Population biology 
Population biology
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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