POPULATION ECOLOGY

MEGHA GUPTAJYOTSNA KAPOOR

What is ECOLOGY ?• Ecology – study of all relationships among

organisms and their abiotic environment• Population- all the members of particular species

that live together in the same area at the same time.

• Community – all the populations of different species that live together in the same area

• Ecosystem – a community and its environment• Biosphere – global ecological system comprises

all the communities on earth & includes all interactions among different communities & earth’s atmosphere, lithosphere and hydrosphere.

Components of Biosphere

POPULATION ECOLOGY ????

• Branch of biology that deals with the number of individuals of a particular species that are found in an area & how and why those numbers change over time.

Attributes of a Population1.1. Natality/ Birth RateNatality/ Birth Rate : ratio of live births in an

area to the population of an area2.2. Mortality/ Death RateMortality/ Death Rate : ratio of deaths in an

area to the population of an area3.3. Population Density :Population Density : number of individuals of

a population present per unit area at a given time

4.4. Sex Ratio Sex Ratio : number of females or males per thousand individuals

5. Age DistributionAge Distribution : percentage of individuals of different ages in a given population. Commonly presented through age pyramids.

Age Pyramid Types

• Expansive population pyramids depict populations that have a larger percentage of people in younger age groups. Populations with this shape usually have high fertility rates with lower life expectancies.

• Constrictive population pyramids are named so because they are constricted at the bottom. There is a lower percentage of younger people. Constrictive population pyramids show declining birth rates, since each succeeding age group is getting smaller and smaller. The United States has a constrictive population pyramid.

• Stationary population pyramids are those that show a somewhat equal proportion of the population in each age group. There is not a decrease or increase in population: it is stable. Austria has a stationary population pyramid.

Age Pyramid Types

6. Population Dispersion : Population Dispersion : refers to the spatial & temporal pattern of individuals of a population. Types are : - Clumped (- Clumped ( Individuals in a population are clustered together, creating some

patches with many individuals and some patches with no individuals.) - Uniform (- Uniform (Individuals are spaced evenly throughout an area. ) - Random (- Random ( Individuals are arranged without any apparent pattern ) )

Population Dispersal• The movement of individuals into or out of the

population or the population area. EMIGRATION – one way outward movement of individuals from an area. IMMIGRATION – one way inward movement of individuals into an area. MIGRATION – periodic departure & return of individuals to same area.

• Population size is affected by :- No. of births (b)- No. of deaths (d)- Immigrants (i)- Emigrants (e)• Growth Rate of Population (r) : a)rate of change in size [r = b – d] b) population ↑’s in size as long as b>d c) r= (b-d) +(i-e) { when migration is a factor}

PopulationPopulation DynamicsDynamics

Survivorship Curves • Survivorship curves can be classified into three general

types:• Type I: low death rates during early and middle

life, then an increase among older age groups E.g. : Humans and many other large mammals

• Type II: the death rate is constant over the organism’s life span E.g.: Some birds and some lizards

• Type III: high death rates for the young, then a slower death rate for survivors. E.g.: Oysters produce millions of eggs, but most larvae die from predation or other causes

Survival CurvesSurvival Curves

Population Growth ModelsPopulation Growth Models• Population growth rate: the change in a population over a unit time

period.

• Population growth rate = (births + immigration) – (deaths + emigration) x 100 initial population

• A positive growth rate indicates that the population is increasing • A negative growth rate indicates that the population is decreasing• A growth rate of zero indicates there was no difference between

birth rates and death rates• Some species reproduce continuously so the sizes of these

populations have the potential to increase exponentially by a contrast ratio per unit of time. Ex, bacteria, virus

Exponential Growth ModelExponential Growth Model• Associated with the name of Thomas Robert

Malthus (1766-1834) who first realized that any species can potentially increase in numbers according to a geometric series.

• Populations that grow exponentially increase in numbers rapidly, resulting in a J-shaped growth curve. As the population gets larger, it grows faster and becomes steeper.

• We can determine the change in population size over time by using the equation

dN = B – D dt

dN dt = the change in population size over time.B=Birth rateD=death rate

• This equation can be used for ANY population if we know the exact number of births and deaths

Limitations

• Environmental limitations prevent populations from experiencing continuous growth

• Per capita birth rate decrease and the per capita death rate increases as competition for resources such as food and shelter occurs

Large sudden birth rates in Canada due to breeding periods (frogs reproduce in fixed

intervals) not continuously

Logistic Growth Model• Environmental limitations create this model. It describes limited

population growth, often due to limited resources or predation.• An environment only provides enough resources to sustain only a

limited amount of any species.• The maximum number of individuals that an environment can

support indefinitely is its carrying capacity (K).

• N=population size• If a population is very small then plenty of resources are available

and therefore the value of (K-N)/K is close to 1. • Vice versa, if the population is large, few resources are available and

the value of (K-N)/K is small and the per capita growth rate is very low.

• Lastly, if the size of the population exceeds the carrying capacity of the habitat then the population would decrease from lack of food and other resources. If this is graphed it produces an S shape curve.

Logistic growth can be seen in a population of fur seals on St. Paul Island, Alaska. In 1911, fur seal hunting was banned on St. Paul Island, since the population had become extremely low. Because their numbers were so severely depressed, the seals had many unused resources to support the recovering population. The population began to grow rapidly until it stabilized around its carrying capacity.

Lag phase: Growth is slow because the population base is small, organisms adapt. Exponential growth phase: Growth is accelerating, that is, many offspring born at a fast rate.Deceleration phase: The rate of population growth slows down – individuals start to die. Stable equilibrium phase: Little growth because births and deaths are about equal. Area has reached its carrying capacity.•Maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water and other necessities available in the environment.

• The logistic population growth model and life histories.– This model predicts different growth rates for

different populations, relative to carrying capacity.• Resource availability depends on the situation.• The life history traits that natural selection favors may

vary with population density and environmental conditions.

• In K-selection, organisms live and reproduce around K, and are sensitive to population density.

• In r-selection, organisms exhibit high rates of reproduction and occur in variable environments in which population densities fluctuate well below K.

K- Selected Species• Good colonizers• Reach sexual maturity

rapidly• Short-lived• High fecundity• Low investment in care

for the young• Generalists• Poor competitors

r-Selected Species

• Poor colonizers• Slow maturity

• Long-lived• Low fecundity• High investment in care

for the young• Specialist• Good competitors

Factors that influence and regulate population growth

• There are two general questions about regulation of population growth:

• What environmental factors stop a population from growing indefinitely?

• Why do some populations show radical fluctuations in size over time, while others remain stable?

Factors that influence and regulate population growth : density

• Density dependent factors : competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors

• Density independent factors : Environmental factors ; ex:

hurricanes, fires etc

Density Dependent Factors1. Competition for Resources• In crowded populations, increasing population

density intensifies competition for resources and results in a lower birth rate.

• Resources like: water, shelter, food, space, access to mates, ecological niches.

2. Territoriality• In many vertebrates and some invertebrates,

competition for territory may limit density• Cheetahs are highly territorial, using chemical

communication to warn other cheetahs of their boundaries.

3. Diseases•Population density can influence the health and survival of organisms•In dense populations, pathogens can spread more rapidly.

4. Predation•As a prey population builds up, predators may feed preferentially on that species.•This will decrease the prey population size – and later decrease the predator population size. – Predator-prey relationship (Graph)

5. Toxic WasteAccumulation of toxic wastes can contribute to density-dependent regulation of population size.

6. Intrinsic / Physiological FactorsFor some populations, intrinsic (physiological) factors appear to regulate population size.