We are now familiar that how a population of an organism can experience an exponential growth increase till the environment imposes a limit at the carrying capacity level. Different other factors come into play to control or regulate the populations of organisms. A few of these regulatory factors will be studied below.
Over and over again a population occupies a new area where space and food are abundant will go through exponential growth to start with. Soon though, the external constraints of the environment affect the rate of increase which starts to decline. Ultimately, in most cases, the population stabilizes. The number in the population at the stabilized state is termed as the carrying capacity of the environment. The carrying capacity has obliged a check or regulation on further growth of the population. The other ways of population regulation will be described as the chapter carry on.
Types of Population Regulation:
Population regulation can easily be classified into density dependent factors and density independent factors. Factors which are dependent on the size of a population to control or regulate the growth of the population are stated to be Density-Dependent. Factors which operate in spite of the population size, like weather or physical disruption are stated to be Density-Independent.
Density dependent factors in general are those that encompass an increasing effect in population as population size rises. As the population grows, the individuals in the population compete by increasing intensity for limited resources. Such resources comprise space, water and nutrients. The more individuals there are competing for the resources, the higher the mortality will be in the population.
Space is the most fundamental necessity of an organism. As most of the animals are not motionless, competition for space might come beneath territory defense. For plants, space is significant especially for plant leaves to catch the sunlight. Apart from a plant is isolated, at a point in growth the plant begins to interfere with or will be interfered with by neighboring plants of the similar species or different species. Therefore, a mature plant will probably have displaced some others to stunted growth or death. Illustrations can be found in some plantations. As seedlings get bigger, they start to compete for space. A few plants are out-competed and die as others take over their space. As such, the density of plants fall in a procedure termed as self-thinning.
2) Food and Water:
Both animals and plants need energy, nutrients and water to survive, grow and reproduce. Food for photosynthetic plants signifies mineral nutrients, carbon-dioxide and the presence of adequate light to synthesize organic molecules.
We are familiar that plants, when clustered altogether, their leaves struggle for space to catch the light and fix energy from the sun. The fundamental nutrients for plant growth especially, potassium, nitrogen and phosphorus are frequently in short supply in soils. Soil nutrient content might limit population size of specific plant species. However, not to be overgrown by the bush foliage of the bigger plants that grow well in the fertile soil, a few low-growing plants favor growing in poor soil. Invariably if plants cluster altogether for water and nutrient, a few outgrow others to bring about stunted growth or death.
Food and water have direct consequence on the health of animals and on their reproductive potential. The lack of adequate food to maximize the reproductive potential might be the most significant regulator of population size in animals. Therefore, where the favored source of food of an organism is available in plenty the population of the organism rises. At a period of shortage, it falls. The life style of some herbivores in temperate areas is influenced by this situation. In summer and autumn food abound as young shoots and fruits however, come winter, foliage becomes infrequent and of low quality. Herbivores then modify their life style to migrate, hibernate or live as best they can, often by considerable loss of life. At such periods as well, carnivores have difficulties which frequently would lead to higher mortality because of the scarcity of herbivore prey to feed on. Several animals react to overcrowding by hormonal changes. If the migratory locusts (that is, short horned grasshoppers) are crowded, they produce hormones which cause them to enter a migratory stage in which the locusts take off as a swarm and fly long distances to latest habitats.
As plants struggle for space to get adequate light, nutrient and water, animals struggle to get territories. Such a territory is an area which comprises of a resource of sufficient value to an animal that it protects. The animal holds or defends the territory by marking the boundaries in some manner and challenging strangers when they approach too closely.
Most of the species comprising birds, mammals, amphibians, reptiles, fish insects and crustaceans are known to hold territories. Territories can be small or big and can be held by solitary animals' example: tiger or whole social groups of animals such as the meerkats (that is, Suricata suricatta) where some individuals keep watch for danger or strangers and the whole adult troop goes to protect the area when required. Small territories are generally for breeding sites as employed by birds. The size and number of territories in an area influences how many breeding individuals can live in the region and so affects population size. In a territory defended for food, the amount the animal requirements and the amount and quality of the food in the defended area will influence the territory size and control or regulate the population size.
4) Predator-Prey Relation:
Predators evidently influence the population size of their prey by limiting the population. A low population of the prey species generally gives insufficient food for the predators and the predator population reduces. As this takes place, the prey population can recover. In this condition, the predator and prey populations follow a cyclical pattern. Though, if numbers of prey are affected by food availability, then the predator might not be controlling or regulating the population at all. Only if predators are very proficient, or the prey population is slow to reproduce is predation probable to regulate the prey population.
Density Independent Factors:
Density independent factors are such factors like weather, fire, volcanic eruption and other disasters or physical disruption of the habitat which operate despite of population size. These unpredictable factors tend to vary population size in a fairly haphazard manner. A flood, fire, hurricane or volcanic eruption might wipe out numerous species in the vicinity of the disaster. New populations might then be established through immigrants from neighboring populations. A huge meteor that crashed unto earth some 65 million years ago is thought to have caused the annihilation of the dinosaurs and numerous other species at the end of the Cretaceous period. New populations are assumed to have re-established at the affected sites.
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