POPULATION BIOLOGY
Everything is connected on earth- states
the first law of ecology. Meaning that
we cant even make one step without disruption the environment. Even a usual
step on the lawn is tens of ruined microorganisms and scared insects that might
be changing their migration paths and reducing their natural efficiency. During
the last century humans have gotten to be alarmed about the destiny of the
planet, however, when we stepped into this century, we have stepped into the
ecological crisis that we ourselves created.
Environmental contamination, exhaustion of
natural resources and infringements of ecological communications in ecosystems
became global problems. And if the mankind will continue to continue obusing
environment, its death is inevitable.
Now, during an approaching ecological
crisis on the planet, it is necessary for us to cooperate and preserve
nature.
By consuming natural resources more
intensively human beings have progressed and improved conditions of development
and the growth of Homo sapiens as a biological species. However, by
"winning" the nature, we have created almost crisis situation in
interaction between the person and the nature, fraught with greater dangers to
the future of our civilization. It could be clearly seen in the problems with
natural resources, power, quality of an environment in its communication with
the further industrial development in the world and growth of the population.
Interdependent changes have led to occurrence of new communications between global
economy and global ecology. In the past we were alarmed about the consequences of an economic growth for an
environment. Now we can not simply ignore the consequences of " ecological
stress " – the deterioration of grounds, a water polution, a condition of
an atmosphere and forests.
Now becomes more clear, that sources and
the reasons of pollution are much more various,
complex and interconnected, and consequences of pollution carry wider,
cumulative and chronic character, than it was considered earlier. Science has
already given a definition of anthropogenous environmental contamination. It is
physical, chemical and biological change of the quality of an environment
(atmospheric air, waters, ground) as a result of the economic or other
activity, exceeding the established specifications of harmful influence on an
environment and creating threat to health of the person and to the conditions
of flora and fauna.
The practical output of ecology can be
first seen in making the decisions in the questions of wildlife management; it
should create a scientific basis of operation of natural resources. We can
ascertain, that neglect of the laws
underlying natural processes has led to the serious conflict between the person
and the nature.
CONCEPT OF THE POPULATION
Population ecology is defined as group of
organisms of one kind (inside of which individual can exchange the genetic
information), occupying concrete space and functioning as a part biotic
community.
The population is a set of individuals of
one kind living in certain territory, freely crossed among themselves and it is
partially or completely isolated from other populations.
The population has its own characteristics:
number, its density, spatial distribution of individuals. It could be
distinguished by age, sexual and dimensional structure.
Structure.
It is possible to allocate three ecological age groups: prereproductive - group
of individuals, which age has not reached ability of reproduction; reproductive
- the group reproducing new individual; postreproductive - the individuals who
have lost ability to participate in reproduction of new generations. Duration
of these ages in relation to the general life expectancy strongly varies
between different organisms.
Number and density express quantitative characteristics of a population as the whole.
Number of a population is expressed by number of individuals of the given kind
living on the unit of the area borrowed by it. Dynamics of the population
numbers in time is defined by a parity
of parameters of birth rate, death rate, survival rate which in turn are
defined by conditions of life.
The density of
a population is the size of population dependant upon the space taken by
it: number of individuals, or biomass,
of the population per unit of an area or volume. The density depends on a
trophic level on which there is a population. The lower a trophic level, the
higher the density.
Many species under those conditions are
able to only have males or females, or sometimes unable to reproduce at all. In
plant louses, for example, generations consisting from one females replace each
other in the summer. Under adverse conditions only males are born. In some
molluscs, worms, fishes and crustations changes in sex occur with age.
FEATURES
OF POPULATIONS
So, what are the conditions of birth and
death ratios depend upon? They are dependant upon many factors from the outside, and also from
its own properties. An objective parameter of an ability of organisms to
increase the number is the maximal speed of a population gain. This parameter
is inversely proportional to the life expectancies of organisms. It is easy to
be convinced of it, having addressed to the hyperbolic dependence between
congenital speed of increase in number of a population and the average time of generation
expressed in days (fig. 1). Smaller organisms have higher values rтах, than larger ones, that
explains shorter time of generation. The reason of this correlation is clear,
because it takes more time for a larger organism to grow. The delay in
reproduction also inevitably leads to the reduction of rтах.
Nevertheless the advantages in having a
larger sizes of a body, should exceed the lacks that have to do with reduction of rтах, otherwise large organisms would never appear in evolution. The
tendency to increase the body size with the flow of the geological time,
tracked on fossils, has formed the basis for introduction of the phyletic
size concept.
Larger body sizes give abundantly clear
advantages: larger organism should attract less potential predators and, hence,
it has more chances to not become a prey and should differ with the best
survival rate; smaller organisms are in close dependence on the physical
environment, and even little changes can appear to be deadly to them. It is
easier for larger organisms to adapt to the surroundings and therefore they are
better protected. However larger
organisms require more food and energy per one individual in unit of time, than
smaller ones. Besides less safer places exist for them.
There are three periods in the life of an
organism: prereproductive, reproductive and postreproductive. Relative duration
of each varies. The first period is the longest in many animals. A very good
example of this are mayflies, which prereproductive period reaches up to 3 years, and reproductive
period takes only from 2-3 hours to a day. American cicada takes 17 years. But
there are species in which individuals start to reproduce intensively once they
are born (the majority of bacteria).
Reproductive opportunities of population depend on its life expectancy.
Life expectancy of individuals of a population can be estimated, using curve
survivals. There are three types of survival curves(fig. 2).
First type (curve 1) corresponds to the
situation when most individuals have identical life expectancy and die during a
very short interval of time. Curves are characterized by the strong convex
form. Such curve survivals are peculiar to the person (fig. 2, 1), however, the
survival curve for men in comparison with
the one for women is less convex, therefore an insurance policy for men in the
majority of the countries in the West is 1,5 times is more expensive, than for
women. For the majority of hoofed animals, survival curve is also convex (fig.
3), however, it is dependant upon the sex of the species. The second type (fig.
2, 2) is peculiar to the kinds which mortality rate coefficient remains
constants during all their life. Therefore the survival curve is transformed to
a direct line. Such form of the survival curve is peculiar to a fresh-water hydra.
The third type (fig. 2, 3) is represented by strongly concaved curves,
reflecting high death rate of an individuals at early age. So that is how the
life expectancy for some birds, fishes, and also many invertebrates is
characterized.
The knowledge the survival curve types enables
us to construct a pyramid of age (fig. 4). It is necessary to distinguish three
types of such pyramids. The pyramid with the wide base that corresponds
to high percent of growth of the young,
is characteristic for a population with great value of factor of birth
rate. The average type of the pyramid corresponds to the uniform
distribution of the individuals based on age in a population with the balanced
factors of birth rate and death rate – a leveled pyramid. The pyramid with the narrow base,
corresponds to the populations with
numerical prevalence of old individuals over young growth, is characteristic
for reduced populations. In such populations the mortality rate coefficient
exceeds factor of birth rate.
The important factor in the change of the
population numbers is the parity of sexes. It is seldom equals to one, as in
most cases one of the sexes prevails over another. In vertebrates, males are born more often then females.
In ducks males often numerically prevail
over females as well.
It is also
important to calculate the energy and resources spent on reproduction in the
population. Not all offsprings are
equivalent: those of them which are born at the end of the vegetative season,
usually have less chances to live up to an adult condition in comparison with
the descendants who have been born earlier.
What are the
efforts that parents should spend for each offspring? At a constant
reproductive effort, average fitness of a given offspring is connected with the
return parity of their number. One extreme tactic of reproduction is to use all
the resources to create one large and fit offspring, another is to produce as
much offspring as possible and not spend much resources. However the best tactics
of reproduction is a compromise between reproduction of a large number of
offspring with high fitness.
The quantity and
quality of offspring is illustrated in
the graphic model (illustrates fig. 5).
In an improbable case, i. е. in case of
linear dependence of offspring fitness
on expenses of their parents, fitness of each separate offspring
decreases with increase of a laying size. Because the fitness of parents or, that the same, the
general fitness of all offspring is a constant, the optimum size of a laying does
not exist, that is believed by the parent. However, initial parental care has
greater contribution to fitness of offspring, than the next ones (5-shaped
character of dependence of fitness of descendants takes place at increase in
the contribution of parents; see fig. 7.6) it is obvious, that there exists
some optimal size of a laying. In the given hypothetical case the parents
spending only 20 % of the reproductive effort to each of their five
descendants, will receive greater feedback from the contribution, than at any
other size of a laying. Similar tactics, being optimum for parents, are not the
best for each separately taken descendant which maximal fitness that is reached
in the event that the unique offspring who has received the full contribution
of efforts from the parents. Hence, we get " the conflict of parents and
children ".
Competitive
conditions are a big influence on the S-shaped curve. In strongly rarefied
environment (competitive vacuum) it is necessary to consider maximal
contributions of energy for the production of maximum offspring in the
shortest time possible. Because the
competition is insignificant, descendants can survive, even if they are very
small in size and have low fitness. However in the sated inhabitancy where
effects of weight are noticeably shown, and the competition is high, optimum strategy
would be to spend plenty of energy on competition, increase of own survival
rate and on the production of more competitive descendants. It is best to have large descendants but since they are
so costly, only few can be brought to life.
So, properties of a population can be
estimated on such parameters such as birth rate, death rate, age structure,
parity of sexes, frequency of genes, genetic variety, speed and the form of a
curve of growth, etc.
The density
of population is defined by its internal
properties, and is also dependant on the outside factors of this population.
FACTORS OF DYNAMICS OF NUMBER OF POPULATIONS
There are three
types of dependence of population from
its density (fig. 6). In the first type (curve 1) growth rate of a population
decreases in process of increase in density. This widespread phenomenon allows us
to understand, why populations of some animals are rather steady. First of all,
as the density of a population increases, decrease in the birth rate is observed.
So, in a population of a big titmouse at a density of less than one pair per 1
hectares on one jack 14 nestlings are necessary; when the density reaches 18
pairs per 1 hectares, offspring is less than 8 nestlings. Secondly, as the
density of a population increases, the age maturity changes.. For example, the
African elephant depending on the density of a population can reach sexual
maturity between the age of 12 -18
years. Besides at low a density it breeds 1 baby per 4 years whereas at high
density - birth rate makes it 1 baby per 7 years.
In the second
type of dependence (a curve 2) growth rate
of a population is maximal at average, instead of at low values of
density. So, some kinds of birds (for example, seagulls) the number of
nestlings increases with the increase of population density, and then, having
reached the greatest size, it starts to decrease. This type of influence of the
population on the speed of duplication
of individuals is characteristic for kinds at which the group effect is noted. In
the third type (curve 3) the rate of growth of a population does not change
until it will not reach its highest density, then it sharply falls.
The similar picture is observed, for
example, with lemmings. At the peak of their number the density of lemmings
becomes superfluous, and they start to migrate. Elton has described migrations of
lemmings in Norway:
animals have passed through villages in such quantities, that dogs and cats
which in the beginning attacked them, have simply ceased to notice them. Having
reached the seas, weak lemmings simply died.
Regulation of the numbers of equilibrium
populations is defined mainly by biotic factors. The primary factor are often
appear to be intraspecific competition. An example of this could be struggle of birds for nesting.
Intraspecific
competition can cause the physiological effect also known as shock illness. It can
be noted in rodents. When the density of
a population becomes too big, shock illness leads to decrease in fruitfulness
and increase in death rate that returns density of a population to its normal
level.
Some adult
species eat their offspring. This phenomenon is known as cannibalism, which reduces
numbers of population. For example, cannibalism can be traced in perches: in the lakes of Western
Siberia, 80 % of grown
perches eat young offspring of the same kind. Young offspring, in turn, eats plankton. Thus, when there is no other kinds
of fish, adult individuals feed off plankton.
Interspecific
interactions also play an essential role in the control of density of a
population. Interactions such as paracite-owner and predator-victim are often density dependant. Illnesses
are also a factor in the regulation of population density. When rabbits are ill
with a virus, the infection spreads faster in the heavily dense population.
Predatoriness as
the limiting factor is of a great
importance. And if the influence of a prey on a number of a predator population
does not cause doubts, the return influence, i. е. Influence on the prey
population, doesn’t always happen. First of all, the predator kills sick
animals, by doing so it improves the average qualitative structure of the
prey’s population. Secondly, a role of a predator is heavily weighted only when
both of kinds possess approximately
identical biotic potential. Otherwise because of low reproduction rate, predator is not able to limit the number of
prey. For example, only one insectivorous birds cannot stop mass production of
insects. In other words, if biotic potential of a predator is much lower
of the biotic potential of a prey,
actions of a predator inherit constant character, not dependent upon the density of its population.
The resulted
differentiation of factors of dynamics of number of populations allows us to
understand their real value in life and
reproduction of populations. The modern concept of automatic control of number
of populations is based on a combination of two essentially various phenomena:
modifications, or casual fluctuations of number, and regulations, operating by
a principle of a cybernetic feedback and levelling fluctuations. According to
this modifying (populations independent of density) and adjusting (populations
depending on density) ecologic factors are allocated, and first ones influence
organisms inderectly or through changes of other components biosenosis.
Actually, modifying factors represent various abiotic factors. Adjusting
factors are connected with existence and activity of alive organisms (biotic
factors), because only live creatures are capable to react to the density of
its population and populations of other kinds base on the principle of a
negative feedback (fig. 7).
For example, the predators-polyphages,
which are able to weaken or strengthen their reaction based upon the prey’s
numbers-functional reaction- they usually act when the pre’s population is
low. Predators - oligophages, unlike
polyphages, they are characterized by the numerical reaction of a population of a
victim, have an effect in a wider range, than polyphaes. Once the prey
population reaches higher number, the conditions for distribution of illnesses occur,
and, at last, the limiting factor of regulation - the intraspecific competition
leading to limiting of accessible resources and development of stressful
reactions in a population of a victim are created. Fig. 8 illustrates the
iterative buffer system of regulation of the number of a population under
influence of biotic factors, which degree of influence depends on density of a
population. In a real life situation the given parameter depends on the large
number of factors, particularly those that do not render adjusting influence on
density of a population by a principle of a feedback. Interaction between
modifying, adjusting, and such specific factors, as the sizes of a body, groups
and individual site, at their influence on density of a population of mammals
it is shown on fig. 9.
So in order to
receive exhaustive information on what factors cause fluctuations of number,
data about physical and chemical conditions, security resources, life cycle of
these organisms and influence of competitors, predators, parasites, etc. is
necessary to know, how all these factors influence birth rate, death rate and
migration. All populations continuously change: new organisms are born or
arrive as immigrants, and former perish or will emigrate. Despite of it,
fluctuations of the size of a population are not boundless. On the one hand, it
cannot grow endlessly, and on the other hand - kinds seldom enough die out.
Hence, one of the basic attributes of population dynamics is a combination of
changes to relative stability. Thus fluctuations of the sizes of populations
strongly differ with different kinds of species.
Individuals in a population cooperate among
themselves, providing the ability to live and steadily reproduce. In animals
leading a "batchelor” life style or
creating families, the adjusting factor is territory, which influences possession of certain food
resources and is of great importance for reproduction. The individual protects
space from intrusion and allows individuals in only during reproduction.
The most
rational use of space is reached in the event that every other species is
expelled from the territory. This way, the owner of a site psychologically
dominates over it, it is enough for the
exile to demonstrate threats, prosecution, the greatest – false attacks
which stop on the borders of a site. In the given animals individual
distinctions between individuals have huge value.
In animals
leading a group way of life and forming flights, herds, colonies, group
protection against enemies and joint care about posterity raises survival rate
of individuals that influences number of a population and its survival rate. Given
animals are organized hierarchically. Hierarchical attitudes are constructed in
such a way that the rank of everyone is known by everyone. As a rule, the
maximum rank belongs to the senior male. The hierarchy controlls all
interactions inside a population:
marriage, individuals of different age, parents and posterity. In animals the
special role is given to "mother-child" relationships.
Parents transfer the genetic information and the information about an
environment to the offspring
SPATIAL ACCOMMODATION OF POPULATIONS
At a level of a population abiotic factors
influence such parameters as birth rate, death rate, average life expectancy of
the individual, growth rate of a population and its sizes, quite often being
the major reasons defining character of dynamics of number of a population and
spatial distribution of individuals in it. The population can adapt to changes
of abiotic factors, first, changing character of the spatial distribution and, secondly,
by adaptive evolution.
The selective
attitude of animals and plants to factors of environment generates selectivity
to habitats, i. е. ecological specialization in relation to sites of an area of
a kind which it tries to occupy. The choice is defined by such factors; it can
be based on acidity, salinity, humidity, etc.
For some kinds zone the change of habitat
is characterized by zone, it would change habitats from one zone to the other.
One
of the important factors in changing habitats is humidity factor.
Wood lice are a very good example of it.
They live on the sea coasts where air is rich with moisture, and where they can
live openly. In high-mountainous areas
with dry air, wood lice spend most of their
time under stones and a bark of trees.
Wood louse Lygia oceanica lives on the sea
coast. Day time of a wood louse is spent in the shelter. But when the
temperature of air raises up to 20 °с outside and up to 30 °с under a pebble,
they leave the shelters and creep out on the rocks turned to the sun. The
reason of such moving is that the given kind is very badly adapted for a ground
habitat, has very thin cuticle.
When humidity of
air is low, wood louse loses a lot of water by evaporation, which occurs on the
rocks under the sun. Intensive evaporation reduces body temperature of an animal which at its
finding on a rock is equal 26 °с (fig. 11). If, the wood louse continues to
hide under a pebble where relative humidity is close to 100 %, and evaporation
is equal to zero, then the body temperature reaches 30 °с.
Another
important factors is acidity. Sour waters of turbaries promote development of mosses,
but they have absolutely no folding
mollusks population in them. Other kinds of moluscs are extremely, and this has
to do with the absence of lime in it.
Fishes bear acidity of water within the limits of Pн from 5 up to 9.At Pн below
5 it is possible to observe their mass destruction, though separate kinds adapt
and to the surroundings, value of which reaches up to 3,7. The efficiency of fresh waters having acidity less 5, is sharply
lowered, that entails significant reduction of fishe.
Other important
factor limiting distribution of water animals and plants is salinity of water.
Many types such as sponges and worms live in the sea.
Often only insignificant shifts in
concentration of salts in water affect distribution of closely related kinds
(fig. 12). Number of inhabitants of salt waters is very great, but kinds of species that live in it structure is
poor. For example, lake with the salinity ranging from 2 to 7 % is inhibited by fresh-water fishes, such as a
carp, pike, pike perch that are quite well adapted to low salinity, and sea
fishes, such as mullet which is tolerant to insufficient salinity.
Abiotic factors
render essential influence on density of populations of animals and plants.
Downturn of temperature often catastrophically affects populations of animals:
in the areas adjoining to northern borders of an area, the kind can become rare
and even disappear completely. Besides, frosts in some cases influence food as
well, because it is being concealled under a thick layer of an ice or a snow, and
it becomes absolutely inaccessible to animals. In the places subject to strong
winds, growth of plants starts late, and the fauna can be partially or is
completely destroyed.
CONCLUSION
Question on how evolution occurs in
ecosystems, it is very important, because it is a key to understanding of an existing variety
of communities of live organisms on our planet, changes of flora and fauna
during its geological history. In a basis of evolution lies the natural selection. But natural selection plays a very important role at a level of ecosystems. It
can be subdivided into mutual selection of autotrophs, that are dependent upon
each other and heterotrophs and group selection which conducts to preservation
of the attributes favorable for ecosystems as a whole even if they are adverse
for specific carriers of these attributes.
There are the
uncountable ways allowing victims to resist to pressure of predators. They can
be reduced to following categories: protective behaviour (flight, затаивание,
use of refuges and т. Item), the protective form and painting (patronizing,
frightening off, warning, a mimicry), inedibility or ядовитость (it is usual in
a combination to warning painting), parental and social behaviour (protection
the posterities warning signals, joint protection of group and т. Item).
Protective means of plants include: rigid
leaves, thorns and prickles, ядовитость, репеллентные and ингибирующие a feed
of animals of substance. Predators and other "exploiters" have not
less refined ways to overtake a victim. We shall recollect, for example, public
hunting behaviour of lions and the wolves, the bent poisonous teeth of snakes,
long sticky languages of frogs, toads and lizards, and also spiders and their
web, a deep-water
fish-Òñ¿½ýÚ¿¬á or boas,
which душат the victims.
The fauna, being a component of an
environment, acts as the integral part in circuits of the ecological systems, a
necessary component during circulation of substances and energy of the nature,
actively influencing on functioning of natural communities, structure and
natural fertility почв, formation of a vegetative cover, biological properties
of water and quality of an environment as a whole, At the same time the fauna
has the big economic value.
Feature of fauna is that the given object
is renewed, but for this purpose observance of the certain conditions, direct
connected with animal protection is necessary. At destruction, infringement of
conditions of their existence the certain kinds of animals can finally
disappear, and their renewal will be impossible.
In the Federal law traditional methods of
protection and use of objects of fauna are stipulated. Persons, whose existence
and incomes are in full or in part based on traditional life-support systems,
including hunting, fishery and collecting, have the right to application of
traditional methods of getting of objects of fauna and products of ability to
live, if such methods directly or indirectly do not conduct to decrease in a
biological variety, do not reduce number and steady reproduction of objects of
fauna, do not break environment of their dwelling and do not represent danger
to the person. The specified persons can carry out this right both individually,
and collectively, creating associations on a various basis (family,
patrimonial, territorially-economic communities, the unions of hunters,
collectors, fishers and others).
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