ECOLOGY OF POPULATION
Ecological succession is defined as the orderly series of changes occuring in plants and animals or microbial communities over a period of time, resulting in the establishment of a final, stable and climax community. In other words, this is the orderly series of changes occuring in plant communities over a given period of time, until a stable/climax community is established. It usually commences in a newly formed open/barren habitat/abandoned farmland, which is usually asa result of human activities/natural disasters/catastrophies. The pioneer/primary colonizers have short life cycles’ span, which get replaced by other plants which thus derive nutrients from their remains. These latter secondary colonizers have longer life cycles/span. The initial pace of a succession is rapid but it gradually slows down until a final/ stable/climax community is reached.
A climax community in ecological succession is established when a stable or unchanging community is attained or established, thus the community is in equilibrium with the environment.In climax community, the vegetation reaches its highest development and the same species of plants and animals re-occur from year to year.
General Characteristics of Succession
The following are the characteristics of succession:
Characteristics of ecological succession of plants:
- Ecological succession of plants takes place in a newly formed open habitat, barren area or land.
- Successional changes are orderly and they develop in a certain sequence.
- Pioneer or earliest colonisers are hardy with short life cycle and are replaced by small herbaceous plants.
- The rate of successional change is rapid initially but, gradually, it slows down until the final climax or stable community is reached.
- The number of serial communities depends on minor differences in edaphic, topographic or other local conditions.
- Succession occurs over a period of time.
- The earliest colonisers are usually very few species.
- During their life, the pioneers make extensive use of the material in their environment to a point of depletion.
- Death of the pioneer communities adds some organic matter to the soil environment,and favours the establishment of new species.
Outcome of Succession
The following are the outcome or results of succession:
- Replacement by complex organisms: Simpler organisms which start the succession are usually replaced by more complex ones in an evolutionary trend sequence.
- Changes in the physical environment: Changes in the physical environment is attained from the structural changes of the species and the activities in the community.
- Fast replacement: As some species make conditions favourable for others,they in turn create unfavourable conditions for themselves, thereby increasing the process of replacement.
- Attainment of equilibrium point: Equilibrium point is attained through colonization of the abandoned farmland by a wide variety of organisms that are just the same with a neighbouring habitat.
- Climax community form the final stage: The final outcome of succession is the climax or stable community.
Types or Forms of Succession
There are two major types of succession. These are primary succession and secondary succession.
Primary succession starts on bare surfaces which have not previously borne vegetation. It usually starts with lower organisms and takes a longer time to reach a climax community. Examples of primary succession are found in ponds, vegetation on rocks, mangrove forest, estuary, etc.
Factors which can give rise to primary bare surfaces include:
- Deposition of sand dunes.
- Volcanic ash.
- Emergence of bare surfaces.
- Extreme drought.
- Mining or construction activities.
Secondary succession starts on already colonized surface. It may start with fairly complex organisms and it takes a shorter time or duration to reach a climax community. Examples of secondary succession are found in abandoned farmlands and grasslands.
Differences Between Primary and Secondary Succession
- It start on a bare surface.
- It is slower or it takes longer time to reach a climax
- It starts with lower organisms.
It starts on already colonized surface.
It is faster or it takes a shorter time to reach a climax community.
It starts with fairly complex organisms.
Major Communities in a Plant Succession
There are three major communities in a plant succession. These include:
- Pioneers or early colonizers: Pioneers or early colonizers include the first group of plants in a succession. They are small, lower plants and highly drought-resistant organisms. They require minimum space, substratum or nutrients. They are highly adapted to growing on bare surfaces, e.g. Algae, lichens, mosses and liverworts.
- Developmental, intermediate or transitory communities: These are series of communities which follow the pioneers with each succeeding communities more complex than the one it replaced. They are less opened than the pioneers. They have more diverse species with greater numbers of individuals per species. Examples are herbivorous plants such as Talinum, Ageratum, Imperata cylindrical, Cyanotus and Sida acuta.
- Climax community: This is the terminal plant succession or stable community which establishes equilibrium with ecological conditions within the locality. They are the most complex in the series of communities in a succession. It has the greatest species diversity and greater species population diversity. It has bigger life forms, e.g. trees and shrubs.
Process of Succession in Estuary (An Example of Primary Succession)
The process of succession can be observed or studied in lakes, ponds or estuary. An estuary is the mouth of a river where it enters the ocean or sea. The estuary has a high salt content and flooding due to tides that take place. The first group of plants to colonize this salty and muddy habitat are the red mangroves. The red mangroves produce seedlings which germinate on the parent plant before dropping in the mud. They continue to grow in it resulting in the production of stilt roots. These roots fix them firmly into the mud. The banks of the river mouth are raised with the accumulation of plants and animal remains and the mangrove frees develop strong roots.
After this stage, the white mangrove trees replace the red mangroves. These white mangroves grow on the more stable and higher ground. The white mangrove trees now form open forest with some herbs which grow among its pneumatophores. Dead parts of plants and animals accumulate and the white mangroves give way to a collection of different trees, shrubs, herbs, grasses, ferns and epiphytes.
As a result of increased accumulation of the remains of these flora, a normal forest fauna and flora soon emerge to give a stable forest community.
The first group of animals that colonized the mangrove swamp are the flying insects, later followed by water birds which feed on them. Other animals include crabs, snails, worms etc. In the white mangrove and the forest proper, animals commonly found are squirrels, monkeys, rats and birds.
Process of Succession in Abandoned Farmland (An Example of Secondary Succession)
In an abandoned farmland, nutrients remain from the destruction of the old farm. The initial pioneers or colonizers are insect eggs, larvae, worms, seeds of plants, underground stems etc. The second stage of succession will now involve the germination and growth of more grasses, the hatching of eggs of insects and other species. The vegetation can now support more animals such as rats and other rodents, snakes and reptiles. The third stage of succession results in the herbs being displaced by shrubs and increases in the diversity of species.
In the fourth stage, more trees grow and species diversity include birds, snakes, reptiles, monkeys etc which live in different strata of the habitat. The population and activities of detritus feeders now increase and climax vegetation is reached. The animal community will now include herbivores, carnivores and detritus feeders which are involved in a complex food web.
Overcrowding is defined as a situation which occurs when a population in a given habitat increases beyond a point where the resources in the habitat such as space and food are not enough to support all the individuals in the population. In other words, overcrowding occurs where there is an increase in the population of a particular species beyond the carrying capacity of a particular area. This reduces the space available for each individual species in the population. For example, a space designed to accommodate only five rats can be said to be overcrowded if ten or more of the rats have to live in the place.
Population is defined as the total number of organisms of the same species inhabiting a particular area or habitat at a particular point in time. In other words, it refers to the total number of organisms or individuals of the same kinds or species living together within a habitat or area at a particular time.
The size of the population varies due to different factors e.g. natality or mortality, immigration or emigration. In an ecosystem, the community is made up of many populations of different species.
Population density is defined as the number of individual organisms per unit area or volume of the habitat. Mathematically, population density is expressed as:
Total population or population size / Area of habitat
Population density can be used to estimate the total number of individual species of a population or population size. Mathematically, population size is = Population density x Area of habitat.
Factors that May Cause Overcrowding
The following factors may be responsible for overcrowding:
- Increase in birth rate (Natality): When there is an increase in the rate at which a particular species gives birth ina restricted area, overcrowding will definitely take place.
- Increase in food supply: With increase in the supply of food to a particular habitat, there will be a corresponding increase in the population which will later result in overcrowding.
- Decrease in death rate (Mortality): If the rate at which organisms die in a habitat is very low compared to the rate at which they are being given birth to, the population will not reduce, hence overcrowding will take place.
- Immigration: The migration or movement of individuals or species into the habitat will increase the population which will later cause overcrowding in the habitat.
- Lack of dispersal: The fruits or seeds of certain plants fall just under the tree and germinate there. This lack of dispersal causes overcrowding.
- Social habits: Most social animals such as termites, ants and bees which continue to live and multiply in a colony increase the population until the area is overcrowded.
- Inadequate space: This occurs if the space occupied by individuals (plants or animals) is such that when compared to their number is small or inadequate.
- Absence of predators: When predators are absent, the population of some species (prey) can grow to such a level that overcrowding occurs.
Effects of Overcrowding
Overcrowding do have effects on the organisms occupying the habitats. The effects of overcrowding include:
- Shortage of space: As a result of increase in the population of species, there would b lack of space for the organisms.
- Shortage of food: The available food in the habitat is rapidly eaten up due to overcrowding and this eventually results in the shortage of food.
- Competition: Due to increase in the population with limited resources such as food and space, individuals have to compete among themselves to get these scarce resources. The stronger ones get these resources while the weaker ones are deprived of them.
- Anti-social behaviour: Most animals due to stress as a result of overcrowding resort to fighting or cannibalism.
- Spread of diseases: Diseases can easily spread in an overcrowded environment. For instance, tuberculosis in human which is an air-borne disease can easily be spread.
- Preying on each other: Animals in overcrowded environment have the tendency to prey or feed on each other especially when food is in short supply.
- Death of organisms: Death of some weaker organisms may occur in an overcrowded area as a result of nonavailability of food and space. The stronger ones survive while the weaker ones die off.
Adaptations to Avoid Overcrowding
Plants and animals have developed various means to avoid overcrowding. Such means and ways include:
- Territorial behaviour: This is a natural means to avoid overcrowding in animals. Some animals especially mammals, lizards, birds etc establish territories. Territories are resting areas which the animals carve out for themselves. They are usually possessive of these areas. They fight for and defend the territories against intruders or other members of the same species. Their ability to claim the territory successfully ensures that they have sufficient space, food, mating partners and parental care. The establishment of territories ensures that there is no overcrowding in the area.
- Dispersal of seeds and fruits: This is also a natural means of avoiding overcrowding in plants. The dispersal of seeds and fruits either by water, wind, animals, insects, explosive mechanism etc reduces the chances of overcrowding. These seeds and fruits are carried far away from parent plants hence overcrowding is avoided or prevented.
- Emigration: Emigration involves the movement of individuals or animals out of their locality to another place for settlement in a new habitat, Such outward movement of animals, even human beings, prevents overcrowding.
- Swarming: The swarming of certain social animals such as termites and bees occur when some members of an old colony separate and fly out to establish a new colony. Following this swarming, a new colony begins at a new location, hence overcrowding is avoided.
- Production of chemicals by plants: The production of chemicals by roots of some plants sometimes prevent the growth of other plants close to them. This device prevents overcrowding.
- Production of canopies: The production of canopies by tall plants helps them to trap enough sunlight and prevent the plants below from getting the light. These lower plants eventually die off and overcrowding is prevented or avoided.
In all habitats, producers provide the food that support all consumers either directly or indirectly. In a well established habitat, population sizes of the various species are adjusted to the quantity of food that is available in the habitat. Certain factors can, however, decrease the food supply to the habitat causing food shortage.
Causes of Food Shortage
The following factors are responsible for shortage of food in a habitat:
- Overpopulation: An increase in population without a corresponding increase in food supply tends to create food shortage.
- Poor storage facilities: Lack of or inadequate storage facilities to store excess produce do result in losses leading to food shortage.
- Flood: The occurrence of flood in a particular year may result in the destruction of crops and farmlands which can also lead to food shortage.
- Drought: Severe drought can lead to poor agricultural activities resulting in the production of little food. This eventually leads to food shortage.
- Pests: Crops may be attacked by pests, e.g. pests like locusts and aphids, grasshoppers and weevils can attack crops leading to low yield. The poor harvest from such crops can lead to food shortage.
- Diseases: Various diseases of plants and animals reduce production hence food shortage will occur.
- Bush burning: Bush burning also lead to the destruction of useful soil organisms, reduces soil fertility and expose the soil to erosion. These conditions of the soil can lead to poor yield and subsequent food shortage.
- Poor harvest: Poor harvest or low yield of crops and animals due to one problem or the other can result in food shortage.
- Infertility of the soil: Soil infertility due to erosion or bush burning leads to poor yield of crops which can also cause food shortage.
- War: During wartime, attention is not given to food production and this leads to food shortage.
Effects of Food Shortage on the Size of a Population
The scarcity of a particular type of food may create population problems. Such effects of food shortage on the size, of a population include:
- Competition: When food is scarce, there will be scrambling for available food thereby leading to survival of the finest in some cases. The individuals may engage in fighting, biting and cannibalism.
- Emigration: If food shortage lasts for a long time, animals or even human beings may migrate from an area of food shortage to places where sufficient food is available.
- Decline in the rate of reproduction: Food shortage can bring about a decline in the rate of reproduction, for example, human beings tend to avoid marriage when they are not sure of how to feed themselves. Again many engage in family planning so as to raise the number of children which they can easily take care of.
- Scarcity of a particular type of food: This may create population problems especially for herbivores and carnivores which feed directly or indirectly on producers. Absence of producers may lead to the death of these group of organisms. However the omnivores can survive this condition because it can feed on a wide varieties of food materials.