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Ecology

Ecology

Overview

Ecology is the study of the relationship of living organism with each other and their nonliving environment. The study of ecology lays a foundation for understanding agriculture, forestry, fisheries, conservation, impact of human activities on ecosystem and how to remedy these impacts.

  1. Components of the environment

Concepts in ecology.

Biosphere

The earth is the only planet on which life is believed to exist.

The part of the earth and its atmosphere inhibited by living organism is called ecosphere / biosphere. The biosphere is subdivided into large areas with different environment conditions called biomes. Large the biome is subdivided into terrestrial biome and aquatic biome.

Terrestrial biomes

The two most important environment variables for life on land are rainfalls and temperature.

Basing on these facts the terrestrial biome is further subdivided into;

 

  1. Tropical rain forests

These are formed whenever, it’s hot [24-280C] and wet throughout the year. The vegetation of a tropical rain forest has a distinctive structure, with very tall tree which project above the rest of the forest canopy, beneath these emergent is a second layer of large tree which, together with emergent make up a continuous canopy. A third tree layer is made up of smaller tree which complete their cycle without ever reaching the canopy. Still nearer the ground are young trees, palms vine and herbs. By the time sunlight reaches the forest floor, most of it has already been intercepted so that it’s dark with relatively sparse vegetation. Many organisms live in the canopy.

The leaves of the tree are tough and impossible for most animals to digest. The most successful leaf-eater in central and tropical south America are sloths.

 

Adaption of the sloth to feed on leaves

–  It eats a large amount of leaves to enable it extract as much food contents as possible.

– has long gut that allows enough time for digestion to complete

– contain mutualistic bacteria that digest cellulose

– Because of low energy content of leaves and necessity to conserve energy; the sloth.

* is a slow-moving animal which rely on camouflage for protection

*its temperature fluctuates, falling several degrees Celsius at night.

 

The soils of tropic rain forest are generally poor probably because decomposition occurs very quickly and any other available nutrient are rapidly taken up by plant or leached.

  1. Temperate deciduous forest.

The north temperature zone lies between tropical of Cancer and artic cycle. The southern temperature zone lies between tropical of Capricorn and Antarctic cycle. In these two zones is where the temperature deciduous forest likely to be found, dominated by oak, hazel, beech

 

The major problem in this biome is that half of the year is winter when the temperature is too low for life. To survive in this condition, organisms have developed the following adaptations.

  1. loss of leaves by the trees in winter to reduce on water loss by guttation.
  2. small animals hibernate
  3. birds migrate to tropics during winter
  4. some animals have accumulated a lot of fast to prevent heat loss in winter
  5. some have short life cycle in of 3-6 months and survive in dormant stage in winter

 

Desert

For example, Sahara Desert in Northern Africa is characteristic by scarce irregular rainfall and sandy soil. To survive in the desert organism have developed the following adaptations.

  1. Small plants and animals have got very short life cycle; For instance, plants may germinate, mature, flower in a few weeks following occasioned rainfall and then survive in form of seeds during the long dry spell.
    1. To overcome shortage of water, the camel; use metabolic water and it’s body is very resistant to dehydration; certain frogs can survive for years without water by burying themselves deep into the sand. When it rains they dig themselves out, mate and lay their eggs in shallow puddle. Here the tadpole grows very quickly, metamorphosing into adults before the puddle disappear.
    1. The camel has got broad feet not to sink in sand.

     

    • Some plant such cactus store water in their fleshly stems.

     

    Tandra

    It occurs in northern Canada and Asia. It is characterized by long winter and very short growing season. For this reason, it contains no tree neither small mammals but plants with very short life cycle and bigger animals with a lot of fats deposits e.g., Reindeer

     

    Aquatic biomes

    Most important variable that affect organism which live in water are

    • Salinity
    • nutrient availability
    • depth of water
    • how permanent is the water body
    • tide strength

     

    Salinity

    Aquatic organisms have got a problem of osmoregulation at different salinities. Consequently, organisms are only adapted differently to this problem in fresh water and marine water. And those that can survive in fresh water cannot survive in marine water and vice versa

     

    Nutrient   availability

    The most important nutrient in water are the nitrates and phosphate. Lake with low phosphate and nitrate (oligotrophic) contain more species than lakes with high levels of nitrates and phosphates (eutrophic lakes)

     

    In eutrophic lakes, the high levels of nitrates and phosphate promotes high growth rates of algae and other photosynthesis organism. This in turn support large number of aerobic bacteria which decompose the dead photosynthetic organisms. However, the aerobic bacteria take up more oxygen from water thus oxygen concentration may fall below that, can support life of big organism.

     

    Depth of water

    Shallow water may not be able to support big animals like whales. Small ponds may dry up which has a profound effect on organism that can only survive in water.

     

    Tide strength

    Distribution of organism on the shores is affected tide strength; very strong tides, it prevents the growth of plant near the lake and big animals in water.

  2. Factor that affect distribution of organism on land.Abiotic factors
    1. Soil

    It provides vital link between the biotic (living) and Abiotic (nonliving components) of the terrestrial ecosystem.

     

    The term soil is applied to a layer of material overlying the rocks of earth crust. A suitable nutrient content and structure essential for successful growth and production in ecosystem.

  3. Soil that contains a mixture of sand, clay, silt. Loams soil a mixture sand and clay soil in equal proportional with humus is the best soil for plant growth.

  • Air and water

Water and air occupy the same space therefore the soil that contain more air will contain less water and vice versa. Aeration is the term used to describe the amount of air in the soil. Drainage is the ability of water to go through the soil. Oxygen is particularly required for respiration of micro organism and plant roots; decomposition of organic matter, germination of seed, root hair formation and growth and water absorption.

 

Soil water is important

  • As a source of water and dissolved mineral salt in plant.
  • Promotes seed germination
  • Excess water (water logging) slows down water absorption (by lowering the respiration of plant roots since it displaced air from the soil) and decay.
  • Water softens soil for easy root penetration

 

  • Organic matter

Dead organic matter is derived from soil organisms and from organism that live above the soil surface. Fungi and bacteria in the soil decompose organic materials to humus. Humus improves the water retention in the soil and therefore the soils containing humus will resist leaching.

 

  • Soil organism.

The soil organism range in size from bacteria and protoctist to fungi, Nematode, insect, worms and a few mammals such as screw:

Role of soil organism

  • Microorganisms such as bacteria and fungi.
  • Promote germination by breaking seed coats
  • Fix nitrogen in the soil
  • Decompose and recycle or organic matter
  • Macro organism such as rodent, termite and worms.
  • Turn the soil and improve aeration and drainage
  • Promote decay by breaking big pieces into small pieces
  • When they die they decay and add humus to the soil
  • Dissolved minerals

The nature of dissolved minerals in the soil depend on the parent rock, organism growing in and above the soil and whether aerobic conditions prevail chalk soil for example are high in calcium carbonate. However, are low in phosphate, nitrogen and iron, again as is the case with oligohophic lakes, species diversity is high. When soils are water logged, ions capable of existing in either oxidized or reduced state are found in their reduced form. Fe 2+ for instance occurs instead of Fe3+ some plants are very sensitive to Fe2+, so the vegetation found on waterlogged soil differs from vegetation in well drain condition

 

  • Soil pH is the acidity or alkalinity of the soil. It usually lies between 3 and 8.

Effect of soil pH

 

  • Acidic soil with pH of less than 4.5 reduce the availability of nitrogen and phosphorus to plant on the other hand,
  • the concentration of toxic Al3+  ions increase as the pH falls. These two factors enable a few plants to grow in acid bogs. Those that do are often carnivorous and this supplies them with extra nutrients.
  • Affect the activity of microorganism which may increase of reduce the rate of decay and nutrient recycling
  • Soil Structure

This is the arrangement of soil particles, it refers to the looseness or compactness of the soil particle.

Its effects are

  • Drainage for loose soil is faster than that of compact soil
  • Erosion for loose soil is faster
  • Aeration is higher in loose soil
  • Soil temperature.
  • High soil temperature increases the rate germination, decay, absorption of water

 

  1. Climate

Climate refers to the predictable long-term pattern of rainfall, temperature and light. Weather is more short term. It may be cold, windy and wet one day and warm, calm and dry the next.

 

  • Water

In general, water is necessary for life. For terrestrial organism annual rain fall is the most important variable. However, it’s predictability and pH are also important organism living in dry places usually have specialized mechanism to reduce water loss.

 

For aquatic organism, the temperature, salinity, oxygen saturation and nutrient content of the water are vital. In addition, the wave action limits the distribution of many aquatic species.

In river the rate of current flow is important. It is noticeable that free- floating aquatic plants are absent from most streams and rivers but wide spread in lakes and ponds

 

Temperature.

Few organisms can grow if the ambient temperature falls outside the range 0 -400C though, remarkably, some thermophilus [heat – loving] bacteria can complete their entire life cycle at temperature in excess of 1000C.

 

Light

Light is need for photosynthesis in plant and vision in animals. Light can vary in its wavelength, intensity and daily duration and these can affect the quality of life.

 

Effects of light on living organism

  1. Photosynthesis in plant
  2. Photoperiodic behavior in plants i.e. influence flowering in plants
  3. Phototropism
  4. Phototaxis
  5. Vision in animals
  6. Migration of animals
  7. Reproduction

Air.

One of the most important component of air is oxygen and few organism can live without it. CO2 is a reagent in photosynthesis

 

Relative humidity

This the measure of the amount of water vapor in air. When it’s low organism lose water through evaporation and therefore face the risk of dehydration, on the other hand, If it’s very hot and humid, a low rate of evaporation may be harmful as evaporation serves to remove heat from an organism and so cool it down.

 

Wind.

  • Occasional stormy wind can flatten trees that are 100 years old.
  • Continuous strong wind can prevent trees from becoming established.
  • Wind serves a useful function in the pollination and seed dispersal in plant
  • Migratory birds may use winds to minimize.

 

Fire

Fire can only burn if organic matter has accumulated. Now days fire is often the result of human carelessness. Natural fires are caused by lightning strike or more rarely by volcanoes

 

Ecological effect of fire

  • kill slow animals & plants
  • some animal migrates
  • encourage soil erosion &leaching
  • release poisonous gases
  • encourages regeneration in some grasses e.g. Cymbopogon
  • destroys or chase away vectors, pests and parasites, e.g. tsetse flies
  • accelerates nitrogen recycling of plastic waste.
  • Enable vision of distant are for prey and predator.
  • Allow growth of fire resistant species which are often not palatable.
  • Destroys humus

 

Topography

By topography is meant the altitude, slope and aspects of a place. As one climbs a mountain, many features of the physical environment change. It may be cold, wetter and windier. The air gets thinner so that oxygen and carbon dioxide become scarcer and more ultraviolet light penetrate. Slope is important because it reduce the chance of becoming waterlogged on very steep slope, soil cannot form, so plant cannot establish themselves.

The aspect of a place is of most important to sessile (immobile) organism. In the northern hemisphere, south facing slope receive more light and heat energy than north- facing one. The reverse is the case in south hemisphere. This influences the distribution of plant in the wild.

Population and niche concept

Population.

A population is group of individuals of the same species in a defined area. The number of individual in a population is called the population size e.g. all Tilapia living in a lake, all fleas on a dog, lion (in a National park). It is important to estimate the population in  a given area,

 

Importance of estimating the population size

  1. To be able to construct food webs and chains, pyramid of numbers, biomass and energy
  2. To understand the existing food relationship within a habitat.
  • To observe population changes with time/seasons so as to understand the way population affect each other at different environmental factors.
  1. To know the population of pests in order to work out control method.
  2. For management purpose i.e. Plan for cropping programs in game and national parks.
  3. To value and conserve biodiversity/ natural reserves/ wild life and national parks.
  • To understand dispersal/spread of organisms
  • To understand the impact of population in the environment.

 

Estimating population size.

  1. Factors to be considered to select a method to be used.
  2. Size of the organism e.g. big organisms such elephants are easily counted directly
  3. Mode of the organism such as
  • motility; plants and sessile or slow-moving organism may be counted directly, while indirect methods may be required for fast- moving organism in large open grass land.
  • Association between organism- some methods such as capture – recapture method requires organism that associate randomly.
  • Hostility hostile organism may require special gadgets for collection and/or counting.

 

  • Size of the area under investigation. For big area, population may be estimated basing on samples
  1. Physical and climatic factors
  2. Climatic condition
  3. Topography
  • Nature of vegetation

 

  1. Method for estimate of population size of large animals living in unconcealed habitats, e.g. Antelopes, Hippopotami, lions etc.

 

  • Direct counting method using a low flying aircraft.

Basic requirements are: an aircraft, survey map of the area and a counter

Procedure

The aircraft is flown on a transect section of the area of a known dimension and the animals in each transect are counted. The aircraft is then flown back along another adjacent transect and counting continues until the whole a rea is covered. Several counts are made and an average is determined. The estimated population in a given unit area is determined.

 

Advantage

  • The method gives a quick estimate of the population of animal in an area.
  • It can also be done concurrently with studies on other aspect of population like feeding habits
  • It reduces the risk of attack from aggressive animals
  • It reduces the risk of counting the same organism more than once
  • There is no disturbance to the environment.

 

Disadvantage

  • It is sophisticated and expensive
  • Aircraft may scare away some animals to be counted
  • It cannot be applied to small animals in concealed habitats such as forest.
  • It is greatly hampered by climate; i.e., cannot work in cloudy or mist climate.
  • Aerial photography

Basic requirement are aircraft and good camera

Procedure

Photographs are taken from a low flying aircraft usually on scale over the whole study area. They are developed and animals counted from the photographs. As in direct counting method, a population density is a given number per unit area

 

Advantages and disadvantages are similar to those for direct count using an aircraft

 

 

  • Drive and count

 

In this method animals are driven by a number of people into a particular spot and counted.

 

Advantage

  • it reduces the chances of counting animals more than once
  • gives accurate results

Disadvantage

  • it can’t be used on aggressive animals
  • it is tedious on fast moving animal
  • it is difficult to apply on animals that do not live in herd

 

  • Strip census

In this method animals are counted a long path while walking or in a vehicle. The population density of an area is determined as the number per unit area (of strips)

Advantage

It gives a quick estimate of the animal population of an animal

It is cheap

 

Disadvantage

  • Some animals such paths and are unavailable for counting
  • People of vehicle may scare away the animals
  • Fist moving animals may be counted more than once

 

  • Direct counting (Census)

Large organisms that are not aggressive and are living in open habitat e.g. buffaloes, Elephants and trees in the forest are counted one by one.

 

Advantage

  • Quick and accurate
  • More than one population can be estimated at the same time
  • Other aspects of an organism ecology, behavior can be taken at the same time.
  • There is minimal disturbance to environment.

 

Disadvantage

  • It may be difficult to count overcrowded acid organism, e.g. flock of weaverbird.
  • Some organisms avoid being seen.
  • Difficult to use on animals that concealed in their habitat.
  • Bad weather may affect visibility and hence the count.
  • Does not take into account the immigrants
  • Tedious

 

  1. Determining the population of small animals

 

  • Quadrat

Suitability: plants, immobile or easily caught animals

This a small area marked out for study. A quadrat flame (1m2) is thrown randomly and the number of organism within a number of quadrats that represent a known fraction of the total area determined to estimate the total number in the whole area by simple proportion multiplication.

 

This method provides a mean of calculating three aspects distribution.

  1. Species   Number of organism per unit area; limitation. It’s time consuming to count each and every individual in a quadrant

 

  1. Species frequency; This is a measure of the probability [chance] of finding a given species with any one throw of a quadrat in a given area. The limitations are quadrat size, plant size and spatial distribution [random, uniform or clumped].

 

  • Specie cover; This is a measure of the proportion of the ground occupied by the species and give an estimate of the area covered by the species as a percentage of the total area. Limitation it’s slow and tedious.
  • Advantage of quadrat method
    • Accurate
    • Enable the comparison of different areas and species
    • It does not have to be completed over a short time
    • It reduces the chances of counting same individual twice
    • It is possible to estimate population of more than on species at the same time

    Disadvantage

    • In practice animals are not randomly distributed therefore random plots may give inconsistent results.
    • Not appropriate for large area
    • Not applicable in water and concealed habitat
    • Time consuming to do well/tedious
    • Causes some level of disturbance to the environment
    • Not suitable for fast moving organism

    Assumptions

    • the quadrats are chosen randomly.
    • The organism do not move from one quadrat to another
    • The samples taken are representative of the population as whole
    • The population is uniformly distributed

    Reliability of the quadrat method depend on

    • The population of each quadrat must be known exactly
    • The area of each quadrat should be the same shape
    • The quadrate size must be appropriate for the organisms being sampled.

     

     

    • Capture – recapture or mark-release methods. Here a sample of individual is caught, counted and marked in some ways. Then these individuals are released.

     

    After being allowed to mix with the rest of un marked population a second sample is caught and the number of marked individual noted. An estimate of population size can then be made from.

     

    Where

    n1   is the number of individuals marked and released

    n2   is the number of individual caught in a second sample

    nm   is the number of marked individual caught in a second sample

     

    Precautions for capture-recapture methods

    1. Organism should mix uniformly within the population.
    2. Sufficient time must elapse between capture and recapture to allow uniform mixing.
    3. There must be no emigration and immigration to the specified area.
    4. Making does not hinder movement of organism or make the conspicuous to predators.
    5. Marking should be permanent.

     

    • Removal method

    The removal method is very suitable for estimating number of small organism particularly insects, within a known area of grass land or volume of water. Using a net in some form of standard sweep; the number animals is recorded and the animal kept. This procedure is repeated a further three times and gradually reducing number recorded. A graph is plotted of number of animal captured per sample against the previous cumulative number of animal captured. By extra plotting the time of the graph to the point at which no further animal would be captured (that is the number in sample = 0) The total population may be estimated, e.g.

Population growth and growth curves

Populations grow and decline in characteristic ways. The size of population increase will be determined by the reproductive potential of the organism concerned and by environmental resistance. A Plot of the number of individuals against time form a growth curve.

Two basic forms of growth curves can be identified by the J-shaped growth curve and the S- shaped or sigmoid growth curves.

The S- shaped or sigmoid growth describes a situation in a new environment:

  • initially the population density of an organism increases slowly because there is a shortage of reproducing individual which may be widely dispersed as it adapts to new conditions and establishes itself;
  • then increases rapidly, approaching an exponential growth rate. During this growth phase there are optimal environmental conditions- No environmental resistance, birth rate exceeds death rate.
  • It then shows a declining rate of increase until a zero-population growth rate is achieved where the reproduction (natality) equals rate of death (mortality), The declining rate of increase reflects increasing environmental resistance, which become proportionately more important at higher population densities. In other words, as the number increase, the competition for essential resource such as food or nesting materials, increases until eventually feedback in terms of increase mortality and reproduction failure [fewer mating, stress induced abortion] reduces population growth to zero with natality and mortality in approximate equilibrium.

 

The J-shaped growth curve describes a situation in which, after initial establish phase (lag phase) population growth continues in an exponential form until stopped abruptly, as environmental resistance becomes suddenly effective. In very general terms the J- shaped growth form may be considered an incomplete sigmoid curve where a sudden limiting effect comes into play before the self- limiting effect within the population assume importance.

The maximum population of an organism that a particular environment can sustain is termed the carrying capacity. This is identified theoretically as the k-value [or upper asymptote] of the sigmoid curve

 

Factors that affect the size of the population

The number of individuals in a population is affected by four factors; birth, deaths, immigration and emigrations. The change in the size of any population over a period of time can be summed by the equation. Change in the population size = B+I- D- E where

B=birth, I= immigration, D=death and E=emigration.

Environmental resistance

The form that the environmental resistance takes depends on the species in question.

Here are the main factors that limit population growth

  • lack of food or water
  • lack of light
  • lack of oxygen
  • predator and parasites
  • Disease
  • lack of shelter
  • Accumulation of toxic waste e.g. CO2 and nitrogenous waste
  • stress in some case overcrowding may excess stress leading to abnormal behaviors. E.g., Female rats kept in capacity at a high population density show a breakdown in normal maternal behavior, failing to build adequate next and abandoning their young ones.

 

It has even been argued by some biologist that in the wild the males of some species of small mammal die prematurely as a result of excessive stress.

 

-Weather and catastrophes. Weather conditions and generally, may drastically reduce population. The effect is perhaps most severe for small organism, but in particularly bad winter even large species may show significant decline in population size

 

-Predator -prey relationship; The population of the prey is usually high when that of the predator is low. That of the predator increase with the population of the prey [which provide food]

 

Survivorship curves

Ignoring for the moment immigration and emigration, birth and death are the two processes which affects population size. However, these processes depend on the age of individuals and on their sex. The crucial factor is the chance of an individual has of surviving to a given age, This can be shown by means of survivorship curve, To understand a survivorship curve, image a population of 100 individuals borne at the same time. The curve shows how many of them are likely to be alive at any particular age. There are three main types of survivorship curve and these are shown below;

Curve (II) is found in many small birds, notice that as the vertical axis is logarithmic, the curve actually shows an exponential decline in the number of individual surviving overtime. The individuals do not die of old that is, there is no senescence.

 

Curve III The lower curve is typical of many plants and fish. Thousands or millions of young are produced, few of which mature into adults. The vast majority die as juveniles.

 

The niche concepts

In 1934 a biologist called G.F. Gause published a book called the struggle for existence. In it he included the results of careful experiments he had carried out on two species of paramecium, a unicellular ciliate. Gause found that he could easily keep paramecium caudatum and p. Aurelia. in separate containers in his laboratory. However, when he tried to keep the two species in the same container, P. caudatum always died out, after a few days. Gause’s results were put into a general statement called the competitive exclusion principle. This state that two species cannot co-exist unless there are significant differences in their ecologies. In the case of Gause’s two species of paramecium, their ecologies are similar.

 

Nowadays Gauss’s competitive exclusion principle is sometime stated as follows; ‘Each species has its own unique niche’. The niche of an organism is what an organism does in its community. Niches are sometime distinguished from habitats [spatial address] because the niche of an organism describes it’s the location and function [address and profession with a particular community or ecosystem. There is a notation of fundamental niche of an organism as opposed to its realized niche. The fundamental niche is the niche a species would occupy in the absence of any competitors, predator or parasite. The niche that the species actually occupies on the other hand is called it’s realized niche.

 

Association between species

There are many situations in which organism form close associate with one another. Such association may occur with the same species [intraspecific associate] or between different species interspecific influenced by the presence of the other.

Intraspecific association, such as communication, mating behavior, form the basis of social organization.

 

Biotic environment

Physical environment of an organism was defined as the sum of the non-living factor that influence that organism- factor such as climate, soil and topography. The biotic environment of an organism, on the other hand, is the sum of living factor that influence the organism

 

Consider a female goat. Her biotic environment includes other goats with which she interacts, such as her offspring, any male which she spends her time. These individuals are all members of her species. However, her biotic environment also includes the plants on which she feeds the mites, tick, flies and other parasites that bother, and the soil organism that will decompose her body when she dies.

Intimate associations

Intimate associations are found when the body of one organism called the host, acts as the habitat for another organism. The host may be habitat in or on which the organism can survive or reproduce, in which case the association is described as obligatory. If the host is not essential, the association is described as facultative. The general term used to describe intimate association between pairs of species is symbiosis which loosely translated from the Greek means ‘living together’. Some older books restrict the terms symbiosis to cases where the association benefits both species. However, most ecologist nowadays use symbiosis in its wider sense and divide the term into three categories distinguished by the consequence of the association to each party.

 

Parasitism;

A parasite is an organism that lives on or in another organism from which feed. A parasite which lives on the surface of its host is called an ecto-parasite and that lives inside it’s called an endo parasite. Most species, including human, harbor parasites that reduce their health and may cause death.

 

Challenges faced by a parasite

  • Locating a new host
  • Overcoming host rejection
  • Entering a host

 

Parasitic adaptations

Parasite show many different adaptations of overcoming these challenges, depending on whether they are ecto-parasites or endo-parasites.

 

  1. Many endoparasites show degeneration, or even total loss of certain organs which reduces their energy and material requirements and hence a reduced burden on their host. For example, gut parasites like the tapeworms lack an alimentary canal.

 

  1. Many parasites especially ecto-parasites have attachment devices such as sucker, hooks or anchors enabling them to cling to the host. Tapeworm has hooks and suckers to anchor on host digestive canal.

 

  1. Some parasites have penetrative devices for gaining entrance into the host and its cells. For example, miracidum larva of the liver fluke, has a slender tip on to which open a group of glands which secrete tissue- digesting enzymes. By softening the tissue, the enzyme enables the larva to bore into the foot of a freshwater snail the intermediate host.

 

  1. Gut parasite live in a particularly hazardous environment. They typically have protective device which protects their being harmed by the host’s digestive processes. These devices include the possession of a thick protective cuticle, the secretion of large quantities of mucus and the production of inhibitory substances which locally inactivate the host’s digestive enzymes.
  2. To protect themselves from the host’s immune system, some parasites such as the blood fluke, schistosoma, that cause, bilharzia, synthesizes chemicals, which switch of the host’s immune system; The parasite coat’s itself with molecules which the host recognizes as self.
    1. Parasite overcome a problem of moving from one host to another by a number of strategies, one of which is to wait until the host mates. The various organism responsible for sexually transmitted diseases in human spread in the same manner

     

    1. Many parasites employ a secondary or intermediate host which conveys the parasite from one primary host to another. Thus, the Anopheles mosquito transfers the malaria parasites from one person to another.

     

    1. To raise the probability of success vast number of offspring are produced

     

    1. The parasites may have a dormant resistant stage in its life cycle to survive adverse conditions until a suitable host is found.

     

    1. Some parasites are closely linked with their host that their tissue are actually interconnected.

    E.g., certain plant parasites such as mistletoes plug into other plants and tap off nutrients from the host’s tissue.

     

    Commensalism

    This is a relationship between two organisms. One of the two organisms, the commensal benefits from the association, while the other organism usually the larger partner, neither lose nor gain.

     

    Mutualism; Here the association benefit both the participant, i.e., the gain is mutual. E.g. lichen is an association between a fungus and an alga. The fungus absorbs water from atmosphere while an alga photosynthesizes for both.

     

    Predation

    In this relationship in which one organism lives the other dies instantly. For example lion the predator kills the zebra (prey) instantly.

     

    Communities and ecosystems

    On this plant no one can exist without another, Human depend on one another and the surroundings. Without plants animals would starve through lack of food and in the long would run out of oxygen. Without bacteria and fungi, decomposition and nutrient cycling would stop. In absence of animals, many plants would be unable to reproduce.

     

    Communities

    This a sum of all living organism found in a specified. Species within a community interact with each other, sometimes these interactions benefit both species. For example. The fruit produced by the plant supply birds with food, however, the birds disperse the seeds of these fruit which benefits the plant.

     

    Ecosystem

    An ecosystem is a self-sustaining unit consisting interacting organisms in area together with the non-living constituents of their environment. Example an oak wood ecosystem consists of livingorganisms such as trees and animals and the physical environment such as rain, the inorganic components of soil, sunlight and atmospheric oxygen and carbon dioxide.

     

    Succession

    This is the change of community overtime, that is replacement of some species by other through time.

     

    Primary succession

    begins with a bare rock where there is not form of organic matter. Usually the first organism (pioneer community) to colonize a bare rock is the lichen followed by mosses and ferns, big plants and animals as organic matter accumulate. It must be noted that the first organisms to colonize a bare rock must be photosynthetic.

     

    Secondary succession

    Occurs when the surface in completely or largely stripped of vegetation but has already been influenced by living organism and has an organic component. For example, a cleared forest or a previous burned or farmed area. Seeds and pores and organs of vegetative reproduction, such as rhizomes, may be present in the ground to influence the succession.

     

    The climax community is often described as having one dominant [those with the greatest collective biomass or productivity] or several co-dominant species

     

    Zonation

    This is the spatial distribution of species with a community according to variations of physical environment. An example of zonation is the vertical zonation that occurs on mountains with increasing altitude superficially zonation may resemble succession, but it is important to recognize the basic differences, names that with zonation the species vary in space[spatially], whereas with succession the species vary in time [temporally]

     

    Productivity and biomass

    1. Gross primary production is the total organic material made by photosynthesis in a specified time s known as the.
    2. Net productivity is amount of organic material produced by synthetic organisms actually available to the herbivores since part of gross primary production is respired or decomposed transferred to herbivore or carried downstream.

     

    i.e., Net primary productivity =  Gross primary   –   respired organic matter+ organic matter

    productivity             decomposed

     

    The percentage of the energy at one trophic level which ends up in the next trophic level is called the trophic efficiency. The trophic efficiency of the herbivores, for instance, equals the percentage of the net primary production that is converted to the herbivore production [i.e. to growth and reproduction of herbivores

     

    Lindeman in 1942 proposed that succession involved increase productivity until a climax community was reached in which the maximum effience of energy conversion occurred.

  3. Evidence shows that the latter stages of succession do not become productive, but that there is usually a decline in the gross productivity associated with the climax community. This older forest which in turn may have lower productivity than young forest, which in turn have lower productivity than the more species- rich herb area.Reason for decrease in production can only be speculated upon but using a forest as an example, older trees might be expected to be less productive than younger trees for several reasons. One is that the accumulation of nutrients in the increasing standing crop biomass may lead to a reduction in nutrient recycling. However, a simple reduction in vigor as the average age of the individuals in the community increases to a constant point would presumably cause a reduction in productivity.

     

    Succession also lead to maximum accumulation of biomass. This is most obvious in the case of forest community, where the plants become larger and larger during succession, but the accumulated biomass of other climax communities is also normally greater than in the successional stages. Changes in gross productivity, respiration and biomass during a typical succession and summarized on the graph below.

Changes in gross productivity, respiration and biomass during a typical succession

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