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
Why
Populations
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
Clumps
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
K
Carrying capacity not fixed
Stable in stable environments, population can hover above K or below K depending on environmental variables
Patterns
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
r-selected
High growth rates—take advantage of infrequent favorable conditions
Pressures are density-independent: physical (abiotic) forces and predation limit growth
Smaller size
r-selected
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
K-selected
Larger size
Pressures are density-dependent: as population increases, food and living space limit growth
Live longer, mature slowly, fewer offspring/generation
Examples
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
Dispersal
Take genes with them—protect species in case of catastrophe
Factors that Regulate Growth
Intrinsic—within organism
Extrinsic—outside the population
Biotic
Abiotic
Density-dependent
Density-independent
Density-independent
Abiotic-weather, climate
Extremes at wrong time of year
Without regard to the number of organisms present
Density-dependent
Decrease natality, increase mortality
Interactions between populations
Some within population
Interspecific—predator-prey population levels, peaks and valleys slightly offset
Cyclical
Density-dependent
Intraspecific—compete for resources
As carrying capacity is reached, resources are limited
First, quickest, clever, lucky
Territoriality
Fighting equipment
Attractiveness-song and dance
Density-dependent
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: