Population & Population Dynamics Ii

Ecology-Population Dynamics II P.1

Eco_POPULATION & POPULATION DYNAMICS II

Regulation of population growth

I)Limits to Population growth

A single oyster can produce one million eggs a year, so the population has the pot______to grow at an en______rate. E.g.If all the oyster offspring reproduced once, after five generations there would be 1030oysters, weighing something like 500 times the mass of the sun. Suppose a female housefly in a kitchen lays 120 eggs, and there are seven generations of houseflies per year. The final number could be something like 5 x 1012 (5 million million) flies after one year.

Fortunately, the pot_____ growth of a population (biotic potential) is never realized; at some stage in its growth there will be checks imposed by the environment, setting a l____ to the size of the population.

The checks / environmental re______ may cause the population to become stabilized, decline, crash, and may even disappear.

A)Fluctuation and Regulation of population size

Despite the enormous biotic potential of most species, population size of most species tends to remain fairly steady (with small fluctuations) after reaching an equil______with the c______c______ of the environment.

A generalized population growth for a newly introduced species can be represented by the graph below :

Population C

size E

D

B

A

Time

A - lag phase, B - exponential phase, C - overshoot, D - plateau, E - oscillations

Populations are thought to be regulated by a form of negative f______mechanism or homeostasis which returns them to anoptimum level. In the case of natural populations, such a mechanism is called reg______, in contrast to the deviations or fluctuations.

II)Population regulating factors

Density Independent Factors

These operate independently of population s____ and exert similar destructive effect on either crowded or sparse population.

/ These may involve aspects of p______environment (abiotic factors), weather, as well as chance occurrences,such as flooding and drought, unusually cold winters or warm, dry summers, volcanic eruptions, or gradual changes such as the onset of an ice age.
Abiotic factors, such as a catastrophe, are likely to cause fluct______ in population-rather than to regulate populations around an optimum level.
Numbers of breeding pairs of grey heron in two areas, fluctuation in population seems related to hard winters

Density Dependent Factors

Their actions are sen_____ to the size of the population and increase its eff______in checking population as population increases.The effect of such factors will be to return a population to the level which the environment can su______.

Factors acting in this way are usually b____, of which there are essentially two types: (i)predator-prey, (ii) competitionbetween species, or between different members of the same species.

/ a State for each of the lines F & G in the figure whether it represents a density-independent or density-dependent relationship.
b Explain why the shooting of a wood-pigeon population is likely to have only a short-term impact on the population size.
c Decide for each of the following factors whether it is likely to be density-dependent or density dependent.
1Death of birds from hypothermia in winter;
2Death of birds from hypothermia in winter if there is only a number of warm places where they can shelter;
3The mortality of locust eggs in dry sand;
4An insect pest population being attacked by certain wasp parasite.

a)Predator-prey interaction

Apredator is an animal which feeds on l_____preys, either plant or animal. Predation is a feeding relationship between the two organisms, in which one k__ and consumes the other as food.

In predation, avoidance mechanisms evolved by the prey are counteracted by more sophisticated hunting methods of the predators.

Q.How do predators evolve to counteract avoidance mechanisms of preys?

Avoidance mechanism of prey / Example of preys / Counteracting hunting technique / Example of predators
alarm calls / rapid and accurate strike
camouflage / camouflage, ambush
social species / social hunting

Despite all these adaptation of the prey, some losses due to predation are inevitable. Prey species usually have higher rate of rep______ to compensate losses.

A population of predators is much smaller than the prey population from which it draws its food. Although there is usually some kind of balance between the two populations, this balance is rarely stable and may show periodic flu______.

A Model of the Predator- Prey Relationship

A simplified version of the predator-prey relationships is well illustrated by laboratory experiments with two mites, one predatoryand one herbivorous. The figure shows the cyclic fluctuations that occur in their numbers, the cycles for the two species being slightly out of phase with each other.

• The explanation for these cycles is that an increase in numbers of the p___ supports a subsequent increase in numbers of the predator.

• The predators then cause a crash in numbers of the prey, followed by an inevitable decline in numbers of predators.

• The cycles are completed when the decline in p______allows an increase in numbers of the prey. Each cycle occurs over a number of generations.

2)Man interfering with the Natural Balance of predator-prey relationship

An insight into the ecological importance of predation to the prey can be gained from the fate of a population of deer on the Kaibab Plateau in Arizona:

In 1906 the area was declared a wild-life refuge, and in order to protect the deer, their predators, such as pumas and wolves, were systematically removed over the next 30 years. Up to 1906 the deer population had remained stable at about 4 000, but subsequently a 'population explosion' occurred, with the result that the carrying capacity of the environment, estimated to be about 30 000 deer, was exceeded.
The population reached an estimated 100 000 by 1924, following a boom-and-bust 'J shaped' curve, but the overgrazing resulted in starvation and, together with disease, a subsequentpopulationcrash.
In addition, the veg______had been seriously damaged and did not recover to its 1906 level with the result that the c______capacity of the environment dropped to 10 000 deer. /

Thus a predator-prey relationships has long-termadvantageto the prey:

• Ecol______,predation plays an important part in reg______ natural populations (as a density dependent factor). Preventing over______of prey beyond carrying capacity of the environment.

• In evol______term, by removing weak and sick individuals from the prey population, predator helps to maintain the gen____ fitness of the pop______; it also promote evolution of better-ad_____ preys in the population.

Q1a. What will happen to predators if their prey becomes scarce? Explain?

b.What could then happen to the prey population? Why?

c.If the predators are too efficient what will happen to both populations? Why?

1. If the predators are entirely removed explain what might happen to the prey population?

e.Apart of course from providing it with food. In what respect do you think predation is also beneficial to the predator species?

f.To be truly successful predators, the predator population must also be able to survive when prey number decline, what can predators do to survive under such situations?

g.Why animals that have become too specialized in their diet are at a higher risk of extinction? Give an example of such an animal.

Q.2Man may upset the natural balance of predator and prey in other ways. Consider the situation where a careless or ill-judged use of insecticide in an ecosystem has destroyed not only most of the population of a plant-eating insect pest, but also all of its predators.

a.What problem could then arise with the pest, and how might it be overcome?

b.On the other hand, suppose the insect pest is completely removed by an insecticide. Its predator is left with not enough food to survive and dies off. What could happen to the populations of other pest species, too small to support the predator on their own, which the predator had previously helped to control?

c.In summary, which of the following would result in the most stable situation?

1. A simple environment with one prey species for each predator.
2.A complex environment with one prey species for each predator.
3.A simple environment with many prey species for each predator.
4.A complex environment with many prey species for each predator.

Ecology-Population Dynamics II P.1

b)Competition as a population regulation factor

It refers to the interaction of two organisms that strive for the same res_____ for survival. There are two conditions for competition to occurs:

• Organisms occupy si______ecological niches*. Competition can only occur where the n_____ of two organisms o______, otherwise they will not actually meet each other in the same habitat.

• The resources for living are l______.

I____specific competition is competition between members of different species, for available resources such as food, space, light and shelter.
I____specific competition occurs between members of the same population.. /

Q. Which type of competition is more intense, interspecific or intraspecific, why ?

i)Studies of Competition

Ecology-Population Dynamics II P.1

Eco-Population Dynamics II P.1

Example 1: Two species of Paramecium reared together vs reared separately

When the two species are cultured together P. aurelia has a competitive adv______ over P. caudatum for gaining food. After five days the numbers of P. caudatum start to decrease until, after about 20 days, the species has become'extinct', that is it has been competitively excluded.

P. aurelia takes longer to reach the stationary phase of growth than when grown in isolation, so is alsoaffected adversely by the competition, even though it is more successful than its competitor.

This helps to explain the sel______pressure for competitors to adapt to sep_____ niches. Under natural circumstances, the less successful competitor rarely becomes extinct, but merely becomes rare and may even increase in numbers again before achieving an equilibrium position.

Q.With reference to the diagram
a)what type of population growth curve is shown by the two species when grown in isolation?
b)what resources might the two species be competing for in the mixed culture? /

c)What factors giveP. aurelia competitive advantage over P. caudatum?

Interspecific Competition

i)Niche and Evidence of Competition

There are two possible niches for a species. The larger niche is the one it can occupy inthe absence of competition, that is, its fund______niche. Competition may 'eat into' the fundamental niche, and reduce it to the niche that the species can defend against competitors. This smaller niche is the real___ niche.

If a species is known to be able to inhabit a particular area and set of conditions (its fundamental niche) and yet is absent, it is natural to suspectthat competition may be occurring.

Eco-Population Dynamics II P.1

Q.The figure shows an example involving small burrowing rodents, called gofers. There are four species with different preferences for soil texture and depth.
All prefer deep soil but can tolerate shallow soils to different extents. Notice that the actual distributions (realised niches) are, in each case, smaller than the fundamental niches.
Assuming these distributions to be controlled by competition, what would happen to the distribution of:
a) Species D, if species C were removed?
b) Species C, if A and B were removed?
c) Species C, if species D were removed? /

ii)The possible Results of Competition:

In a) there is no overlap between the two species, hence no c______ can take place.
In b) there is overlap, and the result depends upon which species is the b_____ competitor within the area of overlap. For example, one species may be more efficient at finding and digesting food, or have more rapid reproduction, than the other.
In c) the niche of species A is totally enclosed within that of species B. Therefore, species A must be the better competitor if it is to sur____.
When individuals or groups of different species are forced into competition, there are two possible outcomes:

• There may be a eq______situation in which both species can live together butin red____ numbers

• or the less competitive species is el______altogether.

The latter phenomenon is known as comp ______
exclusion. /

iii)Competition between Plant Populations

Plants compete for l_____, n______and w____and living space.

Plants compete by increasing the h_____ of stems, shading area of leaves or by extending their roots; one form of competition is that plants and micro-organisms produce a variety of complex organic molecules which affect the growth of other living organisms. A kind of ch______warfare between rival species
They include anti______and growth inhibitors, such as penicillin which is produced by the fungus Penicillium and has antibiotic properties against certain bacteria. /

The odours of ar______plants are volatile secondary /by-products which are sometimes involved in interactions of this kind; it has been shown that some are adsorbed on to surrounding soil particles and inhibit the germ______or growth of surrounding plants.

Phenolic compounds that areleached into the soil from the litter of some plants have a similar effect.

Intraspecific Competition

Intraspecific competition is a very important density dependent factor that re______population sizes.

Results of Overcrowding

Laboratory experiments with rats show that when a certain high population density is reached, fecundity is greatly reduced even if there is no food shortage.

Various hormonal changes occur which affect repro______behaviour in a number of ways; for example, failure to copulate, infertility, number of abortions and eating of young by the parents all increase, and parental care decreases with consequent reduction in chances of survival.

There is also an increase in aggr______behaviour.

It has been shown that the number of eggs laid per day by fruit flies (Drosophila) decreases as the population density of flies increases, but food availability, competition for territory, mutual physical disturbance with possible hormonal changes are all factors which may be involved.

With plants, the number of s____ produced by each plant may be reduced at higher plant densities.

Territorial behaviour
Territorial behaviour or territoriality occurs in a wide range of animals, including certain fish, reptiles, birds, mammals and social insects.
It has been particularly well studied in bird populations. The s___ of the bird and sometimes a vis___ display, are means of asserting terr______claims, and intruders usually retreat, sometimes after a brief 'ritual fight' in which neither competitor is seriously damaged.
This has obvious advantages over a series of 'real' fights. There is little or no overlap between neighbouring territories of the same species. /

II)Human population

A)Historical development of the Human Population

We have become familiar in recent years with articles on the increasing number of people in the world and the potential threat of overpopulation. The increase in numbers is estimated at between 70 and 80 million per year, an annual growth rate of 1.9%.

/ Q.The graph shows rapid increases in the size of the human population at three times in history and pre-history. Say what kinds of development in the human way of life you think these three increases reflect ?

In human populations, checks have happened at various times. In many parts of the world there are hunger, malnutrition and fa_____, often accompanied by di______ such as cholera and typhus. One and a half million people died in the single Indian state of Bengal in 1943 due to famine. The Irish famines of 1845 and 1846 caused about one million deaths and more than a million people to emigrate.

The main cause of death, however, has always beendisease. Malaria, until recently, was a great scourge of mankind. Before 1946 there were 300 million cases of malaria a year, with 3 million deaths. With the massive use of insecticides, malaria has been reduced to 120 million cases a year, of which 100 million are in tropical Africa. In the fourteenth century plaguecarried away a quarter of Europe's population. Infectious diseases used to take a heavy toll, particularly amongst children. Today, however, the killer diseases in industrial countries are not the infectious ones but cancer, heart disease and various diseases of ___ age.

Interspecific competition is hardly a check on human population growth, but none-the-less a frightening amount of food is spoiled in storage by p____ which compete with us, quite favourably to themselves. In addition, our livestock and crops are consumed or attacked by pests, pathogens, consumers, predators of one sort or another.

Intraspecific competition is seen asw___, even in infanticide and abortion, or in crime, can account for large numbers of deaths and thus be a check on population growth. About twenty-two million people, soldiers and civilians, died in five years during the second World war, nine million of them Jews in the gas chambers and other Nazi pogroms. But even this vast number is such a s____ part of the total world population, and more babies were born than normal (the bulge) after the war. Indeed, in a world population which is increasing at the rate of 70 million per year, twenty-two million of these extra births would occur in about four months.

Conscious individual choice on the number of children and when to have them is a recent feature. We shall increasingly rely on it as improved medical and environmental technology continues to reduce the mor____ rate. In some countries, laws and the choice of society control the population to a certain extent.

Space and climate may have set a limit to man's increase in the early days of our evolution, when a large area was needed to support a food-g______and h_____ family. Space or other resources may well prove a l______factor in the future.

B)The Population crisis ?

The recent growth of the human population gives rise to a lot of speculation about the shape of the curve to come. Natural populations show first a sigmoid growth curve and then one or more of a number of possible patterns: a steady equilibrium, oscillations, or a J shape crash.
Is the human population graph a sigmoid curve still in its log phase? Is it a J-shaped curve which will collapse soon or eventually? Will it move into a phase of steady equilibrium or will it show oscillations and fluctuations?
Earlier increases in the human population of the world have probably gone hand in hand with an increase in the c______c______of the environment
The increased carrying capacity may have resulted from new ter______ being made available (by means of travel, perhaps by changes in climate) or greater e______in food production. We do not yet know at what level the human curve will be forced to stop rising.. /

The carrying capacity of the world could be increased by our own efforts, or we could ex____ the carrying capacity for some time, like the deer on the Kaibab Plateau.