Life Constantly Undergoing Change

Life Constantly Undergoing Change

Ecosystem Change

Life constantly undergoing change

Populations change or evolve

Ecosystems evolve also

Change in populations size: population dynamics (more detail later)

Changes may include increase and decreases in population size

Growth Curves

Populations inherent ability to exponentially increase population size

They don’t



Change in size (total number of individuals)

Change in density (number of individuals in a certain space)

Change in age distribution (proportion of individuals in each age group)

Change in habitat distribution


Resources may vary greatly from place to place

Better protection against predators

Some predator species increase chances of securing a meal—wolf pack

Temporary clumps for mating and care for young

Population Measurement

Count directly

Mark the presence of individuals in a representative area and extrapolate to a larger area

Estimated based on sign of presence such as droppings, birdcalls, etc.

Change in Population Size

Increases are exponential, not arithmetic

Increase is proportional to number of individuals already present

J-shaped growth

Exponential growth: slow at first, followed by increasingly faster rates of growth, also called

Geometric growth: growth follows a geometric pattern of increase (2,4,8,16)

Rule of 70: divide 70 by annual percentage yields approximate doubling time in years

S-shaped Growth

Growth model starts off like J-shaped form, but rate of growth slows and ceases altogether

Stabilizes at certain level, denoted as K

Environmental resistance limits growth in S-shaped growth

Logistic growth

Biotic Potential

Capacity for growth

“r”=intrinsic rate of increase if the population has unlimited resources

Most populations grow at rate less than this

Environmental Resistance

All factors that act to limit growth

Abiotic: light, temperature, critical nutrients

Biotic: specialized niche, too many competitors, drought, low repro. rate, inability to fight disease or migrate or adapt

Carrying Capacity

Biotic potential and environmental resistance determine carrying capacity “K”

Maximum # of individuals of given species that can be sustained indefinitely in a given space (area or volume)

Growth rate decreases as size nears K, resources begin to dwindle

Real World

Population growth varies in complex ways

May show J- or S-shaped growth at various times

Growth rates may hover around K


Carrying capacity not fixed

Stable in stable environments, population can hover above K or below K depending on environmental variables


Rate of population decrease is faster than increase=population crash or dieback

Population explosion followed by dieback is called irruptive of Malthusian growth

Growth Rate

Difference between birth and death rate over a period of time

birthrate=# of individuals born in a given time expressed as a proportion of the total population=r

Growth Rate

Death rate=# of individuals who die in a given time

Growth rate (r)=birth rate-death rate

r<0: population shrinks

r>0: population grows

r=0: population at zero population growth

Reproductive Strategies

r-selected or Malthusian:

Opportunistic species in highly variable environments

High growth rates—take advantage of infrequent favorable conditions

Reproduction is rapid, with little care of offspring


High growth rates—take advantage of infrequent favorable conditions

Pressures are density-independent: physical (abiotic) forces and predation limit growth

Smaller size


Reproduction is rapid, with little care of offspring, many offspring

Sheer numbers and effective dispersal

Reproductive Strategies

K-selected or Logistic: relatively stable environment, numbers fluctuate within narrow range around K

Population size limited by carrying capacity

Reproduction slower, considerable care of offspring


Larger size

Pressures are density-dependent: as population increases, food and living space limit growth

Live longer, mature slowly, fewer offspring/generation


r-selected: rodents, many insects, marine invertebrates, weedy annual plants

K-selected: whales, wolves, elephants, primates

What about humans?

+/- Population Size

Emigration—movement of individuals out of population


Take genes with them—protect species in case of catastrophe

Factors that Regulate Growth

Intrinsic—within organism

Extrinsic—outside the population






Abiotic-weather, climate

Extremes at wrong time of year

Without regard to the number of organisms present


Decrease natality, increase mortality

Interactions between populations

Some within population

Interspecific—predator-prey population levels, peaks and valleys slightly offset



Intraspecific—compete for resources

As carrying capacity is reached, resources are limited

First, quickest, clever, lucky


Fighting equipment

Attractiveness-song and dance


Stress—”stress shock”causes physical and psychological and behavioral changes

Combinations—OK food and water, space is lacking, lab animals, humans, chickens

Humane livestock practices—not crowding chickens, livestock, “slow food”movement

Population Size Maintenance

Conservation biology: maintain populations for long term viability

Maintain biodiversity:

Number of species and

Genetic variation