Limiting factors are often classified in two categories: (a) Density Dependent and (b) Density Independent.
The effect of the former on a population increases as the population increases in density. It is essentially a stabilizing influence on growth and causes a population to level off at carrying capacity. A density- independent factor, conversely, affects many or few individuals without reference to the population level.
Food supply is generally density dependent. The more there are to eat it, the less there is for each individual and the greater the effect of food scarcity. By contrast, a flood is density independent, since it may wipe out an entire population of a species, whether there are few or many. The more important limiting factors are as follows:
(a) Climatic and Atmospheric Factors:
These operate in a variety of ways to affect species populations. To begin with, any species has limits of tolerance and some optimum range of tolerance for such factors as sunlight, temperature, humidity, rainfall or wind. If a food climate normally exceeds the limits of tolerance for a species, then the species will not occur in that area, if it exceeds the optimum limits, the species will not thrive there.
If the local climate only rarely exceeds the limits of tolerance, then a species may temporarily occupy that area, but will be eliminated in those years when the climate becomes extreme.
Assuming that the climate is favourable to the establishment of a species, then a number of climatic factors may influence population growth. Changes in temperature-years that are warmer or colder than normal-may permit a species to thrive and increase, or they may cause a decrease and permit its survival only in the most favourable sites.
Changes in rainfall and humidity, dry years (or cycles) and wet years (or cycles), have major effects. Fluctuations in temperature and rainfall tend to be most severe in areas where temperature or moisture is already near the limits of lowlands where rainfall and temperature are high and near the optimum for the growth of the greatest fluctuation in either factor.
In the dry tropics fluctuations in rainfall from year to year may be extreme. In northern temperate regions where winter is cold and the duration of low temperatures restrict the growing season for plants, one finds the greatest variation between temperatures in one year and another.
Climatic and atmospheric factors tend to have their most severe effects upon those introduced populations of plants and animals which man attempts to grow or produce in areas that have climate more extreme than the optimal climate for the species concerned.
Native species are adapted to the range of climate in the area they occupy, and although they may be affected by climatic extremes, these effects are usually much less limiting than for a domesticated species introduced outside its normal climatic range. In the African savannas, a drought will be more likely to wipe out a high percentage of the cattle than of the native antelopes. Climatic and atmospheric factors also have more effect of species that are at, or approaching, the level of subsistence density.
Well- situated animals or plants, with adequate nutrients, water and other essentials are less likely to be affected by drought than populations which have increased and expanded into marginal areas and are already in poor condition because of nutritional limitations.
Certain factors of weather and climate may have catastrophic effects on all populations in an area- hurricanes, torandoes, unusually severe floods or droughts, but usually there are less significant than the more normally occurring changes, except in the case of those species which are highly limited in distribution. With most species, normal fluctuations in climate have their greatest effect when animal population densities are excessive, or when plants are established in sub-optimum environments.
Organisms living at nearly constant temperature cannot tolerate variations.
Organisms which can tolerate wide variations in temperature.
Living at high temperature.
High summer temperature and low winter temperature.
Moderate warmth in summer, low temperature in winter.
Living with brief summer below 10°C and winter prolonged with very low temperature.
Temperature ranges have created different life zones- tropical, subtropical, temperate, arctic/alpine.
Poikilothermal (= Ectothermal = Cold Blooded). Temperature varies with the surroundings.
Homoisothermal: (= Endothermal, Warm Blooded): Temperature constant due to internal regulation. Excessive subcutaneous fat (blubber) protects whale, seal and polar bear from low temperature. Shivering is a mechanism to increase body temperature. ‘Persipration’ and ‘panthing’ reduce body temperature.
Hibernation (- Winter Sleep), aestivation (= Summer Sleep), periodic activity and winter/summer eggs/seeds are adaptations of poikilothermics to avoid unfavourable temperatures. Thick cuticle, corky bark, hair, thick leaves, high solute content, mucilage and tannins are protective mechanisms in plants to tolerate high (55°-73°C) or low (- 20° to – 35°C) temperatures. Generally seeds, spores and cysts tolerate temperature extremes better due to low water content and thick covering.
Thermoperiodicity controls phenology in several cases.
Bergmann’s Rule states that avian and mammalian forms are generally of larger size in colder regions as compared to warmer/hotter regions.
It controls photosynthesis, tissue differentiation, growth, pigmentation, plant movements, period of activity, reproduction in plants and animals. Depending upon the period of activity, animals are diurnal (active during day, e.g. Sparrow, Pigeon, Butterfly), nocturnal (active and dusk) and crepuscular (active at dawn/dusk, e.g. Rabbit) Depending upon period of breeding, animals and plants are called long day (e.g. Turkey, Ferret, Radish), shortday (e.g. Sheep, Deer, Xantheium, Dahlia) or day neutral (e.g. Guinea Pis, Tomato).
(Air Current). It determines cloud formation, pollination and dispersal in many organisms. Wind restricts flight animals, nest formation and height in plants. It enhances soil erosion, transpiration and develops flag trees.
Epiphytes can be found only in humid areas. Arid areas do not have them. Transpiration in inversely proportional to atmospheric humidity. Low atmospheric humidity produces aridity and xerophytism. Periodicity and amount of precipitation determine type of vegetation-evergreen, deciduous, chapparal, savannah, grassland, desert etc.
Here the limiting factors operate directly on plants, and through plants, upon animal populations (other than those of the soil fauna itself, which is often directly limited by soil conditions). The requirements of plants and animals, however, are not identical.
An abundant growth of vegetation does not necessarily mean there will be an abundant food supply for animals, nor that the food supply will have high nutritional value.
Certain plants may produce abundant vegetative growth and high amounts of carbohydrates under conditions where certain soil chemicals are in short supply, however, they may produce relatively small amounts of the proteins and vitamins needed to support an abundance of animal life.
Thus, a shortage of trace elements, such as cobalt, in the soil may have little effect on plant growth. However, cattle feeding on their plants may do poorly or do not survive. Addition of a cobalt top- dressing to the soil, which is then taken up by the plants can open up a rangeland to livestock use.
For another example, a dense forest may produce an abundance of vegetation, but will support very few ground-dwelling grazing or browsing animals. There is no lack of food for them, but plants growing on a forest floor under dense tree cover are commonly lacking in protein.
Animals grazing of browsing them can receive adequate carbohydrates but not enough protein to thrive or reproduce. Thus, in many areas of dense forest, grazing and browsing animals will only be common around natural or man-made clearings.
The abundance of natural vegetation in an area often has little relation to the inherent fertility of a soil. Thus, the ferasols and acrisols of the humid tropics are notoriously infertile, yet they support the most luxuriant vegetation to be found on earth.
Examination of this situation reveals, however, that virtually all the nutritional elements required for the support of life are tied up in the vegetation and animal life of the area, and recalculate quickly from the dead plant or animal back into a new, living individual.
Below the organic surface layer the soil contains few of the elements needed for plant or animal nutrition, and if the vegetation is scraped away with a bulldozer, the soil that remains is often highly infertile. By contrast, some desert soil that supports little plant life, because of lack of water, may have a rich supply of soluble mineral elements needed for the support of life.
When water can be made available, they will support high levels of productivity, although there may be new ill effects such as salinization, water logging and water borne diseases.
Water is an obvious limiting factor for plants and animals. Plants are classified in relation to their tolerance for dry medium or wet conditions as xerophytes, mesophytes and hydrophytes. Most land plants are in the intermediate, mesophytic range, desert plants are usually, but not always xerophytes, hydrophytes grown in water or require an abundance of water in the soil.
All plants require water to support their active growth and metabolism. Some plants cannot tolerate even brief periods of moisture deficiency and that will wilt and die when the soil dries out.
Xerophytes have special adaptations that permit them to survive under prolonged drought, but must have some moisture in order to grows and reproduce.
Where present in abundance, water limits the occurrence of terrestrial organisms and provides a habitant for aquatic forms of life. The latter will relate in their diversity and abundance to the quality of the water, its ability of supply oxygen, its salinity, temperature, velocity and other factors. All of these are capable of being affected, usually adversely, by pollution or siltation.
In relation to terrestrial vegetation, abundance of water or in the soil limits the plant growth primarily by restricting the availability of oxygen. Those plants growing in wet soil or flooded areas have the ability to derive oxygen through special structures, such as PENUMATOPHORES of mangrooves, or the so-called ‘Knees’ of bait cypresses are good examples.
Continuously moist or flooded soils inhibit the bacterial fungal action that would otherwise bring about decay of organic debris and litter-Organic remains will usually accumulate on such soils in the form of organic mucks or peat. Animals, like plants, require moisture for their continued existence, but vary greatly in their ability to endure drought.
Apart from purely aquatic animals, amphibious animals such as the crocodile and hippopotamus cannot long survive away from water. Most land animals must drink water regularly in order to continue to live. However, some desert species can exist indefinitely in the absence of free water as long as the food which is eaten contains sufficient moisture and they can find enough shelter to hold the loss of water from their body surfaces to a minimum.
A severe drought, bringing an absolute shortage of water to an extensive area, can be fatal to all animals that require water for drinking. Nevertheless, much of the mortality attributed to drought results not from a shortage of water, but from a shortage of food within reach of water.
(d) Biotic Factors:
The greatest number of limiting factors influencing plant or animal growth, abundance and distribution are biotic in nature.
Food supply for animals is one of these, and in the most common factor limiting the growth of animal populations, either directly, through being short of requirements, or indirectly, through behavioural responses to food shortage. The number of plant- eating animals in any area is ultimately limited by the abundance of the plants of which the animals feed.
Furthermore, since the plants must survive and reproduce, there must always be a greater abundance of plants than what is actually needed to provide food to the animals. Most plants produce a surplus of vegetative growth and of seed, part of which can be used by animals.
However, each plant, in order to survive, must maintain a metabolic reserve-a minimum amount of leaf age to permit it to store food for its own survival, or set seed for its own reproduction.
Plant growth may be limited by competition from other plants of the same or different species, each drawing on the same reserves of soil and water, or shading one another from essential sunlight. Some plants secrete substances which inhibit the growth or establishment of other plants.
A variety of organisms may prey upon plants, from the seed stage through the life cycle to the mature plant. It is obvious that the number of carnivores or meat-eating animals in any area is limited by the availability of the prey upon which they feed. Also there must be enough prey animals to enable the prey species to survive and reproduce, the predator cannot eat all of prey, or both would become extinct.
Most prey animals have developed behaviour patterns in relation to their habitat conditions that permit some of them to escape predators. Predators seek the most vulnerable and available prey, and usually do not hunt down and capture the more elusive individuals. Often the individuals thus captures are either very young and very old or the most diseased or genetically least vigorous stock, therefore, natural predation tends to have overall beneficial effects on the population of a prey species.
The relationship of the abundance of predators and prey is sometimes illustrated in the form of a biotic pyramid. Green plants occupy the base of the pyramid and must always have a greater total volume (biomass) than the plant-eaters that feed upon them. The latter, the herbivores, form the second step be wider and represent a greater biomass than the third step, representing the carnivores that eat other carnivores they must be fewer in number and contain less biomass than their prey.
Each succeeding level in the biotic pyramid also represents a consistent loss of energy from green plants of herbivores to predators, since at each transfer of energy is necessarily lost. Far more calories are available at the lowest level of the pyramid than at any higher level.
The sequence of plants and animals feeding upon one another is called ‘food chain’. Food chains are interconnected, since more than one herbivore may feed on a plant and more than one kind of carnivore may eat a herbivore. These interconnected chains form a ‘food web’.
As elements pass from soil (or atmosphere) to plants, and are then eatern by herbivores and in turn by carnivores, they are often concentrated in varying degrees by a process known as ‘biological magnification’.
Thus, the element iodine may be present in the soil in small quantities and taken up by plants as it goes into solution and reaches plant roots. It has little direct function in plants, but is essential to plant eating mammals. They must obtain enough iodine from the plants they eat and concentrate it in their thyroid glands for their glands to function properly. Therefore, the level of iodine in an animal’s thyroid is far greater, than in either the plants or the soil.
Although the number of herbivores is limited by the supply of plants, and the number carnivores is determined by the number of their prey, the reverse is also true in varying degrees.
The abundance and distribution of plant can be limited by the presence of herbivores. If a plant is introduced into an area where there are no species that feed upon it, can spread rapidly and become transformed into a weed or pest.
Similarly, the abundance and distribution of plant- eating insects and other animals is often controlled by a wide variety of diseases, insects or other predators that feed upon them.
If the predators are destroyed, their plant eating prey can increase suddenly and explosively, since its reproductive rate in nature is adjusted to the need for survival under constant predation. Sudden increases in number of rodents or other plant-eating mammals have often been related to extermination of the predators that formerly held them in check.
Populations of many animals’ species are limited by the effects of interactions among individuals within a species. The species concerned are described as territorial and comprise a proportion of individuals which persistently or seasonally occupy a particular area to the exclusion of other individuals of the same species.
Commonly, territorial animals will tolerate or encourage the presence of certain other members of their species a mate or mates, their juvenile offspring, and sometimes the members of a larger social group, a herd, troop, flock, or pack. However, they will drive off or otherwise exclude animals that do not belong to the tolerated category.
Territorialism tends to limit the number of animals of any one species that occupy an area and to reserve for the territorial individual or group an adequate habitat and food supply, however, there are many kinds and degrees of territorial behaviour, and not all of them serve directly to limit population increase.
Plants might also be considered to compete for ‘territories’ the well-established individual holds its ground against competitors. Their territorial defence mechanisms include the shading out of competitors and the secretion of chemical which prevent rival from germinating or growing.
Numerous studies have now been made of the effect of overcrowding on a variety of animal species under condition where food supply is abundant, and all other essentials (except space) are present is adequate supply.
All of these studies show adverse effect upon individuals in a population where numbers exceed a certain level. These adverse effects are caused by social interactions, and vary from mild behavioural disturbances to serious forms of beahvioural pathology, leading to increased mortality and a decline in birth rate.
(e) Interaction of Factors:
Virtually all limiting factors interact and either reinforce or diminish their mutual effects. Thus, the effects of animal overcrowding may be seen in the limitations imposed by diminished food supply, increased predation, greater mortality from disease and various behavioural disturbances. Similarly among plants, factors of soil, water, air, nutrients, the presence or absence of intense sunlight, all interact to influence the germination or growth of a plant.
Simple solutions to biological problems are, therefore, seldom likely to produce permanent, positive results. Modifying the impact of one limiting factor may simply increase the operation of others.
For example, in an area of scarce water supply, it may seem sensible to make additional water in order to permit animal populations to increase. Yet doing so has, in many observed instances, quickly led to major dies-offs from malnutrition resulting from an insufficient food-supply for the increased population.
Similarly, destruction of predators may cause protected herbivores to die in an epidemic and the elimination of a disease may cause a crash die-off from starvation. It is, therefore, always necessary in management planning to look at the operation of the total ecosystem and consider how all the factors interact.
Failure to recognize these principles has caused many economic developments activities to fail and much money to be wasted.
Other abiotic components are pH, gases and minerals which determine the types of plants and animals found in an area.
Surface behaviour of earth determines environment including wind, rainfall, light and temperature.
Most animals have the colour of their background-muddy in case of elephant and rhino, sandy in camel and lion, green in praying mantis.