Ecological Life History of Species

Several eminent workers have prepared detailed schemes for a systematic study of autecological life histories.

Stevens and Rock (1952) have suggested scheme for autecological life histories of herbaceous plants, Pelton (1951) has given a scheme for the study of autecological life histories of trees, shrubs and stem succulents; Curtis (1952) for vascular epiphytes, and Cooke (1951) for fungi and they have emphasized the need for study of environment and performance of plants.

The main purpose of such studies should be to uncover the response of various forms of life to stimuli and compulsions of environment, and also to throw light on the limits of its geographical distribution and causes related to that.

The different aspects in the study of autecology of an individual species are outlined below:

(1) In any ecological or botanical study identification of plants is the first requisite. Taxonomy and nomenclature of the species under study are discussed. Under nomenclature scientific valid names, various synonyms and common names are discussed. The confirmation of identification is advisable in a standard regional or central herbarium. The botanical survey of India has developed a chain of herbaria; Central National Herbarium at Sibpur, Howrah, and Regional Herbaria at Poona, Coimbatore, Shillong, Dehradun and Allahabad.

Besides, a small Economic plant herbarium exists at Central Botanical Laboratory, Calcutta. These herbaria maintain large collections of correctly identified and properly arranged plants specimens and run a plant identification service.

Associates:

The other species associated with the plant species under study should be recorded in a number of habitat conditions in order to get a proper range of variations, Associate species indicate the sociability and adaptations of a plant.

(2) Distribution and importance:

The range of distribution, altitudinal and latitudinal limits and its ecological importance in various regions and habitats are noted. The degree of dominance or sub-dominance of species in different types of vegetation; its place in succession, whether pioneer or serial member or a component of climax community; its economic importance for browse, timber, pulp, fruits, in erosion and medicine are emphasized.

(3) Morphology of plants:

In this, the distinguishing features of various plant parts are described. Structural variations are also noted in the plants of same species growing on the different ecological habitats.

(4) Cytogenetics of plant species:

Cytological features, such as the structure of cytoplasm, chromosome morphology and the number and behaviour of chromosomes during mitosis and meiosis are studied in detail. Sometimes plants of the same species growing geographically isolated become so much differentiated and morphologically changed that they are often supposed as separate species. In such cases, interbreeding experiments are made. If the individuals interbreed, it is supposed that they belong to the same species. Variations in nature and factors for segregation are also discussed.

(5) Physiology of plant:

Various physiological processes of the plants of the particular species are studied in detail and the factors influencing the rates of those processes are also taken into consideration.

(6) Environmental complex:

The life cycle of individual species is greatly influenced by a number of environmental factors operating in conjunction as environmental complex. The various stages of life cycle of plant species in nature remain completely embedded in the environmental complex. Different species differ in respect of their response to climatic factors at different stages of their life cycles.

Each species has a definite time (month or season) in the year for seed germination, seedling growth, vegetative growth, flowering and fruiting etc. Study of all these processes of species in relation to different periods or seasons of the year is called its phenology. Since each species has a definite period for a particular stage of its life history, presence of species in that particular stage will indicate the time of the year.

In other words, the phonological behaviour of the species and different environmental factors at different seasons of the year are so much interlinked that the species is said to be biological or ecological clock. In autecology of a species both biotic and abiotic aspects are measured quantitatively at different stages of plant growth at regular intervals in order to study the phenology (germination, leaf- fall, initiation of flower bud, etc.) in relation to different seasons of the year. The environmental complex which is made up of several factors in various combinations affects each stage of plant’s life cycle.

In the foregoing discussion correlations of phenology to the various environmental changes are discussed:

(1) Flowering:

Various environmental factors, such as, light, temperature of the soil and atmosphere, percentage of nutrients in the soil, etc. affect the flowering process, of plants. In autecology of a species, the period of flowering as well as the specific roles of different environmental factors on flower initiation, is noted. The photoperiodic effects and temperature effects on the flowering are studied in detail.

(2) Pollination:

During pollination, the environmental factors help the plant to a great extent. Some plants may be pollinated by wind, some by water and some others by biotic agencies, such as insects, birds and even man.

(3) Fruiting:

The structure and number of fruits, number of seeds per fruit, season of fruiting, etc. are some of the important aspects in study of ecological life cycle of a species. The environmental factors by way of their influence on the fruiting of the plants determine the success or failure of particular species of the community in regeneration and establishment. Biotic factors, especially certain disease causing fungi and insects, damage fruits and thus affect fruit formation to a great deal.

(4) Seed output:

The number of seeds produced by a plant in one reproductive flux is known as seed output.

The seed output is measured by the following methods:

Counts for seed output are made from the plants growing in a number of diverse localities in order to get a correct assessment. In autecology the knowledge of seed output is of great importance. It is commonly observed that annuals reproduce only once in their life, whereas the perennial shrubs and trees produce seeds usually once in a year but many times in their life time. Every species has its own range of seed output. The range of seed output may be affected by many habitat factors, both biotic and abiotic. So while taking into consideration the seed output of a particular species, various factors which affect it are also considered.

(5) Dispersal of seeds:

The shape, size, weight and volume of seeds are important characters which govern the mode and extent of dispersal. Distribution and success in the establishment of plant species on the habitats are governed directly by the distribution of their seeds. If all the seeds scattered around the mother plants are allowed to grow, the new plants developing from them will have little chance for survival and regeneration because of over-crowding.

Movement of plant populations to wider areas ensures success in the establishment of species. Thus, the study of dispersal of seeds of plants is one of the important aspects of autecology. The dispersal mechanisms and the agencies causing the dispersal of seeds and other reproductive bodies are also studied in autecology.

(6) Viability of seeds:

Seeds have life spans of their own. They lose their power of germination after some time if stored under favourable conditions for long period. The period right from their formation up to the time when they begin to lose their germination capacity is called ‘viability period’. This period varies from species to species. In some cases, the viability period is zero and the seeds are totally incapable of germination. To test the viability, seeds are placed in petridishes on sawdust every week from the day of maturation and seed fall. The experiment is continued till the seeds do not germinate at all.

For calculating the percentage of viable seeds in a given seed sample, seeds are cut and dipped in dilute solution of TTC (tetra zolium trichloride) with a little sucrose. After 24 hours, if the embryos of dipped seeds become pink, such seeds are treated as viable.

The seeds of many crop plants remain viable for 5 to 10 years. In Mimosa glomerata. Cassia bicapsularis and Astragalus massibiensis the seeds are reported to be viable even after over one hundred years of storage. It is usually seen among in the seeds that cannot germinate immediately after formation.

Viability is affected by the conditions prevailing in the particular place where the seed is stored. It has been observed that the conditions which reduce the metabolic activities are usually responsible for increasing the seeds longevity. Low temperature, low oxygen and high CO₂ contents in the atmosphere have marked effects in increasing the viability period of seeds.

(7) Dormancy:

In some cases the viable seeds do not germinate up to a certain length of time even if the conditions for germination are favourable. This period is called dormancy period and such seeds are said to be in dormant stage and this phenomenon is known as dormancy.

Dormancy may be on account of physiological immunity for germination or due to impermeability of seed coat to water and gases or due to specific light requirements or due to the presence of some germination inhibiting substances. Recently caumarin and its derivatives have been found to be present in the dormant seeds of some species.

The dormancy can be broken by the following methods:

(a) Mechanical methods:

When the dormancy is caused by hard seed coats that are impermeable either to water or to oxygen or both, it can be broken by abrasion, removal or puncture of seed coats. Coriandrum seeds are pounded mechanically before sowing which gives high percentage of germination.

(b) Temperature:

The following alternating temperatures are usually given to seeds for breaking dormancy: (i) 0°C and 10°C, (ii) 30°C and 45°C and (iii) 0°C and 45°C. High temperature may also break dormancy, e.g., in Cassia occidentals.

(c) Irradiation of seeds:

Irradiation of seeds to red light breaks the dormancy and increases the germination percentage in some plants. X-rays and Gamma-rays have also been found to break dormancy.

(d) Chemical treatments:

Many chemicals are known to affect germination of seeds. Dormancy of seeds can be broken by dipping them in conc. H₂SO₄ for a few minutes (say 2 to 5 minutes) followed by thorough washing with water. Sometimes, it can be broken by putting soaked seeds in solution of copper sulphate or potassium permanganate or hormones.

(e) Removal of Inhibitors of germination:

When the dormancy is caused by the presence of some germination inhibiting substances, it can be removed by washing the seeds in running water for varying periods of time before placing them for germination.

(8) Seed germination and Reproductive capacity:

All the seeds may or may not germinate; some of them are in-viable, some do not get proper conditions for germination and some may be destroyed by living organisms and by excess water. According to Salisbury (1942), the percentage of germinated seeds in the average seed output is called ‘reproductive capacity’.

Reproductive capacity = Average seed output of a plant x Percentage of germinated seeds/100

The reproductive capacity and the seedling establishment are significant for physiognomy (outward appearance) and sociological structures of species population. The process of seed germination is controlled by several factors, such as, temperature, dormancy of seeds, availability of water, concentrations of oxygen and carbon dioxide, pH value of medium and so on. In autecology of a plant species the different factors which influence the seed germination are also taken into account.

(9) Seedling and vegetative growth:

When mature and viable seeds get favourable conditions for germination, they start germinating and after some time they establish their seedlings. Proper development and growth of seedlings are important factors for the survival of plant. Growth performance of shoot and root in relation to climate, physiography, soil character (physical and chemical both), fire, root/shoot ratio, stomatal counts, their physiology of movements are considered.

Besides, the aerial and underground productivity in various habitats and communities, water requirement (the amount of water required per gm of dry weight) are also studied. Several factors influence the growth and establishment of seedlings. Many seedlings of sal develop on moist leaf litter during rainy season but the onset of dry season even for a short period due to interruption of rains may kill many of them and unless the roots of seedlings have penetrated deep enough into the soil establishment of seedlings cannot be achieved.

The aerial shoots of seedling of sal die back year after year but the root growth continues. Shoots regenerate every year during or before the rains. This phenomenon of “die back” continues for 20 to 25 years and only after the roots have become established in deep strata of soil with good supply of water and nutrients the plant shoots up and establishes itself. In autecological studies, thus, the knowledge about seedling growth, establishment of young seedling in a particular area and climate is essential.

The intensity of solar radiations, durations and quality of light, temperature, soil conditions, water, etc. affect vegetative growth of any species. Every species in a population or community has its own requirements for the environmental conditions and has its own ecological amplitude of tolerance to an extent of fluctuation towards higher and lower sides from the optimum. Interspecific and intraspecific competitions for space, nutrients, light, water, etc. may also wipe out or delay the establishment of seedlings.

Dense shade of trees and absence of sufficient light may have lethal effects on seedlings of some species. Interspecific and intraspecific competitions may be studied by growing varying number of seedlings of the same species (intraspecific) and those of associated species (interspecific) in mixed condition in a unit area and recording growth performance (mainly dry weight) of individual plants under all conditions.