controlling their morphological, physiological and behavioural features through either heat gain or heat loss or both. When heat gain is equal to heat loss then the body temperature of the animal or plant will remain same. If heat gain is more than heat loss then body temperature will rise, while the reverse is true when body temperature drops.
Poikilotherms and Homoiotherms:
On the relationship of body temperature to environmental temperature, organisms are of two types — Poikilotherms and Homoiotherms.
(a) Poikilotherms:
Poikilotherms or cold blooded organisms are those whose body temperature tends to match environmental temperature. As the environmental temperature goes up or down, the rates of their body processes also goes up or down — as the case may be. Most lower organisms, particularly those residing in water, are poikilotherms.
Poikilotherms have no internal physiological means to keep body temperature constant. These animals do adjust their body temperature with that of the outer world. However, some animals (lizards) use behavioural rather than physiological means to regulate their body temperature.
(b) Homoiotherms or homeiotherms:
Homeiotherms better known as warm blooded organisms, can keep their body temperature constant even when the environmental temperature changes. Homoiotherms are mainly birds and mammals, who have automatic physiological mechanisms for keeping their body temperature constant despite the changes of the outside or surrounding temperature.
To keep their body temperature high at low environmental temperature, homoiotherms increase their metabolic rate or increase insulation by adding fat or fluffing up feathers or fur. On the other hand, to keep body temperatures low at high environmental temperatures, warm-blooded animals lower their heat production and increase their heat loss by evaporating water from sweat glands or by panting etc.
Poikilotherms and homoiotherms differ drastically in the relationship between the organism and its environment, as can be seen in Fig. 4.17. For example, very low temperature is tolerated in different ways. Poikilotherms gradually become inactive as temperature drops (Antarctic mites are, however, active even at subzero temperature), and they may seek microhabitats (if available).
Their further survival depends on tolerating the cold.
There may be two types of species:
(i) Freeze-tolerating species (survives ice-formation in their extracellular spaces) and freeze-susceptible species (which do not survive on freezing).
Homoiotherms, on the other hand, can survive low temperatures by accumulating enough food in their body to keep heat production high and so as to maintain normal body temperature. If a homoiotherm fails to accumulate enough food, its body temperature will drop and it will die quickly at a body temperature well above freezing.
Poikilotherms are energy conservatives with low power, while homoiotherms are extravagant but powerful. In a powerful food rich habitat, homoiotherm wins but poikilotherms will not get along quite well. When food or water or oxygen disappears for the season, poikilotherms can survive by shutting themselves down.
While homoiotherms, even if they greatly restrict their activity, continue to use large amount of energy. Poikilotherms can be very small and can restrict their energy requirement, while homoiotherms simply require too much energy to exist at sizes much below 5 gram.
Hibernation:
Hibernation or winter sleep is seen in only a few mammals (thirteen-lined ground squirrels, jumping mice etc.) and still fewer birds (woodchucks etc.) The warm-blooded animals that hibernate in winter are termed heterotherms.
The hibernating homoilotherms search for a place to stay (hibernaculum), as winter approaches and become poikilothermic, so that as the temperature in their hibernaculum decreases so also does their body temperature.
In this way they save their energy which otherwise they would have spent in keeping their body temperature constant and in moving about. They, however, differ from poikilotherm in terms that when their hibernaculum gets close to freezing point, their heat production begins to increase.
If temperature falls further, these animals rouse and reverts back to homoiothermic again. If they are lucky they may find a deeper hole and may again go back to winter sleep. Raccoons, possums, foxes, squirrels, bears do not hibernate but spend the winter sleeping in burrows. Their temperature however do not drop.
Eurythermal and stenothermal organisms:
Organisms such as man, lizards, amphibians etc. can tolerate wide range of temperature fluctuations and are referred to as eurythermal organisms. On the other hand, corals, snails etc. cannot tolerate wide ranges of temperature fluctuations as they have no adaptation to adjust themselves to such temperature fluctuations. They are called stenothermal organisms.
Ectotherms and Endotherms:
A classification based on the relationships between organisms and environmental temperature divides organisms into ectotherms and endotherms. Ectotherms are organisms who largely depend on external sources of heat to raise their body temperature.
Examples: protista, plants, fishes, reptiles etc. Endotherms are organisms capable of generating heat internally in order to raise their body temperature. Examples: birds and mammals.
As the temperature moves away from the thermo neutral zone, the endotherms expend more and more energy to maintain body temperature. Endotherms produce heat at a rate controlled by the brain. Heat loss is moderated by insulator material (fur, fat etc.) and by controlling blood flow near the skin surface.
Functions of Temperature:
The effects of temperature on plants and animals are:
1. Effect on metabolism:
As temperature regulates the activity of enzymes, it regulates the metabolic processes of organisms. It affects the rate of transpiration, photosynthesis and seed germination in plants. It also regulates the respiration rates and other metabolic activities in both plants and animals.
2. Effect on reproduction:
(a) Plants:
Temperature affects flowering in plants. It also plays an important role in the phenology (study of periodic phenomena) of plants.
(b) Animals:
The maturation of gonads and liberation of gametes take place at a particular temperature that varies from species to species. Breeding also is affected in some due to temperature. The number of eggs laid by blowfly increases with increasing temperature up to 32.5°C and thereafter decreases. The fecundity of animals is also affected by temperature.
3. Effect on growth and development:
(a) Plants:
Extremes of high and low temperature have adverse effect on the growth of plants. Low temperatures bring about diseases like desiccation, chilling and freezing injury. Extremely high temperature causes stunting and death of plants — called heat injury.
(b) Animals:
Growth and development of animals are affected by temperature. Corals do not flourish when the temperature of water drops below 21°C. In blow-fly, the incubation period of eggs decreases with increasing temperature.
4. Effect on crossing over:
Temperature is seen to affect the crossing over and somatic expression of genes in animals. If the larva or pupa are kept at low or high temperature it affects the development of wings, eyes etc.
5. Effect on sex-ratio:
In rotifers and daphnids, temperature affects sex ratio. Under normal temperature, daphnids lay parthenogenetic eggs that develop into females, while with the increase in temperature they give fertilised eggs that develop into either males or females.
6. Effect on colouration:
Some species of mammals, birds and insects present in warm humid climates, bear darker pigment than the races of species who resides in cool and dry climate. In Hyla (tree frog) and Phrynosoma (horned toad), low temperature induces darkening of pigments.
7. Effect on morphology:
The absolute size of an organism is affected by temperature. Mammals and birds attain larger body sizes in cold regions than in warmer areas. Whereas, poikilotherms are smaller in cold regions. Snout, ear, legs etc. of mammals are relatively shorter in colder areas than in warmer areas. The races of birds occurring in colder regions have relatively narrower and more accuminate wings, while those in warmer areas tend to be broader.
Ecological Factor # 3. Rainfall:
Nearly 97% of the earth’s available water is found in the ocean and the rest 3% on land. Evaporation and transpiration of this water results in the formation of water vapour, which at any one time amounts to only about 0.001%. If this entire water vapour were precipitated (rainfall) at the same time over the whole earth’s surface, it would equal to 2.5 cm of rain.
The annual rainfall averaging over the entire earth’s surface is about 80 cm. Three-fourth of this falls on the ocean. The ocean, however, contributes about five-sixth of the water evaporated in the hydrological cycle The difference falls as rain on land which later joins the ocean through the rivers. This asymmetry is essential; otherwise the land would have been a drier place.
Precipitation or rain falling on the earth’s surface returns back to the atmosphere by two ways:
1. Some water is directly evaporated from the soil, from open waters of ponds, lakes, oceans etc. and from the surfaces of objects (wet from recent rain).
2. Some water returns back via transpiration (process by which the leaves of plants evaporate water taken up from the soil by their roots).
The combination of the above two ways is referred to as evapotranspiration, a process that is dependent on temperature. Evapotranspiration is related to the availability of water in the soil, the leaf area of the local vegetation and the temperature. Evapotranspiration nearly doubles with each 10°C rise in temperature. It is also proportional to the rate of photosynthesis.
From the pathway of rain falling on the vegetated land (Fig. 4.18), much of the water is intercepted by the vegetation and re-evaporated without reaching the ground. A small part of this rain water is generally available for plant growth, of which only 1% is used in photosynthesis.
Rainfall is determined by geography and weather systems (pattern of large air movements). The distribution of rainfall over the year is an important limiting factor for organisms. For example, a 35 inch rainfall evenly distributed throughout the year is quite different from that provided by the same amount of rain falling during a restricted time of the year. In the latter, the flora and fauna will have to survive long droughts or sudden floods.
The amount of rainfall (evenly distributed over the year) gives a rough approximation of the biotic community that may be expected, which is listed below:
Functions of Rainfall:
1. Formation of biomes:
The annual amount of rainfall evenly distributed in temperate latitudes determines the climax biotic communities (as given above).
2. Type of vegetation:
The annual rainfall determines the type of vegetation. For example, heavy rainfall throughout the year in tropical regions results in evergreen forest. Countries having heavy rainfall in winter and less rainfall in summer results in the formation of sclerophyllous forest—made of shrubs with leathery, thick evergreen leaves. Conversely, areas with heavy rainfall during summer and less in winter results in the formation of grassland.
3. Type of animals:
With the change of vegetation due to rain, the animal life also differs in different geographic regions.
4. Flooding and reactions by animals:
Flooding by rainwater results in dam building by beavers. Beaver’s dam contains felled trees, mud, stones and sticks. Terrestrial vegetation in that area dies and aquatic succession begins. Finally the beavers leave, the dam deteriorates and breaks and the valley becomes a dry land again, but with an altered soil consisting of the old lake bed.
5. Effects on metabolism:
Rainfall brings along with it a cooling effect which regulates the activity of various animals.
6. Effect on reproduction:
Rainfall is one of the physical factors that regulate the maturation and liberation of gonads. For example, the croaking of the male frog can be heard at the time of rainfall which attracts the female for mating.
7. Effect on growth:
In case of certain plants, the advent of rainfall witnesses the appearance of shoot and new leaves and branches.