opulations in a habitat may show changes in size or distribution. These changes may be due to;

1. Natality (birth rate): Increase in birth rates especially during breeding periods, lead to increase in population size while a decrease in birth rate results in a decrease in the population.
2. Mortality (death rate):This refers to the rate at which organisms die. An increase in death rate leads to a decrease in the population and vice versa.
3. Immigration:This is the movement of organisms from different habitats into a particular habitat. This increases the population size of the habitat being moved into.
4. Emigration:It is the movement of organisms out of a habitat ant it leads to a decrease in the population. Emigration may be caused by scarcity of food, unfavourable conditions, seasonal climate changes or breeding purposes.
5. Availability of food:An abundance of food in a habitat tends to lead to an increase in the population of organisms due to increase in birth rate (reproduction) and influx of organism from other areas  and vice versa.
6. Seasonal climate changes:Adverse climate changes may lead to a decrease in number of organisms due to death or emigration. Favourable conditions leads to an increase in population.
7. Natural disastersg. fire; flood, drought etc. may lead to a decrease in population due to death and emigration.

(others include availability of water, availability of space, war)

Ecological Factors

These are factors in the environment that influence life in the ecosystem. These factors affect the living organisms or cause changes in the habitat (aquatic or terrestrial).

These factors are grouped into two

1. Biotic factors
2. Abiotic factors
3. Biotic factors

The biotic factors are those concerned with the effects of plants and animals on one another in a given habitat e.g. competition, predation parasitism, etc.

2. Abiotic factors

Abiotic factors include climate topographic (or physiographic) and edaptic (soil factors). Variations in the ecological factors bring about changes in the habitat. Abiotic factors also determine the type of biotic community found in a habitat.

Ecological Factors Affecting Terrestrial Habitats

1. Topographic Factors:These factors are associated with the structure of the habitats e.g. effects of hilts, valleys, plains mountains and rivers. These factors bring about variation in the vegetation and types of animals in an area.

Topographic factors include:

(a) Altitude (elevation): This refers to height of the land above sea level. This affects the growth of plants and the level of erosion in an area. As altitude increases, temperature falls by approximately 1ºc per 150metres, as a result of this, mountain tops are generally cold places cold air causes clouds to condense and fall as rain, thus the annual rainfall on mountains is high especially on the windward side. As one ascends a mountain, the air become less dense, there is less of it to filter the sun’s rays, so organisms at the top of mountains are exposed to intense solar radiation.

(b) Slope: Water flows faster on a steep slope than on a gentle slope as such, run-off  is greater and less water sinks into the soil on steep slopes. Also, erosion tends to be more. Gentle slopes are more favourable to plant growth.

(c) Exposure: This refers to the extent to which living things are not protected from climatic factors such as rainfall, sunshine and wind. Exposure is usually high on mountains and low within a forest. Winds tend to be stronger in exposed habitats and relative humidity is lower than in sheltered habitats. Low degree of exposure ensures the availability of nutrients to plants.

2. Edaphic Factors: These are factors related to the nature of soil particles. The word edaptic refers to the influence of soils on plants and animals. Differences in the soil of a locality usually produce difference in vegetation since plants are dependent on the soil and the type of plants determine the type of animals that will be found in the habitat. Edaphic factors include:

(a) Soil Types: this could be sand, loam or clay. The type of soil determines the fertility of the soil, its porosity and water retaining capacity.

(b) Soil Texture: The amount of sand, silt and clay in a soil affects its water retaining capacity. Soil texture refers to the degree of fineness or coarseness of soil particles. It also affects leaching and erosion.

(c) Soil structure: This refers to the arrangement of the various soil particles in soil. This affects the level of soil aeration and percolation and the type and level of soil organisms in the soil.

(d) Soil pH: The pH of soil also affects the type of plants in the habitat e.g. some plants grow best in acid soils while others prefer alkaline conditions.

3. Atmospheric factors (Relative Humility): This is a measure of the amount of moisture in the air. It affects the rate of transpiration from plants and evaporation from animals. As relative humidity falls, evaporation and transpiration rise as such organisms that live in areas where humidity is low must prevent water loss from their body surfaces. E.g. in deserts, the leaves of plants like the cactus are reduced to spines to prevent loss of water.

Ecological Factors That Affect Aquatic Habitats

1. Salinity: This refers to the concentration of salts in the water. Salinity affects the movement of water and salts across the body tissues of aquatic organisms. Salinity is low in fresh water, high in sea water and moderate in brackish water. Aquatic organisms have to maintain the osmotic balance between their body fluids and their aquatic surroundings in order to survive. Those living in fresh water have adaptive features which enable them get rid of excess water that enters their bodies; those living in sea water have body fluids with almost the same salt concentration as the sea water while those living in brackish water have body tissues that can tolerate wide and sudden fluctuations in salt concentration of their body fluids.
2. Depth Of Water: As a body of water becomes deeper, the amount of light and dissolved oxygen become less, so at the bottom of deep lakes and oceans, there may be too little light for photosynthesis as such no green plants can grow there. Shallow bodies of water such as ponds are usually well supplied with oxygen and light and support a lot of plants and animals. However, these habitats are subject to evaporation and drying up in the dry seasons, the plants and animals therefore have to develop adaptation to survive such conditions. E.g. formation of cysts by some protozoans.
3. Turbidity: This refers to cloudiness of water. It is caused as a result of suspended materials in water. Light penetration is low in cloudy or muddy water and this hinders green plants from growing at some depths.
4. Dissolved Gases: This refers to dissolved oxygen. Oxygen concentration of water decreases with depth. Oxygen is required by most aquatic organisms for respiration as such organisms which live in stagnant or very deep water have to be able to tolerate low levels of oxygen concentration. Organisms that require high oxygen concentration, usually live near the surface of deep water or in fast-flowing rivers and streams e.g. the simulium larva lives in fast flowing streams.
5. Tides and Wave Action:Tidal movement refers to the regular rise and fall in the level of the sea. Organisms which live in the intertidal zone of a seashore have to be able to tolerate being alternately covered by sea-water and then exposed to air twice daily.

Wave action is also important both in the intertidal and splash zones of the seashore. Most organisms in these areas are attached to the substratum or live in burrows. Some attach themselves firmly to rocks and other immovable objects, while some others have hard body covering to prevent evaporation of water from their bodies. Waves cause the aeration of the surface waters of the open sea, thus enabling aquatic organisms to have sufficient supply of dissolved gases for their needs.

6. Speed of Flow (currents):Plants and animals are affected by the rate at which the water flow. Some organism’s e.g. spirogyra prefer to live in slow moving stagnant water while others e.g. Tilapia, prefer fast-flowing water. Many organisms which live in fast-flowing rivers and streams have adaptations which serve to prevent them from being swept away from their support by currents in water. Water currents increase aeration and the turbidity of the water. Currents also carry warm water to colder regions and this affects the distribution of organisms.
7. Density: Density of water varies with the type of habitat. The density of fresh water is about 1.00 while that of sea water is 1.028 at atmospheric pressure and 0ºc. It is easier to move through air than water because water is more dense, as such aquatic organisms have a streamlined shape to help them move easily through water. Some organisms that float on the surface are sensitive to changes in density e.g. eggs of aquatic organisms sink to different depths depending on the density of the water.

Ecological Factors Common To All Habitats

The ecological factors that affect both the terrestrial and aquatic habitats are mainly climatic e.g. temperature, rainfall, relative humidity, wind, high intensity hydrogen ion concentration (pH) and pressure. Of these factors temperature and rainfall determine the type of vegetation in a region.

1. Temperature: This refers to degree of hotness or coldness. Variation in temperature results in hot or cold climate. It affects the terrestrial habitat more than the aquatic habitat. In the terrestrial habitat temperature varies with season, while in the aquatic habitat it decreases with depth.

A rise in temperature usually results in a higher rate of transpiration in plants and higher rate of metabolism in most animals (except homoiotherms). Most living organisms are killed by high temperatures and it reduces the performance of some. Low temperatures lead to inactivity or dormancy. In some organisms (e.g. tadpoles, insect larvae and bacteria) a rise in temperature results in faster rate of growth and shorter length of life-cycle. A higher rate of evaporation of water from the soil, ponds and lakes and a lower relative humidity are also observed.

Too high or too low temperature inhibits the growth and life activities of living things. However most organisms have various adaptive features that allow them to live at low or high temperatures e.g. Bears living in the arctic regions have very thick furs.

2. Rainfall: Rain is the main source of water to most organisms. It also supplies water to soil on which land plants depends. It is also the major source of water in rivers, ponds, lakes, oceans etc. The amount of rainfall in an area has a major effect on the type of vegetation found there. Low amount of rainfall usually causes drought on land and drying up of freshwater habitats (which leads to death of animals). Too much rain causes floods and destruction of vegetation through erosion.

Rainfall increases relative humidity and also increases turbidity of streams, rivers and lakes. Rainfall is necessary for seed germination. It helps to dissolve nutrients in the soil thus making them available to plants. It is also necessary for the vegetative growth of most crops e.g. flowering and proper development of groundnut pods.

Rain water may form puddles and small pools which provide temporary habitats for mosquito larvae, algae and tadpoles. It is also necessary for the start of new termite colonies.

3. Light: Light is necessary for photosynthesis in green plants. It affects the productivity of crops and facilitates flowering and fruiting in some plants. Light is the ultimate source of energy for all organisms. Light affects the activities of animals e.g. some animals are active during the day (butterfly) while others are active at night (cockroaches). The ultra-violet rays of the sun enable animals to manufacture vitamin D.
4. Wind: Winds are important because they cause water currents and waves thus mixing water and making food available in aquatic habitat, Winds carry rain bearing clouds. They also determine a season e.g. In Nigeria, the S/W wind is responsible for the rainy season while the N/E wind brings the harmattan. Wind has drying effects (on land) and so it increases the rate of transpiration in plants. In an area exposed to strong winds only xerophytes can grow there. Winds also aid pollination of flowers and dispersal of seeds and fruits. Winds increase the rate of evaporation from the soil and in savanna and desert areas it is a major cause of soil erosion. Winds also play an important role in the establishment of insects in a given area.
5. Pressure: Atmospheric pressure decreases from the depths of the ocean upwards to the higher attitude of the atmosphere. Plants and animals have special adaptations to a particular level of pressure to enable them survive. For instance in the oceans, the pressure increases by 1.03kg/m² every 10m, so organisms found at depths of about 400m live in conditions of enormous pressure but are well adapted to such conditions and will not survive at levels with lower pressure.
6. Hydrogen ion Concentration (pH):This refers to the acidity or alkalinity of the soil or water in a habitat. pH affects the types of plants and animals in a habitat. E.g. some plants grow best in acidic conditions while others can only grow in alkaline conditions. In aquatic habitats pH varies with the salinity of the water. Freshwater is neutral while sea water is fairly alkaline (pH 8.5). Organisms like the freshwater mollusks (Mytilus) are usually absent in water with a pH less than 6 (i.e. acidic water).

Biotic Factors Affecting the Ecosystem

Biotic factors refer to the effects of plants and animals on themselves or one another. The biotic factors include:

1. Parasitism:One organism called the parasite lives in or on another organism called the host. The parasite benefits while the host suffers harm or may die.
2. Competition:This may occur between organisms of the same species or different species. Competition may be for food, space, mates, etc. One of the organisms will eventually over come the other.
3. Commensalism:This involves two organisms living together. One of the organisms (the commensal) benefits from the association while the other organism neither benefits nor is harmed.
4. Predation: This involves an organism (called the predator), killing / feeding on another organism (the prey).
5. Trampling: Grazing animals trample on plants and invertebrates.
6. Pollinationof flowers by insects (this aids continuity and increase).
7. Aeration of the soilby some animals e.g. earthworms, termites, etc.
8. Support provided to climbing plantsby trees or bigger plants.
9. Shade provided by trees, etc.