Population Ecology

Population Dynamics and Carrying Capacity

Population dynamics- study of how populations change in size, density, and age

distribution- populations respond to their environment- change according to distribution

Dynamics of Natural Populations

• Population growth curves• Biotic potential - the ability to increase

population numbers• Environmental resistance - the combination of all

the biotic and abiotic factors that limit a population’s increase.

• Carrying capacity – the upper limit to the population of any particular organism that an ecosystem can support

Density Dependence And Critical Number

• Environmental resistance factors can be density dependent.– If population density increases, environmental resistance

becomes more intense and causes in increase in mortality.– If population density decreases, environmental resistance

lessens, allowing the population to recover.– Food, Water, Disease, Predation

• Environmental factors that cause mortality can be density independent– A sudden deep freeze in spring– A fire that may kill all small mammals– Natural Disasters

• Biotic Potential– Reproductive rate– Ability to migrate

(animals) or disperse (seeds)

– Ability to invade new habitats

– Defense mechanisms– Ability to cope with

adverse conditions

• Environmental resistance– Lack of food or nutrients– Lack of water– Lack of suitable habitat– Adverse weather– Predators– Disease– Parasites– Competitors

Biotic Potential and Environmental Resistance

Exponential and Logistic Growth

LOGISTIC GROWTH

- Rapid exp. growth followed by steady dec. in pop. Growth w/time until pop. Size levels off

EXPONENTIAL GROWTH-Population w/few resource limitations; grows at a fixed rate

Natural Population Curves

• STABLE– pop. Size fluctuates above or below its carrying capacity– Stable population size– EX: undisturbed tropical rain forests

• IRRUPTIVE– pop. Growth occasionally explodes to a high peak then

crashes to stable low level– EX: Algae, insects

• CYCLIC– Fluctuations occur in cycles over a regular time period– EX: Lynx & snowshoe hare

• IRREGULAR– No recurring pattern in changes of population size

The Role of Predation in Controlling Population Size

Top-down control- lynx preying on hares periodically reduce the hare pop.

Bottom-up control- the hare pop. may cause changes in lynx pop.

Species Interactions

• Niche• Competition

– Interspecific– Intraspecific

• Symbiotic Relationships– Mutualism– Parastism– Commensalism

How do Species Reproduce• ASEXUAL REPRODUCTION

– all offspring are exact genetic copies of a single parent– Common in single celled species (bacteria)– Each cell divides to produce 2 identical cells

• SEXUAL REPRODUCTION– Organisms produce offspring by combining sex cells or

gametes from both parents– Produces offspring with combination of genetic traits from

each parent– Provides greater genetic diversity in offspring

• DISADVANTAGES– Males do not give birth– Increased chance of genetic errors and defects– Courtship & mating rituals consume time & energy

and transmit diseases

Reproductive Patterns and Survival r-selected species vs. K-selected species

Fig. 9-10 p. 170

OBJ 9.10

Survivorship Curves•Shows the % of members in a pop. Surviving at different ages

LATE LOSS-High survivorship to certain age; then high mortality-EX: elephants, rhinos, humans

CONSTANT LOSS-Fairly constant death rate at all ages-EX: songbirds

EARLY LOSS-Survivorship is low early in life-EX: annual plants, bony fish sp.

Age Structure Stages• PREREPRODUCTIVE AGE

- Not mature enough to reproduce• REPRODUCTIVE AGE

- Capable of reproducing• POSTREPRODUCTIVE AGE

- too old to reproduce

Factors Governing Changes in Population Size

• Four variables– births, deaths, immigration and emigration

• Population Change = (births + immigration) – (deaths + emigration)

• Crude Birth Rate = CBR = (births/population)*1000

• Crude Death Rate = CDR = (deaths/population) *1000• Immigration and emigration are calculated the same way

• Crude Growth Rate = CBR = CDR

• Population Growth Rate = CGR * 100

Calculating Population Growth

• N0 is the starting population • N is the population • after a certain time, t , • has elapsed, • r is the rate of natural increase expressed as a percentage

(birth rate - death rate) and • e is the constant 2.71828... (the base of natural

logarithms)

Growth Curves – Two Types J or S

• Exponential growth results in population explosion

• Rule of 70• to find the doubling time of a

quantity growing at a given annual percentage rate, divide the percentage number into 70 to obtain the approximate number of years required to double.

• For example, at a 10% annual growth rate, doubling time is 70 / 10 = 7 years.

• This results in a J curve graph.