Population Growth and Regulation

What is Ecology and Population Ecology?

Ecology

The study of the interactions of organisms with each other and with the physical world

Biotic component- living things

Abiotic component -nonliving (water, soil & weather)

Ecosystem - includes everything within a defined area (e.g. an island)

Community - all interacting populations of organisms

Population Ecology

Relationships that influence size, structure and distribution of populations

Characteristics of Populations

Population - group of individuals of the same species occupying a given area (ecosystem)

The gene pool of the population is the basis of the range of characteristics found in the population

Demographics - vital statistics of a population

Population size, density, distribution and age structure

Population Ranges

No population occurs in all habitats throughout the world

Most species have limited geographical ranges

Devil’s hole pupfish lives in one hot spring in southern Nevada

Common dolphin is found however in all the world’s oceans

Each population has it sown requirements - temperature, humidity, certain types of food and a host of other factors - determine where a population can live & reproduce and where it can’t

The presence of predators, competitors or parasites may also prevent a population from occupying an area

Range Expansions and Contractions

Population ranges are not static

Environmental changes -e.g. as glaciers retreated and the end of the last ice age - plant and animal populations moved north - also observed shifts in elevation at which they could live

Populations expand ranges when they are able to circumvent inhospitable habitat

Cattle egret originally from Africa arrived in South America in the late 1800’s after crossing the Atlantic (2000 miles)

Now are found widely in the US

Population Size

Three factors effect population size

Birth, deaths and migration in and out

Births and immigration (migration in) add to a population

Deaths and emigration (migration out) decrease population size

Thus: (births - deaths) + immigrations - emigrations)= change in population size

Population Size

Migration in many populations has little effect on the change in population size - thus births and deaths are the main factors effecting growth rate

Interaction of biotic potential (max rate of growth under ideal conditions) and environmental resistance (curbs on population growth caused by the living and non living environment)

Population Growth Rate

Growth Rate (r) - measure of the change in population size per individual per unit of time

b= birth rate & d = death rate

Growth Rate: r = b - d

Example: Annual growth rate of a population of 1000 with 150 births and 50 deaths each year

r= 0.15 - 0.05 = 0.1 or 10% per year

Population Growth Per Unit Time

G= r x N

G= population growth per unit time

R= growth rate

N= population size

Example: (population of 1000)

G-= 0.1 x 1000 = 100 (in the first year)

If this growth rate is constant then:

Following year G = 0.1 x 1100 = 110

Third year G = 121 and so on

This pattern of accelerating growth is called exponential growth

Influences on Biotic Potential

The age when the organism first reproduces

Frequency at which reproduction occurs

The average number of offspring produced each time an organisms reproduces

The length of the organisms reproductive lifespan

The death rate of individuals under ideal conditions

Exponential Growth

Thus during a given period of time the population grows by a fixed percentage of its size at the beginning of the time period

Thus an ever increasing number of individuals is added during each succeeding time period

The graph of exponential growth is J shaped

Populations approximate exponential growth whenever births exceed deaths

Exponential Growth in Bacteria

Exponential Growth in Eagles

Effects of Death Rates of Population Growth

Boom and Bust Cycles of Growth

Exponential growth occurs in populations where rapid growth is followed by sudden and often massive die-off

Boom-and-bust cycles

Algae and insects show this with seasonal population cycles

These may temperature or rainfall related cycles

Lemmings and other rodents show four year cycles

Exponential Growth in New, Favorable Habitats

Some populations do not show boom-and-bust cycles but may still have exponential growth

Whooping cranes - protected from hunting and human disturbance in 1940 - exhibit exponential growth

Cane toads introduced into Australia to control beetles destroying sugar cane -encountered few predators

Logistic Growth

Environmental resistance limits population growth

Skin cells growing in a sterile culture dish - they initially show exponential growth that slows as they occupy all the space in the dish and eventually the growth rate would drop to zero & the population would be stable

Logistic formula

G= rN [(K-N)/K]

K= carrying capacity = max population size that can be sustained by an ecosystem for an extended time without damage to the ecosystem

S shaped growth curve

S Curve of Logistic Growth

Logistic Formula

G= rN [(K-N)/K]

K-N in the new multiplier [(K-N)/K] - is where we are subtracting the current population from the carrying capacity - this is the number of individuals that can still be added to the current population

Logistic Formula

Dividing this new number by K (carrying capacity) gives the fraction of the carrying capacity that can still be added to the current population before it stops growing

Thus when N is small (K-N)/K is close to 1 and the equation resembles exponential growth

However as N increases K-N will approach zero - growth rate will slow

When N equals the carrying capacity - growth ceases (G=0)

Population Density

Number of individuals in some specified area or volume of a habitat

Habitat - the type of place where a species normally lives

Habitat characterized by physical features, chemical features and the presence of other species

Population Dispersion

The general pattern in which individuals of the population are dispersed through a specified area

Three patterns

Dispersed in clumps, nearly uniformly or randomly

Population Dispersion

Clumping is the most common pattern

1. Each specie is adapted to a limited set of ecological conditions & these are usually not uniformly distributed

For example - some areas are shady and some are not

2. Social groupings common - for survival &reproduction

3. Adults of many species cannot disperse their seeds larvae or other immature forms very far

e.g. Sponges are sessile& their larvae cannot swim far and settle near the adult

Density-Dependent Factors

Density dependent factors limiting growth

Availability of sustainable resources

Refuge from predators

Living quarters

Pollution-free environment

Density-Independent Factors

Weather

Rampaging weather -diminishes population size (cold winters, hot summers, droughts, storms or even volcanic eruptions)

Fewer births , more deaths

Pesticides

Heavy application kills not only insects but mice birds, and also other larger mammals such as cats and raccoons

Density-Independent Effects in Moths