Quantitative Ecology

8/4/2019 Quantitative Ecology

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Quantitative EcologyQuantitative EcologyQuantitative EcologyQuantitative Ecology


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Popoulation Ecology Biotic potential Natality

Mortality Migration Survivorship r and k strategies

Factors limitingpopulation size anddistribution

Liebigs Law Shelfords Law Carring capacity

Sigmoid shape ofgrowth curve


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Population ecology

is a major sub-field of ecology that deals

with the dynamics of species populationsand how these populations interact withthe environment

Population ecology has also played an

important role in the development of thefield of conservation biology especially inthe development of population viabilityanalysis (PVA) which makes it possible to

predict the long-term probability of aspecies persisting in a given habitat patch(e.g., a national park).


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Population genetics is the study ofgene pools and evolution whereas

population ecology is the study ofhow biotic and abiotic factorsinfluence the density, size,

distribution, and age of a population


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Biotic potential

(Science: biology, ecology)

The potential growth a population ofliving things can expect if it wereliving under ideal environmental

circumstances. It is when thepopulation just keeps on growing andgrowing.


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Biotic potential is the maximumreproductive capacity of a population

under optimum environmentalconditions.

Full expression of the biotic potential

of an organism is restricted byenvironmental resistance, anycondition that inhibits the increase innumber of the population.


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It is generally only reached whenenvironmental conditions are very

favorable. A species reaching its biotic

potential would exhibit exponential

population growth and be said to havea high fertility, that is, how manyoffspring are produced per mother.


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Biotic Potential is a fundamentalspecies characteristic, defined by

Chapman (1925) as "the inherentpower of organisms to reproduce andsurvive".

In 1931, Chapman redescribed it as:"It is a sort of algebraic sum of thenumber of young produced at eachreproduction, number ofreproductions over a period of time,sex ratio of the species, and theirgeneral ability to survive under given

physical conditions."


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Chapman relates to a "vital index": Vital Index = (number of births/number

of deaths)*100 Biotic potential is the highest

possible vital index of a species;

therefore, when the species has itshighest birthrate and lowestmortality rate.


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If the potential value of populationincrease can be determined, the impact of

the environment upon the population alsocan be determined.

Compute the biotic potential (potentialincrease) and subtract the actual or

observed value of decrease; thisdifference represents how effective theenvironment is in preventing the speciesfrom attaining its full potential.

Chapman called difference betweenpotential and actual value theenvironmental resistance.


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Components of Biotic Potential

Reproductive potential - potential natality:

It is the upper limit to biotic potential (inthe absence of mortality)

Survival potential: Because reproductivepotential does not account for the number

of gametes surviving, survival potential is anecessary component of biotic potential; itis the reciprocal of mortality

(in the absence of mortality, bioticpotential = reproductive potential)


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Chapman identified two components:

nutritive potential - the ability to

acquire and utilize food for growthand energy

protective potential - potential

ability of the organism to protectitself against the dynamic forces ofthe environment

assuring successful fertilization(mating)

care of young


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Natality

Definition

noun, plural: natalities

The birthrate, which is the ratio oftotal live births to total population in

a particular area over a specifiedperiod of time; expressed aschildbirths per 1000 people (or

population) peryear


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Noun

natality (plural natalities)

The ratio of live births in an areato the population of that area;expressed per 1000 population per

year. birth rate, fertility rate


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Mortality rate is a measure of thenumber of deaths (in general, or due

to a specific cause) in somepopulation, scaled to the size of thatpopulation, per unit time.


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It is distinct from morbidity rate,which refers to the number of

individuals in poor health during agiven time period (the prevalencerate) or the number who currently

have that disease (the incidencerate), scaled to the size of thepopulation.


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Migration Animals travel in their

billions all over theEarth, sometimes -

like the Arctic Tern -covering globaldistances.

Animal migration

represents amasterpiece ofbiological adaptationand programming.


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Migration refers to directed,regular, or systematic movement of a

group of objects, organisms, orpeople


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What is migration and why do it?

Animals that live in habitats that aredifficult to survive in year round, mustevolve a way to cope with the difficult

time of year. A strategy used by many mammals andother species is hibernation.

Migration is another option for animals

that can move across long distances. They survive by leaving the area for part

of the year or part of their life, and moveto habitats that are more hospitable.


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A population is defined as all the organismwithin an area belonging to the same

species. OR A localized group of individuals of thesame species that can interbreed,producing fertile offspring.


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Population density is the number ofindividuals of a certain species per

unit area or volume, and populationdistribution is the pattern ofdispersal of them within that area.


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The density and distribution of apopulation changes with time, due to

abiotic factors(inorganic factors) aswell as biotic factors(organicfactors).


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Asurvivorship curve is a graphshowing the number or proportion of

individuals surviving at each age for agiven species or group (e.g.males/females).

Survivorship curves can beconstructed for a given cohort (agroup of individuals of roughly thesame age) based on a life table.


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Define survivorship

curve. A plot of the number of members of

a cohort that are still alive at each

age; one way to represent age-specific mortality.


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Describe a Type I survivor shipcurve and what it says about the

population. Flat at the start, reflecting low

death rates during early and middle

life, and then drops down steeply asthe death rates increase among olderage groups; produce few offspringbut provide good care


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Describe a Type II survivor shipcurve and what it says about the

population. Constant death rate over an

organisms life span; predators dont

care whether full grown or baby?


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Describe a Type III survivor shipcurve and what it says about the

population. Drops sharply at the start, reflecting

very high death rates for the young,

but flattens out as death ratedecline for those individuals thatsurvive the early period of die-off;produce large number of offspringbut dont provide good care at all


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There are threegeneralized types

of survivorshipcurve, which aresimply referred toas Type I, Type II

and Type IIIcurves.


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Type I survivorship curves are characterized byhigh survival in early and middle life, followed arapid decline in survivorship in later life. Humans

are one species that show this pattern ofsurvivorship.

Type II curves are an intermediate between TypeI and III, where roughly constant mortality rate

is experienced regardless of age. Some birdsfollow this pattern of survival.

In Type III curves, the greatest mortality isexperienced early on in life, with relatively lowrates of death for those surviving this

bottleneck. This type of curve is characteristic ofspecies that produce a large number of offspring(see r/K selection theory).


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The number orproportion oforganisms surviving isplotted on they-axis,

generally with alogarithmic scalestarting with 1000individuals, while theirage, often as aproportion ofmaximum life span, isplotted on the x-axis.


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Survivorship curves denote mortalitypatterns of a certain population over

a certain period of time. It may varyin abnormal conditions, but in mostcases the pattern stay predictable.


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Curve I is characteristic of a population inwhich most individuals survive well passthe midpoint.

On the contrary, Curve III typifiespopulations wherein most individuals dieyoung.

In the type II curve, survivorshipdecreases at a constant rate throughoutthe lifespan.


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r/K selection theory

In ecology, r/K selection theoryrelates to the selection of

combinations of traits that trade offthe quantity and quality of offspringto promote success in particularenvironments.


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In r/K selection theory, selectivepressures are hypothesised to drive

evolution in one of two generalizeddirections: r- or K-selection.

These terms, r and K, are derived

from standard ecological algebra, asillustrated in the simple Verhulstequation of population dynamics


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where r is the growth rateof the population (N), andK is the carrying capacityof its local environmentalsetting. Typically, r-selected species exploitless-crowded ecologicalniches, and produce manyoffspring, each of which

has a relatively lowprobability of surviving toadulthood.


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In contrast, K-selected species arestrong competitors in crowded

niches, and invest more heavily infewer offspring, each of which has arelatively high probability ofsurviving to adulthood.

In the scientific literature, r-selected species are occasionallyreferred to as "opportunistic", while

K-selected species are described as"equilibrium".


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Carring capacity of

enviromnent (K) Carrying capacity is the total number of organisms that canbe supported by the environmental resources in anecosystem

The area occupied by a population has limited the resourcesand this limits the population growth by maintaining anequilibrium between the natality rate and the mortalityrate.

Population size increases until it reaches saturation withincaring capacity of its ecosystem

The carring capacity of an ecosystem is not constant as it isaffected by environmental conditions.


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For r-selected species, "r" refersto the growth rate term in thelogistic population growth model.For these species, populationsizes and mortality tend to bevariable and unpredictable. Since

populations frequently are farfrom carrying capacity ("K"),intraspecific competition often isweak. Selection tends to favorindividuals with rapiddevelopment, high and earlyreproduction that is notrepeated, small body sizes, highresource requirements, and shortlives. The potential forpopulations of r-selected speciesto grow is large.


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In contrast, k-selectedspecies have moreconstant mortality andpopulation sizes that often

are close to carryingcapacity. Intraspecificcompetition tends to bestrong. Selection favorsslower development, late,repeated reproduction,

long lives, and efficient useof resources.


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Population growth

Population growth is the change inpopulation over time, and can be

quantified as the change in thenumber of individuals in a populationusing "per unit time" formeasurement


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The term population growth can technicallyrefer to any species, but almost alwaysrefers to humans, and it is often usedinformally for the more specificdemographic term population growth rateand is often used to refer specifically to

the growth of the population of the world.


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Population growth rate

In demographics and ecology, Populationgrowth rate (PGR) is the fractional rate atwhich the number of individuals in a

population increases. Specifically, PGR ordinarily refers to thechange in population over a unit timeperiod, often expressed as a percentageof the number of individuals in the

population at the beginning of that period.This can be written as the formula:


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The above formula can be expandedto:

growth rate = crude birth rate -crude death rate + net immigrationrate, or P/P = (B/P) - (D/P) +

(I/P) - (E/P), where P is the total population, B isthe number of births, D is thenumber of deaths, I is the number ofimmigrants, and E is the number ofemigrants.


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In the natural world, limiting factorslike the availability of food, water,

shelter and space can change animaland plant populations. Other limitingfactors like competition forresources, predation and disease canalso impact populations.

If any of the limiting factorschange, animal and plant populations

change, too.


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Limiting Factors Limiting factors are things that prevent a

population from growing any larger. Forexample, 10 rabbits may live in a habitatthat has enough water, cover and space tosupport 20 rabbits, but if there is onlyenough food for ten rabbits, the

population will not grow any larger. In thisexample, food is the limiting factor.


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Food is not the only factor that maylimit population growth. For example,

there may be enough food to supporta thousand birds in a certain area,but only suitable nesting sites forone hundred.

Or perhaps there is plenty of food,water, cover and space to support alarger population of pheasants in an

area, but predators are the limitingfactor.


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Limiting factors are very closely tiedto carrying capacity. Many kinds of

animals can increase in numbers veryquickly, and may temporarily exceedthe carrying capacity of theirhabitat.

This results in stress, starvation,disease, predation and parasites,poor reproductive success and

damage to the habitat.


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For example,

multiplying muskrats can very quickly eat

all the vegetation in a marsh.W

ith thevegetation gone, food becomes the limitingfactor and the muskrats may starve ormove to another area. The marsh now has a

reduced carrying capacity for muskratsuntil the vegetation grows back again.


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imatic an iotic actors


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imatic an iotic actorsaffect the size of a

population Climatic factors:temperature, light

intensity, wind, water current,

oxygen Biotic factors:food source,

competition, predation, parasitism


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The size and density of a populationare affected by various factors.Some of the important ones are:

a) Birth rate orNatality rate b) Death orMortality rate c) Age distribution (Agecomposition)


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d) Immigration

e) Emmigration

f) Carrying capacity (Resources) g) Natural calamities

h) Abiotic and biotic factors

i) Population fluctuations and cycles

Differences between


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Differences betweenNatality rate and Mortality rate

c Age istri ution Age


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c Age istri ution Agecomposition)


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Tutorial 16 (a) Sketch a graph showing the survivorshipcurves for a population of human beings (kstrategists), a population of oysters ( rstrategists) and a population of hydras.(3 marks)

(b) Using the graphs in (a), explain the survivalpatterns of the population of humans, oysters andhydras.(5 marks)

Describe the adaptation characteristics of

organisms which use K and r strategists tosurvive.(7 marks)


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Population ecologists commonly dividethe factors that regulate the size of

populations into density-dependentand density-independent factors.


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Density-independent factors, such asweather and climate, affect the sameproportion of individuals in a populationregardless of population density.

In contrast, the effects of density-dependent factors intensify as the

population increases in size....


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Density dependent

factors Density dependent factors typicallyinvolve biotic factors, such as theavailability of food, parasitism,predation, disease, and migration. Asthe population increases, foodbecome scarce, infectious diseases

can spread easily, and many of itsmembers emigrate.


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Density Independent Factor

Afactor that affects the size of apopulation independent or regardless ofthe population density.

In ecology, density independent factorsare the physical or abiotic factors like

weather, forest fire, pollutant, etc.

What are examples of density


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What are examples of densityindependent factor?

weather, climate


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Interspecific

competition

Interspecific competition, in

ecology, is a form of competition inwhich individuals of different speciesvie for the different resource in anecosystem (e.g. food or living space).


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Intraspecific competition intraspecific competition, which

involves organisms of the same

species sleeping with one from theother.


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Competition is only one of many interactingbiotic and abiotic factors that affectsexual community structure. Moreover,competition is not always astraightforward, direct, interaction.Interspecific competition may occur whenindividuals of two separate species share alimiting resource in the same area.


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Interference competition involvesdirect interactions between

individuals such as fighting overlimiting resources.


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This form of competition is typically, though not always,

detrimental to both individuals and both species involved.

These interactions are usually asymmetric with one specieshaving an advantage over the other resulting in greater lossby one competitor.

For example, large predators like canids have a significant

size advantage over smaller predators such as foxes orweasels.

A violent interaction between two of these species wouldlikely result in victory for the larger predator. The stabilityof some mammalian carnivore populations is maintainedthrough these conflicts over prey resources.


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An example of interference competition in whichonly one party is negatively affected isallelopathy, in which plants of one species releasetoxic chemicals that inhibit the germination andsurvival of other potential competitors.

Some animals utilize a similar strategy. Forexample, algae in the feces of the common frog,

Rana temporaria, inhibit the tadpoles of thecompeting natterjack toad, Bufo calamita.


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Trees in this Bangladeshi forest are in competition for light.


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Population dispersion The spatial distribution at any

particular moment of the individuals

of a species of plant or animal.


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Under natural conditions organismsare distributed either by active

movements, or migrations, or bypassive transport by wind, water, orother organisms.


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The act or process of disseminationis usually termed dispersal, while the

resulting pattern of distribution isbest referred to as dispersion.


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Dispersion is a basic characteristic of populations,controlling various features of their structure and

organization.

It determines population density, that is, the number ofindividuals per unit of area, or volume, and its reciprocalrelationship, mean area, or the average area per individual.

It also determines the frequency, or chance ofencountering one or more individuals of the population in aparticular sample unit of area, or volume.

The ecologist therefore studies not only the fluctuationsin numbers of individuals in a population but also thechanges in their distribution in space.

l f


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Principal types of

dispersion The dispersion pattern of individuals

in a population may conform to any

one of several broad types, such asrandom, uniform, or contagious(clumped).


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This research attempts to determine the environmental factors


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(abiotic factors) and species interactions (biotic factors) that may

explain the observed patterns


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To calculate the dispersion patternthe study area is divided into

quadrats. The number of individualsin each quadrat are then counted andthe mean number of organisms perquadrat and the variance among

quadrats is calculated.


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Any pattern is relative to the spacebeing examined; a population may

appear clumped when a large area isconsidered, but may prove to bedistributed at random with respectto a much smaller area.


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Random or haphazard Random or haphazard implies that the individuals have beendistributed by chance.

In such a distribution, the probability of finding an individual atany point in the area is the same for all points.

Hence a truly random pattern will develop only if each individualhas had an equal and independent opportunity to establish itselfat any given point.

Examples of approximately random dispersions can be found inthe patterns of settlement by free-floating marine larvae andof colonization of bare ground by airborne disseminules ofplants.

Nevertheless, true randomness appears to be relatively rare innature.

U if di t ib ti


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Uniform distribution Uniform distribution implies a regularity of distance

between and among the individuals of a population.

Perfect uniformity exists when the distance from oneindividual to its nearest neighbor is the same for allindividuals.

Patterns approaching uniformity are most obvious in thedispersion of orchard trees and in other artificial plantings,but the tendency to a regular distribution is also found innature, as for example in the relatively even spacing oftrees in forest canopies, the arrangement of shrubs indeserts, and the distribution of territorial animals.


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contagious or clumped, indicating the existence of aggregations or groups in thepopulation.

Clusters and clones of plants, and families, flocks, andherds of animals are common phenomena.

The formation of groups introduces a higher order ofcomplexity in the dispersion pattern, since the severalaggregations may themselves be distributed at random,evenly, or in clumps.

An adequate description of dispersion, therefore, mustinclude not only the determination of the type ofdistribution, but also an assessment of the extent ofaggregation if the latter is present.

Factors affecting


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Factors affectingdispersion

(1) the action of environmental agencies of transport,

(2) the distribution of soil types and other physical features ofthe habitat,

(3) the influence of temporal changes in weather and climate,

(4) the behavior pattern of the population in regard toreproductive processes and dispersal of the young,

(5) the intensity of intra- and interspecific competition,

(6) the various social and antisocial forces that may developamong the members of the population.


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Liebig's law of the minimum

It states that growth is controlled not bythe total of resources available, but by thescarcest resource (limiting factor).

This concept was originally applied to plantor crop growth, where it was found thatincreasing the amount of plentiful

nutrients did not increase plant growth.


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Only by increasing the amount of thelimiting nutrient (the one most scarce inrelation to "need") was the growth of a

plant or crop improved. This principle can be summed up in the

aphorism, "The availability of the mostabundant nutrient in the soil is as available

as the availability of the least abundantnutrient in the soil."


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For example, the growth of anorganism such as a plant may be

dependent on a number of differentfactors, such as sunlight or mineralnutrients (e.g. nitrate or phosphate).


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The availability of these may vary, suchthat at any given time one is more limitingthan the others. Liebig's Law states thatgrowth only occurs at the rate permittedby the most limiting.

For instance, in the equation below, thegrowth of population O is a function of theminimum of three Michaelis-Menten terms

representing limitation by factors I, N andP.


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Shelford's law of tolerance

stating that the presence and success of anorganism depend upon the extent to which acomplex of conditions is satisfied (e.g. theclimatic, topographic, and biological requirementsof plants and animals).

The absence or failure of an organism can becontrolled by the qualitative or quantitativedeficiency or excess of any one of several factorswhich may approach the limits of tolerance forthat organism.


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M d


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Management and

conservation of ecosystems Conservation is important for the

following reasons:

(a) Utilitarian/pragmatic reasons (b) Ecological reasons

(c )Aesthetic reason

(d) ethical/moral reasons

I t f ti


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Importance of conservation

of the ecosystem Contain biological resources;medicine Biogeochemical cycles, extinction of some

species resulting in the destruction of an

ecosystem May lead to global climatic changes Ecosystems in their natural state provides

aesthetic values for humans

For future generations

I t f ti


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Importance of conservation

of the ecosystem Management and conservation

programmes are aimed at sustaining

the biodiversity of ecosystems andat the same time, maintaining orimproving the quality of life.


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Example:forests Contain large biodiversity;contain manyundiscovered species that may haveeconomial or medicinal values.

Producing rain as the transpiration ofplants in forest releases water intoenvironment

Important role in carbon cycle Maintaining soil fertility and in preventing

floods


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Sustainable development Development that can continueindefinitely.

It is achieved by minimising the useof non-renewable resources andcontrolling the use of renewableresources of the earth


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Example; Replanting programmes Creating forest reserves Restoration programmes of damaged

forest Selective logging Enforcement of laws and surveillance to

prevent illegal logging

Recycling of paper to reduce the demandfor new raw materials.


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Quantitative Methods Sampling theories Measures of

central tendency

Mean

Mode

Median

Measures ofdispersion

Range

Standard deviation

Standard error

Variance

sampling


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Types of estimation Objective methods Subjective methods


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Sampling methods Quadrat (i) Frame quadrats

(ii)Point quadrats Capture-recapture/mark-release-

recapture method


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Sampling parameters Species frequency Species density

Species cover

ECOLOGICAL


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ECOLOGICALSAMP

LINGMETHOD

S


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The usual sampling unit is a quadrat.Quadrats normally consist of a squareframe, the most frequently used size being

1m2 (see picture below). The purpose of using a quadrat is to enable

comparable samples to be obtained fromareas of consistent size and shape.


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Rectangular quadrats and even circularquadrats have been used in some surveys.

It does not really matter what shape ofquadrat is used, provided it is a standardsampling unit and its shape andmeasurements are stated in any write-up.


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It may however be better to stick to thetraditional square frame unless there are verygood reasons not to, because this yields data thatis more readily comparable to other published

research.(For instance, you cannot compare data obtainedusing a circular quadrat, with data obtained usinga square quadrat. The difference in shape of thesampling units will introduce variations in theresults obtained.)


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The estimation can be improved by dividing thequadrat into a grid of 100 squares eachrepresenting 1% cover.

This can either be done mentally by imagining 10longitudinal and 10 horizontal lines of equal size

superimposed on the quadrat, or physically byactually dividing the quadrat by means of string orwire attached to the frame at standard intervals.

This is only practical if the vegetation in the area

to be sampled is very short, otherwise thestring/wire will impede the laying down of thequadrat over the vegetation.


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There are three main ways of takingsamples.

1. Random Sampling.

2. Systematic Sampling (includes linetransect and belt transect methods).

3. Stratified Sampling.


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. SYSTEMATIC SAMPLING

a)Line Transect Method

b)Belt Transect Method

Line Transect Method


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Line Transect Method

A transect line can be made using anylon rope marked and numbered at

0.5m, or 1m intervals, all the wayalong its length. This is laid acrossthe area you wish to study.


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A line transect is carried out by unrolling thetransect line along the gradient identified. Thespecies touching the line may be recorded alongthe whole length of the line (continuous sampling).

Alternatively, the presence, or absence of speciesat each marked point is recorded (systematicsampling). If the slope along the transect line ismeasured as well, the results can then be insertedonto this profile.


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Belt Transect Method In this method, the transect line is laidout across the area to be surveyed and aquadrat is placed on the first marked point

on the line.

The plants and/or animals inside thequadrat are then identified and their

abundance estimated.

STRATIFIED


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STRATIFIED

SAMPLING Stratified sampling is used to take

into account different areas (orstrata) which are identified within

the main body of a habitat.

These strata are sampled separately

from the main part of the habitat.


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A Practical Study of theCapture/Recapture Method of

Estimating PopulationS

ize


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The capture/recapture technique (also known as the LincolnIndex) is used to arrive at estimates of the size ofpopulations of mobile animals, like ground beetles andwoodlice.

An initial sample of the population in question is caught, itsindividuals marked and then released back into the wild, anda note taken of the number released.

These marked individuals are allowed to become randomlydispersed throughout the population and then a second

sample is taken. Its size and the number in it of marked,and hence recaptured, individuals is noted.


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Quantitative Ecology