05 - Population growth & age structure

what affects population size

immigration and birth add to population

emigration and mortality take away from population

population in unlimited env

Nt+1=Nt + Bt + It −Dt − Et

population in isolated env

Nt+1 = Nt + Bt - Dt

Population growth rate

change in the number of

individuals (∆N) over time (∆t)

types of growth in closed system

exponential and logistic

Exponential growth

(density independent)

•Growth rate does NOT depend on number of individuals

•Usually occurs when species …

I. Colonize new habitat

II.Recolonize habitat (after local extinction)

per capita

per individual

per capita growth rate equation

∆n/∆t = rN

r = constant (per capita growth rate)

exponential growth equation

Nt = (N0)(e^rt)

Nt = future population size (at time t)

N0 = current population size (at t = 0)

e = base of natural log

r = intrinsic growth rate

t = time

Organisms that have a higher r will grow

faster

Logistic growth

(density dependent)

•Growth rate DOES depend on number of individuals

• Most populations show this type of growth

Carrying capacity (K)

maximum number of individuals in a population that can be supported by that population

K depends on:

o Food

o Space

o Water

o Soil quality

o Resting/nesting sites

o Life history strategies

logistic growth equation

∆n/∆t = rN [k-n/k ]

logistic growth rate___ as N approaches K

decline

Demography

study of factors that determine size and structure of populations through time

Age structure

– the number of individuals alive at each

age within a population

Young-heavy population =

more future reproduction = potential for growth.

Older-heavy population =

declining future population.

Survivorship Curves: Type I:

High survival until old age (e.g., humans).

Survivorship Curves:Type II:

Constant survival rate (e.g., birds).

Survivorship Curves:Type III:

Low early survival, high once matured (e.g., fish, insects).

Life history strategy

the overall pattern in average timing of events

related to growth, development, reproduction, and survival in a population.

life history traits include

1. Age and size at sexual maturity

2. Amount and timing of reproduction

3. Survival and mortality rates

Tradeoffs in traits can involve:

1. Growth versus reproduction

2. Early versus late maturity

3. Few large versus many small offspring

Optimal strategy varies with ecological characteristics:

1. Abiotic conditions

2. Community composition

3. Resource availability

Growth versus reproduction tradeoff

•Individuals that delay maturity and spend more time growing are typically

LARGER as adults

•For many organisms, size is correlated with fecundity and competitive

ability (larger = more offspring / higher fitness)

what happens when mortality risks are high for all age groups

• Organisms that mature early have higher fitness if it increases their chances of reproducing before they die

• Waiting too long to mature → death before reproducing at all (zero fitness)

what happens when juvenile mortality is high but adult mortality is low

• Organisms that delay maturation can become larger / more fecund adults, and have higher fitness through lifetime reproductive efforts (i.e., more children in lifetime)

• They may also have higher fitness because they’re bigger and more competitive, able to access more resources

Opportunistic life history

individuals with high fecundity, grow

quickly, reach sexual maturity at early age, produce many/small

offspring (r-strategists))

R-selected species

High r

Many offspring

Little to no parental care

Early maturity

Thrive in unpredictable environments

Equilibrial life history

– individuals with high survivorship, grow

slowly, invest more energy into protection/survival/acquiring

resources, reach sexual maturity later and have few/large

offspring (i.e., k-strategists)

K-selected species:

Lower r

Fewer offspring

High parental care

Stable, competitive environments

Invest energy in survival and quality