Реферат: Population biology


Everything is connected on earth- statesthe first law of ecology.  Meaning thatwe cant even make one step without disruption the environment. Even a usualstep on the lawn is tens of ruined microorganisms and scared insects that mightbe changing their migration paths and reducing their natural efficiency. Duringthe last century humans have gotten to be alarmed about the destiny of theplanet, however, when we stepped into this century, we have stepped into theecological crisis that we ourselves created.

Environmental contamination, exhaustion ofnatural resources and infringements of ecological communications in ecosystemsbecame global problems. And if the mankind will continue to continue obusingenvironment, its death is inevitable.

Now, during an approaching ecologicalcrisis on the planet, it is necessary for us to cooperate and preservenature. 

By consuming natural resources moreintensively human beings have progressed and improved conditions of developmentand the growth of Homo sapiens as a biological species. However, by«winning» the nature, we have created almost crisis situation ininteraction between the person and the nature, fraught with greater dangers tothe future of our civilization. It could be clearly seen in the problems withnatural resources, power, quality of an environment in its communication withthe further industrial development in the world and growth of the population.Interdependent changes have led to occurrence of new communications between globaleconomy and global ecology. In the past we were alarmed about  the consequences of an economic growth for anenvironment. Now we can not simply ignore the consequences of " ecologicalstress " – the deterioration of grounds, a water polution, a condition ofan atmosphere and forests.

Now becomes more clear, that sources andthe 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 hasalready given a definition of anthropogenous environmental contamination. It isphysical, chemical and biological change of the quality of an environment(atmospheric air, waters, ground) as a result of the economic or otheractivity, exceeding the established specifications of harmful influence on anenvironment and creating threat to health of the person and to the conditionsof flora and fauna.

The practical output of ecology can befirst seen in making the decisions in the questions of wildlife management; itshould create a scientific basis of operation of natural resources. We canascertain, that neglect of  the lawsunderlying natural processes has led to the serious conflict between the personand the nature.


Population ecology is defined as group oforganisms of one kind (inside of which individual can exchange the geneticinformation), occupying concrete space and functioning as a part bioticcommunity.

The population is a set of individuals ofone kind living in certain territory, freely crossed among themselves and it ispartially or completely isolated from other populations.

The population has its own characteristics:number, its density, spatial distribution of individuals. It could bedistinguished by age, sexual and dimensional structure. 

Structure.It is possible to allocate three ecological age groups: prereproductive — groupof individuals, which age has not reached ability of reproduction; reproductive- the group reproducing new individual; postreproductive — the individuals whohave lost ability to participate in reproduction of new generations. Durationof these ages in relation to the general life expectancy strongly variesbetween different organisms.

Number and densityexpress quantitative characteristics of a population as the whole.Number of a population is expressed by number of individuals of the given kindliving on the unit of the area borrowed by it. Dynamics of the populationnumbers  in time is defined by a parityof parameters of birth rate, death rate, survival rate which in turn aredefined by conditions of life.

The density ofa population is the size of population dependant upon the space taken byit:  number of individuals, or biomass,of the population per unit of an area or volume. The density depends on atrophic level on which there is a population. The lower a trophic level, thehigher the density.

Many species under those conditions areable to only have males or females, or sometimes unable to reproduce at all. Inplant louses, for example, generations consisting from one females replace eachother in the summer. Under adverse conditions only males are born. In somemolluscs, worms, fishes and crustations changes in sex occur with age.


So, what are the conditions of birth anddeath ratios depend upon? They are dependant upon  many factors from the outside, and also fromits own properties. An objective parameter of an ability of organisms toincrease the number is the maximal speed of a population gain. This parameteris inversely proportional to the life expectancies of organisms. It is easy tobe convinced of it, having addressed to the hyperbolic dependence betweencongenital speed of increase in number of a population and the average time of generationexpressed in days (fig. 1). Smaller organisms have higher values rтах, than larger ones, thatexplains shorter time of generation. The reason of this correlation is clear,because it takes more time for a larger organism to grow. The delay inreproduction also inevitably leads to the reduction of  rтах.

Nevertheless the advantages in having alarger sizes of a body, should exceed the lacks that have to do  with reduction of rтах, otherwise large organisms would never appear in evolution. Thetendency to increase the body size with the flow of the geological time,tracked on fossils, has formed the basis for introduction of the phyleticsize concept.

Larger body sizes give abundantly clearadvantages: larger organism should attract less potential predators and, hence,it has more chances to not become a prey and should differ with the bestsurvival rate; smaller organisms are in close dependence on the physicalenvironment, and even little changes can appear to be deadly to them. It iseasier for larger organisms to adapt to the surroundings and therefore they arebetter protected.  However largerorganisms require more food and energy per one individual in unit of time, thansmaller ones. Besides less safer places exist for them.

There are three periods in the life of anorganism: prereproductive, reproductive and postreproductive. Relative durationof each varies. The first period is the longest in many animals. A very goodexample of this are mayflies, which prereproductive period  reaches up to 3 years, and reproductiveperiod takes only from 2-3 hours to a day. American cicada takes 17 years. Butthere are species in which individuals start to reproduce intensively once theyare born (the majority of bacteria).

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Reproductive opportunities of  population depend on its life expectancy.Life expectancy of individuals of a population can be estimated, using curvesurvivals. There are three types of  survival curves(fig. 2).

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First type (curve 1) corresponds to thesituation when most individuals have identical life expectancy and die during avery short interval of time. Curves are characterized by the strong convexform. Such curve survivals are peculiar to the person (fig. 2, 1), however, thesurvival curve  for men in comparison withthe one for women is less convex, therefore an insurance policy for men in themajority of the countries in the West is 1,5 times is more expensive, than forwomen. 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 remainsconstants during all their life. Therefore the survival curve is transformed toa 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 thelife expectancy for some birds, fishes, and also many invertebrates ischaracterized.

 The knowledge the survival curve types enablesus to construct a pyramid of age (fig. 4). It is necessary to distinguish threetypes of such pyramids. The pyramid with the wide base that correspondsto high percent of growth of the young, is characteristic for a population with great value of factor of birthrate. The average type of the pyramid corresponds to the uniformdistribution of the individuals based on age in a population with the balancedfactors of birth rate and death rate – a leveled  pyramid. The pyramid with the narrow base,corresponds to the  populations withnumerical prevalence of old individuals over young growth, is characteristicfor reduced populations. In such populations the mortality rate coefficientexceeds factor of birth rate.

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The important factor in the change of thepopulation numbers is the parity of sexes. It is seldom equals to one, as inmost cases one of the sexes prevails over another. In vertebrates,  males are born more often then females.In  ducks males often numerically prevailover females as well.

It is alsoimportant to calculate the energy and resources spent on reproduction in thepopulation.  Not all offsprings areequivalent: 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 withthe descendants who have been born earlier.

What are theefforts that parents should spend for each offspring? At a constantreproductive effort, average fitness of a given offspring is connected with thereturn parity of their number. One extreme tactic of reproduction is to use allthe resources to create one large and fit offspring, another is to produce asmuch offspring as possible and not spend much resources. However the best tacticsof reproduction is a compromise between reproduction of a large number ofoffspring with high fitness.

The quantity andquality of  offspring is illustrated inthe graphic model (illustrates fig. 5).

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In an improbable case, i. е. in case oflinear dependence of offspring fitness on expenses of their parents, fitness of each separate offspringdecreases with increase of a laying size. Because the  fitness of parents or, that the same, thegeneral fitness of all offspring is a constant, the optimum size of a laying doesnot exist, that is believed by the parent. However, initial parental care hasgreater contribution to fitness of offspring, than the next ones (5-shapedcharacter of dependence of fitness of descendants takes place at increase inthe contribution of parents; see fig. 7.6) it is obvious, that there existssome optimal size of a laying. In the given hypothetical case the parentsspending only 20 % of the reproductive effort to each of their fivedescendants, will receive greater feedback from the contribution, than at anyother size of a laying. Similar tactics, being optimum for parents, are not thebest for each separately taken descendant which maximal fitness that is reachedin the event that the unique offspring who has received the full contributionof efforts from the parents. Hence, we get " the conflict of parents andchildren ".

Competitiveconditions are a big influence on the S-shaped curve. In strongly rarefiedenvironment (competitive vacuum) it is necessary to consider maximalcontributions of  energy  for the production of maximum offspring in theshortest time possible.  Because thecompetition is insignificant, descendants can survive, even if they are verysmall in size and have low fitness. However in the sated inhabitancy whereeffects of weight are noticeably shown, and the competition is high, optimum strategywould be to spend plenty of energy on competition, increase of own survivalrate and on the production of more competitive descendants. It is best  to have large descendants but since they areso costly, only few can be brought to life.

 So, properties of a population can beestimated on such parameters such as birth rate, death rate, age structure,parity of sexes, frequency of genes, genetic variety, speed and the form of acurve of growth, etc.

The densityof  population is defined by its internalproperties, and is also dependant on the outside factors of this population.


There are threetypes of dependence of  population fromits density (fig. 6). In the first type (curve 1) growth rate of a populationdecreases in process of increase in density. This widespread phenomenon allows usto 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 1hectares on one jack 14 nestlings are necessary; when the density reaches 18pairs per 1 hectares, offspring is less than 8 nestlings. Secondly, as thedensity of a population increases, the age maturity changes… For example, theAfrican elephant depending on the density of a population can reach sexualmaturity between the age of  12 -18years. Besides at low a density it breeds 1 baby per 4 years whereas at highdensity — birth rate makes it 1 baby per 7 years.

In the secondtype of dependence (a curve 2) growth rate of a population is maximal at average, instead of at low values ofdensity. So, some kinds of birds (for example, seagulls) the number ofnestlings increases with the increase of population density, and then, havingreached the greatest size, it starts to decrease. This type of influence of thepopulation on the  speed of duplicationof individuals is characteristic for kinds at which the group effect is noted. Inthe third type (curve 3) the rate of growth of a population does not changeuntil it will not reach its highest density, then it sharply falls.

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The similar picture is observed, forexample, with lemmings. At the peak of their number the density of lemmingsbecomes superfluous, and they start to migrate. Elton has described migrations oflemmings in <st1:country-region w:st=«on»><st1:place w:st=«on»>Norway</st1:place></st1:country-region>:animals have passed through villages in such quantities, that dogs and catswhich in the beginning attacked them, have simply ceased to notice them. Havingreached the seas, weak lemmings simply died.

 Regulation of the numbers of equilibriumpopulations is defined mainly by biotic factors. The primary factor are oftenappear to be intraspecific competition. An example of this could be  struggle of birds for nesting.

Intraspecificcompetition can cause the physiological effect also known as shock illness. It canbe noted in  rodents. When the density ofa population becomes too big, shock illness leads to decrease in fruitfulnessand increase in death rate that returns density of a population to its normallevel.

Some adultspecies eat their offspring. This phenomenon is known as cannibalism, which reducesnumbers of  population.  For example, cannibalism can be traced in  perches: in the lakes of <st1:place w:st=«on»>Western Siberia</st1:place>,  80 % of grownperches eat young offspring of the same kind. Young offspring, in turn, eats  plankton. Thus, when there is no other kindsof fish, adult individuals feed off plankton.

Interspecificinteractions also play an essential role in the control of density of apopulation. Interactions such as paracite-owner and  predator-victim are often density dependant. Illnessesare also a factor in the regulation of population density. When rabbits are illwith a virus, the infection spreads faster in the heavily dense population.

Predatoriness asthe limiting factor is of a  greatimportance. And if the influence of a prey on a number of a predator populationdoes not cause doubts, the return influence, i. е. Influence on the preypopulation, doesn’t always happen. First of all, the predator kills sickanimals, by doing so it improves the average qualitative structure of theprey’s population. Secondly, a role of a predator is heavily weighted only whenboth of  kinds possess approximatelyidentical biotic potential. Otherwise because of low reproduction rate,  predator is not able to limit the number ofprey. For example, only one insectivorous birds cannot stop mass production ofinsects. In other words, if biotic potential of a predator is much lowerof  the biotic potential of a prey,actions of a predator inherit constant character, not dependent upon the  density of its population.

The resulteddifferentiation of factors of dynamics of number of populations allows us tounderstand their real value in  life andreproduction of populations. The modern concept of automatic control of numberof populations is based on a combination of two essentially various phenomena:modifications, or casual fluctuations of number, and regulations, operating bya principle of a cybernetic feedback and levelling fluctuations. According tothis modifying (populations independent of density) and adjusting (populationsdepending on density) ecologic factors are allocated, and first ones influenceorganisms inderectly or through changes of other components biosenosis.Actually, modifying factors represent various abiotic factors. Adjustingfactors are connected with existence and activity of alive organisms (bioticfactors), because only live creatures are capable to react to the density ofits population and populations of other kinds base on the principle of anegative feedback (fig. 7).

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For example, the predators-polyphages,which are able to weaken or strengthen their reaction based upon the prey’snumbers-functional reaction- they usually act when the pre’s population islow.  Predators — oligophages, unlikepolyphages, they are characterized by the  numerical reaction of a population of avictim, have an effect in a wider range, than polyphaes. Once the preypopulation reaches higher number, the conditions for distribution of illnesses occur,and, at last, the limiting factor of regulation — the intraspecific competitionleading to limiting of accessible resources and development of stressfulreactions in a population of a victim are created. Fig. 8 illustrates theiterative buffer system of regulation of the number of a population underinfluence of biotic factors, which degree of influence depends on density of apopulation. In a real life situation the given parameter depends on the largenumber of factors, particularly those that do not render adjusting influence ondensity of a population by a principle of a feedback. Interaction betweenmodifying, adjusting, and such specific factors, as the sizes of a body, groupsand individual site, at their influence on density of a population of mammalsit is shown on fig. 9.

So in order toreceive exhaustive information on what factors cause fluctuations of number,data about physical and chemical conditions, security resources, life cycle ofthese organisms and influence of competitors, predators, parasites, etc. isnecessary to know, how all these factors influence birth rate, death rate andmigration. All populations continuously change: new organisms are born orarrive 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, itcannot grow endlessly, and on the other hand — kinds seldom enough die out.Hence, one of the basic attributes of population dynamics is a combination ofchanges to relative stability. Thus fluctuations of the sizes of populationsstrongly differ with different kinds of species.

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Individuals in a population cooperate amongthemselves, providing the ability to live and steadily reproduce. In animalsleading a “batchelor” life style  orcreating families, the adjusting factor is territory,  which influences possession of certain foodresources and is of great importance for reproduction. The individual protectsspace from intrusion and allows individuals in only during reproduction.

The mostrational use of space is reached in the event that every other species isexpelled from the territory. This way, the owner of a site psychologicallydominates over it, it is enough for the exile to demonstrate threats, prosecution, the greatest – false attackswhich stop on the borders of a site. In the given animals individualdistinctions between individuals have huge value.

In animalsleading a group way of life and forming flights, herds, colonies, groupprotection against enemies and joint care about posterity raises survival rateof individuals that influences number of a population and its survival rate. Givenanimals are organized hierarchically. Hierarchical attitudes are constructed insuch a way that the rank of everyone is known by everyone. As a rule, themaximum rank belongs to the senior male. The hierarchy controlls allinteractions inside  a population:marriage, individuals of different age, parents and posterity. In animals thespecial role is given to  «mother-child» relationships.Parents transfer the genetic information and the information about anenvironment to the offspring


At a level of a population abiotic factorsinfluence such parameters as birth rate, death rate, average life expectancy ofthe individual, growth rate of a population and its sizes, quite often beingthe major reasons defining character of dynamics of number of a population andspatial distribution of individuals in it. The population can adapt to changesof abiotic factors, first, changing character of the spatial distribution and, secondly,by adaptive evolution.

The selectiveattitude of animals and plants to factors of environment generates selectivityto habitats, i. е. ecological specialization in relation to sites of an area ofa kind which it tries to occupy. The choice is defined by such factors; it canbe based on acidity, salinity, humidity, etc.

For some kinds zone the change of habitatis characterized by zone, it would change habitats from one zone to the other.

            Oneof 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 areaswith dry air,  wood lice spend most of theirtime under stones and a bark of trees.

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Wood louse Lygia oceanica lives on the seacoast. Day time of a wood louse is spent in the shelter. But when thetemperature 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. Thereason of such moving is that the given kind is very badly adapted for a groundhabitat, has very thin cuticle.

When humidity ofair is low, wood louse loses a lot of water by evaporation, which occurs on therocks under the sun. Intensive evaporation reduces  body temperature of an animal which at itsfinding on a rock is equal 26 °с (fig. 11). If, the wood louse continues tohide under a pebble where relative humidity is close to 100 %, and evaporationis equal to zero, then the body temperature reaches 30 °с.

Anotherimportant factors is acidity. Sour waters of turbaries promote development of mosses,but they have absolutely no  foldingmollusks population in them. Other kinds of moluscs are extremely, and this hasto 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 below5 it is possible to observe their mass destruction, though separate kinds adaptand to the surroundings, value of which  reaches up to 3,7. The efficiency of  fresh waters having acidity less 5, is sharplylowered, that entails significant reduction of fishe.

Other importantfactor limiting distribution of water animals and plants is salinity of water.Many types such as sponges and worms live in the sea.

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Often only insignificant shifts inconcentration 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 ispoor. For example, lake with the salinity ranging from 2  to 7 %  is inhibited by fresh-water fishes, such as acarp, pike, pike perch that are quite well adapted to low salinity, and seafishes, such as mullet which is tolerant to insufficient salinity.

Abiotic factorsrender 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 rareand even disappear completely. Besides, frosts in some cases influence food aswell, because it is being concealled under a thick layer of an ice or a snow, andit becomes absolutely inaccessible to animals. In the places subject to strongwinds, growth of plants starts late, and the fauna can be partially or iscompletely destroyed.


Question on how evolution occurs inecosystems, it is very important, because it is  a key to understanding of an existing varietyof communities of live organisms on our planet, changes of flora and faunaduring 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. Itcan be subdivided into mutual selection of autotrophs, that are dependent uponeach other and heterotrophs and group selection which conducts to preservationof the attributes favorable for ecosystems as a whole even if they are adversefor specific carriers of these attributes.

There are theuncountable ways allowing victims to resist to pressure of predators. They canbe 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 ina combination to warning painting), parental and social behaviour (protectionthe posterities warning signals, joint protection of group and т. Item).

Protective means of plants include: rigidleaves, thorns and prickles, ядовитость, репеллентные and ингибирующие a feedof animals of substance. Predators and other «exploiters» have notless refined ways to overtake a victim. We shall recollect, for example, publichunting behaviour of lions and the wolves, the bent poisonous teeth of snakes,long sticky languages of frogs, toads and lizards, and also spiders and theirweb, a deep-waterfish-Òñ¿½ýÚ¿¬á or boas,which душат the victims.

The fauna, being a component of anenvironment, acts as the integral part in circuits of the ecological systems, anecessary component during circulation of substances and energy of the nature,actively influencing on functioning of natural communities, structure andnatural fertility почв, formation of a vegetative cover, biological propertiesof water and quality of an environment as a whole, At the same time the faunahas the big economic value.

Feature of fauna is that the given objectis renewed, but for this purpose observance of the certain conditions, directconnected with animal protection is necessary. At destruction, infringement ofconditions of their existence the certain kinds of animals can finallydisappear, and their renewal will be impossible.

In the Federal law traditional methods ofprotection and use of objects of fauna are stipulated. Persons, whose existenceand incomes are in full or in part based on traditional life-support systems,including hunting, fishery and collecting, have the right to application oftraditional methods of getting of objects of fauna and products of ability tolive, if such methods directly or indirectly do not conduct to decrease in abiological variety, do not reduce number and steady reproduction of objects offauna, do not break environment of their dwelling and do not represent dangerto 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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