BIO 312: GENERAL ECOLOGY

The first phase of this lecture will be concerned with the Ecology of Local Terrestrial Habitats. This will be approached by examining the climatic, edaphic and physiographic factors as well as the fauna and flora of local terrestrial habitats. In addition we shall also look at the primary and secondary successions, terrestrial pollution, conservation of forestry and wildlife resources.

In our first and second years of studying ecology, we were concerned with the introductory ecology of terrestrial communities especially plants vis-à-vis their requirements which guarantee their survival. In the course of this lecture, we shall see how these requirements are utilized to and as such elucidate the factors which control vegetations. Since rainfall supplies are one of the major requirements, it follows that this factor will be of prime importance in the determination of vegetation types. Similarly, temperature influence plant growth rate and has a direct effect upon relative humidity of the micro-environments of these vegetations when taken into consideration with rainfall.

Climate of Tropical Vegetations ofAfrica

Crop growth and production depend on the genetic constitution of the plant, soil characteristics and the climate in which the plant grows. Climate in this case refers to the aggregate of all the external conditions as well as how the influence the life and development of organisms. Climate has an important influence on the nature of natural vegetations, characteristics of the soil, crops that can be grown and type of farming that can be practiced in any region.

The overriding climatic factor in any ecozone is temperature,and it is by this that the world is divided into its major climatic zones; of which the tropics are one. Be that as it may, it is important here to note that in the tropical worlds, rainfall is the most important climatic factor.

Elements of Climate

The climate of any region is composed of a great variety of elements and it is very unlikely that two different places can have the same or identical climate. The followings are the major elements of climate:

  • Temperature
  • Rainfall
  • Humidity
  • Wind and
  • Solar radiation/sunshine/light/cloudiness.

Temperature

In our elementary science, we were told that temperature is the degree of hotness or coldness of a body or system. Before considering temperature in details and how it affects other factors, it may be proper for us to understand the various terms which may be used as the lecture progresses. These include:

a)The mean daily temperature:

In theory, the mean daily temperature is the mean of the twenty-four hourly readings in a day.

In practice however, it is normally the mean of the maximum and minimum temperature recorded in a day.

b)The mean monthly temperature:

This is the mean of the daily temperatures in a given month. The mean monthly maximum (or minimum) temperature is similarly the mean of the daily maximum (or minimum) temperature for that month.

c)The mean annual range of temperature:

This is the difference between the mean temperature of the hottest and coldest months.

d)The diurnal temperature range (in any month):

This is the differences between the daily maximum and minimum temperatures for that month.

From the above, temperature is closely connected to sunshine and radiation and more often than not beginners use these terms synonymously. In Africa, with its relatively small annual variations in radiation, there is a small annual range in temperature. About one-third of the continent experiences an annual range of less than 6oC. The least temperature variation (of less than 3oC) is found within 6oN and S of the equator; because as the latitude increases, temperature variation also increases, reaching maximum values in the desert areas near the tropic of Cancer. The widest temperature variation is found in the areas of least rainfall.

In West Africa, the mean daily temperature usually range from 21 – 32oC except when the harmattan is at its peak. Mean monthly maximum temperatures also increase inland from about 32oC near the coast to about 43oC in the north, but the mean monthly minimum temperatures decreases inland from about 21oC to about 10oC. This increase in temperature range away from the coast is the most important feature of temperature distribution.

Each plant and or animal has its own approximate temperature range, that is, its minimum, optimum and maximum temperatures for growth. Although they are subjected to a rather wide range of temperatures, mostplants (crops) and or animals make the best development between 15oC and 32oC. They cease growth or die when the temperature becomes either too low or high.

Rainfall

Rainfall is the most important climatic factor influencing agriculture and plant growth in the tropics, as it has the biggest effects in determining the potential of any given area (in terms of crops which can be grown, farming systems, the sequence and timing of farming operations). It is the main variable affecting plant growth and seasonality. In fact, it is the main difference between tropical and temperate climates. For agriculture, it is not only the total amount of rainfall per annum that is important but also its seasonal distribution, variability, reliability within and between seasons, intensity and rate of infiltration into the soil, and the balance between rainfall and evapotranspiration (that is, loss of water through evaporation and transpiration).

Rainfall variability

Rainfall in the tropics is generally more variable than in the temperate regions. Variability is particularly important in semi-arid regions where drought can cause great harm. In any dry area, a few years of low rainfall can lead to deterioration in vegetation which cannot always be reversed in succeeding years of high rainfall. Consequently, a decrease in protective vegetation cover in dry areas will make the area more susceptible to flooding and erosion when wetter conditions arise, and the deterioration may be accelerated.

Rainfall distribution, reliability and intensity

Average rainfall figures are of little practical value except as a broad generalization to indicate the type of vegetation and cultivation likely to be found. Rainfall distribution and reliability are crucial and significant factors.

Rainfall and vegetation

In West Africa, the vegetation belts show a general correlation with rainfall. Forest trees form closed canopy and continuous layer of grasses is restricted to regions of higher rainfall in the south.

Assignment 1: Draw a map showing the main vegetation zones of West Africa. Note examination Question 1.

Going northwards from the Atlantic Ocean, is the forest, which gives way to the moist savanna (Guinea) where grasses grow tall in rainy season with their trees quite close together. This is followed by the drier Sudan savanna where grasses are shorter and trees more scattered. Immediately beyond the Sudan, is the Sahel savanna where grass tussocks are short and isolated, while tree cover is sparse and low. Finally, the sub-Deserts and Deserts take over where annual rainfall drops below 200 mm. In general, the amount of vegetation which the soil/land supports depends on the rainfall.

Humidity:Exams. Assume figures & temp and set questions to estimate relative humidity

Within the tropics humidity is more important than temperature. Humidity refers to the amount of water vapour in the air. Normally, it is recorded as relative humidity, which is the vapour pressure in the air in terms of percentage necessary to saturate the atmosphere at a particular temperature. In other words, it is the ratio of actual vapour pressure of the atmosphere to its saturated vapour pressure at that temperature; and is usually expressed as a percentage.Given the above, it is important to note that the relative humidity of two different places within the same locality may be the same but the evaporating powers of their atmospheres will be different if their temperatures are different. The lower the relative humidity at a given temperature the more rapidly the air will take up water transpired by the leaves or evaporated from moist soil surfaces. Evapotranspiration increases with increase in temperature and decrease with relative humidity.

Atmospheric humidity varies both with the absolute amount and the seasonal distribution of rainfall, being uniformly high throughout the year in the wet equatorial and wet seasonal climatic regions; and falling to very low levels in places where there is severe dry season. In Nigeria and most parts of Africa, relative humidity shows very large variations. In coastal regions, the months of June to October have monthly means of over 90% while in the North the mean values are close to 20 or 25% from January to April. Relative humidity therefore has considerable effect on evapotranspiration and hence the water requirements of crops.

Wind

Wind influences plant growth in several ways especially in the arid regions which are characterized by frequent and strong winds. In West Africa, there are two opposing surface air masses. Blowing from the South-west is the equatorial maritime air masses, which, because it comes from the ocean, is humid. Blowing from the North-east is the tropical continental air mass or harmattan, which, because it comes from the desert, is dry. Where these two winds meet is a region of climatic instability and referred to as the Inter-Tropical Front (ITF).

Wind affects both soil and plants. The effects on plants are both mechanical and physiological. Mechanically,the sand and dust particles carried by the wind damage plant tissues and can destroy seedlings either by completely covering their leaf tissues thereby obstructing photosynthesis and transpiration which may ultimately lead to the death of the plants. In some cases, heavy wind effects may lead to uprooting and falling down of standing plants. They also cause considerable losses through branch falling, stalk breakage and grain shedding.

The physiological effects of wind induce evapotranspiration, affecting the soil and plant water balance while hot dry winds affect photosynthesis and transpiration by causing the stomata to close. Similarly, this can lead to death of plants in extreme cases.

Wind can have some beneficial effects; moderate winds can enhance photosynthesis by plants through continuous replacement of the carbon dioxide absorbed by the leaf surfaces.

Solar radiation, sunshine, light and cloudiness

Solar radiation is the ultimate source of terrestrial and atmospheric energy, and thus occupies a central position in climatological studies. Solar radiation is very important in agriculture as it is the source of energy used by plants during photosynthesis.

The amount of solar energy received at the earth’s surfaces each day depends upon the intensity of the radiation, which varies with the sun’s elevation, the amount of cloud cover and day length. The sunshine hours (roughly from sun rise to sun set) vary with season and latitude. Seasonal variation is greatest at high latitudes and least at the equator. However, the actual amount of bright sunshine is always much less, due to the high degree of cloudiness in most parts of the tropics.

The duration of light is of major importance to the growth and development of plants. The relative length of the daily light and dark periods (photoperiodism) has profound influence on such processes as flowering, tuber formation and seed germination.

Edaphic factors

Soils are very complex natural formations which make up the surface of the earth. Soil is a collection of individual particles, each with distinctive profilecharacteristics. Soils are crucial to life on earth because to a greatdegree, the quality of the soil determines the nature of plant ecosystemsand the capacity of land to support animal life and society.They are able to provide suitable environment for plant growth. Five main soil regimes influence the growing conditions of plants viz:

  • Moisture
  • Nutrients
  • Temperature
  • Air
  • Minerals (weatherable and biological)

These regimes are closely related and interdependent.

Factors Determining Soil Formation

There are fivemajor factors that control the formations of soils.These factors are parent materials, climate, biota, topography and time.However, in certain situations one of the factors could have haddominant influence in determining the difference among a set of soils,such a set of soils are referred to as lithosequence, climosequence,biosequence, toposequence, or chronosequence as the case may be. Thatis to say the dominating influence is by the parent material(lithosequence), climate (climosequence), biota (biosequence),topography (toposequence) or time (chronosequence).

Parent Materials

The nature of the parent materials profoundly influences soilcharacteristics. For example, a soil might inherit a sandy texture that iscoarse–grained, quartz–rich parent materials such as granite orsandstone. Furthermore, the chemical and mineralogical composition ofparent material also influence the characteristics of soil formed.

Climate

The most influential factor acting on parent material is perhaps climate.It determines the nature and intensity of the weathering that occurs overlarge geographic areas. Precipitation and temperature both affect the rateof chemical, physical and biological processes.

Biota

Activities of living organisms (flora and fauna) potentially enhanceorganic matter, nutrient cycling and aggregate stability. For example,leading of soil mineral and erosion of surface soil could be slowed downby natural vegetative cover. Moreover, animals play significant role insoil – formation processes, change animals like bush rats, moles andsmaller ones like earthworms bore tunnels in soil, thereby enhancingmovement of water and air into the subsurface layers of the soil.

Topography

The elevation, slope and landscape position may either hasten or retardthe work of climate forces. For example, steep slopes generally encouragerapid soil loss by erosion and allow less rainfall to enter the soil beforerunning off.

Time

Soil forming processes take time to show their effects because timeinteracts with the other factors of soil formation.

Classification of soils of tropical Africa

Soil classification system otherwise called soil taxonomy, provides a hierarchicalgrouping of natural soil bodies. In dealing with soil classification, we shall be looking at soilproperties and nomenclature employed in classification. Classificationbased on soil properties lessens the likelihood of controversy over theclassification of a given soil which can occur when scientists deal withsystems based on presumed mechanism of soil formation.

Soil taxonomy is based on observable properties of soils as they are found in nature. Such properties include moisture, temperaturestatus, soil colour, texture and structure of the soil. Chemical andmineralogical properties, such as organic mattercontents, clay,iron and aluminum oxides, silicate clays, salts, pH, percentagebase saturation (cation exchange capacity), and soil depth are otherimportant criteria for classification. Precise measurements are also employed in defining soil horizons.

Although there are many systems soil classification and nomenclature, none is universally accepted.The different systems of soil classification are based on:

  • The empirical or taxonomic approach, using properties such as texture, base content, color etc.
  • The morphological approach, based on profile characteristics as observed in the field.
  • The genetic approach, using certain formative or environmental factors.
  • The integrated approach, involving varying combinations of the above approaches.

There are six categories of classification in soil taxonomy: viz. Order (the highest categories), Suborder, Great Group, Sub-Group, Family, and Series (the most specific category).

Nomenclature in Soil Taxonomy

The nomenclature system is logical and conveys a great deal of information about the nature of soils. The names of the classification units are combinations of syllables, most of which are derived from Latin or Greek and are root words in several modern languages. Since each part of a soil name conveys a concept of soil character or genesis, the name automatically describes the general kind of soil being classified. For instance, soils of the order Aridisols (from the Latin aridus; dry, and solum, soil) are characteristically dry soils in arid regions. With this brief explanation of the nomenclature in soil taxonomy, we will consider the general nature of soils in each of the soil orders.

Soil Order

Each of the world’s soils is assigned to one of 12 orders, largely on thebasis of soil properties that reflect a major course of development. Ageneral knowledge of the 12 soil orders is essential for understandingthe nature and function of soils in different environments. In the cause of this study, we will consider each of the soil orders, beginning with those characterized by little profile development and progress to those withthe most highly weathered profiles.

Alfisols

Alfisols are formed in cool to hot humid areas, but also found in the semiarid tropics and Mediterranean climates. Most often, Alfisols develop under native deciduous forests and grasslands.