This unit dwells on how to measure size and density and also feedback mechanisms

Total size of a population is the number of individual in the population. The total size is not commonly used because it’s difficult to demarcate boundaries between different population especially ectonal zones. Population density is therefore preferred cause it considers the number of organisms per unit area on volume of the environment.

Density is often related to size with larger organisms being of low density. As well as varying from place to place. Population density also varies in time.

It follows that population may remain constant, fluctuate on they may steadily increase or decrease.

Factors that of interest in as far as population increase or decrease in concerned include:

  1. i) Natality (Birth) These lead to an
  2. ii) Immigration (individuals entering a population increase in pop and density

iii) Matality

  1. iv) Emigration (individuals leaving a population) These lead to a

Decrease in pop

size and density


Population density is population in relation to some units of space. It is usually expressed as the number of individuals on the population biomass per unit area on volume.

A wide variety of attrilates can serve as biomass units ranging from dry weight to NDA or RNA content in a given volume/area. It is important to know whether a population is changing than the know its size at any one moment e.g. the number of birds seen per hour or the numbers of various kinds e.g. the % of sample plots occupied by a species of a plant. The effect that a population exerts on the community and the ecosystem depends on only on the kind of organism indirect but also on how many; e.g. one crown in 100 acres of maize would have little effect on the yield and this would cause the farmer no concern but 1000 crows per 100 acres would be something else.

The figure below shows the range of density reported by common mammals in a conservation are (density is expressed as biomass per hectare)


The population of mammals as a class as reflected by the biomass ranges over 5……of magnitude.

From the figure, the deer, wood chunt and vole which are foliage caters (herbivores) register the highest biomass (quantity if i.e. between 0.9 to 70kg/hectare). This is followed by the seed fruit eaters (ii) the mouse, rat and squired which ranges from 0.9%. This is followed by omnivores, skunk, fox and black bean (0.001 to 1kg/hectare).

The meat eaters register the lowest biomass i.e. the weasel, lynx and congar ranging from 0.0009 to 0.04kg/hectare.

The range for any given spcies in each trophic level is much less compared to the mammals as a class.


The large difference in magnitude regarding biomass for the mammals as a class is possibly due to the fact that these mammals occupy different tropic level.

  • Producers in any ecosystem have adaptations of fixing radiant solar energy into chemical energy in the bonds of the organic cpds formed.
  • Chlorophyll in leaves traps light energy which energy is partly used to build organic molecules such as glucose and starch.
  • The herbivores foliage eaters in the figure obtained greatest amount of energy since they feed directly on the plant. The foliage eaters obtain greater energy that the seed fruit eaters since the leaves and stems provide a larger biomass than the meat eaters since their diet includes plants and possibly thing feed on the herbivores. Energy is lost between the 2 trophic levels and hence a smaller biomass than the herbivores.
  • The meat eaters mainly feed on meat and these receive the lest amount of energy from their prey since the prey lose energy through excretion and respiration. The meat eater does not eat the whole organism.
  •  Energy is lost between successive trophic levels thus excretion and respiration. The nearer the organism is to producers in the food chain; the greater the biomass and because it receives more energy hence the population of organism increase in size.
  • Energy loses during transportation and storage.


This is the spartial pattern of individuals in a population relative to the another. The basic patterns are the;

  • Regular dispersion
  • Random dispersion
  • Clumped dispersion


In regular dispersion, individuals are more or less equidistant from each other. This is rare in nature but common in managed systems where food or free crops are deliberately planted in this manner. This dispersion is also common where competition between individuals is severe.

Population of animals that exhibit territorial behaviour tend towards regular dispersion.



Individuals are aggregated, to a greater or lesser extent. Such dumps may be regular or randomly dispersed. Individuals are aggregated into patches that are interspersed with no’ or a few individuals. Such aggregations result from social interactions such as family groups on certain patches of the environment being more favourable for the individuals developed.

Population growth form

I shaped growth curves; characteristics patterns of increase and these are called population growth forms.

S-shaped growth curves.

With the S-shaped growth form, the population runs out of some resource e.g. food, space on it could be due to fire, frost or diapauses

In sigmoid form the population increases slowly at first (this is the establishment or phase) then population density increases more rapidly or exponentially. It soon shows down gradually as environmental resistances increases until a more or less equilibrium level is reached and maintained.

NB; this is upper level beyond of no major increase can occur is represented by a constant kard it is known as the asymptote of the sigmoid curve i.e


A-B (log phase)

At this point organisms in a population are establishing themselves. The individuals could be in the pre-reproductive stages, increase in widely dispersed (though mature) B-C (expenetial phase)

Population increase is very rapid. Environmental conditions are optimal and therefore favour population growth. Individuals are in the reproductive stages i.e. have matured and have established themselves properly on the substrate, i.e. a bigger portion of the fundamental riches is put to use. Birth rate exceeds death rate. The organisms are acdamatised.

C-D (Deceleration Phase).

At this point, the curve flattens out due to environmental resistance. The environment becomes saturated with this particular species and its full carrying capacity is reached, and it cannot support any more organisms.

Therefore environmental resistance refers to these factors that limit population growth e.g. shortage of food or H20, 02 competitions for these resources intensifies with an increase population.


  1. Competition (intra/interspecific) causes natural selection and the organisms that are less advantaged are eliminated from the population (Gauses exclusion principle).
  2. Lack of light. This is particularly important in the growth of plant populations.
  1. Predators and parasites play a critical part in keeping down populations. (refer to predator prey relationship, parasite relationships).
  2. Lack of shelter. The could refer to shelter from predators, physical factors of the environment e.g. excessive heat which may result in severe determination.
  • Epidemics of cholera, dysentery are mainly due to poor hygiene starting from poor waste management.
  1. Accumulation of toxic wastes. High concern of C02, nitrogenous waste can be a powerful factor limiting the population growth of certain organisms (green house effect; global warming).

Physical factors when the population increase at a certain point, there is reluctance for the number of the population to breed. Overcrowding is usually affected by territoriality. With this type of behaviour, individual acquire and defend their own territory. This ensures that resources are available for its offspring which boasts its chances of survival.

NB, With a S-shaped curve, there is loss action of environmental resistance with this resistance being delayed until near the end of the population increase. It follows that in the S-shaped curve, there may be no equilibrium. With the S-shape curve on the other hand, there is a greater action of detrimental factors such as environmental resistance or factors induced by the organisms themselves.

Therefore such factors are density conditioned.

Some of the above environmental resistance factors are density dependent. Such factors show their effectiveness in controlling the population depending on the density of the population itself. Density dependent action is usually direct in that it intensifies as the upper limit i.e. as the upper limit i.e. as the carrying capacity is approached. These factors are.

HOMEOSTASIS. Greek meaning: “staying the same”

(Homoio = some, stasis- standing)

Is the maintenance of constant internal body environment. Internal environment = intercellular/tissue fluid + cellular environment. The term may be used fluid for any system, biological or non biological at steady state.

Examples of factors to be kept constant.

  1. Chemical constituents e.g. ions like Ca2+
  2. Osmotic pressure
  • Carbondioxide level
  1. Temperature
  2. PH
  • Waste/excretory products e.g. nitrogen

Pathogenic micro-organisms


For homeostasis to occur, a control system/mechanism is required.

(Cybernetics (cybernos = “steers man”)

Is the science of control systems ie self-regulating systems which operate by means of feedback mechanisms.

Basic components of a control system

Reference point (norm/optimal level

  1. Detector
  2. Controller/regulator components of a modulator.
  • Effector
  1. Feedback loop


Reference point (Norm/optimal level.

Is the set level or optimal level of the parameter (controlled variable) at which the system operates.

  1. ii) Detector e.g. receptons

Signals the extent of any deviation from the reference point/norm.

iii) Controller/regulator

Coordinates the information from various detectors and sends out instructions which will correct the deviation e.g. CNS.

  1. iv) Effector

Brings about the necessary change needed o return the system to the reference point/norm.

  1. v) Feedback loop

informs the detector of any change in the system as a result of action by the effector.


The cyoernetic systems that detect and relay back information to a production source, about the level of parameter, so that the system through its effectors adjusts its output accordingly.

A feedback is therefore a circular process which begins with a stimulus and ends with a response the cancels that cancels or intensifies the stimulus.

Components of a feedback mechanism

  1. A stimulus
  2. A recepton which detects the stimulus
  • A processor or interpreter which processes information about the stimulus and compares the information with an internal standard.
  1. An effector which responds to the processed information and feeds back information to the interpreter.
  2. Messengers (nerves and hormones which information from the receptor to the processor and from the processor to the effector.
  3. A response usually involving connection of the in balance to maintain the steady state.

Types of feedback mechanisms

  1. Negative feedback
  2. Positive feedback

Negative feedback

Here a deviation from the norm or set point triggers corrective mechanisms that restore the norm.



In biological system, homeostasis is usually achieved by a negative feedback process.

Examples of negative feedback mechanisms.

  1. Regulation of population of living organisms; where an increase in population above carrying capacity increases environmental resistance; causing death of organisms thus reducing the population.
  2. Regulation of temperature in endotherms.
  • Regulation of sugar / glucose in man
  1. Regulation of gas tension e.g. in co2 blood
  2. Regulation of heart rate and arterial blood pressure
  3. Regulation of hormone, metabolic levels, water and ionic balance. Ph

Positive feed back

here a deviation the norm lead to function deviation from it i.e. if a parameter increase, mechanisms take place that lead to function increase, if it decreases, mechanical take place that lead to increase: if it decreases, mechanisms take place that lead to further decrease from norm.


Positive feedback mechanisms are rare and usually harmful

Usually occur when homeostasis mechanisms breakdown and negative feedback doesn’t occur.

Examples of positive feedback mechanism

  1. In absence of environmental resistance increase in population of organisms leads to further increase and a decrease will lead to further decrease.

When the negative feedback mechanisms in mammalian temperature regulation  breakdown, a rise in environmental temperature may lead to an increase in body temperature which may spiral upwards and may be lethal.

  1. During child birth/parturition; the more oxytocin hormone secreted, the more contraction of uterine wall
  2. During propagation of an impulse (action potential) along a nerve or across synapse, the initial entry of sodium ions triggers the entry of further Na+ ions at a greater rate.
  • During the menstrual cycle:-

LH aids development of follicle

               Secrets LH        pituitary gland   positive feedback   secret oestrogen


Control of thyroxin production as a simple example of a biological control system:-

  1. Thyroxin is a hormone that regulates metallic rate, growth and development in mammals
  2. Negative feedback is the basic homeostatic principle controlling thyroxin release by thyroid glands
  • The modulator consists of the hypothalamus as a detectors; anterior pituitary gland as a regulator (comp orator) and the thyroid gland as an effecter
  1. Low level of thyroid in blood in the stimulus and is detected by hypothalamus which is stimulated to secret thyrotrophic releasing hormone (TRH), which triggers anterior pituitary gland to secret thyroid stimulating hormone (TSH)

Thyrotrophic hormones that in turn stimulates the thyroid gland to release thyroxin into the blood to increase its level to normal

  1. When the increasing level of tyroxine is in excess of that required to maintain the body metabolic rate in a steady state; negative feedback will inhibit the hypothalamus and anterior pituitary gland; leading to a drop in the level of thyroxin in blood






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