Interaction of Ecological Factors (Light Factor)

 An important aspect of the effects of environment on the life of an organism is the interaction of ecological factors. Light is well known for its effects on such basic physiological process of plants as photosynthesis, transpiration, seed germination, flowering etc. and thus of much concern in ecology, where it plays important role in the species composition and development of vegetation. 

Light in relation to plants :

Directly or indirectly light affects the plants' life in following ways :

i. Chlorophyll production - Young fronds of ferns, some mosses and algae which may become green even without light , most of the plants need light for chlorophyll formation, in which plants depend for food is due to light and thus existence of other life forms i.e. photosynthesis.

ii. Heating action - Transpiration rates increase in light raises and correspondingly affect water absorption also. Thus high light intensities are always associated with dry habitats and high transpiration rates. Opening and closing of stomata is regulated by light which is related to transpiration and absorption.

 iii. Distribution of plants - Total amount of light radiation received by the earth's surface differs with latitude (distance from the equator) which make differences in the vegetation at poles and at other parts of the earth. According to effect of light on their overall vegetative development plants are ecologically classified into : Heliophytes - Growing best in full sun light and  also can grow fairly well under shade  i.e called facultative sciophytes . And Sciophytes - which grow best in lower light intensities and also can grow well in full sunlight i.e. known as facultative heliophytes. 

For example, plants have sunny habitats have vertically oriented leaf blades. In moist climates, vegetation is generally a complex series of super imposed layers of tall trees, low trees, shrubs, herbs and mosses etc. such as maples, beech, spruces, firs and hemlocks are facultative sciophytes. In dry climates, species of flora are mostly heliophytes, such as pines, soft maple, willows, tulip tree, birches, larches and juniper. 

iv. Photoperiodism -  Total length of the daily light and dark period to which organisms are exposed i.e photoperiodism. It has effect on vegetative growth as well as flowering of the plans. on the basis of photoperiod , plants may be short-day plants - when photoperiod is between 12 to 14 hours. Examples are Datura stramonimum, Cannabis sativa, Cosmos bipinnatus etc. and long-day plants - when photoperiod more than 14 hours, for example Brassica rapa, Sorghum vulgare etc. Most plants are short-day, which require a relatively long period of uninterrupted darkness for flowering. Some plants are indifferent to above conditions and are called day neutral plants. For example, Nicotiana tabacum, Solanum tuberosum, Poa annua etc. 

v. Succession - The role of light in plant succession, generally mentioned that the light requirements of pioneers are comparatively much more than the climax species. But those could not receive much experimental support. 

Light in relation to animals : 

Light has far-reaching effects on animals also, by affecting their several types of activities like  reproduction, development, growth, locomotion migration etc., which summarised as follows. 

i. Metabolism - Light affects metabolic process of animals by ionization which increase enzymic activities, solubility of salts and minerals, photo-oxidation, respiration rates etc. which animals receiving poor light in caves thus shows slow rate of metabolism. 

ii. Reproduction - In animals like birds, light initiates the breeding activities. In some birds gonads become active with increased light intensity during summer, which are long-day animals. For example spring breeding birds and animals like freest, starlings, turkeys etc. Some sheep, deer and goats are short-day animals which sexual act decreasing in the length of daylight. 

iii. Development - In Salmon larvae undergo normal development only under sufficient light conditions, in the absence of  light their may take place rapid death. In Mytilus larvae their earlier stages grow larger in darkness than in light.

iv. Eyes -  In animal living in caves example Proteus anguinus and in the deep sea fishes, the eyes are absent or rudimentary. In surface dwelling animals as crustaceans and fishes, the ratio of the eyes to head is called as normal. But in the ocean, with increasing depth, size of eyes in animals generally goes on increasing .

v. Vision - Some fishes example Lepomis , depend on eyesight for location of their food. But in higher animals including human are able to see various object only presence of different form of light.

vi. Pigmentation - The process of pigmentation in animals is influenced by light in various ways i.e. a) in cave living animals like amphibians and some fishes skin colour or pigments are lacking due to absence of sunlight, b) leaf insect Phyllium, Australian fish, butterfly etc. have colouration exactly like that of their background which protect them from their enemies, c) some insects, crustaceans, fishes, amphibians, reptiles and cephalopods are able to change their colour or pattern rapidly according to the environmental conditions which help in thermoregulation and give protection from enemies.   

vii. Locomotion -   In some lower animals the speed of locomotion is regulated by light, which is known as photokinesis. Example, the blind larvae of mussel crab move faster if exposed to increased light intensities. Light plays a role in the orientation of locomotion, called phototaxis and movement of animals in response to light known as phototactic. Example, Euglena moving towards light source, positively phototactic and some zooplantons like copepods, earthworms etc. are moving opposite of the light source called negatively phototactic.  Many tubicolous worms and polyps of many coelenterates, only a part of their body shows movement in response to light and this phenomenon known as phototropism. 

viii. Photoperiodism - Migration of eels, salmons and birds is affected by photoperiodism, as some birds are seen migrating towards north during summer when days are longer, and towards south during winter when days are shorter. In some animals development of fur and feathers during wimter is shown to develop in response to photoperiodism. Daily response of animals to light conditions are known as circadian rhythms, whereas the annual rhythms known as circannual rhythms. Circadian rhythms are internally driven or endogenous or innate character. Example Drosophila's larva kept under a constant conditions but they will still emerge from pupae with a regular circadian rhythm. Eggs of chicken and lizards kept under constant condition, produced animals show regular circadian rhythm.